Australia is considered to be free of rabies in our mammalian populations. In countries with terrestrial rabies the dog is the principal reservoir of the virus. Other Canidae responsible for transmission of the rabies virus include foxes, coyotes and wolves. Bats, skunks, monkeys, raccoons and cats can also be infected. Rabies is a significant killer of horses in Central and South America, where the vampire bat is the most common vector.
The Australian Bat Lyssavirus (ABLV) has been found in flying foxes and insectivorous microbats. There have been several fatal human cases in Australia from bites or scratches from bats. Two horses in Queensland developed fatal neurologic signs attributed to ABLV.
The virus multiplies at the site of infection and, after a variable time, it moves within peripheral nerves to reach the sensory or motor neurons in the gray matter of the brainstem and spinal cord. There is rapid spread throughout the rest of the central nervous system and sympathetic chain. The salivary glands usually are infected via cranial nerves before the onset of clinical signs. The incubation period (exposure to signs) ranges from 2 weeks to several months. The rabies virus already is widespread when nervous signs are first seen.
Signs
It is assumed that horses infected with ABLV would display signs consistent with that seen in horses infected mammalian rabies in other countries. The first signs of rabies can be highly variable and can include, either singly or in combination, depression, manic behaviour, self-mutilation, blindness, muscle twitching, colic signs, limb weakness and ataxia, or lameness. Three forms of rabies have been described: a) a furious form where horses are very aggressive and maniacal; b) a paralytic form where horses develop an ascending flaccid paralysis; and c) a dumb form where affected horses have profound obtundation. Irrespective of form rabies is fatal once signs develop, with a disease course of 3 to 10 days.
Diagnosis
In areas where rabies is endemic any rapidly progressive neurologic disease should have rabies as a differential diagnosis. The diagnosis is established after death by applying a direct fluorescent antibody test on brain tissue. The test is considered to be 98% accurate in detecting affected horses. Histologically, there is mild diffuse nonsuppurative polioencephalomyelitis. Large intracytoplasmic inclusion bodies called Negri bodies are found in neurons and ganglion cells.
Prevention
Rabies vaccination is not available for animals in Australia or New Zealand. Horses in high-risk areas may be immunized annually, beginning at 6 months of age with inactivated vaccines. If a previously immunized horse is bitten by a suspect rabid animal, it can be given 3 booster immunizations over 1 week and quarantined for at least 90 days. Post-exposure immunization of a non-vaccinated horse is considered unwise. Animals should either be euthanized immediately or quarantined for 6 months.
Veterinarians who work in endemic regions should be vaccinated against rabies, as should anyone who handles bats in Australia. The vaccine is cross-protective against classic rabies and ABLV.
Tags: Infectious diseases; Neurology