Introduction

 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 and the infectious disease risk from owning pets is considered low. Nevertheless, there is a segment of our clientele that will ask us for advice about some potential zoonotic diseases and we should be prepared to help counsel them AND their physicians about the true nature of risks associated with these agents.

Toxoplasmosis

We get many concerned calls from female clients who are pregnant or want to become pregnant who have been advised by their physician to “get rid of your cat.” Undercooked meat is the most common vehicle for of T. gondii infection. Up to 25% of lamb and pork samples contain Toxoplasma tissue cysts. Therefore, although cats are the definitive host for T. gondii, cat ownership is not associated with an increase in Toxoplasma seroconversion, even in HIV-infected humans. (Wallace et al., 1993)

The hysteria generated by MD’s is therefore unfounded. However, if there is a cat in the household, it is reasonable to take routine sanitation precautions. It takes 48 hours for Toxoplasma oocysts, should they be present, to sporulate into an infective form. The best recommendation to make is to have another person in the household empty the litter box daily. Waste should be placed into a closed outdoor container, not into the yard. Owners should wash their hands well after handling the cat, handling raw vegetables, or digging in soil. Fruits and vegetables should be washed thoroughly before eating and meats should be cooked to at least an internal temperature of 160 F. The cat can be confined to the house to decrease the chance of acquiring Toxoplasma infection through predation.

If the owner has a Toxoplasma titer performed prior to conception and it is positive, there is no risk to the fetus during the pregnancy, regardless of exposure. If the cat has a positive titer, it is extremely unlikely to re-shed oocysts and is therefore an exceptionally safe companion.

At least one biologics manufacturer is working on a Toxoplasma vaccine for cats. Cats do not need this vaccine because clinical toxoplasmosis is quite rare. A toxoplasmosis vaccine would have little to no public health significance, because, as stated previously, pet cats are not associated with human toxoplamosis. The cats that might benefit from vaccination in order to reduce the shedding of oocysts into the environment and contaminating animal feeds are feral and barn cats. These are unlikely to receive veterinary care. Therefore, should this vaccine be developed, it is not appropriate for other than exceptional case use.

Bartonellosis (Cat Scratch Disease, Bacillary Angiomatosis)

Cat scratch disease is the usual result of B. henselae infection in normal, immunocompetent human beings. Bacillary angiomatosis, peliosis hepatis, and Bartonella bacteremia usually occur following B. henselae infection of immunocompromised persons. This latter group includes persons infected with human immunodeficiency virus (HIV) but also may include persons receiving immunosuppressive therapy for cancer chemoprophylaxis, immune-mediated diseases, or to prevent organ transplant rejection. Occasional cases of severe disease have also been described in persons with other chronic illnesses including alcoholism, diabetes mellitus, and lymphocytic leukemia.

Asymptomatic B. henselae bacteremia is quite common in the feline population. Kittens have a higher level of bacteremia than adult cats and thus, are more efficient transmitters of the organism. B. henselae infection has been found in cats in contact with CSD and BA patients as well as in a high proportion of stray and household pets. Fleas are intimately associated with CSD because the organism is ingested by fleas drinking a blood meal and is deposited on the skin in flea excreta where it is then transferred to the claws when the cat scratches due to flea infestation. Fleas also transmit the organism from cat to cat but do not appear to be involved in transmission to human beings. B. henselae infection in humans most often occurs subsequent to cat scratches (hence the name, cat scratch disease), but has also occurred following a cat licking an area of abraded skin, or because of cat bite wounds. There is some evidence that young cats (less than 12 months of age) may be more likely to transmit B. henselae than older cats probably because of a higher level of bacteremia in these young cats. A history of cat contact is found in the majority of human beings with CSD, and although limited studies are available, a history of cat contact is also reported for a high proportion of BA patients. The offending cat may not be a household pet of the patient and affected persons should be questioned about contact with cats outside their own household.

Cats in contact with CSD or BA patients can be tested for the presence of B. henselae bacteremia or antibodies against these organisms. In the case of immunocompetent owners, even if B. henselae bacteremia can be demonstrated in their household pet, it is not necessary to euthanize the cat nor remove it from the household. Recovered CSD patients are likely to be immune for life and it is rare for additional cases to occur in the same household. Because disease caused by B. henselae is potentially life threatening in immunosuppressed persons, greater prudence may be advisable and the owners should consider removing the infected cat from the household or attempting treatment to reduce or eliminate bacteremia. Excellent flea control will also help prevent the cat from infecting other cats and prevent the skin and claws from becoming contaminated with organisms.

