Unexplained Symptoms? Consider Cat-Associated Infections

About 15.7 million cats live in German households. Sleeping together in bed, cuddling, or getting a small scratch during play is part of everyday life for many owners. While these interactions are usually harmless, they can sometimes provide an important diagnostic clue when patients present with unexplained symptoms. Here is an overview of the most clinically relevant cat-associated infections.Cat-Scratch Disease (Bartonella henselae)Cat-scratch disease is the most common cat-associated bacterial infection transmitted to humans. It should be considered in the differential diagnosis, particularly in patients with persistent or unexplained lymph node swelling. Reliable data on its incidence in Germany are lacking as neither the disease nor the pathogen B henselae is subject to nationwide reporting requirements.The disease is caused by the gram-negative, intracellular rod B henselae. Young domestic cats are the main reservoir and often carry the organism asymptomatically in the bloodstream. Transmission among cats occurs predominantly via cat fleas, whereas transmission to humans occurs primarily through scratch or bite wounds, and possibly also through licking preexisting skin lesions. Human-to-human transmission has not been reported to date.After an incubation period of 3-10 days, a small reddish-brown papule, and occasionally a pustule, typically develops at the inoculation site. This is followed 1-3 weeks later by the characteristic regional lymphadenitis. Axillary or cervical lymph nodes are particularly commonly affected; they may be painfully enlarged and persist for several weeks. Fever, fatigue, headaches, and body aches may also occur. Approximately 80% of those affected are children and adolescents between the ages of 2 and 14 years. In immunocompetent patients, however, the disease is usually benign and self-limiting.Diagnosis is based primarily on the typical triad of cat exposure, an inoculation lesion, and regional lymphadenitis, and can be confirmed serologically with an immunofluorescence assay. When the diagnosis is unclear, or in immunosuppressed patients, polymerase chain reaction(PCR) from lymph node aspirate or biopsy material is regarded as the most sensitive method of pathogen detection.Because the disease usually resolves spontaneously in immunocompetent patients, symptomatic treatment with analgesics and local heat is generally sufficient. Antibiotics are not routinely required, although they may shorten the duration of lymphadenitis.Particular caution is warranted in immunocompromised patients, including those receiving chemotherapy, living with HIV infection, or taking posttransplant immunosuppression. In these patients, B henselae can disseminate hematogenously and cause severe organ involvement. The most important complications include bacillary angiomatosis and hepatosplenic infection; neurologic manifestations such as encephalitis with seizures occur less commonly.Toxoplasmosis (Toxoplasma gondii)Few cat-associated infections generate as many questions in prenatal counselling as toxoplasmosis. Yet the risk posed by a household cat is often overestimated. Most human infections are acquired through raw or undercooked meat or through contact with contaminated soil, for example during gardening.In Germany, approximately 50% of all adults are serologically positive for T gondii. Each year, there are an estimated 6400 primary maternal infections during pregnancy, resulting in about 1300 fetal infections. In contrast, only 6-23 cases of congenital toxoplasmosis are reported to the Robert Koch Institute annually — an indication of significant underreporting.Cats are the only definitive hosts of T gondii. After a usually asymptomatic initial infection, they excrete oocysts in their feces for several weeks. These oocysts are not immediately infectious but must first sporulate in the environment over 24 hours to several days. For that reason, daily litter-box cleaning can significantly reduce transmission risk.In immunocompetent adults, postnatal toxoplasmosis is usually asymptomatic. When symptoms do occur, they most commonly include cervical lymphadenopathy, low-grade fever, fatigue, and general malaise. Clinically significant disease occurs mainly in two groups: pregnant women with primary infection and immunocompromised patients. During pregnancy, there is a risk for transplacental transmission to the fetus; in immunosuppressed patients, latent infection may reactivate and cause life-threatening toxoplasma encephalitis.Diagnosis is primarily serologic, based on the detection of specific immunoglobulin G (IgG) and IgM antibodies. In pregnant women, the immune status should be determined as early as possible. If protective IgG antibodies are absent, follow-up serologic testing can detect seroconversion at an early stage and allow for prompt initiation of therapy.Treatment depends on the risk profile. In cases of a primary maternal infection with no evidence of fetal infection, spiramycin is initially administered. If transplacental transmission is confirmed or highly probable, combination therapy with pyrimethamine, sulfadiazine, and folinic acid is initiated, depending on the gestational age. This is also the standard of care for toxoplasma encephalitis in immunosuppressed patients.The potential consequences for the unborn child are particularly severe. Although the risk for transmission is comparatively low in the first trimester, any fetal damage that occurs tends to be particularly severe. Typical manifestations include chorioretinitis, intracerebral calcifications, or hydrocephalus. As pregnancy progresses, the likelihood of fetal transmission