When humans contract diseases from their pets, the consequences, in rare instances, can carry legal ramifications for their veterinarians.

It's a scenario Dr. Larry Glickman says he's seen firsthand. The epidemiology professor at the University of North Carolina at Chapel Hill has served as an expert witness in legal cases involving the transmission of zoonotic diseases. Several cases he encountered concerned children who developed ocular toxocariasis and lost vision.

"Where children are involved, it's very emotional," Glickman recounted.

He noted that some of the cases he worked on involved veterinarians who, as a rule, did not prophylactically deworm puppies for Toxocara canis, a zoonotic parasite commonly known as the canine roundworm. Rather, they waited until a patient's fecal sample tested positive for the parasite. Outcomes of the cases Glickman refers to are unclear; he's never been involved with one that wasn't settled out of court.

Legal conclusions aside, the suits Glickman describes give rise to key questions: What is the liability risk for veterinarians when a patient infects its family with a zoonotic disease? And how frequently do such cases occur?

No one knows how often parasites, bacteria and viruses cross the species divide. Anecdotal reports of people acquiring serious illnesses from their family pets appear to surface sporadically. Even so, experts point out that when humans and animals in the same house become affected with a zoonotic disease pathogen, both the family's veterinarian and physician can be held liable.

That seems especially true in cases where pet owners have contracted T. canis. Larvae shed in the feces of infected dogs (usually puppies) may hatch in soil and grass and infect humans (usually children) who inadvertently ingest the larvae. Because humans are not the parasite's normal host, the larvae follow an atypical migration route and can end up in the eye, causing blindness; or in the liver or lungs, which can be damaged by the migration of parasites and subsequent inflammation.

How can courts determine that a child contracted a zoonotic disease from a pet and not the local park? According to Glickman, they can't. And they don't have to.

The source of zoonotic infection is "always impossible to identify beyond a reasonable doubt," he said. "That's generally not the level of proof one needs to establish in these types of (civil) cases. One only needs to show that it's a possibility that a pet dog in the house could have been involved."

Dr. Charlotte Lacroix, a New Jersey-based veterinarian and lawyer, agrees — to a point. By email, she clarified that the standard of proof for a civil case is “by a preponderance of the evidence." In other words, it's more likely than not that transmission originated with the family dog.

She considers the possibility that a household member might contract a pet's intestinal parasites to be small but serious: "Risk is assessed in gravity and frequency. It is not a frequent risk, but it is a grave risk."

Apart from canine roundworm, other zoonoses of common concern in North America include rabies, leptospirosis and canine hookworm infection. Drs. Linda Ellis and Nina Mouledous of the Professional Liability Insurance Trust (PLIT), a program founded by the American Veterinary Medical Association that offers malpractice coverage to veterinarians, say that the PLIT has seen claims against veterinarians related to rabies exposure. PLIT claims related to zoonses arising from intestinal parasites are far less frequent.

Given the infrequency in which humans contract toxocariasis — the illness caused by canine roundworms — some medical professionals wonder how much emphasis should be placed on prevention, despite the fact that the illness can lead to devastating consequences.

Dr. Jay Keystone, a medical parasitologist and professor at the University of Toronto, reviewed the records of patients at a large tertiary care children’s hospital in Toronto between 1952 and 1978. During the 27-year span of the study, he and his colleagues found “only 18 cases that met at least three out of six criteria for toxocariasis.” He calls the risk of acquiring the disease from a pet “incredibly low.”  

Conversely, Glickman cites a 2004 study that reports a greater level of incidence. "The prevalence of consultant-diagnosed toxocaral eye disease in the U.S. was 6.6 cases per 100,000 persons when only cases regarded as definite by the consultant ophthalmologist were included," the study said. "This increased to 9.7 cases per 100,000 persons when both definite and strongly suspected cases were included.”  

Even so, Keystone and colleagues have found no correlation between T. canis antibodies and dog ownership. They made that determination after checking 114 veterinary workers and 113 personnel from human hospitals in Canada for antibodies to T. canis. Contrary to what might be expected if contact with dogs increased exposure to T. canis, it turned out that 8.8 percent of the veterinary personnel possessed antibodies indicating exposure to the parasite, whereas 9.4 percent of the hospital workers were T. canis positive.

Veterinary, human medicine overlap

It's unclear how much training or education is needed to effectively prevent the transmission of zoonotic disease. What's apparent is that veterinarians and physicians aren't getting enough of it, said Dr. Scott Weese, a veterinarian and expert on zoonotic pathogens.

