Aspergillosis , is an important highly infectious global fungal disease of avians as well as mammals. The disease is mainly caused by Aspergillus fumigatus which occurs as saprobe in the environment. Aspergillus fumigates is known to occur in a wide variety of avian species. The source of infection is exogenous and chicks acquire the infection by inhalation of fungal conidia through the respiratory tract. The contaminated litter is often associated with cases of aspergillosis in chicks and turkeys. Acute aspergillosis in chicks is usually characterized by severe outbreaks with high mortality and morbidity, and results in great financial losses to the poultry industry. The demonstration of the fungal agent in the clinical specimens both by direct microscopy and cultural isolation is considered the gold standard of the diagnosis of aspergillosis. Good hygienic practices should be adopted in the poultry farms to combat this infectious mycosis which carries a high mortality in chicks. Aspergillosis should be included in the differential diagnosis of respiratory diseases of birds. It is recommended that Narayan stain should be widely used in microbiology and public health laboratories to study the detailed morphology of the fungi which are implicated in various clinical disorders of humans as well as animals. In addition, the further research on the development of safe, potent and cheaper vaccine against Aspergillus fumigates infection in chicks will be rewarding.
Poultry production is the most highly developed segment of food animal production in both developed and developing countries of the world. In intensive production system, as large numbers of chickens are kept together, the chickens remain susceptible to many infectious agents, which create challenges in the yearly production of large number of birds (Lister, 2000).The infectious diseases which include multiple etiologies such as viruses, bacteria, fungi, chlamydophilia, rickettsia, protozoa, helminthes and ectoparasites are responsible for high morbidity and mortality in poultry (Jordan et al., 2002; Kahn, 2005; Pal, 2007 and Charlton, et al,2008). The infectious diseases have great impact to the poultry industry in terms of huge financial losses. Among several diseases, aspergillosis is one of the most frequent, highly infectious, global mycotic diseases of avians as well as mammals (Pal, 2007). Aspergillosis is an economically important respiratory disease of poultry. High mortality due to disease results in great financial losses to the poultry enterprises. The present paper delineates the etiology, host, transmission, symtomatology, pathology, epidemiology, diagnosis, and management of aspergillosis in chickens. In addition, public health significance of aspergillosis is also described.
Natural infection has been encountered in a wide variety of avians such as budgerigar, canary, chicken, crow, duck, eagle, falcon, goose, gull, jay, macaw, parakeet, parrot, patridge, penguin, pigeon, quail, sea fowl, swan and turkey (Kaplan et al., 1975; Dyar et al.,1984; Smith,1989; Pal, 1991; Pal et al., 1992; Hafstad et al., 2003; Pal and Dave; 2006 and Pal, 2007).In addition, the disease is also recorded in mammals including man (Smith,1989 and Pal,2007).
There are about 600 species of the genus Aspergillus. Out of which, 27 are implicated in the etiology of the disease in humans and animals including birds. The disease is caused by the fungus Aspergillus which is a saprobic, non-dimorphic, non motile, non-acid fast, Gram-positive, aerobic and filamentous organism. Among several species, A. fumigatus, the principal causative agent of avian aspergillosis, was first found in the lungs of a bustard (Otis tardaga) in 1863, and the species name was attributed to Fresenius ( Castellani,1928). Other species rarely isolated include A. flavus, A. niger, A. nidulans, A.nigrecens, A.amstelodami , A. terreus ,and A.glaucus (Pal et al., 1989; Smith ,1989; Baton et al., 1992; Pal, 2007 and Charlton et al.,2008). Occasionally, dual infection in birds due to A. niger and A. flavus are also observed (Pal, 2007). These organisms are saprophytes and are recovered from diverse types of environmental materials such as the soil, litter, grain, straw, hay, decaying plants, vegetables, fruits, manure, dust, water and air (Pal et al., 1983; Pattron, 2006 and Pal, 2007). Spores are not formed in the tissues, but on the surface exposed to air such as those on the air sacs and the lining of the external orifices and trachea of birds. The organism may produce long, aerial mycelia bearing conidiophores. In tissues, the fungus forms septate hyphae that branch at 45 degree angles (Pal, 2007).
