29th July 2023

Histoplasmosis

Histoplasmosis is caused by Histoplasma capsulatum, a dimorphic fungus, which operates either as a yeast or a mould, depending on the temperature conditions. It is acquired through inhalation of spores and associated with soil, bird and bat droppings. It is not directly spread from person to person.

Although found worldwide, it is endemic in the US’s Ohio and Mississippi river valleys. The general route of infection is the inhalation of spores after disturbance of soil associated with riverbeds, and it is also known as ‘Ohio River Valley Fever’ due to its epidemiology.

Most often asymptomatic, or it causes only a mild flu-like illness. However, in immunocompromised people, it can cause a respiratory illness similar to tuberculosis or pneumonia. Pathophysiologically it is characterised by granulomatous inflammation, which creates either caseating or non-caseating granulomas that are visible on chest x-rays. The typical incubation period is 7-21 days, and the predominant symptoms are a non-productive cough, headache, fever and malaise. Extrapulmonary manifestations are primarily dermatological or rheumatological, including arthralgia, erythema nodosum and erythema multiforme – more common in women.

Diagnosis can be confirmed by combining serum and urinary antigen detection, bronchoalveolar lavage, and blood cultures. Bronchoalveolar lavage is rarely positive but is also used to screen for other diseases. Blood cultures are used to assess for disseminated disease, which is associated with hepatosplenomegaly and haematological complications.

Treatment is only recommended if symptoms persist beyond 4 weeks. The first line treatment for acute infection is 3 months of itraconazole. If the disease develops into its chronic form, then 6 months of treatment is indicated, extending to a year if associated with cavitation.

Disseminated illness requires treatment with liposomal amphotericin B as induction therapy for approximately 4 weeks, followed by a minimum of 1 year of itraconazole as maintenance therapy.