Valley Fever

Introduction | Causes & Prevention | Symptoms & Signs | Diagnosis & Test | Treatment Options

Valley Fever

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Introduction

"Valley Fever" is a common term for coccidioidomycosis, which is an uncommon but extremely severe and potentially fatal systemic disease caused by Coccidioides immitis. This fungus thrives in the hot, dry, dusty areas of the deep southwestern United States – especially in Arizona, California and Texas. It also exists in Mexico and in parts of Central and South America. Not all dogs exposed to the fungi succumb to clinical disease, but many do. People can also become infected with this organism.

Causes & Prevention

Causes of Valley Fever in Dogs
Valley Fever is caused by inhalation of the infectious form of the dimorphic fungus, Coccidioides immitis. This fungus has a mycelial form and a yeast form. The mycelial form persists naturally several inches deep in the soil of endemic areas. It is dormant during periods of drought. After rainfall, earthquakes, excavation or other causes of soil disturbance, the mycelial fungi surface, sporulate and become airborne. The released spores, called arthroconidia, are highly infectious and easily inhaled.

The arthroconidia invade deep portions of the dog's respiratory tract, where they change into the organism's yeast form. The yeast form of the fungus produces endospores, which can spread throughout the dog's body and lodge in distant tissues. It takes several weeks after inhalation of arthroconidia for symptoms of respiratory infection to develop. It can take months for signs of disseminated infection to develop.

Prevention of Valley Fever
No vaccine is currently available for Valley Fever. The disease cannot develop unless a dog has lived in or traveled through a region where the fungal organism is endemic. Contaminated soil in endemic areas – especially during dust or wind storms after rain – should be avoided.

Special Notes
Coccidioidomycosis occurs in people as well as in dogs. Owners can become infected by the fungi at the same source as their dogs. Valley Fever is not considered to be contagious. It is extremely unlikely that people can contract infection from dogs, or that people can cause infection in their pets.

Valley Fever usually can be treated, if the fungal infection is promptly and properly diagnosed. However, successful treatment can take months to years to complete.

Symptoms & Signs

Introduction
Coccidioidomycosis, commonly called "Valley Fever," is an uncommon but potentially dangerous fungal disease in domestic dogs. It develops after a dog inhales infectious spores of the fungus, Coccidioides immitis. This organism thrives in hot, arid areas of the southwestern United States. It is also found in Mexico and in parts of Central and South America. Some dogs become infected subclinically and develop immunity to the organism. Unfortunately, other infected dogs become gravely ill.

Symptoms of Valley Fever
Most cases of Valley Fever are subclinical, meaning that the dog is infected by the fungi but does not develop observable symptoms. These animals probably never will be diagnosed or require treatment. However, when clinical illness does develop, it can be life-threatening. The early signs of Valley Fever are nonspecific and primarily respiratory. Owners of affected dogs may notice one or more of the following signs in the early course of the disease:

Acute onset of respiratory symptoms
Difficulty breathing (dyspnea)
Shortness of breath
Rapid, shallow breathing (tachypnea)
Cough (initially mild; may be dry and hacking or moist and productive)
Loss of appetite (anorexia; inappetence)
Weight loss
Weakness
Lethargy
As the infection spreads (becomes systemic or disseminated), which normally takes more than 4 months after the onset of respiratory signs, affected dogs deteriorate and develop additional symptoms, especially in their eyes, joints, skin and bones. These may include:

Diarrhea (usually intermittent)
Vomiting (usually intermittent)
Chronic cough
Lameness (especially in the long bones of the limbs)
Swollen or enlarged joints
Neck pain
Back pain
Seizures
Vision abnormalities; vision loss
Weeping skin sores; draining skin ulcers
Emaciation
Fever unresponsive to antibiotics

Dogs at Increased Risk
Young, male, medium to large-breed dogs that live or spend a substantial amount of time outdoors in endemic areas have an increased risk of becoming infected by Coccidioides immitis. Boxers, Doberman Pinchers, Australian Shepherds, Beagles, Scottish Terriers, Cocker Spaniels and Pointers are reportedly predisposed to Valley Fever, as are dogs living in endemic areas that tend to "nose around in the dirt." Older and immunocompromised animals are at higher risk of infection, as are those with poor nutritional support.

