Diseases that affect both humans and other animals are called “zoonotic diseases” or “zoonoses” (pronounced zoh-oh-no-sees). This article is a brief overview of some key zoonotic diseases, focusing on risks to humans. In all cases, the summaries and recommendations apply to normal adults within theUnited States, except where otherwise noted. Young children, the elderly, or immunocompromised people (illness, splenectomy, certain medications, etc.) are at increased risk for all infections, zoonotic or otherwise. Pregnant women can have immunocompromise and many infections can affect pregnancy. Risks differ depending upon nation and region.
At-risk humans should consult closely with their doctor and veterinarian to address zoonotic potential. In most cases, zoonotic risks can be managed while maintaining the human-animal bond when it is most needed. General recommendations are common sense and include regular hand-washing, maintaining current vaccinations for all pets, frequently picking up feces, and at least yearly check ups with your veterinarian (sometimes including routine blood and fecal testing).
Gastrointestinal Zoonotic Diseases in Dogs and Cats
Important animal diseases affecting the stomach and intestines of humans include Giardia, Cryptosporidium and Toxoplasma gondii. Salmonella or Campylobacter bacteria may also cause disease in humans. Although Toxocara (cati and canis) and Baylisascaris roundworms are passed through pet feces, they typically migrate through other tissues outside of the intestines in humans.
Gastrointestinal zoonoses are usually passed by ingesting organisms or eggs contained in feces or fecal-contaminated environments (sand boxes, fields, yards, homes if not properly decontaminated). The infectious period and stability within the environment vary by organism. For example, Giardia is a risk in fresh feces, whereas the eggs of Toxocara and Baylisascaris take days to weeks to become infectious, but survive in the environment for months. Toxoplasma gondii requires >24 hours after defecation to become infectious, meaning that cat feces should be removed daily. Gastrointestinal zoonoses are usually a very low risk for adult humans without immunosuppression or pregnancy.
Zoonotic Diseases Caused by Dog or Cat Bites, Scratches, or Direct Contact
Most bite wounds from dogs and cats become infected. However, the majority of bacteria associated with such wounds cause only local problems in adult humans with normal immune systems. More sever infections may develop, potentially affecting the brain, heart, joints, or other organs. The bacteria Pasteurella, Capnocytophaga canimorsus (DF-2), or Capnocytophaga cynodegmi are associated with an increased risk of these systemic signs. Other bacterial infections include Bartonella, Yersinia pestis, and Francisella tularensis, though these are also associated with other animals and insects.
Fungal infections resulting from direct contact with abnormal skin include Sporothrix schenckii and the dermatophytes (“ringworm”). Rabies viral infection can result from any mammal bite, including dogs and cats. Feline retroviruses do not affect humans.
Zoonotic Diseases of the Respiratory Tract
The organism most associated with kennel cough is Bordetella bronchiseptica. This bacterium is not a risk to humans unless severely immunocompromised. Mild coughing may be caused by Chlamydophila felis, which comes from cats. It is possible for cats and dogs to pick up Streptococcus group A bacteria, S. pyogenes and S. pneumoniae (“strep throat”) from people and transmit it to other humans, even without showing signs.
Genital and Urinary Tract Zoonoses
Brucella canis (dogs), Leptospira spp. (dogs) and Coxiella burnetii (dogs and cats) are the most common zoonotic agents in this group. 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 zoonosis.
Zoonoses of Shared Environments
Humans occasionally contract diseases from the same vectors (fleas, ticks, and mosquitoes) or environment as their pets, but not from the pets themselves. Examples of vector-borne diseases include Rickettsia spp. (ticks and fleas), Borrelia burgdorferi (ticks), Bartonella spp. (fleas and ticks), Dipylidium caninum (mosquitoes), Ehrlichia spp. (ticks), and West Nile virus (mosquitoes). Examples of diseases humans can contract from environmental sources include Aspergillus spp., Blastomyces dermatitidis, Coccidioides immitis, Cryptococcus neoformans, Histoplasma capsulatum.
Of course, no discussion of zoonoses would be complete without mentioning rabies. This virus may affect any mammal, but is most common in wildlife. It is always fatal and spreads through bodily secretions, most notably saliva. It is recommended that all dogs and cats be vaccinated for rabies, regardless of lifestyle or region.
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