Zoonotic diseases are defined as being common to, shared by, or naturally transmitted between humans and other vertebrate animals. Humans are infected with zoonotic agents from direct contact with the infected pet, contact via contaminated food or water, from shared vectors, and from the shared environment. Direct contact with animal feces (enteric zoonoses), respiratory secretions, urogenital secretions, or infected skin and exudates, as well as bites and scratches can result in human infections. Some zoonotic agents are transmitted between animals and man by shared vectors like fleas, ticks, or mosquitoes. Rickettsia rickettsii (ticks), Ehrlichia spp. (ticks), Borrelia burgdorferi (ticks), Rickettsia felis (fleas), Bartonella spp. (fleas, ticks), Anaplasma phagocytophilum (ticks), Dirofilaria immitis (mosquitoes), Dipylidium caninum (mosquitoes), and West Nile virus (mosquitoes) are examples of vector borne zoonoses. The pet brings the vector of the organism into the environment resulting in exposure of the human.

Bartonella spp. are notable examples as some species like B. henselae survive in flea feces for days. Flea and tick control should always be maintained on our client’s animals and infested animals that are seen in the clinic should be treated immediately. Use of flea control products have been shown to block transmission of B. henselae amongst research cats and so theoretically could lessen transmission to humans. Neurobartonellosis (headaches, blurred vision) in veterinary health care providers is now recognized as an important zoonotic disease syndrome related to Bartonella spp. in flea frass.

Some zoonotic agents including Sporthrix schenkii, Histoplasma capsulatum, Coccidioides immitis, Blastomyces dermatitidis, Cryptococcus neoformans, and Aspergillus spp. do not usually infect humans from direct with the infected pet but are acquired from the same environmental source.

Most of the agents discussed in this lecture can infect and cause disease in anyone, but disease is generally more prevalent or more severe in those that are immunodeficient. Humans with AIDS are discussed most frequently, but there are many more immunodeficient individuals including the very old, the very young, and those receiving chemotherapy for immune-mediated diseases, organ transplantation, or neoplasia.

Humans are unlikely to contract zoonotic diseases from contact with their pets and so in most cases do not need to relinquish their animals. The Centers for Disease Control of the United States online site, Healthy pets Healthy People (http://www.cdc.gov/healthypets/index.html) is a great resource for veterinarians and owners. The American Association of Feline Practitioners’ Zoonoses Guidelines states ‘All human or animal care providers should provide accurate information to pet owners concerning the risks and benefits of pet ownership so that an informed decision about acquiring and keeping pets can be made’ (https://catvets.com/public/PDFs/PracticeGuidelines/ZoonosisGLS.pdf, VIN editor: :Link modified on 12/18/19). The Companion Animal Parasite Council also provides great information to help with decision making about zoonotic parasitic diseases (https://capcvet.org; https://www.petsandparasites.org). The WSAVA encourages veterinarians and physicians to work closely together to determine the risks associated with pet ownership for individuals and their families.

Enteric Zoonoses

There are multiple infectious agents of the gastrointestinal tract that potentially can be shared between pets and humans. Since some enteric zoonotic agents are infectious when passed with feces (Campylobacter spp., Salmonella spp., Giardia spp., Cryptosporidium spp. and others), direct contact with infected animals can result in human infections. However, it is felt that most enteric zoonoses result from ingestion of the infectious agent in contaminated food, water, or other environmental sources. Giardia spp., Cryptosporidium spp., Toxocara spp., and Toxoplasma gondii are notable examples. Toxoplasma gondii, hookworms, and roundworms require a period outside the host prior to becoming infectious. While on the zoonoses list, Giardia and Cryptosporidium spp. of dogs and cats are rarely detected in people and human strains have not been associated with illness in pets. Prevalence rates for enteric zoonoses have been reported multiple studies of dogs and cats and generally are generally greater in those with diarrhea. These findings emphasize that diagnostic workups for enteric infections are indicated due to potential human health risks. The minimal diagnostic plan to assess for enteric zoonoses in pets with diarrhea includes a fecal flotation, Giardia spp., screening procedure, and fecal wet mount. Fecal culture should be considered if Salmonella spp. or Campylobacter spp. are on the list of differential diagnoses. In the United States, heartworm preventatives that control hookworms and roundworms are recommended year-round (http://www.capcvet.org). Dogs and cats with normal stool are not considered human health risks.

Bite, Scratch, or Exudate Exposure Zoonoses

Approximately 300,000 emergency room visits per year are made by people bitten by animals in the United States.

