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Aspergillosis is a fungal disease of the respiratory tract of birds and mammals usually caused by Aspergillus fumigatus. Aspergillus flavus, A. niger, A. nidulans, A. terreus, A. glaucus and Penicillium sp. have also been identified as pathogenic. Aspergillosis was one of the first mycoses of birds to be identified, being first observed in a wild scaup in 1813. The disease usually occurs either sporadically (chronic) or in an acute epizootic form.
Aspergillosis has a worldwide distribution having been reported in almost all domestic mammals and birds and in numerous wild species. Aspergillus sp. organisms are capable of living both a saprophytic and parasitic way of life and susceptible hosts have numerous opportunities to contact this agent.
Aspergillosis is most often seen in wild and captive birds. Aquatic and scavenging wild species have the highest incidence rate while captive species are most susceptible when first subjected to captivity. This is especially true when an extended transporting is required in overcrowded and unhygienic conditions.
In Michigan, aspergillosis has been identified as a mortality factor in 51 avian and 2 mammalian (white-tailed deer and big brown bat) species. It is a minor mortality factor in white-tailed deer.
Transmission and Development
Aspergillus fumigatus is widely present in nature and produces tremendous numbers of spores in a few days when placed in warm and moist conditions. Favored environments for the mold are damp litter in stubble fields, on moldy grass along riverbanks, on moldy grain or meal, and on rotting plant and animal material, especially damp hay, and straw.
Birds usually become infected by inhaling dust-laden air containing spores or by ingesting spores while feeding on or near the contaminated sources. The inhaled or ingested spores lodge in the air sacs and begin to develop. If the large air sacs are involved, the walls become thickened (3 to 6 cm) and pneumonia develops. If the small air sacs of the bones are involved, an osteoarthritis will result.
Aspergillus sp. organisms are present on the external surface of eggs following laying. Spores of A. fumigatus can penetrate fresh or incubating eggs through the pores and grow on the shell membranes. Newly hatched chicks can become infected in this manner. A. fumigatus is not contagious; that is, it cannot be transmitted from bird to bird.
Transmission, development and mortality of A. fumigatus depend on five conditions: the number of spores in the air, the length of the bird's exposure, the age of the bird, the physiological state of the bird and the opportunistic organisms present.
Small numbers of spores can be tolerated by the body, but large numbers can cause disease. Five million inhaled spores have been found to be necessary to produce fatal infections in day-old chicks and at least 17 million are necessary for an adult bird to become infected. Birds are most susceptible during periods of physiological stress, such as during mating and egg laying, and when resistance to disease has been lowered.
Once the A. fumigatus infection has become established, the fungus produces poisonous substances or toxins which may damage various tissues. The toxin is hemotoxic, neurotoxic, and histotoxic.
Frequently, weak and overcrowded birds or birds kept in unhygienic conditions are most prone to aspergillosis infections. Usually nestlings and old birds are most likely to become ill with this disease.
Aspergillosis infections may be peracute, acute, or chronic and the clinical signs seen with each vary slightly.
Young birds generally have a peracute or acute infection with clinical signs that include: loss of appetite, increased respiration rate, increased temperature, listlessness, foetid diarrhea, rapid loss of condition and in some cases convulsions.
Older birds usually have a chronic, debilitating type of infection with clinical signs that include dyspnea, gasping, inappetence, emaciation, increased thirst, pyrexia, diarrhea, somnolence, and signs of nervous involvement. Respiration may be noiseless or there may be a wheezing, rattling, or clicking sound. Most physical activities are curtailed but light exercise is accomplished with minimal respiratory distress. Affected birds usually will act depressed, separate themselves from the flock, and oftentimes remain in a sitting position with drooping wings.
The pathological changes resulting from an A. fumigatus infection occur primarily in the respiratory system but may involve the joints, eyes, and nearly all of the viscera. The most dramatic changes occur in the respiratory system with the bird's sinuses, trachea, bronchi, lungs, and air sacs being affected. The internal nares and sinuses may fill with a yellow cheesy nodular mass. The spores on the walls of the trachea and bronchi undergo development and a branching of mycelia and a yellow-white nodular mass develops in the early stages of the infection. Eventually the air passages become blocked and the walls of the bronchi and trachea are covered by elevated dirty yellow or green layers.
The disease may take 3 forms; acute diffuse pneumonia form, acute nodular pulmonary form, and chronic nodular pulmonary form. The lungs may show an acute diffuse pneumonia form, marked congestion of the lungs occurs as well as hepatization of the lung tissue.
