Merck Manual

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Professional Version

Transmission of Zoonoses Between Animals and Humans


Anna Rovid Spickler

, DVM, PhD, Center for Food Security & Public Health, College of Veterinary Medicine, Iowa State University

Reviewed/Revised Feb 2023 | Modified Jun 2023

Zoonotic agents are acquired by the same routes responsible for transmitting pathogens between animals. Commonly, transmission involves close contact with an animal, generally via inhalation, ingestion, or other mechanisms, resulting in the contamination of mucous membranes, damaged skin, or in some cases, intact skin.

  • Body fluids, secretions and excretions, and lesions are potential sources of infection. Unprotected contact with tissues during postmortem examination often carries a high risk of transmission.

  • Aerosols are occasionally involved, particularly in confined spaces.

  • Fomites can transmit some agents; the likelihood of this route correlates with the organism’s persistence in the environment.

  • Some organisms are transmitted by ingestion of contaminated food or water and may infect large numbers of humans. Sources of zoonotic pathogens in foodborne disease include undercooked meat or other animal tissues (including seafood and invertebrates), unpasteurized milk and dairy products, and contaminated vegetables.

  • Insect vectors, serving as either biological or mechanical vectors, are important in transmitting some organisms.

The risk of acquiring a zoonosis can be affected by many factors, including susceptibility of the host The Spectrum of Illness and the Role of Comorbidities in Zoonosis The spectrum of zoonotic illness varies from skin eruptions or mild, self-limiting infections (easily misdiagnosed as influenza in humans) to serious, life-threatening disease. Some zoonoses... read more , potential routes of transmission, number of organisms shed by the animal, infective dose, and ability of the agent to cross species barriers. Some pathogens, such as Bacillus anthracis, readily infect humans with suitable exposure; others are uncommon zoonoses even when exposure is frequent.

A person’s leisure and vocational activities, travel, and pet ownership can define their risk for acquiring certain zoonoses:

  • Contact with soil during gardening or childhood play carries a risk of infection with pathogens that reside temporarily or permanently in the soil, such as Toxocara spp or Sporothrix schenckii.

  • Veterinary practice, agricultural activities, and pet ownership are overt hazards.

  • Nontraditional pets have a particularly high risk of being infected with zoonotic agents, especially when captured directly from the wild. During an outbreak of monkeypox in the US, this virus spread from exotic African rodents, imported as pets, to pet prairie dogs and then to humans.

  • Activities that bring humans into closer contact with wildlife, including hunting, fishing, and camping, can result in exposure to organisms carried in wild animals (eg, Francisella tularensis, Yersinia pestis, and Leptospira spp) or transmitted by arthropod vectors (eg, Borrelia burgdorferi or West Nile virus). Hunters, in particular, may contact pathogens in animal tissues during butchering.

  • Dogs, cats, production animals, or birds may also bring some wildlife pathogens into closer proximity to humans. The animal can be infected directly with the agent, either clinically or subclinically, or it may act as a transport host for infected arthropods such as ticks.

  • The popularity of ecotourism has resulted in elevated exposure to some exotic wildlife diseases.

  • Some zoonoses may also be linked to cultural practices such as eating raw meat, fish, gastropods, or mollusks.

Once a zoonotic disease has been acquired by a person, it can sometimes be transmitted from person to person. The risk varies with the specific agent, its ability to spread readily in humans, and the routes of transmission. Often, the humans most at risk are health care workers and close family members. However, diseases such as plague have the potential to spread widely in human populations under some conditions.

Some zoonotic diseases are not contagious during casual contact but can be transmitted by transfusion or organ transplantation, or from mother to fetus in utero. A particularly wide variety of agents, from encysted parasites to latent viruses, are potentially transferable in organ transplants. These agents, which may have been well controlled in the organ donor, can be reactivated in the recipient, who is immunosuppressed by drugs taken to prevent rejection. Transfusion can also bypass normal barriers if the agent is found in the blood at the time of the donation. The bovine spongiform encephalopathy agent, for example, is ordinarily transmitted from host to host only by ingestion of tissues; however, it can be acquired in transfused blood.

Key Points

  • The risk of acquiring a zoonotic disease can be influenced by many factors that range from the intrinsic ability of the agent to cross species barriers to recreational and occupational factors that increase exposure.

  • Some zoonotic agents that infect humans can be transmitted from person to person.

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