Several cats have been successfully cleared of bacteremia following treatment with doxycycline at 25–50 mg PO q12h for 30 days; however, other studies have not demonstrated similar success. Additional recent studies have effectively cleared cats with doxycycline, amoxicillin, or amoxicillin-clavulanate. Once cleared of infection, cats are apparently resistant to rechallenge with the organism and do not redevelop bacteremia. Azithromycin is the current treatment of choice for humans with bacillary angiomatosis and CSD responsive to antibiotics. This has not been tested in cats, but is safe and would be a good alternative to the drugs listed above. Prophylactic treatment of cats in contact with immunocompromised people may also be a good precaution.

Group A Streptococcus in Dogs and Cats

The few published prevalence studies suggest that GAS infection in dogs is rare: Biberstein (J Clin Microbiol, 1980; 11:558-61) recovered 254 isolates of hemolytic streptococci from dogs over a three year period, 45% from the skin, 25% from the genitourinary tract and 13% from the respiratory tract. Eighty-one percent of these isolates were group G; only 1% was GAS. Kurek and Rutkowiak (Epidemiol Res, 1971; 25:234-8) reported 7% of urban pets positive for GAS, while Peterson (USAF Med Service Digest, 1976; 27:21) found 1% positive.

Evidence of an association between GAS in dogs and human disease is limited to case reports: Mayer (Postgrad Medicine, 1983; 74:277-9) reported on a family of four that experienced recurrent GAS pharyngitis. After two unsuccessful simultaneous treatments of the family, their dog was found to be culture-positive for GAS. Family and dog were treated and all were culture negative four weeks after treatment. Copperman (NYS J Med, 1982:1685-7) investigated 72 dogs (along with some cats and birds) over a 16-yr period as possible sources of recurrent strep sore throat in humans; he found that 42% were positive for GAS and after treatment of both family and dog most cases resolved. For comparison, he studied a consecutive dog control group not known to be in contact with SST and found 10% prevalence (2/20).

Analytical studies suggest no association between human GAS disease and infected dogs. Crowder (Int J Zoon, 1978; 5:45-54) found 3/71 (4.2%) dogs from families with GAS disease were positive for GAS, similar to the 4/143 (2.8%) from families free from disease for 30 days. Recently, Wilson (Pediatr Infect Dis J, 1995; 14:372-5) investigated throat-cultured children with acute pharyngitis from 42 households. Although 26 households had a child with GAS, none of the oropharyngeal secretions from 43 household dogs and 25 cats was GAS positive. In addition, no GAS was recovered from 149 dogs and cats surveyed at a veterinary hospital, although 9% of the dogs were positive for group G strep.

Unpublished data (to be presented at ICAAC) from Dr. Ed Kaplan (U. Minnesota) also suggest that carriage of GAS in dogs is rare and not associated with human illness. Throat cultures were done on 295 dogs (from 190 households) undergoing elective surgery: 21% were positive for Beta-hemolytic strep. Ninety percent of isolates were group G, only one isolate was group A. No association between strep-positive dogs and human illness was observed.

While some of the case reports suggest that treating culture-positive dogs can help clear recurrent infection within a household, there seems much more evidence that dogs are not a reservoir of human GAS disease and the consensus from veterinarians was that culturing dogs wasn't worth it. Some suggested that dogs only carry GAS temporarily while in contact with infected humans.

Salmonellosis

Again, like Toxoplasmosis, the most important source for Salmonella typhimurium or other type B Salmonellae infections in human beings is contaminated meat. Although, there have been rare (but highly publicized) instances of human exposure and infection from reptiles (turtles, etc.), and other animals, these not typical. Cats can acquire salmonellosis, most often from exposure to migratory songbirds. It causes a “panleukopenia-like” syndrome sometimes called “song bird fever.” Cats preying on diseased birds or frequenting areas where the birds feed develop an acute febrile illness of two to seven days duration. Clinical signs in infected cats include depression, anorexia, vomiting, hemorrhagic enteritis, and high fever. Laboratory findings include panleukopenia, slight thrombocytopenia in some, and hyperbilirubinemia. Treatment of affected cats consists of isolation and supportive care. Antibiotics are not recommended routinely in enteric cases since it reduces normal gut flora, which enhances colonization of Salmonellae. Antibiotics (gentamicin, ampicillin, chloramphenicol) are recommended if septicemia is present. To prevent cat infection, prevent access to bird feeding areas and infected birds. Stop bird feeding in spring and summer and disinfect feeders with Clorox. Human infections associated with affected cats have not been reported but may be associated with handling infected birds. Assuming Salmonella is involved, the owners should be advised of the risks and proper precautions taken.

