Canine
histoplasmosis is a non-contagious systemic fungus that grows in the
soil in tropical and subtropical regions, especially endemic in the
Mississippi and Ohio River valleys and the Appalachian Mountain range.
Bird, chicken and bat manure provide a rich environment for the fungus,
but other origins have recently been reported. Histoplasmosis
usually affects the lungs, which is where the microorganisms reside
after they are inhaled. The incubation period
after exposure is approximately 12 to 16 days. Dogs often develop
severe signs, and it's not unusual to see an X-ray of a dog's lungs
that are being severely damaged by the fungus. Most cases can be cured
if caught in time, but untreated cases usually result in death. In
dogs, the most common sign is difficult breathing, often due to
pneumonia. Occasionally, Histoplasma infects the eye, causing
conjunctivitis,
granulomatous blepharitis, granulomatous chorioretinitis, retinal
detachment, and optic neuritis. In humans and dogs,
permanent eye damage can result if the organism gets into the eyes.
Medications used for typical treatments, such as ketoconizole, are
generally not effective in eyes. Common symptoms of
Histoplasma
capsulatum infection in canines include inappetence, weight loss, and
fever (unresponsive to antibiotic therapy). In some dogs, the clinical
signs may be limited to the respiratory tree and include coughing and
abnormal lung sounds. However, in most dogs, clinical signs result from
disseminated histoplasmosis with GI involvement. Signs of large-bowel
diarrhea with tenemus, mucus, and freash blood in the stool are the
most common clinical findings .
Some forms of
the disease are mild and self-limiting, or involve only the lungs and
lymph nodes of the chest. The organisms enter the body through the
respiratory tract and become disseminated throughout the body, with the
signs of the disease varying, depending on the various organs involved.
Many dogs have a long course of weight loss due to severe diarrhea,
characterized by fresh blood, straining, and mucous, high fever, and
anemia. The dog may have a chronic cough and show respiratory
difficulty due to obstructions and enlargements of the bronchial lymph
nodes. Dissemination may involve ocular disease, weeping ulcerated
nodules on the skin, ulcerated intestinal walls, enlargement of the
liver and spleen, accompanied by depression, anorexia, lameness and
ocular disease.
Currently, no
consistently reliable immunodiagnostic test is available
for identification of histoplasmosis in companion animals. Intradermal
skin tests for reactivity to histoplasmin are unreliable in companion
animals and cannot be used to confirm the diagnosis of histoplasmosis.
Serologic tests for antibodies directed against Histoplasma antigens
are often falsely negative in animals with active naturally occurring
disease. Also, test results may be false positive in an animal with
prior exposure that has recovered from infection.
Conventionally,
infection by Histoplasma capsulatum could only be
confirmed by culturing the fungus. However, attempts to culture
Histoplasma capsulatum in a routine practice setting are not
recommended because of the pathogenic potential of this organism.
Histoplasma capsulatum can be cultured from tissue specimens,
fine-needle aspirates, and body fluids by specialized laboratories but
the process may take anywhere from 7 to 10 days. The newest generation
of DNA diagnostic tests are much more rapid (2-3 days) then
conventional methods, and avoid the problems associated with
serological testing since antibodies are not used. A DNA test for the
detection of Histoplasma capsulatum in a sample is now available. This
test offers veterinarians a sensitive, accurate and rapid means of
detecting Histoplasma capsulatum, and confirming infection.
Presence of the fungus can be determined by rectal scrapings in dogs
with diarrhea or needle aspirations of the lymph nodes and lungs. The
more acute and widespread the infection is, the less promising is any
drug therapy, but with early diagnosis, antifungal drug therapy with
ketoconazole may improve the outlook. Since Histoplasmosis is primarily
a chronic diarrhea disease, the illness can progress so rapidly that
not even the antifungal drug can help if treatment and diagnosis is
delayed. For severe cases, concurrent treatment with amphotericin has
been used with success.
The duration of antifungal treatment required for each patient is
variable and is determined by the severity of infection and the
patient’s clinical response. Itraconazole (ITZ) is currently the drug
of choice for the treatment of histoplasmosis in animals. Treatment is
usually initiated once daily; however, studies in cats indicate
significant variability regarding per os (by mouth) absorption of ITZ,
and twice daily dosing (at 10 mg/kg) may be required in some to achieve
the desired therapeutic effects (Greene C. – Infectious
diseases of the dog and cat. 1998).
Hypoallergenic
diets must contain ingredients not previously encountered by the
patient and all other potential sources of offending substances should
be excluded, including rawhide chews, table scraps, vitamin and mineral
supplements. Response to the hypoallergenic diet rarely occurs within
the first week. If the dog's skin condition improves by the
twenty-first day, diet is probably at fault, and the dog will be put
back on his original diet. If the animal's itchiness increases, there
is no doubt about what the problem is. In this case you will need to
find a diet that is both nutritionally sound and free of the offending
substances.
There is
no sex predilection and it affects animals of all ages,
however animals under the age of four are most commonly affected with
severe disease.
