I Host-Microbe Relationships
1. Symbiosis and Parasitism
2. Assorted Disease Definitions
3. Normal (Indigenous) Flora
II The Disease Process
1. How Microbes Cause Disease
A) Entry and Exit of Pathogens
B) How Bacteria Cause Disease
C) How Viruses Cause Disease
D) How Other Pathogens Cause Disease
2. Signs, Symptoms and Syndromes
3. Types of Infectious Disease
4. Steps in the Course of an Infectious Disease
III Epidemiology
A. Surveillance
B. Statistics
C. Reservoirs
D. Infectious versus Noninfectious Diseases
1. Classification of Diseases:
2. Communicable and Non-Communicable Diseases
E. Modes of Transmission of Diseases
1) Contact between people
2) Vehicles such as water, air, food.
3) Vectors
4) Special Problems in Disease Transmission
5) Disease Cycles
F. Control of Communicable Diseases
G. Nosocomial Infections
1. How Nosocomial Infections Spread
2. Prevention and Control of Nosocomial Infections
H. Koch's Postulates
*****************************************************
I Host-Microbe Relationships
1. Symbiosis and Parasitism
Symbiosis is when two organisms live together. Usually one organism is larger and is called the host. Several alternatives exist depending on who benefits and who is harmed.
Mutualism is when both organisms benefit.
Commensalism is when one benefits and the other is neither helped nor harmed.
Parasitism is when one benefits and the other is harmed. Well adapted parasites do not harm the host too much, at least not until they can reproduce and their descendents can leave and infect another host.
The dividing lines between mutualism, commensalism and parasitism are often blurred. For example, most microbes living in your intestines or on your skin do little or no harm. By themselves they might be viewed as commensals. However, by occupying space in or on your body, they may prevent dangerous microorganisms from taking up residence. So they do benefit you, indirectly. On the other hand, if you are weakened, some of your normally harmless microbial inhabitants may take advantage to invade your tissues and do real damage.
2. Assorted Disease Definitions
The following terms tend to vary a little in meaning from person to person. Some of these are hard to define precisely.
Sterile = No microorganisms are present.
Contamination = Microorganisms are present. [Some people only consider it "contamination" if the microorganism is "undesirable"]
Infection = When a microorganism is multiplying on or in the host. [Some people only think it is "infection" if the microorganism is "undesirable"]
Disease = Disturbance in health in which the body cannot carry out all its normal functions.
Pathogenicity = Capacity to produce disease.
Virulence = The intensity of the disease produced.
Part of the problem is that our body surfaces and intestines are normally inhabited by harmless microorganisms. Are they "contaminated" or "infected" all the time or only if a dangerous outsider enters?
It is possible to be infected by "undesirable" organisms but not have a disease. In the carrier state a person is infected, usually at a low level, by a potentially harmful organism, but no symptoms occur. For example typhoid (Salmonella typhi) causes severe illness in most people. However, in a few people, the bacteria take up residence in the gall bladder and cause no disease. Carriers are a hazard to others as they can still transmit the infection.
Closely related microorganisms often vary greatly in pathogenicity. For example, some strains of E. coli are pathogenic, some are not. Pathogenic strains usually have extra blocks of genes (pathogenicity islands) that are responsible for the ability to cause disease. Sometimes these extra genes are in the chromosome, sometimes they are carried on plasmids. Sometimes they are carried by bacterial viruses, which in turn may insert themselves into bacterial chromosomes. The virulence also tends to vary greatly from strain to strain of each pathogenic organism.
Some people claim that an organism that has lost the capacity to cause disease should be called "avirulent" whereas an organism that does not cause disease is "nonpathogenic". This assumes you know the history of each microorganism.
Attenuation = When microorganisms used to make "live" vaccines are treated so that their ability to cause disease is decreased or lost. Most live vaccines are viral, not bacterial. Sometimes the viruses are grown in alternate hosts. As the virus adapts to its new host, it often loses virulence against humans. Sometimes viruses are just grown in cell culture for a long time until a less virulent mutant is found. Examples: Sabin polio vaccine, measles, rubella, mumps, chicken pox.
3. Normal (Indigenous) Flora
There are about 1013 human cells in an adult body and about 10 times that many bacterial and fungal cells. These are found on skin, GI tract, respiratory tract, urogenital tract - mostly on mucous membranes (see Fig. 14.2). Mucous membranes are usually moist. In the mouth and intestines there is a good food supply. Other mucous membranes have a less plentiful food supply but remember that microorganisms are small! The skin is not so moist so the organisms that live there must be able to get by with less water. But secretions on the skin supply nutrients.
Not all parts of these systems are inhabited. For example the kidneys are uninhabited. The stomach does not have a normal flora; it may have some microorganisms there briefly after eating but they do not survive the acidic environment. We used to think nothing could live in the stomach but now it is clear that Helicobacter pylori can live there and can cause peptic ulcers.
Some GI tract pathogens survive the acidity of the stomach and the bile in the intestines - especially Salmonella, Shigella, Vibrio and some strains of E. coli, also polio, hepatitis A, echovirus, rotavirus.
