The immune system is made up of a number of different cells, organs, and chemicals which protect us from invading organisms. For some, their immune system either becomes inefficient or is lost completely. This may be transient, while they are undergoing treatment (for example, chemotherapy) or maturing (premature babies) or permanent, due to disease. This article looks at some diseases people can catch from cats as well as ways to reduce the risk of transmission.
Immunosuppression in people can occur for a number of reasons and many medical professionals may recommend rehoming a pet (temporarily or permanently) due to the risks of catching a disease from them. There is no argument that this is possible, however, pets provide many health and psychological benefits to their human owners, and that is especially true for people who have a serious illness. To have to cope with disease and often invasive and physically tough medical treatment and face the risk of losing a family pet is a lot to ask of a person.
What causes people to become immunocompromised?
There are too many possible causes to list in this article, but some common scenarios may include:
Chemotherapy for those undergoing cancer treatment.
Cancer itself can weaken the immune system, or cancer treatment (as above).
HIV/AIDS while treatments for HIV/AIDS have come along in leaps and bounds, this viral infection which affects the immune system can still make infected patients vulnerable.
Organ transplant recipients who need to take lifelong immunosuppressive drugs in order to avoid the body rejecting the don’t tissue.
Metabolic diseases such as chronic kidney disease and diabetes.
Bone marrow disorders which may require chemotherapy to destroy the abnormal cells before a bone marrow transplant is given.
Senior citizens whose immune systems aren’t as strong as those of younger people.
Premature babies whose immune systems haven’t developed yet.
It is believed that up to 20% of North Americans have some degree of immunosuppression.
How do immunocompromised people become infected?
Bites – This may be a play bite or a bite from an aggressive cat or even one who bites when being administered drugs or given a bath.
Scratches – May happen accidentally, during play or aggression.
Licks – Which can potentially pass on infection via the saliva.
Respiratory secretions – Sneezing, coughing.
Urine, faeces, vomit – Can all harbour pathogens. Transmission may occur during cleaning or from an accidental spray of urine.
Fomites – Inanimate objects such as food bowls, floors, bedding, doors etc.
Via the bite from a vector such as a mosquito.
Ingestion of contaminated food.
What diseases pose a risk?
There are potentially dozens of diseases which are zoonotic (transmissible between animals and humans) which encompass bacterial, viral, fungal and parasitic pathogens. Some of these diseases cause almost no outward signs in immunocompetent people, some can make immunocompromised people very sick and some can make be extremely dangerous to both immunocompetent and immunocompromised people.
Caused by an intracellular parasite, toxoplasmosis infects multiple warm-blooded animals, however, the cat is the definitive host to this parasite. Most cats and humans who become infected have mild and self-limiting symptoms (if any), however, the disease can be serious in pregnant women, causing abortion or congenital deformities as well as posing a risk to immunocompromised individuals.
Transmission: Cats become infected when they consume prey or meat containing the infective bradyzoite (an infective cyst) found in the animal’s tissue, or by ingesting infective oocysts which are present in the faeces of an infected animal.
People can become infected when they come into contact with infective oocysts when they clean out an infected cat’s litter tray. It should be noted that it takes between 1-5 days for cysts present in the feces to become infective, which shows the importance of twice daily removal of feces from litter boxes. Cats aren’t the only mode of infection, most people become infected by consuming water or undercooked/unwashed vegetables containing infective oocysts or consuming undercooked meat containing bradyzoites.
Symptoms: Cats and immunocompetent people are generally asymptomatic. Symptoms of toxoplasmosis in immunocompromised people includes localised lymph node tenderness, fever, sore throat, fatigue, general aches, and pains.
Treatment is Pyrimethamine (Daraprim) which is taken alongside the antibiotic Sulfadiazine.
Preventing toxoplasmosis should include having another person scoop out litter trays at least once a day (twice is better), avoid gardening, as well as thoroughly washing vegetables and cooking meat until it is well done.
A bacterial infection which causes enteritis (inflammation of the intestines), Salmonella can infect a wide range of animals including humans, cats, and dogs. Salmonella can make even healthy people quite sick. The annual incidence of salmonella in HIV-infected men is 384 per 100,000 compared to 20 per 100,000 in HIV-negative men with pets being responsible for 3% of cases. 