Diagnosis & Tests

Introduction
Valley Fever is diagnosed by identification of the causative fungal organism, Coccidioides immitis, in blood, fluid or tissue samples. It is not particularly difficult to diagnose.

How Valley Fever is Diagnosed
Valley Fever should be suspected in any dog that lives in or travels through endemic areas and develops chronic upper and/or lower respiratory signs, together with enlarged lymph nodes. Veterinarians presented with such a patient will of course perform a thorough physical examination of the dog and take a complete history from the owner. They also may perform a complete blood count and serum biochemistry panel on a blood sample, the results of which can suggest Valley Fever. A urinalysis will not help this diagnosis. Thoracic radiographs (chest X-rays) often are abnormal in dogs with coccidioidomycosis. Radiographs of the long limb bones of dogs with lameness caused by Valley Fever may be abnormal, as well.

A definitive diagnosis of Valley Fever can only be made by identifying the organism, or antibodies to the organism, in fluid, tissue or serum samples. Agar gel immunodiffusion (AGID) assays are widely used by commercial diagnostic laboratories to detect antibodies to Coccidioides immitis in blood samples taken from dogs showing signs of respiratory or systemic illness.

Coccidioides immitis can also be cultured (grown) from tissue samples taken by biopsy. However, because this fungus is so dangerously infective, only specialized laboratories that are fully equipped to handle such organisms should perform these cultures.

Special Notes
Diagnosis of Valley Fever is complicated by the fact that most laboratories regard the fungus as too dangerous to culture, because of the significant risk of human infection.

Treatment Options

Introduction
Valley Fever, medically called coccidioidomycosis, is a potentially fatal fungal disease in domestic dogs. Successful treatment is possible if the infection is properly and promptly diagnosed. However, treatment can take more than one year to complete. The goals of treating this disease are to prevent further dissemination of the fungal organism, eradicate the fungi from the dog's body and restore the dog's quality of life.

Treatment Options
Most dogs with Valley Fever are treated as outpatients. Supportive care for these dogs includes restricting their activity until clinical signs of the infection resolve. Dogs should be fed a high quality, extremely palatable diet with free access to fresh water. If necessary, patients can be hospitalized and provided with oxygen supplementation.

Diagnosis of Valley Fever should be rapidly followed by oral anti-fungal therapy. The anti-fungal drugs currently available are in the imidazole, or "azole," pharmaceutical group. They include itraconazole, ketoconazole and fluconazole, although of course with time newer medications may become available. Ketoconazole can be effective and is the least costly, but it carries the risk of more adverse side effects, including liver damage, vomiting and loss of appetite. Itraconazole is more expensive than ketoconazole and may or may not have fewer side effects. Fluconazole has the best penetration of the eye and central nervous system and is the most commonly used anti-fungal drug, although it is costly. Amphotericin B is another highly-effective anti-fungal medication that can be used when a dog does not tolerate or respond well to treatment with the azole drugs. It can also be used in combination with an azole drug. However, Amphotericin B is highly nephrotoxic and can cause significant kidney damage. An even newer drug, Lufenuron, has shown promising results in the treatment of disseminated coccidioidomycosis in dogs.

Whichever anti-fungal medications are chosen, they must be administered precisely in accordance with a veterinarian's directions. This will typically be twice a day, for at least six to twelve months, and maybe longer. Medical therapy must continue for several months after the dog's symptoms have resolved, so that long-term suppression of the fungus can be achieved. Anti-fungal medications can be toxic to dogs. They can cause liver dysfunction, kidney damage, gastrointestinal upset and adverse skin reactions, and they require close medical management. Blood testing every 3 to 4 months after completion of drug therapy is recommended to monitor progress. Unfortunately, relapses are common.

Prognosis
The prognosis for dogs that only develop respiratory signs of Valley Fever is fair to good. Sadly, the prognosis for dogs with disseminated systemic disease is guarded to grave.

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