Normal flora are divided into those that are routinely present = resident flora and those that come and go = transient flora. See Table 14.1 for list of major normal flora.
The skin is the site of a lot of transient flora, depending on what you just picked up and how long since you washed your hands and what you washed your hands with - some bars of soap have a lot of bacteria on them. Many of these transient organisms never multiply - they just happen to get picked up and are present briefly.
However, some microorganisms that are normal flora may occasionally cause disease if host defenses are down. Some situations where this may happen:
For example, Streptococcus pneumoniae is around all the time but normal human defenses prevent infection. In the army after long and difficult forced marches (i.e. stress/exhaustion) it is common for some soldiers to come down with pneumonia. Elderly persons who are already sick are liable to get pneumonia.
A normally harmless organism that is capable of causing disease when immunity is impaired is known as an opportunist.
Other possibilities for opportunistic infections:
When a harmless organism gets into a site that it would normally never have access to. For example in a burn victim tissue that is usually covered by skin is now exposed.
When the normal bacterial flora is suppressed or out of balance due to antibiotic treatment fungi, especially Candida, can grow up and take over due to lack of competition for food, water and oxygen.
II The Disease Process
1. How Microbes Cause Disease
Questions that apply to infectious agents that cause disease:
How does it get in
How does it get out
How does it survive and multiply inside the host (i.e. colonize)
How does it cause harm to the host
A) Entry and Exit of Pathogens
How do pathogens get in?
a) Some are already inside &emdash; the natural flora that may cause opportunistic infections.
b) Gastrointestinal tract &emdash; must survive stomach acid on way in.
c) Respiratory tract
d) Urogenital tract
e) Breaks in the skin (these only need to be microscopic).
For many pathogens the point of entry is very specific; however, some can enter more than one way. For example, M. tuberculosis is usually inhaled but can also enter the GI tract. The course of disease is different in the two cases. C diphtheriae can enter via throat or skin. F. tularensis can enter by virtually any way except the urogenital tract.
Similar principles apply to viruses. Mostly they enter through the mucous membranes of the G.I. tract, respiratory tract or urogenital tract. Some may enter directly into the blood. Yellow fever and West Nile viruses are injected into the bloodstream by the mosquito. HIV or hepatitis can enter via transfusions with infected blood or on infected needles.
How pathogens get out:
Sneezing, coughing, talking, breathing, laughing, for TB, flu, measles, chickenpox.
Airborne particles, saliva, mucous, sputum, for mumps, herpes simplex, rabies, pneumonia, whooping cough, scarlet fever, meningococcal meningitis, smallpox.
Shedding skin cells for warts, fungal infections, boils, herpes simplex, smallpox, syphilis.
Feces for Salmonella, Shigella, Vibrio, E. coli, polio
Urine [leptospirosis, S. typhosa, TB, schistosomiasis, hantavirus]
Genital tract - vaginal discharge or semen for syphilis, gonorrhea, chlamydia, herpes simplex, HIV, cytomegalovirus.
Bleeding for HIV, hepatitis, Ebola virus.
Insect bites that transmit blood for Rocky Mountain Spotted Fever, tularemia, malaria.
B) How Bacteria Cause Disease
How Do Bacteria Colonize?
(1)Attachment is often the first step. Some bacteria make specific proteins called adhesins which act to stick the bacteria to the surface of animal cells. Sometimes these are located on the pili or flagella. For example N. gonorrhoeae without pili is non-virulent because it cannot attach.
(2) Colonization then begins if the organism finds a favorable environment and it multiplies. The organism has to find enough food and moisture, an agreeable pH, the right oxygen concentration, and also evade the host defenses. For example Brucella abortus infects cattle. It grows slowly in most tissues but very rapidly in the placenta because there is a high concentration of erythritol in the placenta. This is an especially good nutrient for B. abortus. So it successfully colonizes the placenta and causes abortions.
(3) Invasiveness. A few infectious bacteria remain on the surfaces of tissues. Most penetrate further into the tissues and some actually enter the host cells and live inside them. Most, organisms that are virulent are able to invade tissues. Some secrete enzymes to dissolve the substances that bind cells together.
Collagen is a main part of connective tissues; collagenase dissolves collagen and is made by Clostridium, for example.
Hyaluronic acid is a polysaccharide that holds animal cells together. Hyaluronidase degrades it and is made by staphylococci, clostridia, streptococci, and pneumococci.
Mucin is a protective coating on mucous membranes. Mucinase is made by Vibrio cholerae.
Streptokinase and staphylokinase dissolve fibrin clots and expedite the invasion of damaged tissues.
(4) Some bacteria can invade cells. Some are taken up by epithelial cells in a process much like phagocytosis. They may enter a cell and later leave, and move on to cells in the next layer. So they have moved through a layer of cells - another way to move around in tissues.
Capsules around bacteria protect them from digestion after phagocytosis; therefore an encapsulated strains are usually more invasive than non-encapsulated strains.
How Do Bacteria Cause Harm
(1) By Invading Cells When bacteria invade cells, they usually damage them. Some simply move into cells and stay there as intracellular parasites consuming the cells materials. Others actively degrade cell components.
