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Veterinarians Protect Pets and People
by Celeste Clements, DVM, DACVIM

Veterinarians have a long tradition of protecting the public welfare by controlling disease in livestock and overseeing food safety. While the modern companion animal practitioner’s focus would seem to be nurturing the animal arm of the human-animal bond, veterinarians must also assume responsibility for promoting healthy pet ownership. Physicians may be inadequately prepared to address the many infections that have potential for zoonotic transmission from pets to people. Reverse zoonosis may better explain some cases of concurrent human and animal infection in households, and is likely to be overlooked by most medical professionals. Veterinarians, however, are appropriately educated and uniquely qualified to advocate for maintenance of the human-animal bond when that relationship seems inconvenient or problematic for the pet owner.

Infections with methicillin-resistant Staphylococcus; beta-hemolytic Streptococcus; or protozoa such as Cryptosporidia, Toxoplasma, or Giardia are frequent concerns for physicians, and veterinarians will be asked to test dogs, cat and other companion species for the presence of these organisms, sometimes inappropriately. The results of diagnostic tests must be carefully reviewed and any treatment judiciously prescribed. Indiscriminate use of antimicrobials has been suggested as an important contribution to antimicrobial resistance, one of the most significant challenges facing health care professionals. Practitioners must discriminate between those organisms that are likely to be transient colonizers and those that are more likely to be pathogenic to animals and/or are transmissible to people.

Finding multiple drug resistance in cultures of Staphylococcus pseudointermedius cannot be equated to isolating methicillin-resistant Staphylococcus aureus (MRSA): dogs are natural hosts of S. pseudointermedius, but not typically S. aureus. Dr. Scott Weese of the Ontario Veterinary College's Centre for Public Health and Zoonoses estimates that 5% of dogs and cats may be carrying methicillin-resistant Staphylococcus pseudointermedius (MRSP), and that the percentage may be higher in those previously treated with antibiotics. Human infections with MRSP are uncommon, but have been documented, so MRSP has some public health significance. Immunocompetent pet owners and veterinary personnel who are exposed to MRSP should observe excellent hygiene to minimize their risk.

MRSA is less common in pets and typically originates from people, but can be transmitted from pets to people as a zoonotic infection. The bacteria is carried in the host’s intestinal tract, nose, or on the skin. Efforts to decolonize an asymptomatic pet that cultures positive for MRSA or MRSP is rarely successful, and might actually promote bacterial resistance, rather than protecting the patient or pet-owner. Most dogs and cats will spontaneously clear MRSA if removed from the contaminated environment for several weeks. Livestock-associated strains of MRSA, as recovered from pigs and horses, are likely different in their behavior, and are the subject of ongoing investigation.

MRSP is a common cause of skin infections in dogs, and in that scenario, should be treated, because it is responsible for disease.

Beta-hemolytic Steptococcal infections, in Lancefield group A, are responsible for pharyngitis (Strep throat), necrotizing fasciitis (“flesh-eating bacteria”) and toxic shock syndrome, amongst others. Dogs and cats are often tested for the presence of these bacteria, at the request of physicians, but rarely are positive. Clinicians should be sure their laboratory is testing for the specific bacterial group, and not Streptococcus in general. Dogs and cats, with their nose to ground behavior, are prime candidates for colonization from a bacteria-laden home, and positive pets may be harboring bacteria that originated in the sick family member. A trial period with the pet removed from the home, as is suggested for MRSA–positive pets, could be attempted, to see if spontaneous clearance of the bacteria occurs rather than antimicrobial treatment of an asymptomatic dog or cat.

The availability of highly accurate advanced molecular diagnostic tests such as PCR (polymerase chain reaction) continues to challenge clinicians to make smart assessments of any positive result. Finding protozoal DNA does not predict disease nor does it predict sufficient shedding to spread infection. Giardia duodenalis (lamblia) is spread by fecal-oral transmission and can be harbored in a variety of animal species. The CDC (Centers for Disease Control) does specify that people who have contact with infected animals are at high risk for being infected, but the highest rates of human infection are actually associated with consumption of contaminated water.

Treatment of Giardia in asymptomatic dogs and cats is certainly controversial. The Companion Animal Parasite Council website states that treatment of asymptomatic carriers may not be necessary, and they emphasize that dogs and cats are not to be treated for Giardiasis to prevent zoonotic transmission. The CAPC notes that the host specificity of the different strains or assemblages of Giardia that infect cats and dogs limits risk to immunocompetent persons. As with other intestinal parasites good hygiene practices such as picking up feces and consistent hand washing limit the spread of Giardia to susceptible individuals.

Resources for veterinarians to address these and other complex issues of promoting healthy pets and healthy pet owners are supported by such institutions as the Centers for Disease Control and the Ontario Veterinary College's Centre for Public Health and Zoonoses. These groups offer information to veterinarians, physicians and pet owners, and make available printable materials for physicians and veterinary offices to carry. The Companion Animal Parasite Council also provides detailed information for veterinary and medical professionals at www.capcvet.org. 

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Important Links from this article

Centers for Disease Control

Ontario Veterinary College's Centre for Public Health and Zoonoses

Companion Animal Parasite Council