Transmission: Cats become infected by consuming and eating infected prey, eating contaminated meat, coming into contact with saliva or feces from an infected animal or from contaminated objects.
People become infected much the same way as cats.
Symptoms of salmonella in people and cats include vomiting, diarrhea, poor appetite, fever, dehydration, abdominal pain, lethargy, weight loss.
Treatment of salmonella in immunocompromised people is antibiotics, supportive care such as fluids to treat dehydration may also be necessary.
This infection is caused by single-celled protozoa which live in the small intestine of vertebrates and is passed out of the body via the faeces.
Transmission: Cats and people can become infected with Giardia by ingesting cysts from contaminated faeces, the environment (fecal-oral route), fomites (objects such as door handles), eating food washed in contaminated water, eating uncooked food or undercooked food and from drinking contaminated water.
Symptoms of giardia in people and cats include foul-smelling diarrhea which is often yellowish in colour and frothy, flatulence, abdominal pain and distension and weight loss.
Treatment: There are a number of medications to treat giardia in both humans and cats. Most commonly antibiotics which may include Furoxone or Flagyl. Drontal Plus or Panacur are anti-worming medications which may be used to treat giardia in cats.
Another protozoal infection is cryptosporidium which attaches to and replicates in the intestinal epithelium. Non-infective cysts are passed out of the infected animal via the faeces, once in the environment, they mature, becoming infective.
Transmission: Infection occurs when infective cysts are ingested, which may be via contaminated faeces (handling litter trays, changing nappies of an infected child), the environment, fomites, eating food washed in contaminated water, eating uncooked food or undercooked food and from drinking contaminated water.
Symptoms: Diarrhea, which may have blood and/or mucus, loss of appetite and weight loss are the characteristic symptoms of cryptosporidium.
Treatment: There is no cure for cryptosporidium, healthy individuals and cats usually don’t require treatment and will make a full recovery on their own once their immune system has fought off the infection. Plenty of fluids should be consumed to replace those lost in the faeces. People who are immunocompromised may need to be given anti-retral therapy to help boost their immune systems.
This fatal viral infection can infect almost all warm-blooded mammals. It affects the central nervous system of those it infects. Skunks and raccoons are a major source of infection in the United States.
Transmission: The virus is shed in the saliva and is spread via the bite from an infected animal, injecting the virus into the skin in the process. The virus affects the brain by causing inflammation, which produces psychosis and aggression. So a usually docile cat will become aggressive once infected.
Symptoms: There are three stages to rabies, prodromal, furious and dumb. Prodromal is the first stage and symptoms in cats may include low-grade fever, restlessness, loss of appetite. In humans, symptoms include low-grade fever, headache, loss of appetite, vomiting. The next stage, furious brings about signs of aggression, irritability, excitement, incoordination, and convulsions in cats. People may become anxious and agitated. The final stage, dumb causes paralysis, hypersalivation, paralysis and coma in both cats and humans.
Treatment: Once symptoms appear in both cats and humans, there is no treatment and supportive care will be given to infected people. Even cats without symptoms will need to be euthanised. People who have recently been bitten by a rabid animal will be given a fast-acting injection close to the site of the bite, this will be followed up by a series of rabies vaccinations.
Dogs and cats in rabies-affected areas should always be vaccinated.
Caused by the bacteria Yersinia Pestis, the plague comes in three forms. Bubonic, affecting the lymph nodes, septicemic which affects the blood and pneumonic, which affects the lungs. Pneumonic plague is the most fatal form with a 90% mortality rate. The plague (otherwise known as the black death) was responsible for the deaths of 50 million people in the 14th century.
Transmission: Pneumonic and septicemic are spread via the bite of an infected flea carrying the bacteria. Pneumonic plague is spread via the aerosol droplets of an infected person coughing and sneezing. Cats may also become infected when hunting.
Symptoms: Symptoms of the plague in both cats and humans depend on the type but may include anorexia (loss of appetite), fever, swollen lymph nodes which may become abscessed, coughing and sneezing in cats and people with the pneumonic form of the disease.
Treatment: Antibiotic therapy is used to treat the plague. Streptomycin, gentamicin are most commonly prescribed. While antibiotics are available to treat the plague, it is still extremely serious and if you suspect you or your cat have been exposed, immediate medical care should be sought.