(2) Immune Overreaction Sometimes the damage is due to an over-energetic response of our immune system to the pathogen &emdash; immune hypersensitivity. We will return to this after studying the immune system.
(3) Toxins Toxins = poisonous substances, often but not always proteins. A lot of damage is done by toxins. Note that toxins can cause damage a long distance away from the bacteria that make them. Toxins are usually classified into two groups: endotoxins and exotoxins.
Endotoxins
Endotoxins are made of lipopolysaccharide and are produced by gram negative bacteria. They are originally part of the outer membrane around the bacterial wall and are released in small amounts when the bacteria divide and in larger amounts when they die and disintegrate. They are called endotoxins because they are not secreted but are part of the cell itself. Produced by E. coli, Proteus, Salmonella, Shigella, Neisseria, Hemophilus, Yersinia etc.
Relatively non-specific. Damage tissues, cause fever, and shock (low blood pressure, nausea, tachycardia, slow pulse, cold clammy skin). Amount needed to make you sick is relatively high. Most serious situation is if you have an infection where there are high numbers of gram negative bacteria in you blood. If many of the bacteria die at the same time (e.g. due to sudden antibiotic treatment), the endotoxic shock may be serious or even fatal.
Endotoxins are not protein and are not destroyed by heating. Also chemical treatment of endotoxins does not convert them to toxoids, so we don't get immunized against them.
Exotoxins
Exotoxins are proteins. They are made by most G+, and some G-. They are called exotoxins because they are toxins that are secreted - they exit the bacterial cell that made them. Exotoxins can be chemically altered so that they do not cause harm but are still immunogenic. These are called toxoids and can be used as to immunize against the toxin. Example - tetanus toxoid.
Vibrio cholerae (G-) secretes an enterotoxin, that is, a toxin that affects the intestines. It causes huge losses of fluids - water plus salts (sodium, potassium, bicarbonate and chloride) - and some mucous but no blood, Can lose up to 20 or 30 liters per day (a liter is about a quart). If untreated about 60% of affected individuals die. Treatment by replacement of fluids and electrolytes brings mortality down to about 1%.
Vibrio must cross the acidic stomach to reach the intestines where it colonizes. It remains on the surface of the epithelial cells & does not invade. Virulence factors = motility & adhesion as well as the cholera toxin. Host defenses take over within a few days and eliminate the organism. Cholera is self-limiting if you can survive the effects of the exotoxin. Tetracycline will speed up elimination.
Clostridium botulinum (G+) produces a neurotoxin known as botulinum toxin. It is the most powerful biological toxin known; a lethal dose for a human is a bit over one microgram. It is now being used in cosmetic treatments for wrinkles &emdash; as "botox".
Clostridium botulinum grows in food where it produces toxins. Then we eat the food with the toxins in it - botulism. Because exotoxins are protein they can be inactivated by heat. That is why you can protect yourself by boiling home canned foods for 10 minutes. Most cases of botulism in the US are from home canned vegetables. Occasionally you actually get Clostridium botulinum infections, mostly in infants.
Some lysogenic strains of Streptococcus pyogenes (G+) carry a gene for a toxin that results in a scarlet rash (erythrogenic toxin). A rash plus streptococcal sore throat is called scarlet fever. Immunization against scarlet fever is against the toxin that causes the rash (and so does not prevent infection by the strep).
Some exotoxins act as enzymes:
Hemolysins kill red blood cells and white cells also.
Leukocidins kill neutrophils. They could also be listed under invasiveness because they decrease the host defenses and so increase the chances for the invading organism to survive.
Coagulases (staph) cause blood (or plasma = the fluid portion without cells) to clot. This helps protect the bacteria from phagocytes.
C) How Viruses Cause Disease
How Do Viruses Colonize?
Some viruses spread, some don't. Viruses like influenza or the viruses causing colds don't have to spread. They are usually taken in to the respiratory tract and infect tissues there. Other viruses start out in one paticular tissue and replicate there. Then they may spread and cause most damage somewhere else. Poliovirus enters the GI tract but eventually damages the nervous system. Some viruses spread through the circulatory system - blood or lymph. Rabies and herpes simplex spread through the nervous system from neuron to neuron.
Viruses generally recognize specific receptors on the surface of the cell. This limits a given virus to a group of related host animals, sometimes only one (e.g. smallpox does not infect anything but people). In addition, most viruses can only invade the cells of one or a few kinds of tissue.
How Do Viruses Cause Harm?
Viruses don't make toxins. They do damage cells. Some cause lysis others cause cell fusion or the production of inclusion bodies (clumps of nucleic acids and proteins not properly assembled). Others prevent cells from reproducing. Our immune system kills virus infected cells and sometimes causes other damage during the response to viral infection.
Symptoms depend on the tissues damaged and the responses the body makes to the virus, including fever and swollen glands. A lot of viruses infect but don't cause clinical disease.
Variations on viral infections include cancer, latent infections (viruses are present but not produced most of the time; occasionally they become active) and slow viruses.
