Pathogens and Host Species in Zoonoses

Zoonotic diseases are due to pathogens that are maintained in animals but can be transmitted to humans, causing sporadic illnesses and outbreaks. In rare instances, an agent may even become adapted to circulate in human populations, as demonstrated by the 2009 H1N1 influenza pandemic in humans, due to a virus from swine. Veterinarians should be aware of common zoonotic diseases to advise their clients as well as protect their staff.

Causative agents can include bacteria, viruses, fungi, parasites, or prions. Because organisms are more readily transmitted between closely related hosts, most of these agents are pathogens of mammals. Humans and nonhuman primates share a particularly large number of diseases. Birds, reptiles, amphibians, fish, and invertebrates can also be sources of infection. Many of the zoonotic agents in poikilotherms are parasites; however, these species can also carry zoonotic bacteria and viruses, such as Salmonella, West Nile virus, and opportunistic Mycobacterium spp.

Humans are incidental hosts for many zoonoses; however, some agents have both human and animal reservoirs. In some cases, animal reservoirs are revealed after a disease is controlled in humans. Yellow fever, for example, is known to have a zoonotic sylvatic (jungle) cycle in nonhuman primates as well as an urban cycle maintained in humans. Zika virus, likewise, appears to have a zoonotic sylvatic cycle in Africa in addition to the urban cycle of its Asian strains. Wildlife are increasingly recognized as reservoirs for zoonoses, including some zoonoses once thought to be strictly pathogens of production animals.

The term reverse zoonosis refers to disease from pathogens that are reservoired in humans and can be transmitted to animals. In some cases, these agents can later infect humans. For example, Mycobacterium tuberculosis, the agent of tuberculosis in humans, can colonize the bovine udder and be shed in milk.

The occurrence of a pathogen in both humans and animals does not always mean it is an important zoonosis. Some diseases are acquired from the environment, and transmission between animal or human hosts is either absent or occurs very rarely and under unusual conditions. These are considered infections common to humans and animals rather than true zoonoses. Histoplasmosis, blastomycosis, and coccidioidomycosis, for example, are acquired by inhaling the microconidia of soil fungi; however, the organisms exist as yeasts in tissues. Some agents, such as Candida spp, are widespread commensals in healthy humans and animals and can cause disease when the host becomes debilitated. Although such organisms might be transmitted from animals to humans, this transfer likewise has little epidemiological importance. Nevertheless, rare reports of zoonotic transmission for some of these organisms indicate that ordinary infection control precautions should not be neglected, especially during exposure to high concentrations of the pathogen in tissues.

Understanding of which infectious agents are zoonotic is continually evolving. In some cases, the perception that certain organisms were zoonotic has changed, particularly through the use of genetic techniques. For example, Streptococcus agalactiae was once thought to be acquired from animals, but it is now known that most infections in humans are due to human-adapted S agalactiae distinct from animal strains. However, this organism recently reemerged as a zoonotic concern when it caused a large disease outbreak linked to consumption of raw fish. The causative organism appears to be a particular genotype of S agalactiae (ST283) that might be associated with freshwater fish. Some farmers have also been found to be colonized with S agalactiae isolates that appear identical to those in their cattle.