Histoplasmosis is a systemic (multiple organ) fungal disease caused by the organism, Histoplasma capsulatum. The first human case was reported in 1906, but it was not reported in the cat until 1949. While it is uncommon in the Downingtown, PA area, the doctors at the Animal Clinic at Thorndale keep a watchful eye out for this potentially devastating fungal infection.
Contributing Factors
Establishment of histoplasmosis in cats is thought to depend upon a weakness in one particular branch of the immune system. A combination of environmental exposure and a weak immune system provides the right conditions for successful infection of the cat.
Prevalence
This fungal organism has a worldwide distribution. In North America, certain areas are endemic for histoplasmosis. This means that infection in more common in these locales. These areas include the drainage and tributary systems for the Ohio, Missouri, and Mississippi rivers. It is also common in certain parts of the Southwest.
The organism thrives in topsoil enriched with the feces of birds and bats. Birds do not become infected because of their higher body temperature. It is not possible to rid the environment of the fungus. The organism is ubiquitous, meaning it lives everywhere.
Cats are infected with this fungus at least as often as dogs and many more cats are infected than are diagnosed. Subclinical or inapparent infections occur frequently. Most reported cases of feline histoplasmosis have involved domestic shorthair cats younger than four years of age, although cats of any breed and age may become infected. Male and female cats are infected in about equal numbers.
Clinical Signs
The organism seems to have preferences for certain body systems, although it is usually disseminated (spread) throughout the entire body. The most common signs are nonspecific – fever, depression, weight loss, and anorexia. Respiratory signs, such as labored breathing, are seen in many cases. The fungus may also invade the lymph nodes, eyes, skin, and bone.
Causes/Transmission
This fungus most commonly infects humans and animals through the respiratory tract. After spores are inhaled, they settle in the small airways of the lungs and begin to reproduce. Subsequent to this, the organism spreads throughout the body to involve many organs. Cats with a healthy immune system may never reach the point of an established infection.
Diagnosis
The only tests that offer a conclusive diagnosis of histoplasmosis are cytology and histopathology. Cytology, the microscopic study of cells, can be performed in the veterinarian’s office on some of the fluid draining from an open wound or aspirated from a nodule or lymph node. Rarely, the organism may be seen within white blood cells on a blood smear. Histopathology is the study of cells and tissue architecture. A tissue sample is submitted to a veterinary pathologist for microscopic examination. Because the organism is usually present in large numbers, histoplasmosis is often diagnosed in the office with cytology.
The accuracy of serologic (blood) tests for histoplasmosis is unknown. This type of test is not considered very useful for providing a definitive diagnosis.
Treatment
Histoplasmosis is a treatable disease, although not all cats will survive. Fortunately, anti-fungal agents being used are well-tolerated by most cats and have relatively few side effects. The drug, itraconazole (Sporanox), is quite expensive. It is given daily with food; several months of therapy will usually be required.
For cats with lung involvement, breathing problems may worsen once treatment is started. Prior to treatment, the lung can harbor a large number of fungal organisms. A severe inflammatory response may occur as treatment takes effect and the organisms begin to die in the lungs. This critical period comes in the first 24-72 hours after the initial dose of medication.
Prognosis
The prognosis cannot always be determined, although the cat in poor body condition and with advanced disease is less likely to survive. The animal’s chest will be x-rayed prior to therapy to determine the presence and significance of a fungal pneumonia; however the chest x-ray cannot predict the outcome of treatment.
Many cats are treated and recover completely. However, because a weakened immune system is present in many cats with histoplasmosis, the prognosis for their long-term survival should be considered guarded. To optimize the chance for recovery, it is essential not to withdraw medication before you are advised to do so. Many different parameters must be monitored in order to decide when it is appropriate to stop treatment.
Transmission to Humans
Studies on this fungus have found that once an animal is infected, the organism enters a different form or phase; this does not appear to be directly infectious to other animals or to humans. Therefore, it is considered noncontagious. However, common sense would dictate that strict hygiene should be followed in handling any observed lesions. Thorough hand washing should follow contact with the cat.
The infected cat does not need to be segregated from the owner or other household pets. The true risk of infection to humans comes from sharing the same environment that infected the pet (i.e., soil, etc.). Because the Histoplasma organism may be harbored near your home, we would recommend that you advise your family physician of your pet’s diagnosis. Also, if anyone in your family falls into one of the following categories, we would recommend that you consult with your physician:
- Infants or small children
- Transplant patients
- Chemotherapy patients
- HIV/AIDS
- Elderly family members
- Anyone with a known immunosuppressed state