TREATMENT DURATION AND MONITORING

While the duration of therapy is variable, most patients treated with antifungal drugs will receive these medications for a minimum of four to six months or two months after resolution of all clinical signs.¹⁰ Patients should be rechecked at monthly intervals. In addition to physical and ocular examinations, a serum chemistry profile is recommended to monitor hepatic enzyme activity while an animal is receiving azole antifungal therapy.¹¹ In a study of 112 dogs treated with itraconazole, anorexia was the most common adverse effect and was usually associated with an increase in ALT activity. The anorexia resolved within three or four days after the medication was stopped, and therapy was reinstituted one week after discontinuation.⁷⁹ In dogs, if the serum ALT activity is > 250 IU/L, consider discontinuing itraconazole therapy.⁸⁰ In dogs and cats with anorexia and increased ALT activity, treatment can be resumed at half the original dose once the anorexia resolves or the hepatic enzyme activity returns to reference values.^22,81 Similar recommendations apply for animals exhibiting signs of cutaneous vasculitis.⁸¹

Response to therapy should be evaluated through the resolution of clinical signs, hematologic and biochemical abnormalities, and radiographic lesions.¹⁰ Treatment is continued for a minimum of one month past resolution of clinical signs, and animals should be reevaluated for evidence of relapse three and six months after therapy is discontinued.¹¹

The prognosis depends primarily on the extent and severity of infection. Pulmonary histoplasmosis may resolve without specific antifungal therapy,⁸² and dogs and cats with pulmonary histoplasmosis can have a good prognosis. The prognosis for disseminated histoplasmosis depends on the degree of dissemination and severity of clinical signs.^10,11 While central nervous system involvement often carries an unfavorable outcome,^33,34 recovery of neurologic function is possible.⁴⁵

With ocular involvement, the prognosis for return of vision is guarded to poor.³⁷ One recent report of feline ocular histoplasmosis documented return of vision with complete resolution of retinal hemorrhages six months after the initiation of systemic itraconazole therapy in a cat.³⁹

PREVENTION

Vaccination against histoplasmosis is not available in cats and dogs, so the recommended method of protection is preventing exposure to potentially Histoplasma species-laden areas, such as chicken coops, bird and bat roosts, and construction and excavation sites.^4,10

SUMMARY

Histoplasma capsulatum is a saprophytic, dimorphic fungus that affects people and animals. The primary route of transmission is by inhaling conidia from the soil. It is thought that most infections are self-limiting, but pulmonary or disseminated disease may occur. Clinical signs in cats are often chronic and nonspecific, while dogs often display gastrointestinal signs. Definitive diagnosis is based on cytologic or histologic identification of H. capsulatum. The treatment of choice in cats and dogs is the azole antifungal agent itraconazole. The prognosis depends on the extent of disease and can range from guarded to excellent. The best method of prevention is to avoid exposure to contaminated areas. Horizontal or zoonotic transmission has not been documented, but common-source infection has been observed.

ACKNOWLEDGMENTS

The authors wish to thank Dr. Craig Thompson, DACVP, for obtaining and providing the digital photomicrographs and Dr. Jacob Rohleder, DACVR, for interpreting the radiographic images used in this manuscript.

Kelly Gingerich, DVM
Lynn Guptill, DVM, PhD, DACVIM
Department of Veterinary Clinical Sciences
School of Veterinary Medicine
Purdue University
West Lafayette, IN 47907