Caused by a bacterium which causes enteritis in most warm-blooded animals, including people and cats. Many cats can be infected with campylobacter without displaying outward symptoms. Kittens, cats in shelters and crowded conditions and pregnant cats are all at greater risk of developing symptoms.
Transmission: The main route of transmission is foodborne, in particular, chicken. Other modes of transmission include raw milk, contaminated water, and fomites. Asymptomatic carriers can pass the bacteria in their faeces without any outward signs of infection.
Symptoms: Healthy, adult cats often don’t show any symptoms of infection. Presenting symptoms in both cats and humans include diarrhea, which may contain blood or mucus, abdominal pain, fever, nausea, and vomiting.
Treatment: In cats and people who are healthy, no treatment may be necessary other than replacing lost fluids. Immunocompromised cats or people may be prescribed antibiotics.
One of the most common zoonotic infections passed between cats and people, caused by the Pasteurella bacteria which is part of the normal oral cavity flora of cats. Between 75-80% of cats have the Pasteurella bacteria in their mouths without causing any harm. Cats with tartar and gum disease are particularly at risk of carrying the bacteria.
Transmission: Cat bites are the most common mode of infection, but you can also acquire the bacteria from cat licks, scratches, and respiratory secretions, for example, if a cat gets close to the face of an immunosuppressed person.
Symptoms: In humans, symptoms include cellulitis or abscess at the site of the bite, localised lymph nodes may also become swollen and painful. Complications in people such as infection of the joints, bones, lungs, and blood may also occur. For cats, symptoms may include abscess, pyothorax, respiratory infection, joint infection, and meningitis.
Treatment: Generally a course of penicillin, tetracycline, or cephalosporin. If an abscess has formed, your veterinarian or doctor will need to lance it and clean it out.
Cat scratch disease
Caused by a gram-negative bacterium Bartonella henslae, is an infection cats acquire when they are bitten via the faeces of fleas. One study has shown that up to 80% of cats have positive antibodies to the disease.
Transmission: As the name suggests, transmission in humans often occurs via a cat scratch. Dogs, thorns, and splinters may also be reservoirs of the bacteria.
Symptoms: Most cats are asymptomatic to the disease, however in healthy humans, symptoms may include fever, swollen lymph nodes, muscle soreness, headache, lethargy, and uveitis (inflammation of the uvea of the eye). Immunocompromised individuals can develop more serious symptoms such as serious skin lesions, vomiting, bone lesions, inflammation of the brain, liver and spleen involvement, inflammation of the optic nerve and conjunctivitis.
Treatment: In cats and immunocompetent people treatment isn’t necessary. In immunocompromised patients, antibiotics will be prescribed.
An extremely contagious and common fungal infection which can infect a number of animals. Ringworm is seen most commonly in kittens and cats in stressful or over-crowded environments.
Transmission: Cats and humans can become infected by direct contact with an infected animal or person, or indirect contact via spores on bedding, grooming equipment, soft furnishings etc.
Symptoms:Cats typically develop circular patches of broken or lost hair with a scaly redness on the skin. In humans, circular red patches appear on the skin.
Treatment: Treatment in cats is lime sulfur dips or anti-fungal medications such as Itraconazole or Griseofulvin. Humans can be treated with over the counter anti-fungal creams such as Canesten.
Also known as ‘rabbit fever’, tularemia is a bacterial infection which can be found in the environment. It predominantly lives in the macrophages, a type of white blood cell. The disease can infect dozens of species of mammal and is particularly widespread throughout rabbit and hare populations, hence the name.
Transmission: Infection can occur from eating animals infected with the bacteria, inhaling the bacteria in contaminated soil, drinking contaminated water or from tick and mosquito bites. Cats can pass the infection on to humans via contact with respiratory secretions, bites, and scratches.
Symptoms: Cats and humans may develop a respiratory infection, skin ulcers, swollen and painful lymph nodes, conjunctivitis, fever, and lethargy.
Treatment: Antibiotics are used to treat tularemia in cats and humans. Additional supportive care such as fluids to treat dehydration may also be necessary.
How can immunocompromised people reduce their chances of infection?
