by Arnold Plotnick MS, DVM, ACVIM, ABVP

Millions of Americans have conditions that compromise their immune system, including over 1 million people infected with the Human Immunodeficiency Virus (HIV), the virus that causes AIDS.  Immunocompromised people are susceptible to a number of infections.  Pets are a potential source of opportunistic infections.  In the United States, it is estimated that 30 to 40% of immunocompromised people own companion animals.  Unfortunately, a lack of knowledge about zoonotic diseases – diseases that can be spread from animals to humans – have led some health care providers to suggest that people with HIV should not own pets. How likely are immunocompromised people to acquire an illness from their pet(s)?   How can the risk of acquiring infections be minimized?

HIV profoundly affects the immune system.  The susceptibility for acquiring infections is measured by monitoring the T-cell count, especially the CD4 lymphocyte levels.  As the CD4 count decreases, the risk of developing infections increases, especially when counts are less than 200 cells/ml.  Fortunately, recent developments in treatment of HIV (combination drug “cocktails”) have helped millions of HIV patients maintain good levels of CD4 cells, reducing the risk of opportunistic infections.  However, people with HIV or other disorders that compromise the immune system need to be aware of illnesses they may be exposed to via their pets.

The main infectious diseases that pose a threat to immunocompromised people are bartonellosis (cat-scratch disease), campylobacteriosis, cryptosporidiosis, mycobacteriosis, salmonellosis, and toxoplasmosis.  While pets may be a source of infection, they are not the most common source for most of these diseases.  Contact with raw or undercooked meat, or from an environmental infectious source are more commonly implicated in transmission.

For example, most people (approximately 70% of the population) have already been exposed to the organism that causes toxoplasmosis and they harbor the infection in their bodies.  The organism causes no problems if the immune system is competent.  In people with weakened immune systems, however, toxoplasmosis can be deadly.  The AIDS patient is prone to developing toxoplasmic encephalitis, an inflammation of the brain.  In the United States, an estimated 20 to 25% of AIDS patients will suffer some symptoms of toxoplasmosis during the course of their illness, and 3 to 20% of AIDS patients will die from the disease.  When HIV-positive people develop cerebral toxoplasmosis, most cases are shown to be re-activations of existing infections, and are not due to new exposure from animals.  Studies have shown, in fact, that development of cerebral toxoplasmosis in HIV-positive people occurred independently of whether or not a person owned a pet.  Transmission to people usually occurs through the eating of raw or undercooked meat, or handling of contaminated soil, for example, during gardening.

Bartonellosis (cat scratch disease), however, is transmitted directly from cats to people through scratches and/or bites.  It is usually of no major consequence in immunocompetent people, but may be life-threatening in people with compromised immune systems.

Immunosuppressed patients are much higher risk for three infectious diseases that cause severe diarrhea:  salmonellosis, cryptosporidiosis, and campylobacteriosis.    Salmonellosis is a common infectious disease that may be acquired via contact with domestic and wild animals, including cats and dogs, however, most cases are not due to contact with animals.  Contaminated food and water are the most important sources for infection; only 3% of salmonella exposure is from pets.  Reptiles such as snakes, lizards, and turtles are common sources of salmonellosis, and immunocompetent people should probably not own these animals.

Cryptosporidium is a protozoan that can cause severe, persistent diarrhea in immunocompromised people, often with a fatal outcome.  Cryptosporidium is most often transmitted through contaminated water.  Cattle are a major source of cryptosporidiosis.

Campylobacter is a bacterium that, in immunocompromised people, is associated with acute, severe, often bloody diarrhea and abdominal pain.  The most common mode of transmission is via contaminated food and water; studies have shown that only about 6% of cases of campylobacteriosis is due to exposure to animals with diarrhea (puppies and kittens especially).

Pet birds pose a low risk to immunocompromised people.  Concerns about two main avian infectious diseases, cryptococcosis and  mycobacteriosis, are largely unwarranted.  Cryptococcus neoformans, the organism that causes cryptococcosis, is commonly isolated from soil and droppings of wild birds, especially pigeons, but seldom from pet birds.  Pet birds are an unlikely source of Cryptococcus infection for human beings. 

Marine exposure may also be an issue for immunocompromised pet owners.  Owners of aquarium fish may want to consider having someone else clean their fish tanks, to prevent infection with Mycobacterium marinum, an organism that can cause disseminated granulomatous infection in immunosupressed individuals.  Human infection, though uncommon, can occur if abraded skin is exposed to contaminated water.  If cleaning tanks themselves, one should wear durable gloves. 

There are several ways that immunocompromised people can protect themselves from infections spread by animals.  Giving up one’s pets is not the answer.  On the contrary, according to Dr. Deborah Marriott, of the St. Vincent’s Hospital in Sydney Australia, the health benefits of pets outweigh the risks of zoonotic disease transmission for most HIV-positive people.  Dr. Marriott presented her findings at the XIII International AIDS Conference in Durban, South Africa.  “The overall risk of transmission from contact with pets is extremely low, and I feel that the benefits of animal ownership to the infected person outweigh the risks”, says Marriott.  Health benefits of owning pets include decreased cholesterol, decreased blood pressure, and better ability to deal with loneliness. Data from the multicenter AIDS cohort study (MACS) showed that owning a pet decreased the chance that someone with HIV or AIDS would experience depression. 

To protect oneself from infections spread by animals, the Centers for Disease Control recommends:

·       Washing your hands well with soap and water after playing with or handling animals, especially before eating or handling food.

·        Feed pets only pet food, or cook all meat thoroughly before giving it to your pet.  Do not feed raw or undercooked meat.  Don’t let pets drink from toilet bowls or get into garbage.  Don’t let pets hunt or eat another animal’s feces.

·        Do not handle animals that have diarrhea.  If your pet has diarrhea that lasts more than 1 or 2 days, have an HIV-negative friend or relative take the pet to your veterinarian and have the animal screened for infectious causes of diarrhea.

·        If you’re thinking about acquiring a pet, make sure you check out the sanitary conditions and licenses of the establishment you plan on using, whether a pet store, breeder, or animal shelter.  Do not bring home an unhealthy pet, and avoid kittens and puppies; kittens and puppies are more likely to have infectious causes of diarrhea, and are more likely to scratch or bite.

·         Do not touch or handle stray animals, as they are more likely to scratch and/or bite, and can carry many infections. 

·         NEVER touch the stool of any animal.

·        Ask or have an immunocompetent individual (who is NOT pregnant) to change your cat’s litter box daily.  If you must clean the box yourself, wear vinyl or household cleaning gloves and wash your hands with soap and water right afterward.

·         Keep your cat’s nails trimmed so that you will not get scratched.  Discuss ways to prevent being scratched with your veterinarian, such as declawing or the use of plastic nail covers.

·         Don’t let your pet lick you on your mouth or on any open cuts or wounds you may have.

·         Don’t kiss your pet

·        Avoid owning reptiles such as snakes, lizards, and turtles.  Wash your hands with soap and water immediately after handling any reptiles.

·          Practice good flea control.

·         Avoid exotic pets such as monkeys, ferrets, or wild animals such as raccoons and skunks.

·         Use caution and wear gloves when cleaning aquariums and fishbowls, or have an immunocompetent friend or relative do it for you.

·         Consider keeping pets indoors to limit their exposure to other animals or their feces.

Get regular veterinary check ups for your pets.

Updated 2/9/06