Helicobacteriosis

The discovery of the spiral bacterium Helicobacter pylori and its causative role in gastric disease in humans has raised concern among physicians about whether these organisms are shared between humans and animals. Although spiral bacteria have been known for more than a century to infect the stomachs of dogs and cats, H. pylori has not been found in dogs and only very rarely in cats—zoonotic risk is minimal. In a recent survey, 25 stray cats were examined for the presence of H. pylori by histological examination, culture and two polymerase chain reaction (PCR) assays. Histologically, the gastric biopsy specimens from all cats showed large spiral organisms typical of H. felis and not H. pylori. Samples from 23 cats yielded bacterial growth and two had no growth. Colonies grossly similar to H. pylori were tested for catalase, oxidase, urease and Gram's stain reactions. None was H. pylori. Therefore, isolations of H. pylori from animals are more likely a case of an anthroponosis—animals acquiring a pathogen from humans or “man bites dog” (or cat) rather than the reverse. Most therapeutic studies in pets have not shown long-term eradication of Helicobacter spp.; whether this is due to reinfection or recrudescence has not been established.

Campylobacteriosis

The most common risk factor for acquiring Campylobacteriosis is from raw or undercooked meat, especially chicken, and unpasteurized milk. It can however be acquired from dogs or cats (especially puppies and kittens that are strays or from kennels, shelters, catteries), or from ferrets, hamsters, birds, or rabbits as well as farm animals. Animals that are shedding the organism may or may not have diarrhea, although usually they usually do. Fecal cultures should be taken from a fresh specimen or rectal swab and put into a transport media made for enterics. Call your laboratory regarding how to do this. Campylobacter is not considered a “normal” flora in dogs or cats, but there are asymptomatic shedders/carriers. In a cross-sectional study was performed in Denmark between August and December 1996, 72 healthy puppies and 42 healthy kittens, aged between 11 and 17 weeks, were sampled for fecal Campylobacter shedding by culture of rectal swab specimens. Twenty-one (29%) of the puppies were positive for Campylobacter spp., with a species distribution of 76% C. jejuni, 5% C. coli, and 19% C. upsaliensis. Of the kittens examined, two (5%) excreted campylobacters; both strains were C. upsaliensis. In another study, Campylobacter jejuni was isolated from 0–49% of animals tested. The prevalence was higher in dog populations (average of 27.3%) than in cat populations (average of 11.2%) tested. If a pet is diagnosed with clinical campylobacteriosis, the animal should be isolated when treated, and preferably hospitalized to prevent transmission. Quinolone antibiotics are reported to be effective against Campylobacter. Anyone in contact with an area, surface, etc., that is potentially contaminated with infected feces should wash thoroughly with soap and water. All contaminated equipment should be disinfected or washed and dried.

Giardiasis (Beaver fever)

The infectious agent of concern to human beings is Giardia lamblia. Natural hosts for this organism are beaver, muskrat, and waterfowl. Transmission is by fecal contamination of water and hand-to-mouth transfer of cysts from feces of an infected animal. Dogs and cats may be temporary hosts for this organism and acquire infection by the same route as human beings. Treatment is with metronidazole or fenbendazole. Prevention consists of avoiding hand-to-mouth contact while handling any wildlife species, good personal hygiene and hand washing.

Cryptosporidiosis

Cryptosporidium parvum is a coccidian parasite with a wide host range including wild and domestic birds, cats, dogs, ferrets, horses, swine, ruminants, and primates including humans. Infections in dogs and cats are rare and are usually subclinical. Fecal specimens from 200 stray dogs impounded at the San Bernardino City and County animal shelters were screened for Cryptosporidium sp oocysts and only 4 (2%) dogs were found to be oocyst positive. A similar survey of 206 cats revealed oocyst shedding in 5.4%. Humans are most likely to be infected by contact with food animal species. Paromomycin is reported to be effective in treating patients with Cryptosporidium but may cause renal failure in cats. Prevention consists of avoiding contact with infected feces, good sanitation, and hand washing.

Bordetellosis

There have been a very few rare instances of B. bronchiseptica pneumonia in humans, usually in immunocompromised individuals. In several of these occurrences, the isotype of B. bronchiseptica was related most closely to a human or swine strain, not a dog or cat strain of the organism.

Human and Animal Parvovirus

Animal parvoviruses are significantly different from the human strain. Animal viruses are not contagious to human beings and human parvovirus is not contagious to animals.

Human Cytomegalovirus

Cytomegaloviruses of humans and animals are highly species specific and are not shared across species.