Introduction

In recent years, fungal infections have been recognized more frequently in both human and veterinary patients, and the use of antifungal drugs has increased, especially with the use of more potent immunosuppressive drug treatments for longer periods of time. Efforts have focused on the development of new, less toxic and more efficacious antifungal drugs, and antifungal drugs with novel mechanisms of action, and early diagnosis for more effective treatment. The purpose of this presentation is to provide an update on new diagnostic tests for fungal infections, to review the use of some of the major antifungal drugs, and introduce some new human antifungal drugs with potential applications in veterinary medicine.

Diagnosis of Fungal Infections

Classically, dogs with deep mycotic infections are large-breed, middle-age to younger, male dogs. In North America, American Cocker Spaniels are predisposed to cryptococcosis, and German Shepherds and Rhodesian Ridgebacks to disseminated mold infections such as aspergillosis. However, infections can occur in dogs of any signalment, especially when there has been a history of immunosuppressive drug treatment. A history of travel to endemic areas may assist diagnosis in regions where the disease is not endemic.

Although different fungal species vary in their organ tropism, fungal infections should be highly suspected in dogs with fever, nodular, cavitary, or lobar pulmonary lesions, osteolytic bone lesions, ocular inflammation such as uveitis or chorioretinitis, diskospondylitis, enlarged lymph nodes, or cutaneous nodules. Enlargement of the bridge of the nose or lesions of the planum nasale in cats should also raise suspicion for a fungal infection. In dogs, the nailbed is a site of predilection for many fungal infections. A lack of response to antibacterial drugs may also suggest underlying fungal disease. Rather than trialing multiple different antibacterials when an animal fails to respond to treatment, a diagnostic workup should be initiated that includes appropriate tests for fungal infection.

Major progress has been made in diagnostic testing for fungal disease in recent years. Cytology, histopathology and culture are useful but sometimes organisms are undetectable with cytology and histopathology, and culture may be dangerous to laboratory personnel. A history of antifungal drug use may render these assays falsely negative. In addition, these techniques may require invasive specimen collection techniques. Previously, with the exception of cryptococcosis, the only alternative tests available were assays that detect antibodies. These assays are largely only useful for diagnosis of coccidioidomycosis and nasal aspergillosis, provided they are performed and interpreted correctly by the laboratory that performs the assays. A positive agar gel immunodiffusion assay titer in a dog suspected to have nasal aspergillosis is strongly suggestive of the disease, but a negative titer does not rule out the possibility of nasal aspergillosis. Assays that detect antibodies to Histoplasma capsulatum and Blastomyces dermatitidis have also not shown to be useful for diagnosis, because of low sensitivity and specificity.

In the last few years, additional assays have become available that detect fungal antigens. These have been used extensively for diagnosis of disseminated mycoses in humans but have only recently been validated in dogs and cats. They appear to be useful for diagnosis of histoplasmosis, blastomycosis, and disseminated (but not nasal) aspergillosis and are available in North America through MiraVista Diagnostics, Indianapolis, IN. Cross-reactivity between different fungal infections can occur. A rapid line immunoassay (IMMY) for cryptococcal antigen has recently become available and shows promise for in-practice diagnosis of cryptococcosis. Finally, PCR assays for detection of fungal pathogens are becoming increasingly available, but these will require validation in dogs and cats.

Treatment of Fungal Infections

Cure with antifungal drug treatment is most likely to be achieved in dogs with localized histoplasmosis, blastomycosis, and coccidioidomycosis, or in dogs with mold infections that have occurred secondary to immunosuppressive drug treatment and the immunosuppression can be reversed. Prognosis is poorest for dogs with disseminated mycoses that are predisposed breeds or that lack a clear underlying cause for their disease. The inflammatory response that follows organism lysis with treatment may lead to significant worsening of clinical signs in the first few days of treatment. Use of NSAIDs or, when the brain is involved, a short period of anti-inflammatory doses of glucocorticoids, may be required until the signs begin to resolve.

Some fungal infections respond to treatment with single agent fluconazole or itraconazole. The cost of fluconazole has reduced considerably, but itraconazole remains relatively expensive. Itraconazole is often more effective than fluconazole, especially when bone lesions are present. Also, molds such as Aspergillus have intrinsic resistance to fluconazole, so fluconazole should not be used to treat mold infections. Fluconazole penetrates the brain and the urinary tract, whereas itraconazole may not, unless significant inflammation is present. Practitioners are cautioned that some compounded formulations of fluconazole and itraconazole may lack efficacy. When treatment failure occurs, serum drug levels can be performed to ensure adequate drug concentrations are present. Itraconazole levels should be performed 2 weeks after initiating treatment, because itraconazole can take this long to reach maximum serum concentrations.

