Coccidioidomycosis, Biology


Coccidioidomycosis is a soil-borne infection of pet, domestic, wild animals and  man. The disease is caused by Coccidioides immitis, a dimorphic fungus which occurs commonly in the soil as saprobe. The fungus is extremely virulent. The fungus has a environmental or saprobic form and a parasitic form in the tissues. The infection is usually localized and benign, but may sometimes become disseminated and fatal. Both humans and animals acquire the infection by inhalation of infective arthoconidia. The affinity for the lung tissue is strong. In some cases haematogenus or lymphatic dissemination results in infection of the bone, skin, lymph nodes, and central nervous system.

Symptoms: The affected dog shows fever, dyspnoea, acute or chronic cough, gradual emaciation, difficulty in walking and intermittent diarrhoea. The infection in cattle and pig is usually benign. The diagnosis is made either at necropsy or in abattoir. The lungs, and bronchial, mediastinal, mesenteric and pharyngeal lymphnodes are commonly involved. They show granulomatous lesions which contain cream-coloured pus. Fever and abscesses in peripheral lymphnodes are observed in sheep. The clinical signs in horse include fluctuating temperature, oedema of legs, severe weakness, intermittent colic, anaemia and leucocytosis. The death occurs due to rupture of liver. The dissemination of infection seems to be common in dogs than in cattle.

Diagnosis is confirmed on culture, histopathologic or molecular evidence of the presence of fungus. The Gomori methanamine silver or periodic-acid-Schiff (PAS) stained sections show thick-walled spherules (20-10 um in diameter) containing many small globose endospores (2-5 u in diameter). Direct microscopical examination of smear from abscess or spinal fluid by wet mount in saline containing 10% KOH for spherules also helps in diagnosis. Thick-walled, barrel-shaped arthrospores appear alternatively in chains, separated by empty (disjunctor) cells. The fungus may be isolated from exudate or tissues on Sabouraud‘s medium with cycloheximide- chloramphenicol agar or brain heart infusion agar. The causative organism appears in the tissues as a round, doubly walled structures called the spherule. The host generates cutaneous delayed-type hypersensitivity reaction to intradermal inoculation of test c o c c id io id in (sup e r na ta nts o f myce lia l  Co c cid io ide s b r o th c ultur e  la rge ly polysaccharide containing some amino acid nitrogen). This skin test is also employed for diagnostic; however, it is more useful in epidemiological survey. Several tests like immunodiffusion, complement-fixation, enzyme immunoassay, latex agglutination, ELISA can detect circulating antibodies in serum or cerebrospinal fluid after 2-4 weeks post-infection. Animal pathogenicity tests are conducted in mice and guinea-pigs.

Treatment and prevention:
Since the mild or benign form of the disease is self- limiting, no therapy is required. However, in progressive and disseminated cases Amphotericin-B and Saramycetin may be used. Ketoconazole remains the drug of choice in veterinary medicine. Live attenuated and killed vaccines are also tried in animals with variable success although vaccines are not available.

Posted Date: 9/17/2012 8:11:41 AM | Location : United States

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