Preventive Medicine in Humanitarian Emergencies

Modes of Disease Transmission

Post-disaster living conditions frequently enhance the transmission of infectious diseases. Understanding how these conditions pose health threats will help set priorities for public health interventions. The most common modes of transmission after a disaster are fecal-oral, respiratory, and vector borne. Fecal-oral transmission can occur by having human waste enter into the water supply by indiscriminant defecation or by flooding, by flies carrying feces on their feet to food sources, or from unwashed hands.

The respiratory route is enhanced by the crowded conditions that frequently follow a disaster. Also, respiratory irritants such as smoke from open cooking fires may increase predisposition towards the spread of respiratory pathogens. Vector-borne illnesses such as malaria and dengue frequently increase following disasters, particularly floods or hurricanes because standing water increases mosquito breeding.

Infections are usually referred to when the notion of transmission is being discussed, because infection transmission is easier to prove objectively. In contrast, the epidemiologic evidence of causality in the transmission of noninfectious ­diseases is more difficult to demonstrate. An example is lung cancer associated with cigarette smoking.

There are four modes of transmission of infectious diseases: contact, common source, airway transmission, and transmission by vectors.

Contact

This necessitates a connection between the host and the infectious agent that causes the disease. The contact can be direct, indirect or through droplets. Direct contact involves person-to-person contact, as in the case of the fecal-oral route, in which the microorganism contained in the feces of an infected person is transmitted, due to defective hygiene conditions, to another person, usually another household member. Examples: hepatitis A, Salmonella, Shigella. An additional example is the contact with a Staphylococcus aureus infected wound. Indirect contact is through an inanimate object. Example: hepatitis B, due to the shared utilization by family members of objects such as tooth brushes. Finally, the contact through droplets is the person-to-person transmission through droplets emitted by the mouth or nostrils. The longest distance that droplets travel in the air is approximately 3 yards. Examples: measles, chickenpox, streptococcal disease.

Common Source

A microorganism or a toxin can cause disease in one or several persons from a common source that contains the infective agent. Example: gastroenteritis outbreaks resulting from contaminated food (ice cream, mushroom sauce) (Box 9).

Transmission through the Air

Microorganisms can travel more than 3 yards in the air from the source of infection. Traveling microparticles usually result from the evaporation of drops emitted by the source of the disease. Examples: tuberculosis (through the cough of a diseased person), psittacosis (from a diseased fowl to a person), Q fever (from contaminated products; can travel several miles), and Legionella (through air-conditioning systems).

Transmission by Vectors

Transmission by vectors can be external or internal. In the external transmission, the vector carries the unmodified infective agent on its body. Example: flies carry Salmonella acquired from contaminated feces and deposit the bacteria on food that is later ingested by the host (human being or animal). In the internal transmission, the microorganism travels inside the body of the vector, where it may remain unaltered (Yersinia pestis is ingested and then eliminated unchanged) or may be modified, as in Plasmodium falciparum transmission by the mosquito.

Recognize that transmission can occur by one or several modes (Table 1).

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