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What Is Zoonosis?

By Melissa Barrett
Updated May 17, 2024
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Throughout history, humans have lived closely with animals. Sometimes, the contact has been as impersonal as sharing the same geographic area. Often though, we share our homes, food, and families with animals. Logically, we would also share our illnesses. Any illness that is shared in this way is called a zoonosis.

Many human parasitic diseases are spread through contact with animal feces. A parasitic zoonosis of digestive roundworms may occur by playing with a family pet and then eating without properly washing one’s hands. Sandworms are found in animal excrement and thrive in sandy areas. These parasites are spread from feces through skin contact and can be contracted by simply cleaning a litter box.

Additionally, insects that feed on both animals and humans are often vectors of zoonosis. Most notably, fleas that fed on both rats and humans are largely believed to be the source of most historical outbreaks of bubonic plague. In the 1300s, such a plague, dubbed Black Death, swept through most of Europe. It is estimated that half of the population died from that outbreak. Today, when small bubonic plague outbreaks occur, they are still generally tracked to plague-carrying rats and fleas.

Much more direct examples of zoonosis occur in food-bourne illnesses. In these cases, humans become ill from ingesting meat from sick animals. Mad cow disease is believed to be transferable to humans by ingestion of tainted meat. This disease may present in humans as a variant of Creutzfeldt-Jakob disease. While this disease is rare, it is incurable and almost always fatal.

Other examples of zoonosis by ingestion are salmonella and Escherichia coli (E. Coli) infections. These bacteria may be present in cows and chickens that appear completely healthy, and during slaughter, the flesh of several animals can become cross-contaminated by one individual. If the chicken and beef that these bacteria infect are not cooked at high enough temperatures, the live bacteria can be transferred to humans, resulting in illness. Care should be taken while handling raw meat, and surfaces and cooking utensils should be properly sanitized after use. Meat should always be thoroughly cooked.

Humans can also spread diseases to animals through reverse zoonosis. The influenza virus, for example, can be spread from humans to many domesticated animals. This can be particularly dangerous in areas where the animals are closely gathered and the virus is likely to spread.

Among captive elephants, reverse zoonosis of tuberculosis is particularly worrisome. Members of the pachyderm family seem particularly susceptible to contracting the disease. Tuberculosis in elephants is unusually virulent, progressing faster and causing more damage than is typical in humans. The presentation of the disease has led some scientists to investigate the disease as a possible contributing cause in the extinction of the mastodon.

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