THE MERCK VETERINARY MANUAL
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Mycotic Diseases of Pet Birds

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Candidiasis

Candida albicans is an opportunistic yeast and is not generally considered a primary pathogen. Small numbers of Candida are commonly found in the digestive tract of birds and may become pathologic when normal digestive flora are disrupted by immunosuppression. (Also see Candidiasis.)

Candidiasis most commonly affects unweaned chicks. Infection may be totally endogenous, with overgrowth due to the above-mentioned factors. It may also be caused by oral inoculation of large numbers of Candida, either by parental feeding or by hand-feeding with utensils that are inadequately cleaned.

Delayed crop emptying with a thickened crop is the most common finding in hand-fed birds. Regurgitation, weight loss, and depression may follow. See Differential Diagnoses for Regurgitation in Pet BirdsTables for differential diagnoses for regurgitation in birds. Adult birds may harbor low-grade candidiasis with few overt clinical signs.

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Diagnosis is usually based on cytology. In more severe cases, when tissue invasion has occurred, the budding yeast will produce hyphae that can be seen in scrapings obtained from the crop or pharynx, or from the feces.

If a reservoir of exogenous Candida is present (eg, poor nest box or feeding tube hygiene), then eliminating the source of the Candida is critical. In neonates, the crop must often be emptied and smaller amounts fed until crop stasis has been resolved. Metoclopramide may aid in crop motility and preventing regurgitation. Nystatin (300,000 U/kg) is the most commonly used medication for candidiasis. Because it is fungistatic and is only effective when it directly contacts infected tissue, it is often administered tid, prior to feeding. Some Candida infections are resistant to nystatin, and in some birds immunosuppression precludes clearing of the infection by use of only a fungistatic agent. In these cases, systemic medications such as fluconazole (10 mg/kg, bid) are often used.

Flock treatment has historically been accomplished with the use of chlorhexidine at 10 mL/gal. of drinking water for 1–3 wk. Because chlorhexidine is a disinfectant, its use will also deplete the normal digestive flora. Acidification of the upper GI tract by use of apple cider vinegar has also been reported to resolve Candida overgrowth.

See Antifungals Used in Pet BirdsTables for some antifungals used in pet birds.

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Aspergillosis

Fungal infections in pet birds are generally caused by Aspergillus fumigatus. An opportunistic organism, it is often found in the same locations and under the same conditions as many bacterial secondary invaders. Malnutrition, especially vitamin A deficiency, is a predisposing factor. Poor hygiene and inadequate ventilation, especially in warm, humid climates, can increase the incidence of this disease.

Rhinitis caused by Aspergillus is similar in appearance to bacterial rhinitis or sinusitis. A Gram's stain or modified Wright's stain of lesions or debris often demonstrates the fungal hyphae. Infraorbital sinusitis involving aspergillosis often must be surgically debrided prior to effective therapy. Extensive or chronic fungal sinusitis may lead to osseous changes and permanent malformation of the upper respiratory architecture.

Tracheitis due to aspergillosis can occur in immunocompromised birds. Aspergillus granulomas often form in the syrinx of psittacines and raptors and are particularly challenging to treat. Changes in vocalization may occur prior to dyspnea. Often these birds will stretch out their necks in an attempt to get more oxygen.

Lower respiratory disease, including air sacculitis, often involves invasion by Aspergillus. Granulomas of the air sacs or coelomic cavity are also common, usually in the caudal thoracic or abdominal air sacs. These lesions may require surgical resection.

Antibody titers are of use in some species for diagnosis of this condition. Antigen tests may also be helpful. Galactomannan is an Aspergillus sp antigen that shows promise in the diagnosis of aspergillosis in psittacines. False negatives and false positives may occur with these tests, so consultation with the laboratory regarding interpretation is important. Serum electrophoresis with elevated β-globulins is consistent with aspergillosis. An absolute monocytosis and heterophilia with a significantly increased total WBC count are usually present. Direct visualization, cytology, and fungal culture may be necessary to confirm the diagnosis. Low fungal viability may yield a negative culture despite confirmation by cytology.