D) How Other Pathogens Cause Disease
Fungi produce enzymes and/or toxins that damage cells. Most fungi enter as spores and secrete digestive enzymes that break down cells. Immune system responses, including allergies are often important.
Probably the most glamorous toxin is ergot, which is produced by a fungus Claviceps purpurea that grows on rye when conditions are right. It contains lysergic acid, also in LSD, causes an intoxication, including delusions, convulsions, a burning sensation, gangrene and was the cause of death of millions of people in the Middle Ages. It has been suggested that some of the curious events surrounding the witchcraft trials in Salem may have been the result of moist weather favoring the growth of the fungus and ingestion of ergot.
The other notorious fungal toxin is aflatoxin, which is produced by Aspergillus flavus growing on corn, grains and peanuts. Peanuts, beer, grains, nuts, vegetable oils, animal feed and milk are monitored for this toxin because it is carcinogenic and toxic to the liver. In droughts infection of the fungus increases, presumably because when plants are dry they tend to crack open and the fungus, which is in the soil, then gains entry.
2. Signs, Symptoms and Syndromes
Sign: something like redness, swelling, fever, number of white cells - can be observed by someone else by examination of the patient.
Symptom: how I feel if I'm the patient. I hurt, I feel dizzy, I hear a ringing in my ears, I'm tired.
Sometimes the causative agent can be deduced from the signs and symptoms. But often two or more diseases may look rather similar. If the physician needs to know, the organism will have to be isolated and identified. Much more often done with bacteria than with viruses. For one thing it is much easier, and also there is much more reason to do it, since a narrow spectrum antibiotic may be indicated.
Syndrome: literally "running together" - a group of signs and symptoms that occur together. E.g. Down Syndrome, where a set of characteristics occur together when an individual has three copies of chromosome 21.
Many infectious diseases share characteristics that usually occur together due to operation of the immune system. Fever, feeling bad (malaise), increase in the white count, swelling of lymph nodes. not all They may not all always occur together but if several do, you've probably got some kind of infection.
3. Types of Infectious Disease
Acute: happens fast, like measles, colds, smallpox
Chronic: usually develops more slowly and lasts a long time. E.g. tuberculosis or leprosy
Subacute: in between acute and chronic
Latent: disease may show periods of inactivity and there is no evidence of infection for a while. E.g. herpes
Local infection: in one place; boils, lung infections
Focal infection: the organisms establish an infection in one place but the organisms or maybe just the toxins spread to other sites.
Systemic infection: the causative agent is spread everywhere, probably by the circulatory system. [Can start as a local infection, become a focal infection and end up as a systemic infection]
Septicemia: organisms are multiplying in the blood
Bacteremia or viremia: bacteria or viruses are present in the blood but not multiplying. There is usually a transient bacteremia after dental procedures.
Primary infection: the first infection, e.g. a viral respiratory tract infection like a common cold.
Secondary infection: an infection that follows, presumably because the first infection set up conditions that favor it, e.g. a bacterial sinus infection following the cold.
Superinfection: infection which occurs because the normal flora was wiped out or decreased or put out of balance, as after broad spectrum antibiotic treatment.
Mixed infection: two or more organisms
Inapparent or subclinical infection: organisms are present but not causing noticeable symptoms.
4. Steps in the Course of an Infectious Disease
1. Incubation Period. You've come in contact with the organism, it has entered and is beginning to multiply, but you don't know you've got it yet. You can be contagious during this time. This is one of the problems day care centers can't really do anything about - a child has something but isn't yet showing symptoms, but can spread it to others.
2. Prodromal Phase. You feel bad and extra tired and you wonder if you're getting sick.
3. Invasive Phase. You show characteristic set of symptoms, maybe your nose runs and you sneeze and your head aches and your temperature is up and you feel even worse. You now realize you have a cold. The organisms are multiplying, spreading, damaging cells.
4. Acme. Your symptoms reach a maximum.
5. Decline Phase. As the invader begins to lose the battle, the symptoms begin to fade and you start to get better. Sometimes this can happen very quickly--one morning you wake up and feel a lot better. But there is still evidence of the pathogen.
6. Convalescence. If it was a serious illness you may still tire easily & need extra rest.
III Epidemiology
Epidemiology = the study of how diseases are transmitted through populations.
Sporadic disease = a case here, a case there.
Endemic = disease is always present in a population or particular geographical area. (Sometimes the term is applied to just the disease itself, sometimes to the disease causing organism).
Epidemic = sudden increase of a disease beyond normal levels.
Pandemic = an epidemic that spreads across several continents and may become worldwide.
Rabies is endemic in bats around here but not in people. Rabies among people is sporadic in the U.S.
SARS (severe acute respiratory syndrome) is an emerging epidemic. Influenza epidemics are relatively common &emdash; a new variant of the virus emerges and makes the rounds.
Plague is endemic among wild rodents in 15 western states (gophers, chipmunks, packrats, prairie dogs, ground squirrels). If it becomes endemic in city rats, it isn't clear what will happen. The organism that causes plague is Yersinia pestis and it usually spreads between animals by flea bites.