For dogs with disseminated disease, treatment with amphotericin B (AMB) should be considered, although this can be expensive, especially when lipid formulations are used. Amphotericin B irreversibly binds sterols in fungal cell membranes, forming pores with subsequent leakage of ions. Amphotericin B is virtually unabsorbed from the gastrointestinal tract, so it is formulated for IV infusion (Fungizone®, AMB-D) as a complex with the bile salt deoxycholate. Addition of electrolyte to the solution causes it to aggregate, so it is administered in D5W. Penetration of the CSF and vitreous humor is poor.

The major adverse effect of AMB-D is nephrotoxicity. Loading with IV 0.9% NaCl for 1 hour before the infusion decreases nephrotoxicity. Slow administration in a large volume of fluid also decreases nephrotoxicity. Fever, inappetence, and vomiting also appear to occur in some dogs treated with AMB-D. Treatment with nonsteroidal anti-inflammatory drugs can be used to decrease pyrexia during therapy.

Lipid Formulations of Amphotericin B (AMB)

Currently, three lipid formulations of AMB are marketed in the US. These are associated with a considerable reduction in nephrotoxicity compared with AMB-D. This allows administration of a higher dose of the drug, sometimes with improved treatment efficacy. Most experience in veterinary medicine has been reported with Abelcet®, which is a mixture of AMB, dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol that forms ribbon-like sheets. Abelcet® has been used in small animal patients to treat cryptococcal meningitis, disseminated coccidioidomycosis, blastomycosis, histoplasmosis, and pythiosis.

Flucytosine (5-Fluorocytosine) (Cats Only)

Flucytosine has activity only against Cryptococcus neoformans and Candida spp. These fungi deaminate flucytosine to 5-fluorouracil, a potent antimetabolite. Drug resistance arising during therapy is very common, and so flucytosine must always be used in combination with other drugs. Penetration of the CSF is excellent and high urinary concentrations can be achieved. The most common adverse effects are myelosuppression and gastrointestinal upset. The drug should be avoided when renal failure is present. Administration of flucytosine should be avoided in dogs, as development of a severe (but reversible) drug eruption within 2–3 weeks of starting treatment is common. Unfortunately, flucytosine can be unaffordable for many pet owners.

New Triazoles

Newer triazoles are expensive and include voriconazole and posaconazole. Voriconazole is an excellent choice for treatment of refractory invasive mold infections in dogs. It penetrates the CNS and uncommonly causes hallucinogenic-like neurologic signs in dogs. Cats are very predisposed to neurologic adverse effects and so its use is not currently recommended in cats. Posaconazole is an itraconazole analog and has also shown efficacy for treatment of refractory fungal infections in dogs and cats.

Pneumocandins/Echinocandins

The echinocandins inhibit formation of beta(1,3)-D-glucans in the fungal cell wall. The prototype drug is caspofungin acetate. Other drugs in this class are micafungin and anidulafungin. Caspofungin is given once daily as a slow IV infusion. Caspofungin is effective against resistant Candida albicans, and it also has efficacy against Aspergillus. It is ineffective against Cryptococcus. Cost is slightly less than for lipid formulations of AMB, but nevertheless the need for daily IV infusion together with the cost means it is rarely used in veterinary medicine at this time. In addition, the optimum dose for dogs and cats and its adverse effects in these species are not known.

Terbinafine

Terbinafine (Lamisil®) inhibits fungal squalene epoxidase, blocking fungal ergosterol synthesis. Terbinafine is well absorbed, although there is a high hepatic first-pass effect in humans. It accumulates in skin, nails, and fat. In veterinary medicine, it has been most commonly used to treat dermatophytosis, and it is well tolerated. Its efficacy for invasive fungal infections has not been well investigated, although there are a few published and some anecdotal reports of its use to successfully treat some deep mycoses, especially when used in combination with other drugs.

Table 1. Commonly used drugs for treatment of deep mycoses in dogs and cats

* Check BUN and creatinine prior to each administration. Discontinue if azotemia occurs.