Sources of infections

Sources of infections

Mainly four- Humans, Animals, Insects, Environment (soil, food and water, fomites)

1.     Human reservoir

The most important source or reservoir of infection for humans is the man himself. He may be a case or carrier

a. Case (Patient)

A case is defined as “a person having the particular disease, health disorder or condition under investigation”.

b. Carriers

A carrier is defined as a person (or animal) that harbours a specific pathogen and serves as a potential source of infection for others.

“Typhoid Mary” is a classic example of a carrier

Carriers may be classified as:

(a) Healthy: Healthy carriers carrying the pathogens without suffering from the disease.

(b) Convalescent: A person who has recovered from the disease but continue to harbour the pathogens in the body and shed them (during the period of convalescence).

(c) Contact Carrier- Carrier who acquires the pathogen from a patient

(d) Paradoxical Carrier- Carrier who acquires a pathogen from another carrier

Depending on the duration of carriage, carriers can be

(e) Temporary: Temporary carriers are those who shed the infectious agent for short periods of time (less than six months)

(f) Chronic: A chronic carrier is one who excretes the infectious agent for indefinite periods- many years to life-long. Chronic carrier state occurs in a number of diseases, e.g., typhoid fever, hepatitis B, dysentery, malaria, gonorrhoea, etc.

Chronic carriers are far more important sources of infection than cases.

2. Animals

Animals are a good source of infection, they cause zoonoses (animal acquired infections in man). Some common examples:

Zoonoses

Bacterial- Plague, from rats

Viral- Rabies, from dogs

Protozoal – Toxoplasmosis, from cats

Helminthic – Tapeworm from cattle/pig

Fungal – Zoophilic dermatophytes from cats & dogs

Some animals act as reservoirs of human infections- they maintain the parasite/microorganism in nature but do not show any symptoms themselves. They are called reservoir hosts.

3. Insects

Insects are a good Sources of infection. Some germs rely on insect carriers — such as mosquitoes, fleas, lice or ticks — to move from host to host. These carriers are known as vectors. Mosquitoes can carry the malaria parasite or West Nile virus, and deer ticks may carry the bacterium that causes Lyme disease. Blood-sucking insects cause many arthropod-borne diseases.

Transmission may be

Mechanical- pathogen transmitted mechanically by the vector. Eg; Transmision of dysentery or typhoid bacilli by the domestic fly

Biological- Pathogen multiplies in the body of the vector, a part of their lifecycle completed in the body of insect vector eg. Aedes aegypti mosquito in dengue, Anopheles mosquito in malaria.

Extrinsic incubation period is the time period between the entry of the pathogen into a vector and till the vector become infective.

4. Food and water

Another source of infections are contaminated food and water. Cholera, Hepatitis A can be caused by drinking contaminated water. Salmonella contaminates water and food and symptoms occur in one to three days after consuming. Staphylococci can cause food poisoning by producing toxins.

5. Soil

Soil is a good Source of infection. Clostridium tetani is a common soil bacterium and the causative agent of tetanus. Fungi such as Histoplasma and parasites such as round worm, hookworm survive in soil and cause human infection.

6. Fomites- Inanimate objects such as unsterile instruments, contaminated objects, accidental injuries with materials or equipments used in patient care/diagnosis which can cause infections.