Immunocompromised Pet Owners

Several million people in the United States have compromised immune systems, including an estimated 1 million people infected with human immunodeficiency virus (HIV). Immunocompromised persons are susceptible to a number of opportunistic infections. Opportunistic infections may be acquired from pets, and 30 to 40% of immunocompromised persons may own pets. Therefore it is important for these individuals to have an understanding of the risks associated with the different species of animals commonly kept as pets.

Conditions Compromising the Immune System in Human Beings

The immune system may become suppressed by diseases or by drugs used in the treatment of specific diseases. Systemic diseases associated with immune suppression in people include congenital immunodeficiencies, diabetes mellitus, chronic renal failure, alcoholism and liver cirrhosis, malnutrition, and certain types of cancers. Persons likely to receive immunosuppressive treatments are cancer patients, organ- or bone marrow-transplant recipients, and patients with autoimmune diseases. Other immunosuppressive treatments include splenectomy and long-term dialysis. Although many infectious diseases may cause immune suppression, HIV causes the most profound suppression. Persons infected with HIV may be free of clinical signs or symptoms for many years. The acquired immunodeficiency syndrome is a severe, life-threatening condition. The median time from HIV infection to development of acquired immunodeficiency syndrome is approximately 10 years, although some individuals have remained asymptomatic 15 years after infection.

HIV-infected persons are usually monitored through measurement of CD4 lymphocyte counts. The risk of developing opportunistic infections increases as CD4 counts decrease, especially when counts are less than 200 cells per microliterl. When CD4 counts are greater than 500 cells per microliter, routine veterinary care is probably adequate for companion animals of HIV-infected clients. However, as CD4 counts decrease, implementing an expanded zoonoses (zoonoses is an infection transmitted to humans from animals) prevention program becomes increasingly important.

Guidelines

Although the health risk of owning a companion animal is thought to be low for a person with an impaired immune system, the risk can be further reduced with simple precautions. If someone with a compromised immune system elects to own a companion animal, he or she should be more vigilant of the pet's health than other pet owners are expected to be. Immunocompromised pet owners should be willing to seek veterinary care early in the clinical course of illness in their pet. They also should recognize that an expanded zoonoses prevention program might be more expensive than routine veterinary care.

Many of the opportunistic infections that infect immunocompromised persons also infect animals, but the extent to which infected pets contribute to the transmission of these diseases to immunocompromised persons varies widely. The most common infections associated with animals include Toxoplasma gondii, Cryptosporidium , Salmonella , Campylobacter , Giardia , Rhodococcus, Bartonella , Mycobacterium , and Bordetella . Each of these infections, except for possibly Bartonella, may be acquired from sources other than pets. The proportion of these infections acquired from pets is not known, but is thought to be small. When considering their prevalence and the proportion of cases originating from pet sources, the most common infections acquired by immunocompromised persons from pets apparently are those caused by Salmonella and Campylobacter.

General Recommendations

Immunosuppressed pet owners should be particularly careful about what their pets eat and drink. We recommend they feed their pets only high-quality commercial pet foods. If immunocompromised persons insist on supplementing their pets' diets, they should ensure that all egg, poultry, and meat products have been adequately cooked before feeding. Only pasteurized dairy products should be used. Tap water is adequate for most pets, unless it has been shown to be unhealthy for human consumption, in which case commercial bottled water is preferred. Pets should not be allowed to drink out of toilet bowls or have access to garbage. They should be closely supervised while outside, unless they are in their own fenced yard. Pets should not be allowed to scavenge, hunt, or eat other animals' feces. Immunocompromised persons should wash their hands after handling pets, especially before eating, and should avoid contact with their pet's feces. Cats and dogs should receive routine annual veterinary care, including a physical examination, standard immunizations, and a fecal examination for intestinal parasites. Pets owned by immunocompromised persons should receive veterinary care early in course of the pet's illness. In animals with diarrhea, a fecal sample should be obtained for Salmonella and Campylobacter culture and should be examined for Cryptosporidium and Giardia (screening for Cryptosporidium and Giardia is not necessary for birds). Animals with diarrhea should not be allowed contact with immunocompromised persons.