The first recorded major plague epidemic was in 542 in the Mediterranean region and killed millions. Then in 1347 plague emerged from the Asian steppes and moved into Italy, went on to France, England, northern Europe. There were cycles of plague over the next 300 years. Probably half the European population died as well as vast numbers in Asia and Africa. This was a pandemic.
When the number of children being immunized against a communicable disease like measles or whooping cough, decreases, about there is a possibility of a new epidemic.
A. Surveillance
Headquarters for a lot of public health activities is the CDC = Centers for Disease Control, in Atlanta. They maintain records of certain diseases, called notifiable diseases, and publish Morbidity and Mortality Weekly Report.
URL = http://www.cdc.gov/
Internationally, the WHO = Word Health Organization
URL = http://www.who.int/en/
Illinois Department of Public Health, Headquarters in Springfield, local lab on Chautauqua St. They mostly take samples from public health agencies, not just individuals. They do not do animal testing, other than for rabies.
US Public Health Service which includes the Centers for Disease Control (CDC) in Atlanta Used to be called the Communicable Disease Center but now does more than communicable diseases. Deals with problems that are national. Follows infectious diseases (statistics for about 36 notifiable diseases are published in Morbidity and Mortality Report) and makes predictions when possible. Follows spread of antibiotic resistance. Makes guidelines for immunizations.
World Health Organization (WHO) deals with health issues on a world-wide basis. Since travel has become relatively easy and international travel frequent, infectious diseases spread faster than they used to. Cooperation between WHO and other national organizations were crucial in eradicating smallpox.
Notifiable diseases = those that physicians are required to report. Some are reportable case by case and some reportable only if epidemic. Examples for USA in 2000 Gonorrhea - over 300,000 cases, AIDS - 36,000, tuberculosis 13,000, Lyme disease 13,000. Some vary from State to State in USA.
Notifiable diseases in the US include: AIDS, amebiasis, anthrax, arboviral infections, botulism, brucellosis, chancroid, chickenpox, chlamydia (genital) cholera, diphtheria, encephalitis, enterovirus, gonorrhea, hepatitis A & B & other, influenza, legionellosis, leprosy, leptospirosis, Lyme disease, lymphogranuloma venereum, malaria, measles, meningococcal infections, mumps, pertussis, plague, poliomyelitis, psittacosis, Q fever, rabies, rheumatic fever, rocky mountain spotted fever, rubella salmonellosis, shigellosis, syphilis, tetanus, trichinosis, tuberculosis, tularemia, typhoid fever, typhus, yellow fever.
Should health care workers be tested for AIDS? For hepatitis B? What else? How would you feel sitting in a dentist's chair and having a dentist or dental hygienist working on you who has AIDS? On the other hand, how would you feel about working with a patient who has AIDS? Should surgical patients be tested before surgery so extraordinary precautions be taken? How do you feel about the possibility that the previous patient in the operating room had AIDS? Who should be tested and why? How much protection is reasonable for people who don't have AIDS. If testing is voluntary, what do you do about someone who refuses to be tested?
B. Statistics
Incidence = number of new cases per given period of time (month, year, etc.) in a given population.
Prevalence = number of people who have the disease.
The incidence for this month is only the new cases. But prevalence includes everybody who came down with the disease before this month and still has it. For a long-lasting disease (like tuberculosis or AIDS) the prevalence can be much higher than the incidence .
Morbidity = number of people who have the disease per number of people in the population. Usually reported per 100,000 people.
Mortality = number of people who die of a disease per number of people in the population. Usually reported per 100,000 people.
C. Reservoirs
Smallpox could be eliminated by the WHO because there is no reservoir for the virus except people. Once enough people were immunized the spread of the virus was stopped. This is the only disease that has been eliminated deliberately, so far.
Why haven't we wiped out more diseases? A major problem is finding all sources of the infectious agent and eliminating them.
1) Human reservoirs = People that have the disease, whether they know it or not, including people who are carriers.
Typhoid Mary - Mary Mallon came from Ireland to New York and got work as a cook in early 1900's. 7 or 8 families she worked for over a 7 year period got typhoid and several people died. A physician tried to track down the source and checked water, food, environment and then tried to find the cook. Finally the police found her and she checked out positive for S. typhi. But she never believed she was the source of the illnesses. When she was allowed to go, she went back to cooking, in spite of her promise not to, and more people got sick and died. Finally she was caught again and spent the rest of her life as "a ward of the hospital."
2) Animal Reservoirs = animals that carry the disease, whether sick or symptomless.
Livestock, cats and dogs, squirrels and other rodents, bats etc can be reservoirs for various infectious agents.
Bacteria: Brucella, Bacillus anthracis, some Salmonella (not typhoid) Pasteurella, Yersinia enterocolitica (causes enteric disease), and Yersinia pestis (in wild rodents).
Viruses: rabies, equine encephalitis.
Fungal diseases and parasitic worms can also be carried by domestic animals.
3) Nonliving Reservoirs = contaminated soil, water, food.
