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EQUINE VETERINARY EDUCATION
Equine vet. Educ. (2022) 34 (7) 352-354 doi: 10.1111/eve.13586
Clinical Commentary
Coccidiomycosis: Evidence from human medicine to diagnose and treat equids
A. J. Stewart School of Veterinary Science, Equine Specialist Hospital, University of Queensland, Gatton, Queensland, Australia Corresponding author email:
stewaaj@gmail.com
Keywords: horse; coccidiomycosis
Coccidiomycosis is a fungal infection that can infect any mammalian species, with reports in people, dogs and cats, horses and now a donkey (Sorum et al., 2022). Coccidioides spp. is a cause of pulmonary and systemic infections in the desert regions of the Americas. Diagnosis is usually made by serology, although culture is possible. In human patients, there is frequently disease resolution in cases of pneumonia without antifungal therapy. When antifungal therapy is required, fluconazole and itraconazole are recommended for the treatment of coccidioides. Response to treatment is indicated by resolution of clinical signs, bloodwork abnormalities and reductions in the serological titre. After treatment discontinuation, monitoring of serological titres is recommended to diagnose disease recrudescence. Coccidiomycosis is a problem in immunocompromised people with lifetime antifungal therapy often required.
Endemic regions
Coccidioides spp. are soil saprophytes found in semiarid areas with alkaline sandy soils (Ziemer et al., 1992). The disease is acquired from specific regions of the southern deserts of Arizona, central valley and southern California, southern tip of Nevada, southern Utah and New Mexico, western Texas, northern and western Mexico and areas of Central and South America. Recently, a pocket of Coccidioides was identified in people in Washington State (Chastain et al., 2017). Two species have been differentiated based on genetic analysis: C. immitis and C. posadasii. Most C. immitis cases originate from California while most C. posadasii infections are acquired in other south-western US states (Galgiani et al., 2005).
Organism characteristics
Coccidioides spp. survive as soil mycelia, which proliferate within the aerobic topsoil layers during rainy periods. The branched mycelia produce arthroconidia. As the soils dry, single cell, barrel-shaped arthroconidia are released. As these arthroconidia are 3–5 µm in diameter, they can become suspended in the air after soil disruption, with inhalation deep into the lungs being the most common route of infection in people. The arthroconidium transforms and enlarges into a multi-celled sphere, which ruptures releasing numerous endospore progeny, each of which can develop into another spherule. If these spherules are obtained by lung biopsy and cultured in a laboratory, the fungus will revert to its mycelial form (Galgiani et al., 2005).
Epidemiology and pathogenesis people
Although approximately 150,000 people are infected annually, only 50,000 have illness that warrants medical attention (Huang et al., 2012). It is estimated that 1% of the human population living in endemic areas are treated annually for coccidiomycosis, which is known as Valley Fever (Campion et al., 2003). Inhaled arthroconidia enlarge to form nonbudding
spherules, which incite an inflammatory reaction in the lungs and lymph nodes. The incubation period of coccidioidal infection in human patients ranges from 7 to 21 days. Most human cases resolve without treatment due to induction of immunity, although the historical illness may have lasted many weeks to months.
Clinical signs
Clinical signs in people consist of fatigue, fever, cough, chest pain, headache, joint pain and skin rashes (Galgiani & Thompson, 2016). The disease can be mild and insidious, with human patients experiencing clinical signs on average for 44 days prior to seeking medical care for mild to moderate respiratory signs, similar to pneumonia. Approximately 5–10% of human infections have serious pulmonary sequelae, and <1% spread to tissues beyond the lungs resulting in lesions within bones, joints, skin, meninges or other organs (disseminated disease) (Galgiani & Thompson, 2016). Approximately 160 people die per year of Valley fever (Huang et al., 2012). Once infected, animals or human patients are not considered contagious to others (Galgiani & Thompson, 2016). Horses diagnosed with Coccidioides commonly have
weight loss, fever, abdominal pain and signs of respiratory disease. Localised, recurring nasal granulomas also have been reported. Diffuse infections, with granulomas in the lungs, liver, kidney or spleen have a grave prognosis (Hodgin et al., 1984). Przewalskii horses may be more susceptible (Terio et al., 2003).
Diagnosis
Coccidioides immitis is difficult to culture, and spherules may be missed on histology from antemortem lung biopsies. However, serology is very useful for diagnosing infection, and decreasing titres are associated with clinical improvement in horses and human patients (Higgins et al., 2005a) (Galgiani & Thompson, 2016; Ziemer et al., 1992). Various serological tests exist and measurement of IgM and IgG antibodies is available. In humans, up to 90% of patients will have tube
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