THE MERCK MANUAL MEDICAL LIBRARY: The Merck Manual of Diagnosis and Therapy
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Mycotic Diseases of Pet Birds

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Fungal diseases are common in companion birds. Often, they are a secondary infection in an already ill or immunocompromised bird. The most common fungal diseases are respiratory tract infections caused by Aspergillus spp and GI tract infections caused by Candida spp. Macrorhabdus is an unusual fungus that affects the GI tract and occurs most commonly in small pet birds (budgerigars, lovebirds, finches, parrotlets, and cockatiels).

Candidiasis

Candidiasis is a common fungal disease seen in pet birds. The etiologic agent is the opportunistic yeast Candida albicans, which commonly affects the GI tract. Although C albicans is the most common isolate, others including C tropicalis, C parapsilosis, C glabrata, and Hansenula spp may also be found and may be refractory to treatment.

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

Candidiasis most commonly affects unweaned chicks. Those on broad-spectrum antibiotics are most at risk. Neonatal cockatiels are considered most susceptible. Infection may be endogenous, because of yeast overgrowth, or caused by oral inoculation of large numbers of Candida, either by parental feeding or by hand feeding with utensils that are inadequately cleaned.

Clinical signs in adult birds are typically mild and may include mild weight loss, lethargy, and dull plumage. Young birds may have more severe disease, especially if they are immunocompromised. Clinical signs in juvenile birds are anorexia, crop stasis, white plaques in the oral cavity, regurgitation, and weight loss. Localized infections in the oral cavity can lead to difficulty swallowing or halitosis. With severe infections, there may be complete crop and GI stasis. Thickening of the crop may develop (Turkish-towel appearance). Infection of the proventriculus and ventriculus can also occur and may lead to more severe clinical signs such as weight loss, regurgitation, diarrhea, and depression. See Table: Differential Diagnoses for Regurgitation in Pet BirdsTables for differential diagnoses for regurgitation in birds.

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Diagnosis of candidiasis is by identifying Candida spp on a Gram, Romanowsky-type, or new methylene blue stain of the feces, crop contents, or regurgitated material. Scrapings or impression smears may also be performed to diagnose suspected yeast infections of the skin. In 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.

Often candidiasis is secondary to poor husbandry and an unclean environment. 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 with crop stasis, the crop must be emptied and smaller amounts fed until the crop stasis has resolved. Metoclopramide may help crop motility and prevent regurgitation. After identifying and resolving any predisposing factors, treatment with nystatin or fluconazole should be initiated. Nystatin (300,000 IU/kg, PO, bid) is commonly used for treatment because of its low cost and low toxicity. Disadvantages are poor taste and large volume required. Because it is fungistatic and not absorbed from the GI tract, it is only effective when in direct contact with infected tissue, so it is often administered tid before feeding. If the yeast is resistant to nystatin or the bird is difficult to medicate, then fluconazole (20 mg/kg, PO, every 48 hr) is available for systemic treatment.

Flock treatment has historically been accomplished with 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 Table: Antifungals Used in Pet BirdsTables for some antifungal drugs used in pet birds.

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Aspergillosis

Aspergillosis is an opportunistic infection that typically occurs in immunocompromised hosts (malnutrition, especially vitamin A deficiency) or when birds are exposed to large numbers of aerosolized spores. It is not transmitted bird-to-bird. Aspergillus fumigatus is the most common species isolated; A flavus andA niger are also found. Rhizopus, Penicillium, Mucor, and Scedosporium apiospernum can cause similar signs and are more difficult to diagnose and treat. Predisposing factors for developing infection include species predilection (African grey parrots, Amazon parrots, cockatiels, and macaws), aspiration of food or medications, immunosuppression (underlying disease), moldy bedding or feed, and use of corticosteroids. Poor hygiene and inadequate ventilation, especially in warm, humid climates, can also increase the incidence of disease.

Clinical signs depend on the site of infection. Most infections occur in the upper respiratory tract, air sacs, lungs, trachea, and syrinx. Systemic infections occur when the infection spreads to internal organs, including bone, liver, kidneys, or brain. Eye and skin infections can also occur. Anorexia and weight loss are often present. Respiratory signs may include dyspnea, tail bobbing, exercise intolerance, and voice change. Rhinitis caused by Aspergillus appears similar to bacterial rhinitis or sinusitis. A Gram stain or modified Wright's stain of lesions or debris often demonstrates fungal hyphae. Infraorbital sinusitis caused by aspergillosis often must be surgically debrided before therapy is effective. Extensive or chronic fungal sinusitis may lead to osseous changes and permanent malformation of the upper respiratory architecture. Extension of infection to the brain can cause CNS signs.