When infected animals - both rodents and fleas - die, Yersinia pestis is deposited in the soil and can survive for months and infect new hosts. Anthrax spores survive for an extremely long time in soil.
D. Infectious versus Noninfectious Diseases
This is a microbiology course so will only cover disease due to microorganisms, together with the responses from the human immune system.
1. Classification of Diseases:
Infectious disease = a disease caused by an infectious agent such as a virus, bacterium, fungus, protozoan or worm.
Noninfectious diseases = all other diseases.
Inherited disease = genetic defects. Note that certain genetic defects alter your susceptibility to infection by microorganisms. Sickle cell anaemia protects against malaria and cystic fibrosis confers resistance to Salmonella and Shigella.
Neoplastic diseases = abnormal cell growth due to genetic damage within the body. Differs from inherited disease in not being passed on to your descendents. Includes both cancers and harmless neoplasms. A few viruses cause cancer.
Congenital disease = defect present at birth. Obviously, this includes inherited disease. However, it also includes other damage to the fetus before birth due to drugs, X-rays or infection of the mother. Rubella (German measles) and syphilis both get across the placenta and can harm the developing fetus.
Immunological disease = due to malfunction of the immune system. Some immune defects make you more sensitive to infection. Conversely, some infections cripple the immune system, e.g. measles (briefly and partially) or AIDS (severe immune deficiency).
Nutritional deficiency = due to lack of essential nutrient. Often alters susceptibility to infection by microorganisms. For example, lack of iron makes you more sensitive to diphtheria but more resistant to plague. Diphtheria bacteria make more toxin when iron is low. Many other bacteria need iron to grow and so divide more slowly when iron is in short supply.
Iatrogenic disease = disease caused by medical procedures.
Nosocomial infection = infection acquired due to treatment in hospital. E.g. Staphylococcus aureus infecting wounds.
Mental disease = disease affecting brain and behavior. May be caused by infection or genetic defects, in addition to poisoning and psychological factors.
2. Communicable and Non-Communicable Diseases
Infectious diseases are divided into:
Communicable or Contagious = can be caught from another person or animal. Most common infections fall into this group.
Noncommunicable or Non-contagious = cannot be transmitted from person to person. Microrganisms from the environment may sometimes cause infections.
Tetanus &emdash; spores in the soil may infect a dirty wound. Clostridium tetani grows and makes tetanus toxin.
Legionaire’s disease (Legionella pneumophila) is carried by the water aerosols prodiced by some air conditioning units.
It is not always clear what is communicable and what is not. For example, malaria is transmitted by mosquitoes and cholera by waste-contaminated water. And what about anthrax? This may be transmitted from one animal to another and sometimes to humans. It may also be caught by breathing in spores that have lain in the soil for years. Generally these diseases are regarded as communicable.
A vector is an agent that carries disease indirectly from one person to another. Mosquitoes carry malaria, West Nile virus and yellow fever. Fleas carry plague. Dirty water can be viewed as a vector for cholera, dysentery, polio etc.
Food poisoning occurs in two ways:
a) When you eat food containing certain bacteria or viruses which then cause intestinal infections.
b) When bacteria grow in the food and make a toxin before you eat the food. There may be no bacteria still alive if you cook the food. But you will still fall sick if the toxin survives cooking.
Some people claim that (a) is a food-borne infection and only (b) is truly food poisoning. In practice we often do not know which happened.
E. Modes of Transmission of Diseases
1) Contact between people
Direct: person to person. Horizontal contact is from an infected person to another person. Vertical transmission is when infection crosses from mother to unborn child through the placenta or as the child passes through the birth canal.
Indirect: from person to object to person. Pencils, doorknobs, coins, banknotes.
Droplets: Not the tiny particles that float around in the air (over 1 meter) but droplets of mucous that contain microorganisms produced by sneezing or coughing.
2) Vehicles such as water, air, food.
a) The Water Supply.
The English physician, Dr. John Snow, first recognized the connection between cholera and water supply in the early 1800s. At that time water was supplied to people in London by water pumps. You took your container to the pump and carried water home. John Snow realized that people with cholera were clustered around a water pump in Broad Street.
Even in modern times in this country a lot of illness has resulted from contaminated wells. Outhouses are sometimes located too close to wells or animal waste enters the water supply. City water supplies are made safe by water treatment plants.
Pure water is not a good growth medium, but water before treatment isn't pure. It can contain enough nutrients for a lot of organisms to survive for significant periods and even to grow, depending on the organism and the water quality.
b) Airborne Transmission
Tiny particles of dust can float around in air for quite a long time. Relatively fragile organisms do not survive well in air. Spores from bacteria (e.g. Bacillus including anthrax) or fungi are often transported by air. Some viruses, especially non-enveloped viruses, and some tougher bacteria and fungal cells are transmitted through the air.
c) Foodborne Transmission
Most things we eat have bacteria in them. Many are killed by cooking but we still eat a lot. Most don't hurt us.
3) Vectors
Vectors are organisms that transmit infectious microorganisms. The best known vectors are insects, together with other arthropods such as ticks and mites.