The acute nodular pulmonary form is characterized by soft, yellow-white caseous necrotic miliary nodules measuring 1-3 mm in diameter, distributed evenly throughout the lung tissue. The nodule is surrounded by a dark infiltrated zone. Nodular formation is the result of hyphae infiltration into the lung parenchyma causing cellular infiltration and the formation of a wall around the center of the radiating hyphae.
The chronic nodular pulmonary form of the disease is characterized by areas of consolidation and coalescence of several nodules or enlargement of single nodules to form masses of necrotic or granulomatous lesions. Calcification of the masses may result from these changes with death being the usual termination of the mycotic pneumonia.
The serous membranes of the air sacs become thickened due to the development of aspirated spores. Small, yellow-white, flattened plaques of dense composition form on the surface of the air sacs. These plaques quickly enlarge and coalesce to form a complete membrane or caseous lining on the air sac. The membrane on the air sacs soon exhibit a white moldy growth which turns green to black as the typical fruiting bodies (spores) develop. The plaque formation results in loss of tissue effectiveness and in actual tissue damage caused by the fungal hyphae.
Systemically, the infection may be spread via the serous surfaces to the kidney, liver, spleen, ovaries, gizzard, and brain with resulting nodular formation. Encephalitic aspergillosis has been diagnosed from young domestic turkey poults, possibly infected by egg invasion by the organism.
Histologically, the air passages are filled with mucus, stained fibrin, nuclear fragments, detritus, mycelia and inflammatory cells. The nodules in the lung parenchyma have a central core of caseous necrosis in which the organisms are found surrounded by a wide zone of epithelioid granulation tissue. There are giant cells, lymphocytes and fibroblasts present around these nodules. In deer, lesions observed consist of small, tan, spherical, necrotic masses (nodules) lying in the parenchyma of the lungs. These lesions can sometimes be mistaken for bovine tuberculosis.
A diagnosis of aspergillosis can be demonstrated by histopathological examination of preserved tissue (lung and air sac), by microscopic examination of fresh tissue, and by cultural methods. It is possible to demonstrate the presence of the characteristic organisms in the typical respiratory tract lesions. Identification can be made by dissecting out and crushing a nodule on a slide beneath a cover slip in a drop of 20% potassium hydroxide and lactophenol cotton blue. The lactophenol cotton blue stains the hyphae projecting from the edges of the mass of caseous material. The hyphae are slender, dichotomously branching, and septate.
Sections preserved in 10% normal saline can be dehydrated and sectioned and stained with differential stains. The organisms stain poorly with hematoxylin and eosin but special stains such as P.A.S., Bauer's and Gridley's differentiate the hyphae and allow for easy identification of the hyphae and mycelia.
To identify the species properly, the pathogenic organism must be isolated by culturing it on differential media. Small pieces of lesions should be removed aseptically and placed onto plates or slants containing malt agar, Sabouraud's glucose agar, or antibacterial antibiotics. Growth is rapid when the culture is incubated at 37º to 45º C for 24 hours.
Hyphae growth is present at this time. Within 48 hours after the culture was started the characteristic spores are seen. The spores are greenish-yellow at first and in time turn dark green in color, giving the colony a dark green and dusty appearance.
Treatment and Control
In a game farm situation preventative measures should be practiced so that this disease does not become established in the flock. A high standard of nutrition, hygiene, and housing should be maintained at all times. Houses and yards should be kept clean and mold-free ration should be fed. Food and water utensils and incubators should be frequently cleaned and disinfected.
If an outbreak develops in a captive setting, sick and dead birds should be removed and disposed of. All litter should be removed from the house and the premises, feeders, and waterers disinfected.
There is no effective treatment for aspergillosis once a bird has contracted an infection. Treatments that have been used are tar vapors, chlorine or sulphur dust, 0.1 to 0.2% potassium iodide in the drinking water, amphotericin B given intravenously, and nystatin either in the feed, injected, or in an aerosol. Nycostatin has been used successfully in the past in the water as a preventative.
There is no practical treatment for free-living birds. When wild birds are captured and transplanted, care should be taken to limit the stress involved and the method by which they are moved.
Aspergillosis is significant to both the game farm manager and the wildlife manager as the disease can be found in birds both in captivity and in the wild. Deaths frequently result following a capture and transplanting operation of wild bird species but can also occur in isolated and epizootic cases when birds have had access to moldy food material or vegetation.
Birds with aspergillosis would not be considered edible due to the extremely poor condition. While it is not possible for humans to contract aspergillosis from eating the meat of an infected bird, it is possible for humans to contract this disease from inhaling the spores that are present on the air sacs. Because of this, infected birds should be discarded and not consumed. Aspergillosis in deer is a relatively rare occurrence and this disease is a minor mortality factor in the deer herd in the state.