Amphotericin B is used in nebulization, nasal flushes, and intratracheal and IV administration and is the only completely fungicidal agent available. For nebulization, it is used at a concentration of 0.25–1 mg/mL of sterile water. The concentration for nasal and sinus flushes is generally more dilute (0.05 mg/mL of sterile water). Amphotericin should not be diluted with NaCl, because this decreases its potency.

The addition of hyaluronidase to the nasal flush may increase the penetration of the antimicrobial agent by its action on hyaluronic acid in the caseated debris in the sinus. The usual dosage for hyaluronidase is 75–150 IU/10 mL of flush. The appropriate antimicrobial may be added to this flush solution. Several flushes of unmedicated warm isotonic saline or sterile water should be employed prior to a final infusion of the medicated mixture. Organic debris obtained by these preliminary flushes can be used for cytology and culture. Care must be exercised to maintain the bird's head in a downward position to avoid the potential for aspiration of the infected debris into the lower respiratory tract.

Itraconazole (5–10 mg/kg, PO, every 24–48 hr) is the most commonly used azole for systemic Aspergillus infections in the USA. African Grey parrots (Psittacus erithacus) seem particularly sensitive to regurgitation and anorexia and generally receive a lower dosage—2.5–5 mg/kg, sid. Clotrimazole (10 mg/mL) is being used more frequently for nebulization in birds. Terbinafine (10 mg/kg, PO, sid) is used with increasing frequency in conjunction with or in lieu of itraconazole. Newer azoles such as vorconizole may be effective against resistant strains of Aspergillus.

Many birds infected with aspergillosis have underlying problems that complicate treatment and recovery. Chronic vitamin A deficiency and squamous metaplasia, immunocompromise, and the scarring and thickening of air sacs that occur following an infection all provide an environment for reinfection, which is common.

If Aspergillus granulomas have formed, surgical removal (with placement of an abdominal breathing tube) can be attempted, as can flushing or aspiration of the granuloma using endoscopic equipment. Recurrence of the granuloma, secondary inflammatory changes, and the production of hyaline membranes subsequent to infection in the trachea and syrinx are common.

Macrorhabdus ornithogaster Infection

(Macrorhabdosis, Megabacteria, Avian gastric yeast)

Previously described as a bacterium, this organism has a worldwide distribution and a wide variation in pathogenicity. Its site of colonization is the GI tract, primarily the proventriculus.

The most common presentation of macrorhabdosis is chronic weight loss. Regurgitation is common, and polyphagia followed by decreased food intake often occurs. Stools may contain undigested seeds or pellets. These clinical signs may mimic proventricular dilatation disease. Mortality may be high, but recovery may occur. In recovered birds, both relapses and potential shedding of the organism in the stool are likely. This disease is often seen in conjunction with immunosuppression (eg, circovirus infection).

Wet mount, modified Wright's, or Gram's stain preparations of feces often reveal organisms. Macrorhabdus ornithogaster appears as a large, gram-positive rod, with mottling or stippling throughout its length. Although the size and length may vary, organisms recovered from the stool are generally several magnitudes larger than the normal digestive bacilli found in birds. This organism is difficult to culture. Selected veterinary laboratories offer both visual identification and PCR testing.

The goals of treatment are to reduce the number of organisms and improve the general health and immunocompetence of the bird. Amphotericin B has been used most often in the treatment of macrorhabdosis. Various azoles may also be effective. Acidification of the proventriculus has been reported to create an environment less conducive to the proliferation of Macrorhabdus.

Malassezia sp

Malassezia sp may exist asymptomatically on the skin of birds. To determine whether the presence of Malassezia is significant in birds with dermatitis or feather loss, histopathology rather than culture is necessary. The recommended treatment is fluconazole (5–10 mg/kg, PO, every 1–2 days). Topical therapy with either dilute chlorhexidine spray (0.1%) or clotrimazole has been used.

Miscellaneous Mycoses

Dermatophytosis, including Trichophyton and Microsporum spp, is occasionally reported in pet birds. Treatment protocols for dogs and cats are used (see Dermatophytosis in Dogs and Cats). Cryptococcus has been occasionally reported to cause facial dermatitis in birds. Therapy includes debridement and longterm itraconazole. This organism may cause zoonotic infections. Histoplasmosis and mucormycosis are also occasionally reported in pet birds.

Last full review/revision July 2011 by Teresa L. Lightfoot, DVM, DABVP (Avian)

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