Fleas are vectors of Yersinia pestis, ticks are vectors of Rickettsia rickettsii which causes Rocky Mountain Spotted Fever and Borrelia burgdorferi which causes Lyme disease, mosquitoes carry Yellow Fever virus, West Nile virus and malaria. In these cases the disease agents are transmitted by bites. However, insects can feed on animal feces or other infected materials and carry microorganisms to their next landing place. This is why it is a good idea to keep picnic food covered.
4) Special Problems in Disease Transmission
Obviously, novel diseases are more difficult to deal with as at first nobody knows either the agent or the means of transmission.
Why has AIDS been so hard to control compared to say cholera or plague? Two aspects. First the mode of transmission - through behavior people are reluctant to discuss. It may be difficult to get a list of sexual contacts or of those sharing infected needles and difficult to track these people down. Second because AIDS develops slowly and an individual can be infected and contagious for a long time before even recognizing they have the disease.
A curious case of transmission concerns the prion diseases that affect the brain. Prions are mis-folded proteins and contain no DNA or RNA. These diseases include Creutzfeld-Jacob disease, kuru, mad cow disease (bovine spongiform encephalopathy = BSE), wasting disease of elk and scrapie. Prion diseases are very unusual because they may also be inherited in certain families.
Kuru is a disease of cannibals in New Guinea that was spread by eating uncooked brains. It was very difficult to track down the transmission of this diseases because the incubation period is very long often 15 years or even longer after exposure. Cannibalism is no longer practiced and kuru is now a diminishing problem. In this case analyzing the custom of cannibalism within a group of people was crucial to the epidemiology. Gajdusek won the Nobel prize for these studies.
Creutzfeld-Jakob disease has been transmitted via corneal transplants and growth hormone derived from human tissue and through scalpel nicks to a surgeon performing an autopsy.
Scrapie affects sheep and has been known for centuries. Mad cow disease (bovine spongiform encephalopathy) emerged in England in 1986. It affects cows and a few people. Recycling of animal parts into feeds and butchering practices have been changed and some countries will not import beef or beef products from England. Chronic wasting disease is found affecting elks and deer in the Northern USA.
5) Disease Cycles
Diseases tend to come and go. Some cycles are predictable, like the increase in respiratory infections during the winter when buildings are more closed up and we tend to crowd together more. Because we understand some of the factors in how the disease spreads, we understand that kind of cycle. Other cycles are less predictable but there are several factors that help explain why diseases disappear and reappear.
Group (Herd) Immunity is very important. Before measles vaccines many children were susceptible to measles at school age. So when one child came down with measles there would be a little mini-epidemic of measles mostly in the five to seven year olds. After most of the susceptible kids had had measles the mini-epidemic would die down.
The first factor is the way measles virus is spread by the respiratory route. So crowding children together facilitates the spread. That accounts for the sudden increase. But what stops the epidemic? As most children get measles and recover they become immune. So the susceptible part of the population becomes smaller and smaller. Therefore the chances of a susceptible child coming into close contact with a contagious individual becomes less and less. The higher the percentage of immune individuals the harder it gets fro the disease to spread.
When group immunity goes down, the public health people worry about what could happen if the causative organism is re-introduced. Since the 1940's people have traveled more, especially by air, and this may re-introduce organisms into susceptible populations. SIU students are required to be immunized against tetanus, diphtheria, rubeola (measles), mumps, rubella (German measles).
F. Control of Communicable Diseases
1) Information. In the middle ages lepers had to carry bells and shout warnings to other people to keep their distance. Before the age of antibiotics people used to put signs on their houses when they had someone with a serious infectious disease there. Scarlet fever, typhoid, measles, chickenpox, diphtheria, whooping cough were treated this way. If you don't have antibiotics, it is very important not to get an infectious disease. In addition houses would often be quarantined.
2) Isolation. This involves more than just keeping people from circulating. The point is to keep people who show symptoms plus those who might be infected but are not yet sick from mingling with other people and spreading it by casual contact. Various levels of precautions are taken to isolate people with various infectious diseases. CDC guidelines exist for many notifiable diseases. Their strictness depends on the virulence of the disease and how easy it is to spread.
Quarantine = "separation of humans or animals from the general population when they have a communicable disease or have been exposed to one."
Traditionally, the four internationally quarantinable diseases were plague, yellow fever, cholera and smallpox.
Reverse isolation, in which the patient is the one at risk. Patients immunosuppressed by cancer therapy, surgery, genetic defects, burns, prematurity or AIDS are all at extra risk of infection.
Example: Respiratory precautions - "Private room with closed door is necessary, gown and gloves not required, masks usually indicated; items contaminated with secretions must be disinfected."
Gowns and gloves are required for all strict isolations. In these cases personnel and visitors must wash their hands when they go in and when they leave the room.
Formerly routines in hospitals, physician's offices and dental offices, etc. were disease specific. If a patient was known to have disease X all materials from or in contact with the patient were handled according to that disease. If something was labeled "infectious" it would get much greater care than usual. With the spread of AIDS that perspective has changed. Now all patients and blood products, saliva, etc., are handled as though they are infectious. The present guidelines, laid down by the CDC, are called universal precautions. These precautions are taken to protect everybody - the health care worker, the patient and the general public.
