Zoonotic pathogens can be acquired during close contact with an animal, generally through inhalation, ingestion, or other mechanisms resulting in the contamination of mucous membranes, damaged skin, or in some cases, intact skin. Sources of organisms include body fluids, secretions and excretions, and lesions. Unprotected contact with tissues during necropsies 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 spread by ingestion of contaminated food or water and may infect large numbers of people. 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 biologic or mechanical vectors, are important in transmitting some organisms.
The risk of acquiring a zoonosis can be affected by many factors, including the susceptibility of the host (see Role of Immunosuppression in Zoonoses), the potential route(s) of transmission, the number of organisms shed by the animal, and the ability of the agent to cross species barriers. Some pathogens, such as Bacillus anthracis, readily infect people with appropriate exposure; others are uncommon zoonoses even when exposure is frequent. Certain occupations or activities can significantly increase the probability of exposure. 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 obvious hazards. Dogs, cats, livestock, or birds may also bring wildlife pathogens into closer proximity to people. 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. Nontraditional pets have a relatively high probability of being infected with zoonotic agents, especially when captured directly from the wild. During an outbreak of monkeypox in the USA, the virus spread from exotic African rodents, imported as pets, to pet prairie dogs and then to people. Activities that bring people 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 and West Nile virus). Hunters, in particular, may contact pathogens in animal tissues during butchering. The growing popularity of ecotourism has resulted in human exposure to some exotic wildlife diseases. The prevalence of other zoonoses may be linked to cultural practices such as eating raw fish, gastropods, or mollusks. Knowledge of a person’s leisure and vocational activities, travel, and pet ownership can sometimes raise the index of suspicion for zoonoses that are uncommon in urban populations.
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 disease, the agent’s ability to spread readily in people, and the routes of transmission. Often, the people 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 spread 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 organs. Agents that were 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, but it can be acquired in transfused blood.