Zoonotic risks of pets: How to handle questions
Recently, researchers asked 327 physicians how often they consulted veterinarians about zoonoses and how often veterinarians sought out their opinions. Then, 322 veterinarians answered the same question with the situation reversed. On a scale of one to five, with one being several times a day and five being never, the researchers discovered that both groups scored between roughly 4.2 and 4.5 for each question, a fairly poor score.
"Clearly, we and our physician counterparts have a large communication gap to bridge regarding the zoonotic aspects of animal and human disease," said Dr. Alice Wolf, a professor at Texas A&M University.
Dr. Wolf spoke at the recent American Animal Hospital Association meeting and used the statistics from a 1999 University of Wisconsin study to explain why patients who get their information on zoonoses from misinformed physicians may have the wrong idea about pet risks. Her presentation focused on certain animal diseases that are unfairly implicated in human infections.
Allaying fears
In some cases, for example, physicians tell parents that a pet may be to blame for their child's recurrent Streptococcus infections. Evidence, however, doesn't support this. In a study of approximately 200 dogs, researchers found that pet owners who had dogs with Streptococcus infections were no more likely to have such infections themselves than owners of infection-free dogs, she said. "There is more evidence that pets carry group A Strep[tococcus] temporarily and only when in contact with an infected person. So, tell your kids with Strep[tococcus] not to kiss the kitty."
Another agent that is more likely to be an anthroponosis, a disease communicable from humans to animals, is Helicobacter pylori, which plays a role in human gastric disease. H pylori has not been found in dogs and only rarely in cats. In a recent survey of 25 stray cats, 23 had spiral bacteria, but none were H pylori, Dr. Wolf said.
Veterinarians can also allay clients' fears about a variety of other agents. Dogs and cats are rarely infected with Salmonella, and to date, no human infections have been associated with cats. Salmonella infection of humans by reptiles has occurred but is atypical. Dogs and cats may be temporary hosts of Giardia, but they acquire infection by the same route as people. You are not going to get this protozoal infection directly from your pet, Dr. Wolf said. And clients need not be concerned with parvovirus or cytomegalovirus at all—animal strains of these viruses cannot infect humans, and human strains cannot infect animals.
According to Dr. Wolf, one of the latest scare tactics in the vaccine wars is the implication that prevention of Bordetella infection in dogs and cats is important because of human infection risks. B bronchiseptica pneumonia in humans, however, is rare. "There have been cases reported in humans but usually in immunosuppressed individuals, [and these] strains have more in common with swine and human strains than dog and cat strains. There is no real rationale for vaccinating pets to protect humans."
Veterinarians may also get questions about pets and Cryptosporidium parvum, but people are more likely to get this infection from food animals than from pets. "Infections in dogs and cats are rare and usually subclinical," Dr. Wolf said. For example, fecal specimens from 200 stray dogs impounded at the San Bernadino City and County animal shelters were screened for Cryptosporidium, and only two percent had the parasite. In a similar survey of 206 cats, 5.4 percent had shed oocysts.
Campylobacter infection, which is usually contracted from undercooked meat or unpasteurized milk, has also been contracted from cats and dogs. In a study from Denmark looking at 72 puppies and kittens between 11 and 17 years of age, 29 percent of fecal samples from the puppies were positive for Campylobacter and many had C jejuni infection, Dr. Wolf said. Results of only two of the samples were positive, but not with C jejuni. "Among dogs and cats, it seems to be carried more by young animals and animals that originate in a multiple-animal environment [such as shelters]," she said. Birds, small pocket pets, and large exotics such as ferrets and rabbits are also known to carry Campylobacter.
Veterinarians can tell clients to decrease their risk of contracting zoonoses by implementing preventive measures such as avoiding contact with feces, hand washing, and good sanitation, cleaning the house with agents such as Lysol, for houses without cats, or Clorox. In addition, flooding areas of a yard where animals defecate can reduce the number of organisms that are concentrated in a particular location. "Dilution is the solution to pollution," Dr. Wolf said.
Precautions for cat scratch disease and toxoplasmosis
Perhaps the most popular zoonosis discussed at veterinary clinics is toxoplasmosis, an infection transmitted to humans primarily from undercooked meat. Obstetricians often recommend that women relinquish their cat if they are pregnant, but this is certainly not the only option. According to a 1993 Journal of the American Medical Association study of 700 HIV-infected men, researchers found that those who owned or had contact with cats were no more likely to be infected with Toxoplasma spp than individuals who had no contact.
Suggesting that titers be obtained for the client as well as the cat may help allay fears. "If the cat is seropositive, it has already gone through acute infection, and that cat is quite safe," Dr. Wolf said. "It only goes through intestinal shedding once during acute infection." In addition, if the owner already has a titer, the fetus is not at risk when she becomes pregnant, because she has already gone through acute infection.
If individuals are seronegative, various precautions can decrease risk. Because it takes 48 hours for Toxoplasma oocysts to sporulate into an infective form, a nonpregnant person should empty the litter box daily, disposing of the waste in a closed outdoor container. Cats should be confined to the house to decrease the chance of acquiring infection through predation, and, of course, individuals should wash their hands thoroughly after handling a cat.
Bartonellosis, formerly known as cat scratch disease, is another disease that can be risky, especially for immunocompromised persons, those who are HIV positive, or who are receiving immunosuppressive therapy. Although removing an infected cat from a household is not necessary for healthy people, prudence is advised for immunosuppressed people.
First, however, an individual must determine whether the household cat is the source; perhaps, a neighborhood cat is the offending one. If the household cat is the source, Dr. Wolf recommends that veterinarians do their best to help clients keep the cat, especially in cases where the individual is HIV positive. These individuals may lead isolated lives, and their pets mean the world to them.
If a cat is already infected, Dr. Wolf recommends treatment with azithromycin, doxycycline, amoxicillin, or ampicillin-clavulanate. Once cleared of infection, cats are apparently resistant to rechallenge with the organism and do not redevelop bacteremia.
But, pets also mean the world to many healthy individuals. It is a veterinarian's job to educate clients and doctors about the real risks of pets in zoonoses, Dr. Wolf said. "With the exception of Bartonella henselae and zoophilic dermatophytes, infections in humans by so-called zoonotic agents are more likely to be acquired from sources other than pets."