Merck Manual

Please confirm that you are a health care professional

honeypot link
Professional Version

Pathogens and Host Species in Zoonoses


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 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 Global Zoonotic Diseases: Bacterial and Rickettsial Diseases Global Zoonotic Diseases: Bacterial and Rickettsial Diseases , viruses Global Zoonotic Diseases: Viral Diseases and Prion Disease Global Zoonotic Diseases: Viral Diseases and Prion Disease , fungi Global Zoonotic Diseases: Fungal Diseases Global Zoonotic Diseases: Fungal Diseases , parasites Zoonotic Diseases: Parasitic Diseases Zoonotic Diseases: Parasitic Diseases , or prions Global Zoonotic Diseases: Viral Diseases and Prion Disease Global Zoonotic Diseases: Viral Diseases and Prion Disease . 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 Histoplasmosis in Animals Histoplasmosis is a chronic, noncontagious, disseminated, granulomatous disease of humans and other animals due to the dimorphic fungus Histoplasma capsulatum var capsulatum. The... read more Histoplasmosis in Animals , blastomycosis Blastomycosis in Animals Blastomycosis is a multifocal fungal infection due to the dimorphic fungi Blastomyces spp. The fungi are often found in soil or decomposing organic matter, such as leaves. Infection is... read more Blastomycosis in Animals , and coccidioidomycosis Coccidioidomycosis in Animals Coccidioidomycosis (valley fever) is a dustborne, noncontagious infection due to Coccidioides spp fungi. These fungi form mycelial mats in shallow arid soil; the mycelium fragments form... read more Coccidioidomycosis in Animals , 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.

Key Points

  • Zoonotic diseases are shared among humans and animals and can be due to bacteria, viruses, fungi, parasites, and prions.

  • Zoonotic agents occur in mammals, marsupials, birds, reptiles, amphibians, fish, and invertebrates.

  • Organisms carried by humans can be transmitted to animals (reverse zoonoses) and may be reacquired from that source.

  • Certain microorganisms have some genotypes that are primarily maintained in humans, and other genotypes that are maintained in animals but may occasionally be zoonotic.

For More Information

quiz link

Test your knowledge

Take a Quiz!