Zoonotic Disease

Zoonotic Disease Classification

All classes of disease agents cause zoonotic disease. These include bacteria (e.g., listeriosis), chlamydia (e.g., psittacosis), rickettsia (e.g., Rocky Mountain spotted fever), viruses (e.g., Hendra), parasites (e.g., leishmaniasis), and fungi (e.g., histoplasmosis).

Zoonoses can be subdivided into those transmitted from animals to humans (zooanthroponoses) or from humans to animals (anthropozoonoses, also called reverse zoonoses). Mycobacterium tuberculosis has been spread from humans to cattle and elephants, and methacillin-resistant Staphylococcus aureus (MRSA) has been transmitted from people to horses and then back to people. Diseases that are rarely transmitted between animals and humans are sometimes included, such as foot-and-mouth disease in cattle.

Zoonoses transmitted through direct contact are orthozoonoses (e.g., rabies). Cyclozoonoses (e.g., echinococcosis) require more than one vertebrate host for development. Metazoonoses are transmitted by an infected invertebrate vector (e.g., scrub typhus from mite bites). Zoonoses transmitted through physical contact with food, soil, or vegetation are saprozoonoses, sapronoses, or geonoses (e.g., fungal infections). Some diseases fit more than one category (e.g., tularemia from fly or tick bites, direct contact, or environmental exposure).

As noted by Enserink in Science in 2000, of 1,709 identified human disease agents, 832 (49%) are classified as zoonotic. Of the 156 ‘emerging’ diseases, 114 (73%) are zoonotic. Thus, zoonotic diseases are disproportionately represented among those spreading into new areas.

Zoonotic disease agents account for most of the organisms in Categories A, B, and C of the U.S. government's Select Agent List of likely organisms for bioterrorism attacks. The diseases caused by Category A select agents include smallpox, anthrax, plague, tularemia, botulism, and viral hemorrhagic fevers.

Select agents in Categories B and C cause bacterial, chlamydial, and rickettsial diseases, including brucellosis, Q Fever, glanders, melioidosis, foodborne/waterborne disease, psittacosis, and typhus. Select viral agents include smallpox, Nipah, hanta, and the [Page 1108]encephalitides viruses. Select agents that can lead to intoxications include Staphylococcus enterotoxin B, ricin, and Clostridium perfringens Epsilon toxin. All these select agents are considered zoonotic except for smallpox.

Populations at Increased Risk

Anyone who comes into contact with infected animals, vectors, or contaminated areas can become infected with zoonotic diseases; however, the risk of clinical signs and death is not uniformly distributed. The proportion who remain asymptomatic and the case fatality rate (proportion of ill persons who die) vary with certain risk factors.

Age is often associated with disease severity. Of those infected with Escherichia coli O157:H7 from contact with animals or their environment, children are more likely to develop potentially fatal hemolytic uremic syndrome (HUS). Younger, healthier people appear to be more susceptible to serious illness from the highly pathogenic avian influenza (HPAI) strain of Asian H5N1, compared with human influenza strains that differentially cause severe illness and death in older people. Similarly, hantavirus infection was first identified in physically fit young adults, and the very young and very old still seem to be relatively unaffected. Although the factors leading to these age differences are not understood, infection with both the Asian H5N1 HPAI virus and hantaviruses lead to pathologic changes caused by the body's own immune response to the viruses.

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