“Your average veterinarian isn’t really trained in zoonotic disease prevention," said Weese, associate professor in the University of Guelph's pathobiology department as well as a microbiologist for the university's Centre for Public Health and Zoonoses. “MDs aren’t, either. The medical curriculum is really packed. The veterinarian assumes that zoonotic disease belongs on the human side, and the human side doesn’t want to do it, either. If a physician or veterinarian isn’t comfortable with their knowledge of the topic, they are less likely to engage their patients or clients, or (more likely to) offer bad advice.”  

While in school, veterinarians learn about diseases but not the "whole story" about disease prevention, Weese said. Post graduation, veterinarians and physicians tend to focus their continuing education opportunities on issues seen in everyday practice rather than public health and zoonotic disease control sessions, he added.

Still, veterinarians receive more training and education on zoonotic disease prevention than their physician counterparts, Glickman said. "It’s amazing how often people get wrong advice from the physician about the role of the pet in conditions such as ringworm, allergies or pinworms."

A 2008 commentary published in the Journal of the American Veterinary Medical Association put it this way: “Because human physicians are minimally concerned with animal disease and veterinarians are minimally concerned with human disease, there is a gap in the effective control of zoonotic diseases.”   

What is a veterinarian’s responsibility in the human medical arena? That's "hard to say," Weese said.

“No one’s really defined what our role is," he said. "If you look at our oath, it’s reasonable to think that we’re part of the health team.”

Nevertheless, veterinarians may be reluctant to engage in client education when it involves human health. "I think we’ve resisted being the authority when, in fact, we often know more than the physician does,” Glickman said. “Clearly we shouldn’t give advice when it comes to treating people, but when the pet is a risk factor for human disease, we should be involved in educating owners."

Lacroix emphasizes the need for the veterinarian to engage in the human health discussion: “Veterinarians need to communicate as educators not as advisers (relative to human health). They shouldn’t be giving advice; they should be educating.”  

A veterinarian's understanding about the role pets play in households provide a critical perspective to the discussion of zoonoses, she added.

“Physicians tend to look at the risks and not the benefits of pet ownership," Lacroix said. "It’s not their world. The veterinarian needs to be part of the equation because we appreciate the risks but also the benefits of the human-animal bond. It should be a collaborative discussion.”

Mitigating liability

Veterinary-client communication can reduce a practitioner's risk of legal liability when pet owners contract zoonotic diseases, Glickman said. He references instances where parents of a child blinded by Toxacara sued everyone but their veterinarian. When asked why the veterinarian was not named in the lawsuit, they noted the good communication and respect they had for their veterinarian.  

But positive client relationships alone won't protect veterinarians from legal action, experts warn. The PLIT's Mouledous says that "courts are not really sympathetic to veterinarians with regard to human injuries."  

She advises veterinarians to educate clients about zoonotic diseases, and document that such information was relayed. “You really need to educate and document with any zoonoses,” Mouledous says.

Lacroix suggests that veterinarians should explain to clients that “there are diseases and parasites that go from animals to people, and that certain individuals are more susceptible than others (to contracting them).”

Lacroix and Glickman recommend guidelines laid out by the Companion Animal Parasite Council that advise veterinarians to suggest monthly pet deworming to clients when the conversation turns to zoonotic disease prevention.  

Weese, however, suggests shifting the discussion's focus away from antiparasitic agents.

“Public health discussion tends to revolve too much around products,” he said. “These products (such as prophylactic dewormers) are important but only a part of zoonotic disease prevention. Dewormers are only effective for a limited number of zoonotic diseases, and deworming is only one component of control for those. Veterinarians also need to be engaged in talking about other aspects such as fecal handling and hygiene. If we are just focusing on products, without talking about the big picture and the day-to-day infection prevention practices people should do, that is pretty superficial.”  

Weese doesn't agree that every dog and cat should be dewormed on a monthly basis. Instead, he suggests basing recommendations around risk factors such as age of the pet (puppies and kittens are most likely to shed parasite larvae), geography and medical condition of members of the household.  

Despite his concerns that using antibacterial and antiparasitic agents can lead to drug resistance, Keystone, the medical parasitologist and University of Toronto professor, acknowledges that “deworming is the most practical” strategy for preventing the transmission of certain parasitic diseases to people. It’s “easier than behavioral therapy for humans,” he said.

Whatever strategy veterinarians decide to use, Lacroix stresses that “ultimately, the duty of the veterinarian is to inform the clients and to practice standard of care medicine.”