3) Immunization. Prevention is better than cure.
Immunizations do prevent a very large burden of disease in the population. So immunization avoids large numbers of children sick and dying because of preventable infectious diseases. In the advanced nations there is almost no polio, diphtheria, whooping cough any more. These were once all common childhood diseases.
Immunizations are not without risk. From time to time a child suffers significant problems or even dies from an immunization. It is not always clear why a child will have an adverse reaction. There have been cases where the vaccine was at fault and others where the child's immune system may have been deficient, and the child would surely have died from the disease.
4) Vector control. Very effective if done properly.
Vectors such as mosquitoes (yellow fever, malaria), fleas (bubonic plague and typhus) ticks, flies, cockroaches, rats and mice can be killed or kept out of contact with humans.
G. Nosocomial Infections
Nosocomial Infection = infection caught during health care procedures. Includes infections caught at a hospital or dentist’s office or other health care facility. Hospital infection rates will vary. This depends partly on the quality of the precautions taken to prevent transmission. It also depends on how many infected patients are in the hospital and how serious their infections are. sick the patients in a given hospital are. On average, about 5% of admitted patients get a nosocomial infection.
1. How Nosocomial Infections Spread
Before we understood how disease agents are transmitted, the risk of childbed fever in the hospital, for example, was very high. But why are hospital-acquired infections a problem now?
1) Sources of Infection and Susceptibility of Hosts
In addition to the risk factors that have always been present, we now do more surgery than we did a hundred years ago, and more sophisticated surgery. In other words, there are a lot more invasive procedures nowadays and these always carry a risk of allowing access by a bacterial or viral agent.
Nowadays there are far more people whose immune systems are not in good shape because of cancer therapy, immunosuppressive drugs after organ transplant, AIDS, and occasional children with genetic defects in their immune systems. We can now keep alive more people who are seriously ill. In past ages people used to die before they could pick up nosocomial infections.
2) Modes of Transmission
As you would expect: direct contact between individuals, through the air, through inanimate objects which have disease agents on them. Exposure to vector carried diseases is rare in hospitals.
3) Equipment and Procedures that Contribute to Infection
Respiratory therapy equipment, dental drills or probes, anything used to inject can all be the source of infection. Any equipment that allows fluid into the body, including IVs, kidney dialysis equipment, urinary catheters, etc. There may be problems with an IV bag, its connections or the site of injection.
Infection can occur because equipment was not properly sterilized or disinfected, it was not kept aseptic after sterilization but during storage, or it was used improperly.
3) Sites of Infection
Most common sites of entry are the urinary tract, surgical wounds, lungs, bloodstream. Organisms can be resident normal flora that get into the wrong place or organisms picked up from the environment.
Most common organisms are E. coli (urinary tract), Klebsiella spp. and Pseudomonas aeruginosa (respiratory tract), Staphylococci (surgical wounds) , Group D. Streptococci which include enterococci such as Strep faecalis (surgical wounds and lower respiratory tract).
Because antibiotic resistant strains are more common in hospitals than elsewhere, nosocomial infections are more likely to be resistant.
2. Prevention and Control of Nosocomial Infections
Hospitals now are very aware of the problem and routinely have someone in charge of monitoring and preventing nosocomial infections. A good understanding of asepsis and disease transmission is the best defense.
Nosocomial infections happen to patients and also to health care personnel. We've mostly focused on patients but remember it is important for people working in health care to be protected from infection. Needle sticks, handling infectious materials, direct contact with contagious patients can all transfer disease agents to health care personnel.
The Universal Precautions mentioned above were designed to decrease the possibility of transmitting HIV but are effective for preventing the transmission of other disease agents as well.
One important general practice is hand-washing. It is important for this to be thorough. While you will not rid your hands of all normal flora, you will remove a lot of the transients, which includes most of the potential pathogens.
Proper sterilization procedures are important. Remember, nothing is "partly" sterile. Proper handling of anything that could be contaminated.
Remembering how microorganisms can be transferred - through direct contact, through the air, from inanimate objects, especially through open places in the skin or mucous membranes.
H. Koch's Postulates
How do you know whether a microorganism is causing a disease? In the early part of the 20th century there was a major influenza epidemic, killing about 50 million people. A bacterium was isolated from many of those who fell ill. It was named Haemophilus influenzae because it was assumed that this was the organism causing influenza. Later it was found that this was a secondary infection, due to an opportunistic invader, and the real cause was a virus.
In the late 1800's Robert Koch proposed a set of steps that would demonstrate what was causing an illness:
It is not always possible to carry out all these steps, especially the third step. If the organism infects only humans, who is going to be the new host? Sometimes volunteers have been used, but for very serious diseases there is an ethical question to consider. Also some organisms, especially obligate intracellular parasites, will not grow in pure culture.
So for some organisms all four steps have never been carried out but there is really no question about the cause--researchers are satisfied. Some individuals have raised questions about the relationship between HIV and AIDS because step three has not been done.