increases, while the disease tends to take a less severe course.Doctors should advise pregnant women not to feed cats raw meat and, if possible, to have someone else clean the litter box. If that is not possible, disposable gloves should be worn and hands washed thoroughly afterward. It is equally important to stress that avoiding raw or undercooked meat and using careful hygiene when handling soil often reduces risk more effectively than avoiding contact with a pet cat.Pasteurellosis and Infected Cat Bite Wounds (Pasteurella multocida)From an infectious disease perspective, cat bites are among the most problematic animal bite injuries, as reported by Medscape’s German edition. While around 5%-20% of dog bite wounds become infected, up to 80% of cat bite wounds do so. Cats’ long, pointed teeth create deep puncture wounds that can inoculate bacteria into tendons, joints, and bone.Reliable nationwide data on the frequency of cat bites are lacking as animal bites are not reportable. It is estimated that 30,000-50,000 animal bite injuries occur annually in Germany, of which about 5%-10% are caused by cats.The most common pathogen is P multocida, a gram-negative rod-shaped bacterium found in the normal oral flora of cats. A characteristic feature is the rapid onset: Pain, redness, and swelling develop within just a few hours of the injury — significantly sooner than with many other bacterial wound infections. If left untreated, the infection can spread to deeper tissues and cause soft tissue infections, septic arthritis, or osteomyelitis. Bites to the hand and those near joints or tendon sheaths are particularly critical.Diagnosis is primarily clinical. If clear signs of infection are present, a specimen for microbiology should be obtained before antibiotics are started. If involvement of deeper structures is suspected, imaging such as x-ray or MRI may be needed.In addition to thorough wound cleansing and, if necessary, surgical debridement, early antibiotic therapy is crucial. The first-line treatment is amoxicillin/clavulanic acid, as this combination reliably targets both P multocida and the rest of the polymicrobial oral flora.Routine antibiotic prophylaxis after every cat bite is not recommended. However, it should be considered liberally in deep puncture wounds; bites involving the hand, joints, or tendon sheaths; and in patients with diabetes, asplenia, or immunosuppression. Tetanus vaccination status should also be checked and, depending on the exposure, rabies postexposure prophylaxis considered.Rarely, hematogenous spread leads to serious complications such as sepsis, endocarditis, or meningitis. Patients with immunosuppression, valvular heart disease, or other serious comorbidities are at greatest risk.Capnocytophaga canimorsus — Rare but Potentially FatalInfections caused by C canimorsus are significantly rarer than those caused by P multocida but are associated with a higher risk for severe disease. This gram-negative rod-shaped bacterium is part of the normal oral flora of dogs and cats. Most infections occur following dog bites, but cat bites and scratches, or contact between animal saliva and compromised skin, can also lead to transmission. If an invasive infection develops, it can progress rapidly; the case-fatality rate is reported to be approximately 10%-30%, depending on the patient population.Because C canimorsus infections are not notifiable in Germany, nationwide epidemiologic data are lacking. Their rarity also means the diagnosis is often delayed. This makes a careful exposure history particularly important because even apparently minor scratches, bites, or saliva contact can be enough.After an incubation period of usually 2-7 days, symptoms often begin nonspecifically with fever, myalgia, and malaise. Within hours, however, patients may deteriorate into severe sepsis with septic shock, disseminated intravascular coagulation, purpura fulminans, and multiorgan failure. Less commonly, meningitis, endocarditis, or ocular infections occur.Diagnosis is challenging because the organism grows slowly in culture. Modern methods such as MALDI-TOF mass spectrometry can speed identification from positive blood cultures. Even so, the history remains critical: in any patient with gram-negative sepsis of unclear origin, clinicians should ask specifically about recent contact with dogs or cats.First-line treatment is amoxicillin/clavulanic acid, which also covers the commonly accompanying polymicrobial oral flora. Penicillin G or third-generation cephalosporins may be alternatives. Fulminant cases also require intensive care support, including organ replacement therapy where needed.Patients with asplenia, marked immunosuppression, or chronic alcohol misuse are especially vulnerable. In such patients, C canimorsus should be considered even after apparently trivial injuries, and the threshold for antibiotic therapy should be low.Dermatophytosis — Ringworm (Microsporum canis)Dermatophytosis is one of the most common skin infections transmitted by cats. The primary causative agent is the zoophilic dermatophyte M canis. Cats are the main reservoir, particularly kittens as well as animals from shelters or breeding facilities. Transmission occurs directly via infected hair or dandruff. Prevention is complicated by the fact that cats often carry and shed the pathogen asymptomatically.Nationwide incidence data on cat-associated dermatophytosis are not available for Germany. However, in German multicenter studies, M canis remains one of the most common zoophilic dermatophytes and, particularly in cases of tinea capitis, one of the major causative agents. Children are affected significantly more often than adults.Typical