Cats and infectious disease

Definition

Feline-to-human infections are zoonotic diseases that are transmitted through bites and scratches, through contact with shared vectors (such as ticks, fleas, or mosquitoes), through shared environments (such as contaminated soil), and through direct contact with infected skin.

Bites and scratches. In 2022, about 1 percent of emergency room visits were attributed to injuries caused by animal bites. Out of those cases, the majority (60 to 90 percent) involved dog bites and a small minority (5 to 20 percent) involved cat bites. Despite the disparities, few dog-related bites result in infectious complications; 50 to 80 percent of cat bites (depending on the source) become infected. Although a dog can exert 450 pounds of pressure per inch when biting, canine teeth are relatively dull. A feline’s long and sharp teeth, in contrast, can penetrate human skin, create deep puncture wounds, and penetrate tissue surrounding bones. Consequently, a cat-related bite wound is more likely to result in an infection.

Pasteurellamultocida is a gram-negative bacteria found in the mouths of most cats. The bacteria are very common and can be transmitted through cat scratches, bites, or saliva (by licking). The first signs of infection (pain, swelling, and redness) usually occur within two to twelve hours of being bitten. Pasteurellosis, the disease caused by the bite, can spread quickly through the body from the wound site, so one should seek medical attention immediately. Bites to the hand require special attention. If left untreated, the infection can cause complications, such as upper respiratory problems, pneumonia, prosthetic-valve endocarditis, and, less often, meningitis and brain abscesses.

Sporotrichosis is a severe skin infection caused by the fungus Sporothrix, which lives on plants and in soil. It can be spread from cats to humans via cat bites and scratches and through contact with lesions. While rare in the United States, sporotrichosis has caused a zoonotic epidemic in Brazil. In humans, the fungus enters the body through a cut or a scrape. It most commonly appears on the skin on the hands and arms.

Shared vectors. Cat scratch fever (CSF), or bartonellosis, is transmitted through cat scratches and bites. In the twenty-first century, between twenty thousand and twenty-five thousand people in the United States are diagnosed with this infection every year, but the disease is most prevalent in warm, humid climates where fleas thrive. Although 30 to 40 percent of healthy-looking cats may be carriers, kittens most often transmit CSF. When fleas bite an infected cat, the bacteria are spread through flea excretions onto the cat’s skin and, ultimately, to the cat’s claws or saliva.

CSF is more common among persons with suppressed immune systems (persons with HIV or who are undergoing chemotherapy) and with children because their immunity is less developed and because they are more likely to roughhouse with cats. Symptoms include a rash or blister at the wound site; swollen lymph nodes around the head, neck, and upper limbs; fever; headache; and sore muscles and joints. The infection usually disappears in four to eight weeks. In immunosuppressed persons, however, complications, often severe, include high fever, sweats, chills, vomiting, and weight loss. Flea control is the key to preventing this disease, so one should consult a veterinarian.

Shared environment. Salmonellosis is a common bacterial disease of the intestinal tract of many animals. It is usually contracted by eating undercooked meat and eggs or contaminated vegetables. When cats eat raw meat or wild prey, they are more likely to contract this bacteria and transmit it through contaminated stools. Salmonellosis is one of the few infections that can be passed from humans to cats and back to humans.

Following a twelve- to thirty-six-hour incubation period, symptoms include headache, fever, diarrhea, nausea, and dehydration. The best prevention is to feed cats processed foods and to wash hands thoroughly after cleaning litter boxes. Also, a person who has a cat as a pet should be extra vigilant if the cat has diarrhea.

Another form of transmission in a shared environment is contact with cat feces contaminated with Toxoplasma gondii, a single-celled protozoal organism that infects animals and birds. Only in the cat, however, does this organism find an ideal host to reproduce and complete its life cycle. Cats become targets when they ingest contaminated prey or raw meat, or infected soil. Once ingested, the bacteria burrow into the cat’s intestine, and early-stage cells, called oocytes, are eliminated in cat feces. The bacteria can also foul soil, water, gardens, sandboxes, or any location where an infected cat defecates. Because toxoplasma oocysts require one to four days to incubate to become infective, it is important to empty litter daily and dispose of waste properly to prevent this serious parasitic disease.

Typically, symptoms include body aches, swollen lymph nodes, headaches, fever, fatigue, and eye infections. However, in immunosuppressed persons or in pregnant women, complications can be serious. Pregnant women who contract toxoplasmosis have a 30 percent chance of passing this infection to their fetus, which can result in stillborn births or miscarriage. Even children who survive may develop complications, such as seizures, an enlarged spleen, jaundice, and eye infections. Additionally, research has linked toxoplasma to mental illness such as schizophrenia and bipolar disorder in adults.

Pregnant or immunocompromised persons should ask another person to change a litter box daily, and those who clean the litter box should wear gloves and wash hands thoroughly afterward. Cat owners can limit exposure by keeping cats indoors.

Direct contact. Ringworm is a fungal infection that may manifest in cats as, most commonly, Microsporum canis; Trichophyton, through rodent contact; and M. gypseum, through contact with contaminated soil. Ringworm is highly contagious and is spread through direct contact with an infected animal or through spores shed in carpets, furniture, bedding, and air filters. Spores can last eighteen months, so treatment must include thorough house cleaning. Most common among kittens, the resulting ringworm lesions consist of localized hair loss, scaling, and crusting, although some cats are asymptomatic carriers. Treating cats with medication is highly recommended. To reduce environmental contamination, infected cats should be confined to one room until they are ringworm free and until the household can be disinfected.

Impact

Although there are thirty or more zoonotic infectious diseases, people are more likely to contract infections from other people than from cats. Also, the principal cause of certain common infections, such as those caused by Salmonella and T. gondii, is not a cat scratch or bite; instead, Salmonella and T. gondii are spread most often through contaminated food and soil and through undercooked meat.

Most cat-to-human infections can be prevented by keeping cats indoors and by practicing good hygiene, which includes washing hands after handling pets, disinfecting contaminated areas, wearing rubber gloves when disposing of cat litter, and following recommended practices, such as regular veterinary checkups, rabies vaccinations, and flea control. Lastly, because kittens are more likely to harbor certain bacteria and to engage in play-stalk behavior, which can result in bites or scratches, immunocompromised persons who want a pet cat should adopt or otherwise obtain an adult cat and not a kitten.

Bibliography

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Nolen, R. Scott. "CDC Issues Warning About Spread of Cat-Transmitted Sporotrichosis." American Veterinary Medical Association (AVMA), 29 Mar. 2023, www.avma.org/news/cdc-issues-warning-about-spread-cat-transmitted-sporotrichosis. Accessed 2 Feb. 2025.

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