Can Diseases Be Transmitted from Pets to Humans: Zoonosis?

Updated: Oct 19, 2021

Introduction & Overview


Zoonotic diseases (also known as zoonoses) are diseases that are caused by germs that spread between animals and people.

Zoonotic diseases are the hallmark of public health hence a challenge to veterinarians and all professions concerned with public health. Zoonotic diseases are best controlled when veterinarians and public health physicians cooperate in zoonosis control programs.

This enables the eradication of and continued surveillance of zoonotic diseases in animals, humans and the environment. Though some zoonoses have been eradicated in developed countries, they are still a major concern in the developing world, e.g. bovine tuberculosis, bovine and porcine brucellosis, and rabies. Zoonotic diseases also reemerge in areas where they have been eradicated, e.g. Hendra and Nipah viruses are reemerging, while many other zoonoses remain a constant concern.

What are the common causes of Zoonotic diseases?

vector illustration of human and animal symbols spreading virus for contagious disease caution
Illustration of human and animal symbols spreading viruses for contagious diseases

The causes of zoonotic diseases are bacteria, viruses, fungi, parasites, or prions. Most of these agents are disease-causing organisms of mammals, with most of the diseases by people and nonhuman primates. Birds, reptiles, amphibians, fish, and invertebrates can also be sources of infection, and some agents have both human and animal reservoirs including wildlife. Reverse zoonoses are caused by human pathogens transmitted to animals. In some cases, these agents can later infect people, e.g., Mycobacterium tuberculosis, the agent of human tuberculosis, can colonize the bovine udder and be shed in milk.

The occurrence of a pathogen in both people and animals does not always mean it is a significant 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 people and animals rather than true zoonoses, e.g. yeasts.

How are Zoonoses Transmitted?


Zoonotic pathogens are acquired through the following routes:-

  1. Close contact with an animal 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.

  2. Aerosols are occasionally involved, particularly in confined spaces.

  3. Fomites (inanimate objects) transmit some agents that correlate with the organism’s persistence in the environment.

  4. Ingestion of contaminated food or water infects large numbers of people. Sources of zoonotic pathogens in foodborne disease include:-

  5. Undercooked meat or other animal tissues (including seafood and invertebrates),

  6. Unpasteurized milk and dairy products, and

  7. Contaminated vegetables.

  8. Insect vectors, serving as either biologic or mechanical vectors, are important in transmitting some organisms.

How are the risks of acquiring a zoonosis?


The risk of acquiring a zoonosis can be affected by many factors that include:-

  1. The immune susceptibility of the host

  2. The potential route(s) of transmission,

  3. The number of organisms shed by the animal, and

  4. The ability of the agent to cross species barriers.

  5. Certain occupations or activities can significantly increase the probability of exposure e.g.,

  6. 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.

  7. Veterinary practice,

  8. Agricultural activities,

  9. Pet ownership.

  10. Dogs, cats, livestock, or birds may also bring wildlife pathogens into closer proximity to people.

  11. Human activities can also bring people into closer contact with wildlife, e.g. hunting, fishing, and camping, which 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).

  12. Ecotourism also has resulted in human exposure to some exotic wildlife diseases.

  13. Cultural practices such as eating raw fish, gastropods, or molluscs expose humans to zoonotic diseases.

What is the role of Immunosuppression in Zoonoses?


Zoonotic illnesses range from mild, asymptomatic and self-limiting cases to chronic debilitating ones. They run the whole gamut from skin eruptions to life-threatening diseases. These diseases also rely on the strength of the individual's immune system. In a healthy host, the disease can go unnoticed while in an immuno-compromised one the condition can have an unusual presentation including delayed diagnosis.

Immunodeficiencies can either be

  1. Primary immunodeficiencies or

  2. Secondary immunodeficiencies.

Primary immunodeficiencies are mainly genetic defects that may increase susceptibility to certain categories of pathogens or broadly suppress the body defences to certain illnesses.

Secondary immunodeficiencies are caused by conditions that compromise the immune system, e.g. splenectomy (removal of the spleen), diabetes, cancer, and infections such as malaria or AIDS, chronic lung disease, injuries