Obtaining a pet

When obtaining a new pet, immunocompromised persons should choose a healthy adult pet and avoid young animals, especially those with diarrhea, since young animals with diarrhea are more likely to be shedding Salmonella, Campylobacter, Cryptosporidium, or Giardia . Because the hygienic and sanitary conditions in facilities of pet breeders, pet stores, and animal shelters are highly variable, immunocompromised persons should exercise caution if obtaining a pet from these sources. Immediately after a pet is obtained, it should be examined by a veterinarian. If the pet's health is questionable, it should not be allowed contact with an immunosuppressed person. Examination of fecal samples from healthy animals for pathogens may be advisable if the client's immune suppression is severe (e.g., an HIV-infected client with a CD4 count below 200 cells per microliter).

Birds

Pet birds pose a low risk to immunocompromised persons. A high proportion of HIV-infected persons have heard they should avoid owning pet birds but these concerns are largely unwarranted. Immunocompromised persons should avoid contact with bird droppings. To further reduce the risk of salmonellosis and campylobacteriosis, immunocompromised persons should feed their birds only a high-quality diet, avoiding old seed that may become contaminated during storage. A reliable pet store may be helpful in selection of good-quality feed. Ideally, an immune-competent person should clean the cage, or the immunocompromised person should wear gloves during cleaning and immediately wash afterwards. In ill birds, samples of droppings should be obtained for culture of Salmonella and Campylobacter. Immunocompromised persons should avoid handling wild birds because they are more likely than pet birds to shed Salmonella and Campylobacter. In addition, because Cryptococcus is transmitted via inhalation and is commonly found in pigeon droppings, these persons should avoid places where pigeon droppings are abundant.

Cats

Many immunocompromised persons have been told they should not own cats to avoid developing cerebral toxoplasmosis, a common opportunistic infection among immunosuppressed persons. However, in 1 study of HIV-infected persons, an association between cerebral toxoplasmosis and cat ownership was not found. Most cases of cerebral toxoplasmosis are caused by reactivation of a previous infection, and not by an acute infection. In addition, most acute infections apparently are acquired through eating undercooked meats. Nevertheless, immunosuppressed persons should take precautions to prevent ingestion of infective cysts from cat feces.

Cats are the definitive host of Toxoplasma gondii; they are the only animals that pass cysts in their feces. Once cysts are shed, they require 1 to 5 days to sporulate and become infective. Cats pass cysts for only 2 to 3 weeks following primary infection. We do not recommend examination of feces for Toxoplasma or testing of serum for Toxoplasma antibodies in healthy cats. Fecal examination is seldom rewarding, because cysts are shed only transiently and are so small they are easily missed. Current serologic tests cannot accurately predict which cats are shedding cysts. Most, but not all infected cats have completed cyst shedding by the time antibodies are detectable in their serum. Thus, a small proportion of seropositive cats may still be shedding cysts. In contrast, a seronegative cat may be uninfected or may be recently infected and currently shedding cysts. Given the uncertainties in the interpretation of these laboratory tests and the relative ease in implementing measures to prevent zoonotic disease, these tests should not be used to modify recommended precautions. Preventing transmission of toxoplasmosis from cats should focus on litter box hygiene. Litter boxes should be cleaned daily and not placed in kitchen or dining areas. An immune-competent person should clean the litter boxes. If an immunocompromised person must clean the litter box, he or she should wear gloves during cleaning and immediately wash afterwards. Direct contact with cats is unlikely to result in transmission of toxoplasmosis because most cats do not leave feces on their fur for the 1 to 5 days required for cyst sporulation. To prevent infection with Toxoplasma, cats should not be allowed to hunt and should not be fed raw or undercooked meat. Because outdoor cats frequently defecate in gardens and Toxoplasma cysts may survive for months under appropriate conditions, immunosuppressed persons should wear gloves when gardening or working with soil and should immediately wash their hands afterwards.

Although infrequently acquired from cats, salmonellosis and campylobacteriosis may be the most common infections acquired from cats in immunocompromised persons. The previously mentioned litter box and feeding guidelines for preventing transmission of toxoplasmosis also will help prevent transmission of these enteric pathogens.

