Modules
B. Communicable Diseases
1. communicable transmission
disease: of an agent (or its by-product)
from reservoir
to host.
In order to fully understand this definition we must define the four terms within the above definition (i.e., transmission, agent, reservoir, and host)
2. agent: a factor that must be present for a
disease to occur in a susceptible host.
Notice that this definition is fairly broad -- the agent could be biological, chemical, or physical. This could include microbes, parasitic worms, chemicals, radiation, and so on. The one requirement is that it must be present.
3. infection: growth of a pathogenic microbe in a host
(with or without evidence of disease)
(also called "sepsis").
An inapparent infection is when growth occurs but there are no symptoms. When signs or symptoms occur, we call this a case.
4. pathogenicity: capable of producing disease.
Pathogenicity is normally measured as: (# of cases) / (# infected) This reflects the fact that not all individuals who are infected with an agent may actually become a case.
5. virulence: harmfulness of a disease.
Virulence is typically measured by the case fatality rate, defined as: (# of deaths) / (# of cases)
6. reservoir: any place where an infectious agent
depends primarily for survival.
Examples of reservoirs might include the soil, ocean, or certain animals, However, it can also include humans. For example, some agents only exist within humans.
7. host: any animal infected by an agent;
may be diseased or
may be intermediate host.
Typically, we are most concerned with human hosts. However, some diseases are quite tricky in that they may invade a number of different hosts. Their control depends on a good understanding of these hosts.
8. incubation time interval between exposure
period: to infectious agent and first
sign or symptom of disease.
A good example of this is foodborne illness. The tmie of exposure is when you eat contaminated food. The sign or symptom of diseaese typically includes vomiting or diarrhea.
9. carrier: person or animal that harbors an organism
of disease without showing symptoms.
Carriers are very importnat to transmitting disease because they may not be aware that they are transmitting.
10. asymptomatic
carrier: never shows symptoms
(also called "inapparent infection").
Actually, there are three types of carriers. The convalescing carrier is someone who had the illness and appears to be recovered, but in fact may still be transmitting. People who return to work too early after having the flu may fall into this category. The second type is the incubatory carrier, which is the indiviudal who is still within the incubation period of the disease. That is, they appear fine but will soon start exhibiting symptoms of disease. This individual has no way of knowing that they are tranmitting until it is too late. The third type is the most dangerous, because they never show symptoms. They are never aware that they are transmitting unless there is some specific lab test to identify the problem.
11. transmission: any mechanism by which a susceptible human
host is exposed to an infectious agent.
Examples of transmission include: airborne, waterborne, food, and so on.
12. fomites: inanimate objects (other than food, water)
which harbor or transmit infectious organisms.
Examples include: dishes, utensils, blankets, towels, toys (since children are susceptible to so many illnesses), and so on.
13. vector: insect or other animal that may transfer
pathogens to humans.
14. infestation:
humans, lodgment, development, and reproduction
animals: of arthropods on the surface of the
body or in clothing.
articles, harboring or sheltering animals
premises: (especially arthropods or rodents).
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Objective: The fundamental purpose of environmental health is to prevent disease from occurring. The purpose of this section is to discuss the different types of prevention and to start applying them in the analysis of the various disease in this section.
From the standpoint of our model of communicable disease, we can divide controls into 3 basic categories. This approach may actually be more helpful in divising strategies for controlling the spread of communicable disease.
1. Control the agent:
For example, we can remove agents before their entry into air, water, and soil. If it is a chemical agent, this may involve simple changes in production processes. If it is a microbial agent, it may involve prohibiting consumption of affected foods, or use of bactericides on preparation surfaces. Other examples are listed on pp. 15-16 of the textbook.
2. Control the environment:
For example, we can control control vectors, or treat polluted air, water, and soil. We can also prevent access to an area. Other examples are listed on pp. 16-17 of the textbook.
3. Control the host:
For example, we can take steps to protect the young, the old, and the sick, each of whom may be the high risk individuals. This may involve personal hygiene, immunizations, or health education.
