Aspergillosis, Biology


Aspergillosis is the most common chronic, granulomatous, necrotizing and cavitary disease of lungs, and is characterized by formation of white yellowish caseous nodules in most of the organs of the body. Occasionally congenital infection may occur in day- old lamb. The disease is most frequently observed in avian species; however, sporadic cases have also been recorded in man and many species of domestic, pet, wild and zoo animals. Aspergillus fumigatus is the principal actiological agent of aspergillosis; the other species of Aspergillus such as A. niger, A. flavus, A. terreus, A. nidulans and A. glaucus have also been associated with the disease. The fungus exists and proliferates preferentially in soil, bagasse, saw dust, litter vegetables, fruits, woods, hay, straw, plant and other organic material undergoing biological heating. The infection is usually by inhalation of fungal spores from the saprobic environment. Sometimes, trauma may also help in the introduction of pathogen in the body of host. The aetiological significance of A. fumigatus has been established in pneumonia, bronchopneumonia, sinusitis, rhinitis, corneal ulcer, conjunctivitis, gastro-enteritis, mastitis and abortion. A. fumigatus accounts for more than 90 % of all affections. The primary infection occurs in the lungs following the inhalation of large numbers of fungal spores from mouldy feed. The fungus may germinate, grow and penetrate into contagious tissue, and later due to metastasis, infected emboli may settle in various organs of the body producing nodular lesions which bear resemblance to tuberculosis. The disease has been recorded in fowl, turkey, parrot, pigeon, canary, duck, goose, penguin, quail, partridge, cattle, horse, buffalo, sheep, goat, pig, mule, camel, dog, cat, monkey, deer, bison, rabbit, rat, guinea-pig and also man. In human A. fumigatus is the chief cause of allergic bronchopulmonary aspergillosis. In domestic animals, about 60-80 % of mycotic abortions are due to Aspergillus spp and most of these are associated with A. fumigatus. The incidence of bovine mycotic abortions vary from 1 to 30 % cent, and
 is much greater in winter season in housed cows than in grazing cows. The fungus is also greatly associated with sinusitis in dogs. A. fumigatus being thermophilic in nature, grows at higher temperature, i.e 50° C. Because of this property, the fungus grows excellently in the lungs and air-sacs of birds producing high mortality and morbidity.

Symptoms: The disease is invariably fatal in birds. Mortality varies from 10 to 41.2 % . The sick birds exhibit signs of dullness, anorexia, difficult respiration, fever, discharge from nostril and eye, loose faeces with offensive smell, oedema of eyelids, torticollis, convulsion , coma and death, Cases of aspergillois occur throughout the year with maximum numbers in winter months. Poor ventilation, overcrowding and poor management practices may act as predisposing factors in the flare up of Aspergillosis infection. The infection is more severe and common in young chicks below two weeks of age. Both acute and chronic form of disease is observed in birds. The necropsy findings show congestion and blockage of trachea with cheesy exudate, yellowish-white miliary nodules on lung surface, thickening of air-sacs, enlargement of liver with calcified, greenish-yellow nodules, and nodules in the spleen and kidneys besides mucoid enteritis. Sometimes congestion, oedema and thickening of the brain tissue is also observed in systematic aspergillosis. In mammals disease is sporadic in occurrence. In pulmonary form the main symptoms are dullness, reduced appetite, severe dyspnoea, laboured breathing, coughing, mild fever, foetid diarrhoea with blood and weight loss. The dogs usually suffer from sinusitis and show depression, bilateral mucopurulent discharge, softening of nasal and frontal bones, and slight eosinophilia. In dairy animals abortions usually occur during 3-7 months of pregnancy. Aborted foetuses show discrete, raised, ringworm type patches on the skin of head, neck and back. The placentae are thickened, the eyes show lacrymation, oedema of eyelids, hypopyon, congestion, ulceration of cornea and panophthalmia.

Diagnosis: In clinical cases complete non-response to broad spectrum antibiotics and corticosteroids suggest the possibility of fungal infection. The clinical signs are not characteristic to conclude a presumptive diagnosis. The diagnosis is therefore based on the demonstration o septate hyphae 4-6 um in diameter in exudate, pus, milk, nodules etc. under microscope, after digestion in 10 % potassium hydroxide. The tissue sections stained with Gomori‘s silver methanamine are examined for thin, branched, septate, dichotomous, 3-4 u wide hyphae. The pathogen should be isolated from the infected clinical material on Sabouraud‘s dextrose agar with antibiotics at 37 C. Agar-gel precipitation and complement-fixation test are sensitive serological methods for the detection of antibodies in the serum of affected animals against A. Fumigatus. Allergic test with aspergillin for skin sensitivity is successfully employed in man for the diagnosis of aspergillosis; however, its use has not been indicated in animals. The pathogenicity test of the isolate recovered from clinical material is conducted in mice and rabbits.
Treatment and prevention: There seems to be no effective treatment of aspergillosis in mammals. However, in avian aspergillosis, a number of chemotherapeutic agents have been tried with varying success. The use of hamycin at the rate of 20 mg/ ml in drinking water for 7 days may reduce the mortality in acute outbreaks. A combined  mixture of boric acid 2 % and iodide 0.5 - 1.0 % may be used in the prophylaxis of Aspergillus infection in birds. The discharges, excreta etc. of all seriously affected and dead birds should be removed and burnt in an incinerator. The litter should be sprayed with 5 % copper sulphate solution to confine the spores of fungus and later removed and burnt. The poultry pen, incubator, brooder, feed and water utensils should be sprayed with 1 % solution of copper sulphate to kill the fungal spores. The pregnant animals should not be fed with mouldy hay, straw and grass. To reduce the chances of mouldy growth, hay straw etc. should be treated with fungicides during hay making. Animals should be always housed in good, ventilated dry cow sheds as humidity favours the growth of Aspergillus. Immunization against Aspergillus infection though tried in small animals has not given significant results. The attempts of producing a potent and safe vaccine against this most important mycosis are still continuing.

Posted Date: 9/17/2012 8:07:00 AM | Location : United States

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