Firstly, work with both your doctor and your veterinarian. Let your doctor know you have a pet, and what your routine is. He will be able to give you personalised advice on how to reduce your chances of picking up an infection from your cat. Also, let your veterinarian know because he will be able to work with you to reduce your cat’s chances of developing an infection which can be passed on to you.
I realise that for some, their immune status may change for the better once they have undergone treatment. Others may remain immunocompromised for life. So please be aware that this advice is general in nature.
Adopting a new pet – It is not recommended that you adopt a new pet while you are immunocompromised, but if you must, consider adopting an older cat who is over one year old, from a reputable source and in good health. He should be health checked by a veterinarian prior to bringing him home. Common tests include FIV and FeLV, which are not contagious to you, but these diseases can compromise your cat’s immune system making him more vulnerable to other infections, which could possibly be passed on. Other tests include toxoplasmosis and internal parasites.
Desex your cat – This will reduce the chances of your cat roaming and fighting, both of which increase his risks of catching a contagious disease.
Litter trays – This job should be given to another member of the household. Litter trays should have urine and faeces removed twice a day and be fully emptied out and disinfected at least once a week.
Feeding your cat – Cats living with an immunocompromised patient should not be fed raw meat. Cooked or high-quality commercial diets are safer.
Wash food and water bowls – This should be done daily, in hot soapy water. Use a separate sponge when cleaning food bowls and wear rubber gloves.
Water – Cats should be provided with clean drinking water every day. They should not be allowed to drink from toilets or other sources such as dams, fish tanks, outside bowls etc.
Vaccinations – Make sure your cat is up to date on his vaccinations, killed vaccines are safer than live ones.
Annual health checks – These are a must for all cats, but especially cats living in a home with an immunocompromised person. Ask your veterinarian to check for FIV, FeLV as well as stool examinations for Giardia, Salmonella, Toxoplasmosis, Campylobacter, and Cryptosporidium.
Regular flea treatment – While fleas shouldn’t be a problem to the immunocompromised person, they can transmit diseases to cats which are zoonotic. Speak to your veterinarian about the most suitable flea treatment products. See here for more information on flea treatment products.
Worm control – In addition to treating your cat for fleas, worming products should also be given to your cat.
Keep pets indoors – This prevents your cat coming into contact with other animals carrying infection. This may be by social contact, fighting, or hunting. A number of diseases and parasites can be caught in cats who hunt.
Don’t kiss your pet, always wash your hands after petting.
Be careful when playing with your cat – Don’t roughhouse with your cat to reduce your chances of being bitten or scratched during play.
Litter trays should be kept well away from food preparation areas.
What else can immunocompromised people do to avoid infection?
Wash hands frequently, especially after touching animals, before and after handling food, prior to eating after being in public areas.
Keep your hands out of your mouth. Many infections (a number of gastro type infections as well as colds and flu to name a few) can be spread when you touch your nose or mouth with contaminated hands.
Clean and disinfect any cat-related bites or scratches and wounds immediately, and seek medical advice. Your doctor may choose to put you on a prophylactic course of antibiotics just to be safe.
Avoid crowded areas.
Don’t garden and especially don’t handle soil or potting mix. These contain many pathogens.
Avoid renovating if a person in the home is immunocompromised as this can expose them to building dust.
Keep your cat’s claws trimmed, or apply Soft Paws to the claws. These are plastic covers which are glued over the top of the cat’s nails.
Avoid touching farm animals or visiting petting zoos.
Tell friends and family to stay away if they have an infection, even something as mild as a cold can be life-threatening to an immunocompromised immune system.
Speak to your doctor about safe swimming. Many lakes, rivers, dams, water parks, swimming pools and oceans can be contaminated with parasites which can easily be swallowed.
Avoid changing nappies (diapers) if possible. If you are the sole carer for a child, wash your hands thoroughly after changing nappies.
Stay away from chicken coops, areas where birds congregate and don’t clean out bird cages.
Be diligent with mould in your home. Ensure there is adequate ventilation to discourage growth.
Don’t share towels or other personal care items such as hairbrushes and toothbrushes.
Cover all scratches and wounds.
1) Celum CL, Chaisson RE, Rutherford GW, Barn-hart JL, Echenberg DF. Incidence of Salmonellosis in patients with AIDS.
J Infect Dis
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