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1. Common cold
agent: rhinoviruses, coronaviruses, others
reservoir: human
transmission: direct contact
2. Influenza:
agent: influenza viruses (3 major serotypes):
type A: widespread, pandemics
type B: local epidemics
type C: only in sporadic cases
reservoir: human
transmission: same as cold
3. Tuberculosis:
agent: Mycobacterium
reservoir: primarily human
transmission: primarily droplet nuclei
4. Coccidioidomycosis:
agent: Coccidioides immitis (fungus)
reservoir: soil
transmission: airborne
5. Pneumonia: inflammation of the lungs with congestion
(not a specific disease, but a pathological
term -- i.e., symptoms)
reservoir: essentially human
transmission generally direct, sometimes airborne
various agents:
5. pneumococcal
pneumonia (Streptococcus pneumoniae)
6. mycoplasmal
pneumonia (Mycoplasma pneumoniae)
7. pneumocystis
pneumonia (Pneumocystis carinii)
8. chlamydial
pneumonia (Chlamydia trachomatis)
9. others: various viruses,
rikettsias (Q fever),
Legionella,
worms (nematodes),
etc.
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4. Selected Water-borne Diseases
1. typhoid fever
agent: Salmonella typhi
reservoir: human
transmission: indirect, vectorborne (flies)
symptoms: can be asymptomatic
fever, gastroenteritis, nosebleeds
rose spots (red patches on abdomen)
2. paratyphoid
fever
agent: 3 main groups of Salmonella paratyphi
reservoir: primarily human
transmission: same as typhoid fever
symptoms: similar to typhoid, but much less virulent
3. Cholera
agent: Vibrio cholera
reservoir: human; perhaps environmental reservoirs
transmission: fecal (or vomitus)
[direct, indirect (food, water, flies)]
symptoms: sudden and severe diarrhea
vomiting, dehydration, death
4. Shigellosis (bacillary dysentary)
(dysentary) = acute colitis with diarrhea
agent: 4 groups of genus Shigella (bacteria)
reservoir: humans; primates
transmission: fecal [direct, indirect (food, water, flies)]
symptoms: diarrhea, fever, nausea
stools contain blood, mucus, and pus
5. Amebiasis (amebic dysentary)
agent: Entamoeba histolytica (a protozoon)
reservoir: human; usually a chronically ill
or asymptomatic carrier
transmission: fecal [direct, indirect (food, water, flies)]
acute patients pose limited danger
(absence of cysts)
symptoms: often asymptomatic; fever, chills, diarrhea
stools contain blood, mucus
6. Giardiasis
agent: Giardia lamblia (protozoan)
reservoir: human; possibly other animals
transmission: fecal (water or food) (cysts resist treatment)
symptoms: often asymptomatic; diarrhea, cramps, fatigue
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5. Selected Helminthic Diseases
1. Trichinosis
agent: Trichanella spiralis
reservoir: pigs, wild boar, bears, foxes, wolves, rats
transmission: eating infected animals
symptoms: nausea, gastroenteritis
(must thoroughly cook pork >150 deg. F)
2. Ascariasis (roundworms)
agent: primarily Ascaris lumbricoides
reservoir: humans, infected soils
transmission: not person to person
ingestion of infective eggs (soil)
symptoms: live worms in stool
varied: coughing, fever, nutrition
3. Schistosomiasis (blood flukes)
agent: Schistosoma (4 species)
reservoir: humans (and various animals)
transmission: vectors (biological)
larvae from snail infested waters
symptoms: varied
4. Other helminthic diseases
a. beef tapeworm Taenia saginata
b. pork tapeworm Taenia solium
c. fish tapeworm Diphyllobothrium latum
d. dracunculiasis
(dracontiasis) Dracunculus medinensis
e. ancylostomiasis
(hookworm) Ancylostoma
f. enterobiasis
(pinworm) Enterobias
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6. Selected Miscellaneous Diseases
1. Ringworm
agent: Microsporum, Trichophyton, others (fungi)
reservoir: infected human or animal
transmission: physical contact
symptoms: dermatitis (athlete's foot,jock itch, etc.)
control: cleaning floors, showers, clothing
2. Tetanus
agent: Clostridium tetani
reservoir: soil, street dust, animal feces
transmission: entrance in a wound
symptoms: muscular contractions, spasms
case fatality can be high
control: immunization, cleansing wounds
3. Anthrax
agent: Bacillus anthracis
reservoir: cattle, sheep, goats, horses, swine
transmission: contact with hides or meat, flies
symptoms: itchy skin; complications if untreated
control: vaccine, sanitation, dust control
4. Leprosy
agent: Mycobacterium leprae
reservoir: humans
transmission: not clearly established
prolonged contact is important
symptoms: disease of skin, nerves, nasal musosa
symptoms vary
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Page updated: 1/20/02