The respiratory tract is considered the chief portal of entry for Aspergillus. The chicks acquire the infection by inhalation of fungal spores adhering to the hatchery trays, feed residues, litter and wood savings, etc.( Pal, 2007). The small size (2-3 um in diameter) of A. fumigatus facilitates its transmission into the lung. Later, the infection may disseminate to various organs of the body through haematogenous route. The contaminated egg when broken releases a “cloud” of Aspergillus spores which are readily inhaled in high concentration by the newly hatched chicks in the poultry farm (Pal, 2007). The chicks become infected usually when they are in brooder stage of growth and hence the term “brooder’s pneumonia”. The fungus has a tendency to invade the blood vessels and cause infraction.
Aspergillosis is common in chicks and turkey poults during first week or so of life. Acute aspergillosis usually is characterized by severe outbreaks in young birds with high morbidity and mortality. The clinical signs and symptoms in young birds (chicks) include dyspnoea, fever, anorexia, dullness, coughing, sneezing, nasal discharge, lacrymation, oedema of eyelid, gasping, increased thirst, foetid diarrhoea, inappetence, emaciation, torticollis, ataxia, convulsion and death (Pal, 2007). Affected chicks die within 24-48 hours of first appearing sick. The disease in chicks may occur as an epidemic with high mortality rate which reaches from 45 to 90 per cent. The disease in adult birds (hens) runs a chronic course that results in decreased egg production. The fowl shows difficulty in respiration, diarrhoea, exhaustion, weakness and death from suffocation. Ocular aspergillosis is characterized by extensive keratoconjunctivitis and panophthalmitis. Mycotic tracheitis due to A. fumigatus and A. flavus has been recorded in chicks (Pal et al., 1990 and Baton et al., 1992).
The pathological changes resulting from 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 (most often the caudal thoracic and abdominal) being affected. The lungs may show an acute diffuse pneumonic form besides, marked congestion of the lungs as well as hepatization of the lung tissue. 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 (Kunkle, 2003 and Cacciuttolo et al., 2009). The serous membranes of the air sacs become thickened due to the development of aspirated spores. Small, yellow-white, flattened plaques formed on the surface of the air sacs. Systemically, the infection may spread via the serous surfaces to the kidney, liver, spleen, ovaries, gizzard, and brain resulting nodular formation. Microscopically, the nodules in the lung parenchyma have a central core of caseation necrosis in which the organisms are found surrounded by a wide zone of epithelioid granulation tissue. Giant cells, lymphocytes and fibroblasts are commonly observed surrounding the nodules. Air passages are filled with mucus, stained fibrin, nuclear fragments, detritus, mycelia and inflammatory cells.
Aspergillosis, primarily a respiratory mycosis of the chicks, is caused chiefly by Aspergillus fumigatus which is a thermotolerant fungus. The acute disease is observed in chicks whereas hens suffer from chronic form. The use of contaminated rice husk, saw dust or sugar cane bagasse used as litter in the poultry farms was found to be associated with severe epidemics of aspergillosis in chicks (Dyar et al.,1984; Sajid et al.,2006 and Pal, 2007).The outbreaks in chicks are usually seen during the first 3 or 4 weeks of rearing and may give rise to a high mortality of 95% in a poultry flock. Contaminated poultry litter is often the source of Aspergillus conidia. A number of factors which include unhygienic conditions of the poultry pen, overcrowding, dampness, humidity, poor ventilation, inadequate nutrition, immunosuppressive compound and concurrent disease are said to predispose the chicks to Aspergillus infection (Pal, 2007). The outbreaks of aspergillosis in chicks are more common in winter months (Kunkle and Rimler, 1996).The outbreaks of disease occur when the fungus is present in sufficient quantity to establish the disease. High incidence of aspergillosis is reported in young birds than in old birds. The outbreaks of disease are frequently encountered in birds particularly in young chicks of first two weeks of age (Sajid et al.,2006). The high mortality in chicks results into great economic losses to the poultry industry. The first author observed two severe outbreaks of aspergillosis in 7 to 9 days old White Leghorn chicks in private poultry farms at Delhi, India, and the chicks in these farms were reared on the deep litter system. The mortality reached from 70 to 90%. The epidemiological investigation established the high concentration of A. fumigatus in the brooder pen where the chicks were kept (M. Pal, Personal Communication). The common source of egg borne aspergillosis appears to be contamination of eggs with mouldy nest box litter and dirty egg storage. An outbreak of egg infection occurred in which the nests were heavily contaminated with A.fumigatus ( Pal, 2007).