Most of the aerobic and anaerobic bacteria associated with bite or scratch wounds only cause local infection in immunocompetent individuals. However, 28–80% of cat bites become infected and severe sequelae including meningitis, endocarditis, septic arthritis, osteoarthritis, and septic shock can occur.

Immunodeficient humans or humans exposed to Pasteurella spp., Capnocytophaga canimorsus (DF-2), or Capnocytophaga cynodegmi more consistently develop systemic clinical illness. Splenectomized humans are at increased risk of developing bacteremia. Mycoplasma spp. and L-form bacterial infections of people has been associated secondary to dog or cat bites. Bartonella spp., Yersinia pestis, and Francisella tularensis infections of humans can be associated with bites and scratches but these agents are also vector-associated zoonoses. Of the many fungal agents that infect both humans and animals, only Sporothrix spp., and the dermatophytes have been shown to infect humans upon direct exposure. Histoplasma, Blastomyces, Coccidioides, Aspergillus, and Cryptococcus infections of humans and animals can occur in the same household, but infection of humans generally results from a common environmental exposure rather than by direct contact with an infected animal. Rabies is still the only significant small animal viral zoonosis in the United States. Pseudorabies is a herpesvirus that infects pigs; dogs and humans can develop self-limiting pruritic skin disease following exposure. Feline retroviruses are not zoonotic.

Respiratory and Ocular Zoonoses

Bordetella bronchiseptica and Chlamydia felis cause mild respiratory disease and C. felis has been associated with conjunctivitis in people. Most people with Bordetella infections are infected by B. pertussis but some immunocompromised people develop infection by B. bronchiseptica. Humans are the principal natural hosts for Streptococcus group A bacteria, S. pyogenes and S. pneumoniae, which cause “strep throat” in people. Dogs or cats in close contact with infected humans on rarely develop transient, subclinical colonization of pharyngeal tissues and so theoretically can transmit the infection to other humans. Yersinia pestis and F. tularensis can be transmitted from cats or dogs to people in respiratory secretions.

Genital and Urinary Tract Zoonoses

Leptospira spp. (dogs more than cats), Brucella canis (dogs), and Coxiella burnetii (cats more than dogs) are the most common zoonotic agents in this group. Whether Leptospira spp. of cats are associated with illness in people has not been studied extensively. Coxiella burnetii is a rickettsial agent found throughout the world, including North America.

Many ticks, including Rhipicephalus sanguineus, are naturally infected with C. burnetii and so this agent is also a shared vector zoonoses. It most commonly is associated with respiratory disease in infected humans that come in contact by inhaling the organism which is shed in high numbers in some cats during parturition. Brucella canis (dogs) is not known to infect cats.

Table 1. General guidelines for veterinarians

References

1.  Ballweber LR, Xiao L, Bowman DD, Kahn G, Cama VA. Giardiasis in dogs and cats: update on epidemiology and public health significance. Trends Parasitol. 2010;26(4):180–189.

2.  Breitschwerdt EB. Bartonellosis: One Health perspectives for an emerging infectious disease. ILAR J. 2014;55(1):46–58.

3.  Brunt J, Guptill L, Kordick DL, Kudrak S, Lappin MR. Association of Feline Practitioners 2006 Panel report on diagnosis, treatment, and prevention of Bartonella spp. infections. J Feline Med Surg. 2006;8:213–226.

4.  Cairns K, Brewer M, Lappin MR. Prevalence of Coxiella burnetii DNA in vaginal and uterine samples from healthy cats of north-central Colorado. J Feline Med Surg. 2007;9:196–201.

5.  Day MJ. Pet-related infections. Am Fam Physician. 2016;94:794–802.

6.  Lappin MR, Elston T, Evans, L, Glaser C, Jarboe L, Karczmar P, Lund C, Ray M. The 2019 American Association of Feline Practitioners Feline Zoonoses Panel Report. J Feline Med Surg. In review, 2019.

7.  Lucio-Forster A, Griffiths JK, Cama VA, Xiao L, Bowman DD. Minimal zoonotic risk of cryptosporidiosis from pet dogs and cats. Trends Parasitol. 2010;26:174–179.

8.  Scorza V, Ballweber LR, Tangtrongsup S, Panuska C, Lappin MR. Comparisons of mammalian Giardia duodenalis assemblages based on the β-giardin, glutamate dehydrogenase and triose phosphate isomerase genes. Vet Parasitol. 2012;189(2–4):182–188.