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Background
Coccidioidomycosis, also known as Desert Fever, San Joaquin Valley Fever, and Valley Fever, is caused by the fungus Coccidioides.
Coccidioides lives in the soil of arid and semi-arid regions in the southwestern United States and northern Mexico, as well as in parts of Central and South America. All mammals (including dogs, cats, horses, and humans) and some reptiles can be infected by this organism. It is the most common fungal infection of dogs and cats in endemic regions.
While this disease is predominant to the southwestern United States, its geographic distribution may be changing. Reports of endemic areas outside of the southwest are increasing (Washington state, for example). Human infections have also occurred in parts of Oregon, Idaho, Colorado, and Wyoming.
How the Disease Spreads
Because it is a soil-based fungus, infections often occur after dust storms, heavy rainfall during dry seasons, earthquakes, or when soil has been disrupted. In regions where the fungus is concentrated (such as Arizona), cases are more likely to be associated with rainfall/drought cycles than with specific dust storms.
Infection occurs more frequently in dogs than in cats, possibly because dogs are more likely to disturb the soil, allowing the fungus to become airborne and be inhaled by the dog.
Outdoor dogs (e.g., large, sporting, working breeds) are more likely to contract infections than dogs that spend most of their time indoors. Pets that are walked on sidewalks, as opposed to being allowed to roam, are less likely to develop infections. Basically, the less contact the pet has with soil, the less likely they are to contract this disease.
Inhaled spores infect the lungs, causing inflammation and nodules to form, then the organisms spread to other parts of the body (bones, skin, eyes, liver, kidneys, etc.).
Many infections in dogs go unnoticed and/or are transient. Lung disease is the most common sign of illness. However, if the disease spreads to other parts of the body, additional signs will become apparent.
If the fungus gets into the central nervous system, the patient may develop encephalitis (inflammation of the brain). The two most common species that develop central nervous signs are dogs and humans.
Effects of Disease and Diagnosis
Cutaneous (skin) signs occur in approximately 20% of affected dogs. Localized skin lesions usually occur in animals that have disseminated (spread through the body) disease. Skin signs often occur over infected bone.
Bone or joint disease can show as lameness, often due to osteomyelitis (bone infection).
Glomerulonephritis (kidney infection) has been reported in some cases of coccidioidomycosis.
Heart and eye problems have also been documented in the dog.
Diagnostic methods/tests may include:
- a physical examination
- blood work
- serology (antibody testing)
- radiographs
- MRI
- antigen testing
- cytology examination of drainage, the contents of nodules, etc.
A combination of tests may be necessary to confirm the diagnosis, as no single laboratory test is reliable, sensitive, or specific enough to definitively detect an active infection.
Treatment
Treatment is recommended for all clinically affected animals, because spontaneous remission is unlikely. Medications prescribed for treatment may include one or more of the following:
Regardless of the therapy chosen, long-term treatment (a minimum of one year with disseminated disease and three to six months after clinical remission) or life-long treatment is often necessary, because relapse is common after treatment has been stopped.
Supportive care for clinical signs may be needed. Drugs used will depend on the body systems affected. Surgical removal of wounds may be beneficial when draining tracts are present. Surgery may be advised to remove solitary lesions. Enucleation (surgical removal) of infected eyes may be necessary to remove a source of further infection. Castration to remove infected testicles may be advised to improve clinical signs and/or remove an infected area.
Monitoring
Monitoring involves repeated physical and eye exams, radiographs, and measurement of vital signs and body weight. During drug therapy, monthly bloodwork is usually recommended.
Response to therapy can be assessed by improvement in clinical signs and repeated measurement of titers (measurement of antibodies in the patient’s blood).
Antigen testing may be used to monitor response to therapy, to make decisions on when treatment can be stopped, and to diagnose relapses. Serology is commonly repeated at three to four months, and treatment is continued until titers are at acceptable levels. A marked decrease in titers, or several consecutive negative serial titers, can indicate that the disease is resolved, although some titers may persist for more than one year following therapy.
Once therapy has been stopped, your veterinarian will probably recommend that your pet be rechecked at three and six months for evidence of relapse. In central nervous system cases, the absence of MRI lesions is considered the gold standard for disease resolution. Your veterinarian may recommend that the MRIs be repeated at intervals to help identify early relapse.
Prognosis
The prognosis varies depending on the body systems affected and the immune system response your pet develops. For animals with localized lung infection, the prognosis is good.
The prognosis is less optimistic if the disease has affected more than the lungs. Survival rates as high as 60% have been reported; however, the response to therapy varies, and relapse is common when therapy has been stopped. In one report, relapse occurred in 15-30% of cases.
Prognosis is guarded to poor for patients that have central nervous system signs or have multiple bones infected. Life-long treatment may be necessary.
In endemic areas, outdoor activities (e.g., digging, walking in the desert) are associated with increased risk of infection. Reducing these activities may decrease the likelihood of disease. Daytime indoor housing may also reduce the risk of infection.
A vaccine for both dogs and people is currently under development, but is not yet licensed or commercially available.
Zoonotic Consideration
Humans can develop coccidioidomycosis, but transmission from animals to people is extremely rare. Few cases of zoonotic transmission (the transfer of disease from one species to another) are documented in the literature. Cases in humans are usually caused by contaminated soil, just as they are in animals.
In areas where this disease is less common, cases in dogs and cats may alert physicians and veterinarians to the possibility of this disease in both human and animal populations.