1. Learning Objectives

• Know the clinical significance of yeast infections in pet birds and poultry
• Be able to recognize, diagnose and treat avian gastric yeast in a bird
• Understand the relationship of stress and disease to the development of aspergillosis in wild and pet birds
• Be able to develop diagnostic and treatment plans for suspected aspergillosis infection in a bird

2. Candidiasis

(Candida albicans)

Candidiasis is an intestinal tract disease primarily affecting the upper intestinal tract (especially the crop) of all species of birds and is caused by Candida albicans. It is often seen secondary to other debilitating disease and is more common in young birds. C. albicans is a normal inhabitant of the digestive tract and results in disease when there has been some derangement of the normal floral inhabitants. The main causes of candidiasis are parasitism, nutritional deficiency, starvation, other infectious diseases, poor sanitation and antibiotic use. Although the crop is the most frequent site of infection, the mouth, pharynx and esophagus are sometimes involved as well as lower reaches of the GI tract (infrequently).

2.1. Candidiasis in pet bird species

Candidiasis is most important in young unweaned birds. It is often referred to as "sour crop". It most often develops secondary to a crop motility disorder, antibiotic therapy, and/or systemic disease. One should routinely use antifungal agents when using antibiotics in baby birds. Candidiasis is also seen as a secondary infection with tetracycline therapy for chlamydiosis. It is most often seen with tetracycline and chlortetracycline, but is less of a problem with doxycycline.

Clinical signs in pet birds usually include anorexia, weight loss, regurgitation and crop stasis. Diagnosis is based on clinical signs and demonstration of the organism on cytology and/or culture of a crop wash, pharyngeal swab, or fecal smear. Budding yeast will be seen as strongly Gram (+) with a gram stain.

Principles of treatment dictate that one must resolve the underlying disorder, not just treat the fungal infection. Effective drugs for candidiasis include: Nystatin, ketoconazole, itraconazole and fluconazole.

An oral plaque in a young pigeon, possible candidiasis

3. Avian gastric yeast

Macrorhabdus ornithogaster? Cryptococcus magnus?

Avian gastric yeast, as yet not definitively classified, is an enteric fungal disease seen in budgies, other psittacines, canaries, finches, turkeys, quails, ducks, geese, ibis and ostriches producing chronic wasting and eventual death. The organism was originally classified as a giant bacteria called "megabacteria" and very recently confirmed as a fungi. The organism is thought to colonize the proventriculus, elevating the pH and altering the physiology of the stomach disrupting the thick, acellular koilin layer found in this portion of the avian GI tract. Malabsorption/maldigestion results in a wasting syndrome.

Avian gastric yeast on a fecal gram stain

Koilin is a carbohydrate-protein complex secreted by the mucosal glands and surface epithelium of the ventriculus. This material hardens in the upper and middle layer of the secretion as a result of exposure to hydrochloric acid secreted by the proventriculus. In the absence of normal HCl secretion, the koilin layer does not form properly.

The organism is Gram positive and can easily be seen on gram stain preparations of feces, or a proventricular wash. They are very long (1 x 90 mm), very difficult to culture, and resistant to all known antibiotics. Treatment with acidification of the GI tract (vinegar, grapefruit juice) and the antifungal agent amphotericin B orally (100 mg/kg PO) has been effective in some cases. Spontaneous recovery has also been reported but prognosis may be very poor.

4. Aspergillosis

(most often Aspergillus fumigatus)

Aspergillosis is a serious disease in captive raptors, captive penguins, marine birds and waterfowl, as well as psittacines and poultry. Fungal diseases in birds are almost always opportunistic and secondary to immunosuppression from chronic stress, other primary diseases (e.g. bacterial, viral, nutritional), severe debilitation, excessive use of antibiotics, or an underlying anatomic or functional disorder (GI motility problem). It can also occur in the presence of high concentrations of the organism in the environment (dirty chicken house, organic litter).

Many marine birds are predisposed to the development of aspergillosis in captivity due to a combination of factors including stress and immune suppression, housing in facilities with inadequate air exchange and overcrowding. These birds should be given prophylactic intraconazole while in captive care.

In poultry aspergillosis is most commonly associated with baby chicks and turkey poults less than 3 weeks of age. Infection usually follows the inhalation of fungal spores from dirty incubators (brooder pneumonia), wet saw dust litter or contaminated feed. Respiratory infection is most common under these conditions but conjunctival infections can sometimes occur if the cornea has been injured by high ammonia levels in the air close to the litter. In addition to the lung and air sacs, the brain and posterior chamber of the eye may be colonized with fungal hyphae.

4.2. Diagnosis

Diagnosis is based on clinical signs, hematology, radiographs, laparoscopy, serology (antigen and antibody), and finally culture. Serology tests are not foolproof but are extremely helpful especially when used with protein electrophoresis. Paired samples are recommended. Currently, making a clinical diagnosis of aspergillosis can be very difficult!

5. Ancillary Material