Cryptococcosis, Biology

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Cryptococcosis


Cryptococcosis is a pulmonary, meningeal or systemic mycotic disease of human beings and animals. It may be acute, subacute or chronic. The disease is caused by  Cryptococcus neoformans, a non-fermenting, non-sporulating, encapsulated yeast- like fungus which occurs abundantly in old pigeon excreta. The pulmonary form is usually mild, benign, and transitory and often goes unnoticed. Though the pathogen has a tendency to invade central nervous system, lesions may also occur in bone, liver, spleen, kidney, skin and other organs during dissemination. The natural infection of central nervous system has been more frequently recorded in small animals. The affections of lung, lymphnodes, nose, eye, mouth, skin, intestinal tract and mammary gland have also been noticed in many animals. This fungus has been recognized as the most important pathogen of mycotic mastitis in the cow, buffalo, sheep and goat. The infection is often serious and may occur both in sporadic and epidemic forms.


Cryptococcosis has a broad host range. The disease has been reported in cattle, horse, dog, goat, monkey, sheep, pig, buffalo, cat, mink, cheetah, baboon, ferret and bat. Though the disease is world-wide in distribution, the source of sporadic infections has not been fully established. It has been presumed that human and animals acquire the infection from the saprobic reservoirs in which fungus grows preferentially. Presently, pigeon droppings are considered the main source of infection and respiratory tract is the most common route of entry. C. neoformans lacks the ability to form a true mycelium.
Symptoms: Clinical signs of cryptococcosis are not well defined. In cats the lesions have been reported in brain, lymphnodes, lung, nostril, tonsil and skin. The wound is usually non-suppurative and contains tenacious, translucent and gelatinous fluid. In dogs, cryptococcal infections are mainly recorded in central nervous system and lungs. The dog shows anorexia, listlessness, emaciation, dilatation of pupils, blindness, epileptic form seizures besides severe dysponea and nasal discharge. Autopsy reveals consolidation of lungs, congestion of meninges, enlargement of spleen and nodules in the liver. Monkeys also suffer from pulmonary and cerebral cryptococcosis; the clinical signs more over resemble with dog. In horses, infection usually involves upper respiratory tract particularly nose and is characterized by persistent nasal discharge and myxoma-like growth in nasal passage. The yeast has also been isolated from the lip, brain and intestinal tract. Among the milch animals, cows are most commonly affected with C. neoformans and suffer from mastitis. There is swelling of one or more quarter, reduction in milk yield, swelling of supramammary lymphnodes, reduced appetite and mild fever. On palpation the mammary gland appears full, firm and indurated. Mucilagenous greyish white, stringly discharge comes out from the teats. In addition, the pathogen has also been isolated from the lymphnode, abomasum and endocardium of cattle.


Diagnosis: The diagnosis may be made by direct microscopic examination of circular, yeast-like encapsulated cells in the nasal discharge, milk, cerebrospinal fluid, urine or other body discharge. The affected organs show slimy, mucilagenous appearance on autopsy. Histological examination of brain, lung, liver, spleen, lymphnode and kidney reveals the presence of yeast-like encapsulated cells with or without budding, tissue reaction takes place when stained with mucicarmine and silver methanamine. C. neoformans differs culturally from the dimorphic systemic fungi in  that it forms smoth, mucoid, yeastlike colonies when incubated at 20 °C and 37°C.Identification is confirmed by characteristic growth on cornmeal agar, rapid urease production, inability to reduce nitrate and brown pigmentation on nigerseed creatinine agar (Bird’s seed agar). In the genus Cryptococcus, only C. neoformans assimilates the purine creatinine. C. neoformans selectively absorbs a pigment derived from Guizotia abyssinica seeds (nigerseed), thereby causing the colony to become brown. The morphology of the isolate is studied in lactophenol mount. Capsule of C. neoformans can be demonstrated in India ink preparation. Animal inoculation test is conducted in mice to find out the pathogenicity of the isolate recovered from clinical specimens. Mice can be infected by intraperitoneal, intracerebral or intramuscular routes. Serological tests, particularly latex agglutination and tube agglutination, are also helpful in cryptococcal diagnosis. Skin test with cryptococcin is not of much si gn if ic a n c e .  Di ffe r e nt ia l  d i a g no sis s ho ul d  b e  ma d e  fr o m b l a s to myc o si s, coccidioidomycosis, histoplasmosis and tuberculosis.


Treatment and prevention: There is no satisfactory treatment for cryptococcosis in large animals. However, human beings and lower animals may be treated with 5- flurocytosine at 15-150 mg/ kg body weight intravenously. The severe form of cryptococcal mastitis is rather difficult to treat; affected animals may be destroyed. In mild form, cycloheximide at 2 mg/ quarter for 9 days may be tried, proper sterilisation of dairy utensils, syringes and needles used for intramammary infusion, segregation of the affected animal and personal hygiene of dairy attendants may also help to minimize the cases of cryptococcal mastitis in dairy animals.


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