The disease of aspergilosis in birds should be confirmed by detection of hyphal elements in pathologic material and isolation of fungus in pure culture on Sabouraud dextrose agar with chloramphenicol ( Pal , 2007). The fungus grows rapidly on Sabouraud medium with colony having a diameter of 3-4 cm in 7 days. Colonies are white at first and then bluish-green as conidia begin to mature. As the colony matures, the conidial masses become gray-green and colony edge remains white. The reverse of the colony is usually colourless. Cycloheximide should not be added in the medium as it inhibits the growth of Aspergillus species. The histopathological examination of autopsied tissues by Gomori methanamine silver (GMS) technique can demonstrate the presence of dichotomously branched septate hyphae measuring 3-7 u in diameter. The detailed morphology of Aspergillus isolates is done in the recently developed Narayan stain (Pal, 2004). The new stain contains methylene blue (3%) 0.5 ml, glycerine 4.0 ml and dimethylsulfoxide ( DMSO ) 6.0 ml. The disease should be differentiated with other poultry diseases such as Newcastle, salmonellosis, pasturellosis, infectious bronchitis, infectious laryngotracheitis, mycoplasmosis, colibacillosis, mycobacteriosis and dactylariosis (Pal, 2007 and Pal et al., 2010). Immunofluorescent can be used to identify hyphae in tissue sections. An immunodiffusion kit is commercially available for the detection of antibodies to A. fumigatus. The application of molecular tool should be tried to make an early diagnosis of aspergilosis.
Aspergillosis in birds is generally not treated. Hamycin (200 mg/ ml) in drinking water for 7 days for the chicks during an acute outbreak of A. fumigatus can reduce the mortality rate (Barbras and Radhakrishnan, 1967). Proper ventilation in poultry houses, avoidance of overcrowding of birds, removal of wet or moist litter, thorough disposal of contaminated litter, daily cleaning and periodical disinfection of all farm premises, incineration of dead birds and maintenance of good hygienic practices in hatchery, brooder house and poultry pen will certainly help to minimize the incidence of aspergillosis in birds. Propionic acid may be used to prevent mould growth in feed. Periodical spraying of 1% copper sulphate solution in poultry pen, brooder, feed hopper, incubator, water utensil and other equipment is found beneficial to kill the spores of Aspergillus. Enilconazole has been tried to control aspergillosis in the poultry environment. Nest hygiene, frequent collection of egg, and proper storage of egg can prevent egg borne aspergillosis. Attempts should be made to develop the safe, effective and cheap vaccine to immunize the chicks against Aspergillus fumigatus infection.
Public health significance: Aspergillus infection in humans has been reported by many investigators from different parts of the world (Rosenberg and Patterson ,1977; Denning,1991; Latge,1999; Pal, 2007 and Pal et al; 2011). Allergic pneumonitis can develop in those subjects who have frequent exposure to mouldy hay, straw and litter in which Aspergillus organisms thrive. Fatal pulmonary aspergillosis has been diagnosed in a child who had close contact with the infected pigeons (Greif et al.,1981). Pal and Torres-Rodrigues (1990) described a case of pulmonary aspergillosis in a poultry worker who was occupationally exposed to the spores of Aspergillus fungus. It is, therefore, advised that employees working in poultry industry must use face mask to prevent the inhalation of air borne spores of aspergilli from the saprobic environment. Moreover, severely immunocompromised individuals are at great risk for developing opportunistic infection due to Aspergillus spp. ( Denning,1991) , and hence they should not be allowed work in poultry farms as Aspergillus is a well recognized opportunistic fungal pathogen of global significance.
We wish to express our sincere thanks to Prof. Ram Mohan Keshav for going through the manuscript, and Dr. Govind Narayan for sending some of the literature on the subject.
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