lesions are sharply demarcated, ring-shaped erythematous patches with peripheral scaling and central clearing, usually accompanied by pruritus. Exposed areas such as the face, neck, and arms are most often involved. More severe or atypical presentations may occur in children and immunosuppressed patients.The diagnosis can often be suspected clinically. Wood’s lamp examination may provide an early clue, particularly in M canis infection, but its sensitivity is limited. The diagnostic gold standard remains direct microscopy of skin scales or hair in a KOH preparation together with fungal culture; PCR-based methods are also being used increasingly.Localized disease can usually be treated topically with terbinafine or clotrimazole. In extensive disease, inadequate treatment response, or tinea capitis, systemic therapy — for example with terbinafine or itraconazole — is required because topical agents do not adequately penetrate the hair follicles. Treatment usually lasts several weeks.More severe or chronic cases occur mainly in immunocompromised patients and may progress to deep dermatophytosis or pseudomycetoma. In recurrent cases, an asymptomatically infected cat should be considered as the source. Veterinary assessment and, where needed, treatment of the animal can help prevent reinfection.Toxocariasis (Toxocara cati) — The Cat RoundwormThe cat roundworm T cati is particularly common in outdoor cats. Young cats are an important reservoir because they are more frequently infected and excrete large numbers of eggs in their feces. These eggs become infectious only after maturing in the environment for 2-4 weeks. Humans are accidental hosts and acquire infection by ingesting embryonated eggs, for example from contaminated soil or sand or because of poor hand hygiene after gardening.In a laboratory study of 6425 feline fecal samples, T cati was detected in 3.8%; young cats were affected significantly more often. Earlier studies in stray and shelter cats found much higher prevalences, up to 27.1%. Human incidence data are lacking because toxocariasis is not a notifiable disease.After ingestion, larvae hatch in the small intestine and migrate via the bloodstream to various organs, especially the liver and lungs, and less often the muscles, eyes, or central nervous system. Two main syndromes are recognized: visceral larva migrans, characterized by fever, hepatomegaly, cough or wheeze, and marked eosinophilia; and ocular larva migrans. The ocular form is usually unilateral and, if untreated, can lead to permanent visual impairment or blindness.Diagnosis is based on exposure history, clinical findings, and peripheral eosinophilia, with serologic confirmation by ELISA. In ocular disease, ophthalmologic findings of typical granulomatous lesions are particularly informative; serology may remain negative or inconspicuous despite localized infection.Symptomatic patients are treated with albendazole or, alternatively, mebendazole. In severe inflammatory organ involvement or ocular disease, systemic glucocorticoids are also used. Patients with ocular toxocariasis should be managed jointly with ophthalmology.Young children are at particular risk because they may ingest contaminated soil or sand during play. Prevention therefore focuses on regular deworming of outdoor cats, proper disposal of cat feces, covering sandboxes, and careful hand hygiene after soil or sand exposure.Rabies — Rare in Germany, but Still a Concern AbroadGermany has been considered free of terrestrial rabies since 2008. The last confirmed infection in a wild animal, excluding bats, was detected in 2006. The risk for rabies transmission from domestic cats within Germany is therefore extremely low. The situation is different, however, for cat bites sustained in rabies-endemic regions or from illegally imported animals infected with classic rabies virus.After an incubation period of weeks to months, nonspecific symptoms such as fever, malaise, or paresthesia at the bite site may appear first. These are followed by the classic neurologic manifestations, including hydrophobia, aerophobia, seizures, and progressive impairment of consciousness. Once clinical symptoms develop, rabies is almost invariably fatal.Early risk assessment is therefore essential. After cat bites of unknown rabies status, or after relevant animal contact abroad, careful travel and exposure history should be taken. Because reliable diagnosis before symptom onset is practically impossible in humans, the decision to give postexposure prophylaxis depends on the nature of the exposure and the individual’s infection risk.If there is reasonable suspicion, action must be immediate. In addition to prompt and thorough wound cleansing, postexposure prophylaxis may include active vaccination and, where indicated, rabies immunoglobulin, depending on immune status and exposure type.Conclusion for Clinical PracticeMost cat-associated infections are mild and self-limiting in immunocompetent people. Even so, a history of cat contact can provide an important diagnostic clue, particularly in unexplained lymphadenopathy, treatment-resistant skin lesions, or rapidly progressive wound infection.Pregnant women, children, and immunosuppressed or asplenic patients require special attention. In these higher-risk groups, even apparently trivial scratches or bites can have serious consequences.A medical history that explicitly asks about pet exposure therefore remains a simple but important part of the diagnostic work-up. In primary care, emergency medicine, and infectious disease practice, asking about cats may provide the clue that shortens the path to the correct diagnosis.This story was translated from Medscape’s German edition.