Tracheitis due to aspergillosis can occur in immunocompromised birds. Aspergillus granulomas often form in the syrinx of psittacine birds and raptors and are particularly challenging to treat. Changes in vocalization may occur before dyspnea is observed, and often these birds will stretch out their necks while breathing. Lower respiratory disease, including airsacculitis, often involves invasion by Aspergillus spp. Granulomas of the air sacs or the coelomic cavity are also common, usually in the caudal thoracic or abdominal air sacs. These lesions may require surgical resection.

Diagnosis can be challenging but should be pursued because treatment is longterm and costly. Aspergillosis should be considered in any pet bird presenting with signs of upper or lower respiratory disease, wasting disease, or marked leukocytosis/monocytosis. A leukocytosis/monocytosis combined with clinical signs and radiographic findings can provide a strong presumptive diagnosis in suspect cases. Often birds with chronic disease will be anemic. A radiograph may reveal airsacculitis, granulomas, or severe pulmonary disease. A CT scan or MRI is more likely to reveal more subtle or diffuse disease. Serologic testing is of limited use; antibody and antigen titers, including galactomannan antigen testing, often result in false-negative or false-positive results. A false-negative often occurs because of immunosuppression, so consultation with the laboratory regarding interpretation is important. PCR testing to detect specific Aspergillus DNA is a much more sensitive method to confirm infection and is more suitable for immunocompromised birds because detection of antibodies is not involved. Plasma protein electrophoresis showing increased β-globulin levels is consistent with aspergillosis. Definitive diagnosis may require direct visualization of lesions either by surgery or endoscopy and confirmation by biopsy, cytology, and/or fungal culture of lesions. Low fungal viability may yield a negative culture despite confirmation by cytologic results.

Treatment varies depending on the site of infection. An acute tracheal infection with obstruction by an aspergilloma is an emergency that may require placing an air sac tube. After tube placement, syringeal or tracheal plaques are removed with a rigid endoscope. Amphotericin B can then be instilled intratracheally at 1 mg/kg through the glottis, and treatment with systemic antifungals and nebulization should be started. Amphotericin B is the only fungicidal agent available and can be used in nebulization, as a nasal flush, intratracheally, and in IV administration. For nebulization, a concentration of 0.25–1 mg/mL of sterile water can be used. Nasal and sinus flushes are generally more dilute at 0.05 mg/mL of sterile water. Amphotericin should not be diluted with saline, because this decreases its potency.

Flushing the nares and sinus with unmedicated sterile water or saline before medicating may allow samples to be obtained for cytologic examination and culture. Several flushes of unmedicated warm isotonic saline or sterile water should be done before a final infusion of the medicated mixture. 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, once to twice daily) is the most commonly used azole for systemic infection. African grey parrots are more sensitive to adverse effects of itraconazole, especially regurgitation and anorexia, and should be dosed at 5 mg/kg/day, PO. Terbinafine (10–15 mg/kg, PO, bid) can be used in lieu of or in conjunction with itraconazole. Clotrimazole can be used for nebulization in conjunction with systemic therapy (10 mg/mL, nebulized 15–30 min 2–4 times/day). Voriconazole (12–18 mg/kg, PO, bid) is being used for resistant strains of Aspergillus.

If fungal granulomas are identified and the lesions are accessible, debulking endosurgically or treating the lesions topically with amphotericin B may improve the outcome. Birds undergoing treatment should be monitored closely for adverse effects of many antifungal drugs, which can include depression, anorexia, and liver dysfunction. Hepatic enzyme, bile acids, and uric acid concentrations should be monitored every 4 wk during treatment. Treatment success may be difficult to determine, but serial CBCs and radiographs may be helpful. Treatment with oral drugs should be continued for 2–4 wk after clinical signs have resolved. Birds with aspergillosis often have underlying disease problems and may be immunocompromised, which may affect treatment and recovery. Thickened and scarred air sacs that develop during and persist after infection can provide an ideal environment for disease recurrence.

Macrorhabdus ornithogaster Infection

(Macrorhabdosis, Megabacteria, Avian gastric yeast)

Macrorhabdus ornithogaster most often affects the proventriculus and ventriculus of smaller companion birds (eg, budgerigars, parrotlets, lovebirds, cockatiels, and finches). Previously described as a bacterium, this organism has a worldwide distribution and varies widely in pathogenicity.

Clinical signs of infection are weight loss, regurgitation, lethargy, passage of undigested food, and diarrhea. These clinical signs may mimic proventricular dilatation disease. Mortality may be high, but birds may recover. In birds that recover, relapses and potential shedding of the organism in the feces are likely. This disease is often seen in conjunction with immunosuppression (eg, polyomavirus and circovirus infection, or associated with poor husbandry). Asymptomatic infection is common.