Bacillary angiomatosis is a newly recognized disease syndrome that has been diagnosed primarily among immunocompromised persons. Much remains to be learned regarding the spectrum of human disease, prevalence of infection, and methods for prevention. Disease signs among immunocompromised persons are variable and include undiagnosed febrile disease, serious skin lesions, and life-threatening systemic disease. Although antibiotic treatment appears to be effective for immunocompromised patients, such treatment may need to be prolonged. Bacillary angiomatosis seems to be caused by the same bacteria that causes Cat Scratch Disease. Cat scratch disease is diagnosed in an estimated 22,000 persons/year in the United States; however, the prevalence of Bartonella infections among immunocompromised persons has not yet been determined. Risk factors for humans acquiring Bartonella infections include owning a cat (especially a kitten), being scratched by a cat, and having a cat with fleas. Bartonella also has been isolated from the blood samples of 41% of apparently healthy cats, and from fleas from some of the same cats. However, the potential role of fleas as effective vectors has not yet been established. Likewise, typically 40 to 50% of healthy cats have evidence of past or current Bartonella infection, and antibody prevalence is higher among cats associated with infected human beings. Because the mechanisms of Bartonella transmission are not yet known, recommendations for prevention are limited to common-sense precautions. Kittens appear to pose a greater risk for disease than adult cats do. Flea control may be beneficial in preventing transmission, but this has not been proven. Persons with immune suppression should avoid activities that may result in cat scratches or bites. All cat scratches or bites should be washed immediately with soap and water. The effect of declawing of cats on the likelihood of transmission is unknown. Similarly, the effect of claw covers for reducing transmission has not been evaluated, but may be beneficial. Treating cats with antimicrobial agents has been suggested to clear the infection, but further controlled studies are necessary. The value of Bartonella antibody testing in cats for individuals making decisions regarding cat ownership remains unclear, especially in environments where the prevalence of infection and rate of transmission in cats are high. Because early antimicrobial treatment is effective in bacillary angiomatosis, all immunocompromised persons should inform their physician about the presence of cats in the household.

Dogs

Bordetella infections have been reported in several immunocompromised persons, but routine use of the Bordetella (kennel cough) vaccine is not recommended except for dogs entering kennels or on show circuits. In lieu of having their dogs vaccinated, immunocompromised persons, especially those with advanced disease, should avoid exposing themselves and their dogs to environments known to represent a source of Bordetella (e.g., boarding kennels, dog shows, or any situation in which dogs from several sources are housed together).

Although infrequently acquired from dogs, salmonellosis and campylobacteriosis may be the most common infections acquired from dogs in immunocompromised persons. Dogs are probably more likely than cats are to transmit these intestinal infections to human beings, because rates of shedding of Salmonella, Campylobacter, Cryptosporidium, and Giardia organisms in feces are usually higher in dogs, and dogs are generally less fastidious about grooming themselves.

Other Animals

Immunocompromised individuals should not own reptiles because of the high rates of carriage and shedding of Salmonella organisms in reptiles. Persons who insist on keeping reptilian pets should wear gloves when cleaning the cages and handling the pet, and should feed it a diet specifically processed for reptiles (without raw meat or egg products). Treatment of reptiles for Salmonella infection is not effective. Small rodents, such as hamsters and gerbils, can also transmit Salmonella, Campylobacter, Cryptosporidium, and Giardia infections. Immunocompromised persons should be diligent about washing their hands after handling the animals or cleaning cages.

A small number of Mycobacterium infections have been reported among HIV-infected persons. All of these patients acquired their infection from contact with pet fish usually when cleaning the aquarium. Gloves should be worn when cleaning an aquarium or when handling fish. Fish infected with Mycobacterium should be killed and the aquarium should be disinfected before new fish are introduced.

Immunocompromised persons should avoid contact with young farm animals, especially animals with diarrhea, because farm animals can act as a reservoir for several infective agents, including Cryptosporidium. Immunocompromised persons also should avoid exposure to swine, which are a known source of Bordetella. Although Rhodococcus infection has been reported in several immunosuppressed people, most patients do not report a history of recently visiting farms or rural areas, and risk does not seem to be increased by having contact with adult horses.

Conclusion

Pets can be vital to the overall mental well being of humans. The emotional damage done by preventing an immunocompromised individual from keeping a pet is often greater that the risk of them acquiring an infection from that pet. By following the above listed recommendations and precautions and with the help of their doctor and veterinarian, a person with a compromised immune system should be able to enjoy their pet without seriously risking their health.