Rabies, Biology


It is also known as lyssa, and hydrophobia in human beings. This fatal disease is characterized by altered behavior, deranged consciousness, laryngeal paralysis, inability to swallow and ascending paralysis.

Etiology: It is caused by a rhabdovirus producing lesions in nervous tissue. The virus is easily killed by common disinfectants, heat treatment and drying. Excessive washing of fresh infected wound with plenty of water and soap helps in killing of organism. The virus is also excreted in milk but the quantity is low and may not be all the diseased lactating cows. WHO reported that 91% cases of rabies in man are caused by dog biting and 3% by other domestic animals.

Pathogenesis: After deep penetration, virus multiplies in the muscle cells and enters into nervous system through neuromuscular motor end plates. Then it enters into brain by passive movement through axons. There is neuronal dysfunction and development of nervous symptoms and paralysis. The affected animals die due to respiratory failure.

Clinical signs: In cows and buffaloes, the disease occurs in paralytic or furious forms. In paralytic form, animals reveal flaccid tail, knuckling of hind fetlock joint and swaying of hind quarter during walking in initial stages. Later on, the animals develop tenesmus, paralysis of anus, frequent urination, excessive salivation, paralysis and recumbency. These recumbent animals die in 2-3 days. In furious form, the animals are alert but tense and hypersensitive to touch or any sound. They attack inanimate objects and loud bellowing occurs. Sexual excitement is also commonly noticed. Such symptoms persist for 36-48 h and then animal dies.

In horses, intense itching, rubbing, fear, restlessness, continuous ear move­ments, development of mania and frequent attacks on inanimate objects are noticed. The animal develops generalized paralysis and dies within few days. In less furious form, ataxia and paresis of hind quarters, lameness, recumbency, pharyngeal paralysis and colic are commonly noticed.

In sheep, symptoms similar to cows are seen but sexual excitement and attacking on inanimate objects are less. Affected sheep reveal vigorous wool pulling, muscle tremors and salivation.

Diagnosis: It is diagnosed by clinical sign and confirmed by animal inoculation test using mice. Impression smears prepared from brain can be tested by fluorescent a ntib o dy te st (FAT ) fo r c onfirma tion. Othe r se r olo gica l  tests like p a ssive haemagglutination, complement fixation and ELISA are also useful in its confirmation. The suspected dogs should be kept in isolation for 10 days which die if suffering from rabies. The disease in these dogs can be confirmed by demonstration of Negri bodies on histopathological examination of brain.

Nervous symptoms differentiate this disease from other diseases. In lead toxicity, blindness and signs of motor irritation occur commonly. In lactation tetany and deficiency of vitamin A, severe convulsions occur while polioencephalomalacia is characterized by blindness, nystagmus and convulsions. Listeriosis is manifested by circling and paralysis of facial nerve.

Treatment: The animals which are bitten by a rabid dog should be treated immediately. The wound should be washed properly with water and antiseptic solution which helps in killing of organisms. Earlier in India, the dog-bitten animals were given post bite vaccination using 5% BPL inactivated sheep brain antirabies vaccine

@ 5 ml daily for 14 days in animals weighing 15-100 kg and @ 15 ml daily for 14 days subcutaneously in animals weighing 100-800 kg. If the animals are previously immunized, vaccination should be done for 7 days. Such vaccines are not available in India. Cell culture vaccine are now administered as the dose rate of 1ml intramuscularly or subcutaneously on day 0, 3, 7, 17, 28 and 90 days post exposure.

Control: For the prevention of rabies in animals there are two useful techniques-prevention of exposure of domestic animals and vaccination by inactivated vaccine.

Posted Date: 9/20/2012 1:16:53 AM | Location : United States

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