Diagnosis is made by examining a wet mount of a fresh dropping at 10–50X magnification with the stage condenser mostly closed to increase contrast. The large, rod-shaped organisms are approximately 2–4 µm wide and 60–90 µm long. Many birds are asymptomatic and shed low numbers of organisms, whereas sick birds tend to shed large numbers. Birds may shed the organism intermittently, so a negative fecal examination does not exclude infection. Wet mount, modified Wright's, or Gram stain of a fecal sample often reveal organisms. M 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 droppings are generally several magnitudes larger than the normal digestive bacilli found in birds. Selected veterinary laboratories offer both visual identification and PCR testing. M ornithogaster does not grow on conventional fungal media. Radiographs may reveal a dilated proventriculus. Necropsy lesions may include thinning of the isthmus, proventricular dilation and ulceration, thickening of the proventricular wall, mucus production, and softening of the koilin layer of the ventriculus.

The goals of treatment are to reduce the number of organisms and improve the general health and immunocompetence of the bird. Amphotericin (100 mg/kg, PO, bid for 30 days) has had the highest treatment success rate, but failures are common, especially with a shorter duration of treatment. Acidification of the proventriculus (apple cider vinegar, vitamin C) has been reported to create an environment less conducive to proliferation of Macrorhabdus. Voriconazole has been successful (anecdotal) at 10 mg/kg, PO, bid.

Treatment with sodium benzoate in the drinking water has been anecdotally reported to be successful but still experimental. Sodium benzoate at 1 tsp/L water for 5 wk cleared the infection in nonbreeding budgerigars, but in budgerigars that were rearing chicks in high environmental temperatures >90°F, treatment with ½ tsp/L water resulted in neurologic signs and death of the adult budgerigars because of their increased water intake. The current recommendation for treatment of Macrorhabdus with sodium benzoate is ½ tsp (2.5 g) of sodium benzoate powder/L of water (used as only water source and made fresh daily). If the birds are not drinking the medicated water, the dose should be decreased to ¼ tsp (1.25 g)/L of water, and slowly increased back to ½ tsp over the next few days. Feces should be rechecked at 14 days; if Macrorhabdus organisms are still present, the dose should be increased over several days to 1 tsp (5 g)/L. Feces should be rechecked at 30 days. The lower dose of ¼ to ½ tsp powder/L of water should be used in birds housed outdoors in summer (temperatures >90°F [32.2°C]) and in birds feeding chicks. Gloves should be worn when handling medication.

Asymptomatic carriers are common. Artificial incubation of eggs and hand feeding nestlings can help establish a pathogen-free flock.

Malassezia spp

Malassezia spp have been reported in birds with feather picking and dermatologic conditions. In suspect cases, biopsy or cytology should be performed. Culture is often unrewarding. The recommended treatment is fluconazole at 5–10 mg/kg, PO, once to twice daily. Topical therapy with either dilute chlorhexidine 0.1% spray or clotrimazole may be used alone or in conjunction with systemic treatment.

Cryptococcus neoformans

Cryptococcosis is worldwide in occurrence and caused by a saprophytic fungus, Cryptococcus neoformans. This fungus is uncommon in avian species, but it has been isolated from the fecal droppings of wild birds, particularly pigeons, and from feces of canaries and psittacine birds. C neoformans infection has been reported in Columbiformes, a Moluccan cockatoo, a thick-billed parrot, a green-wing macaw, and an African gray parrot. Veterinarians should be aware that not only can birds be carriers of the disease and become ill but also that C neoformans is zoonotic. Most human infections with C neoformans occur in immunocompromised individuals and are associated with environmental exposure, although cases have been reported in which contact with a pet bird appeared to be the source of infection.

Clinical signs of Cryptococcus infection include weakness, lethargy, anorexia, diarrhea, and dyspnea. With CNS involvement, neurologic signs such as blindness and paralysis may occur. Clinical diagnosis can be difficult. A definitive diagnosis requires demonstrating the organism on cytologic or histologic examination or culture. The organism is a round to oval yeast with a mucopolysaccharide capsule. A mucoid gelatinous exudate may be noted within the respiratory tract, coelomic cavity, sinuses, brain, or within the long bones.

Treatment recommendations for birds include amphotericin B, itraconazole, ketoconazole, fluconazole, or voriconazole. Fluconazole or voriconazole are the drugs of choice for CNS or ocular infections.

Rhodotorula mucilaginosis

Rhodotorula mucilaginosis is a yeast occasionally seen in skin infection in raptors (falcons). It causes yellowish brown crusty areas of the skin in the axillary or inguinal areas. Untreated lesions may develop into hyperkeratotic proliferative growths. Diagnosis is based on physical examination findings and results of cytologic or histopathologic examination and culture of infected tissues.

Treatment involves excision of proliferative lesions and topical antifungal therapy. Antibiotics may be necessary to prevent secondary infections, and analgesics or anti-inflammatory drugs may be warranted for pain.

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). Histoplasmosis and mucormycosis are also occasionally reported in pet birds.

Last full review/revision October 2015 by Sharman M. Hoppes, DVM, ABVP (Avian)

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