67       Rhabdoviridae

 

Members of the family Rhabdoviridae (Greek rhabdos, rod) have characteristic rod shapes. Rhabdoviruses possess a linear, non-segmented RNA genome of negative polarity encased in a ribonucleoprotein complex. This large family contains viruses of vertebrates, invertebrates and plants. Rhabdoviruses of vertebrates appear bullet- or cone-shaped. The family Rhabdoviridae comprises 11 genera. The genera Vesiculovirus, Lyssavirus and Ephemerovirus contain viruses of veterinary significance. Rhabdoviruses of importance in fish belong to the genera Novirhabdovirus, Vesiculovirus and Perhabdovirus. Replication occurs in the cytoplasm (with the exception of nucleorhabdoviruses of plants). Newly synthesized nucleocapsids acquire envelopes from the plasma membrane as virions bud from the cell. Virions (100 to 430 nm × 45 to 100 nm) are stable in the pH range 5 to 10. They are rapidly inactivated by heating at 56°C, by treatment with lipid solvents and by exposure to UV light.

Clinical infections

Rhabdoviruses of veterinary importance are presented in Tables 67.1 and 67.2. They can be transmitted by bites of mammals, arthropod vectors or direct contact. Infection may also be acquired through environmental contamination. The best-known and most important member of the Rhabdoviridae is rabies virus, a Lyssavirus (Greek lyssa, rage or fury). A number of distinct lyssaviruses, many isolated from bats, produce clinical signs indistinguishable from rabies. Novel lyssaviruses continue to be isolated from wildlife sources. The most important vesiculoviruses which infect domestic animals are the vesicular stomatitis Indiana virus and the vesicular stomatitis New Jersey virus. Bovine ephemeral fever virus, of significance in the tropics and subtropics of Africa, Asia and Australia, is the type species of the genus Ephemerovirus.

Table 67.1 Lyssaviruses which cause rabies and rabies-like diseases.

Virus	Phylogroup	Genotype	Serotype	Geographical distribution	Comments
Rabies virus	1	1	1	Apart from Australia and Antarctica, rabies virus (genotype 1) occurs on all continents. Many island countries are free of the disease	Causes fatal encephalitis in many mammalian species. Transmitted by wildlife species, including foxes, racoons and bats; domestic carnivores also involved in transmission. Rabies is a major zoonotic disease with more than 50,000 human fatalities worldwide each year
Lagos bat virus	2	2	2	Africa	Isolated initially from fruit bats; also isolated from domestic animals with encephalitis
Mokola virus	2	3	3	Africa	Isolated initially from shrews; also isolated from domestic animals. Human infection reported
Duvenhage virus	1	4	4	Africa	Originally isolated from a human bitten by an insectivorous bat; additional cases reported in humans. Not reported in domestic animals
European bat lyssavirus 1	1	5	—	Europe	Identified with increasing frequency in insectivorous bats. Human infection reported
European bat lyssavirus 2	1	6	—	Europe	Present in insectivorous bats. Initially isolated from a human with symptoms of rabies and subsequently from other human cases; not reported in domestic animals
Australian bat lyssavirus	1	7	—	Australia	Identified in fruit bats and in insectivorous bats; human infection reported

Table 67.2 Viruses of veterinary significance in the genera Vesiculovirus and Ephemerovirus.

Genus/Virus	Hosts	Comments
Vesiculovirus
Vesicular stomatitis Indiana virus	Cattle, horses, pigs, humans	Causes febrile disease with vesicular lesions; resembles foot-and-mouth disease clinically. Occurs in North and South America
Vesicular stomatitis New Jersey virus	Cattle, horses, pigs, humans	Causes febrile disease with vesicular lesions; infection more severe than that caused by the Indiana virus. Occurs in North and South America
Vesicular stomatitis Alagoas virus (Brazil virus)	Horses, mules, cattle, humans	Originally isolated from mules in Brazil
Cocal virus (Argentina virus)	Horses	Isolated initially from mites in Trinidad; occurs in South America
Ephemerovirus
Bovine ephemeral fever virus	Cattle	Arbovirus that causes febrile illness of short duration; occurs in Africa, Asia and Australia

Rabies

This viral infection, which affects the central nervous system of most mammals including humans, is invariably fatal. However, mammalian species vary widely in their susceptibility. Most clinical cases are due to infection with rabies virus (genotype 1). A number of other neurotropic lyssaviruses, closely related to the rabies virus, produce clinical signs indistinguishable from rabies. Classical rabies caused by rabies virus is endemic on continental land masses, with the exception of Australia and Antarctica. Many island countries are also free of the disease.

Several species-adapted genotypes or strains of rabies virus have been described. Strains affecting a particular species are transmitted more readily to members of that species than to other animal species. In a given geographical region, rabies is usually maintained and transmitted by particular mammalian reservoir hosts. Two epidemiologically important infectious cycles are recognized, urban rabies in dogs and sylvatic rabies in wildlife. More than 95% of human cases in developing countries are as a result of bites from rabid dogs. Racoons, skunks, foxes and bats are important reservoirs of rabies virus in North America. In continental Europe, the principal reservoir is the red fox. The vampire bat is an important reservoir of the virus in Central and South America and in the Caribbean islands. Although virus may be transmitted through scratching and licking, transmission usually occurs through bites. The saliva of infected animals may contain rabies virus for some time before the onset of clinical signs.

The incubation period, which is highly variable and can be as long as six months or more, is influenced by various factors including host species, virus strain, the amount of inoculum and the site of introduction of the virus. The clinical course in domestic carnivores, which usually lasts for days or for a few weeks, may encompass prodromal, furious (excitative) and dumb (paralytic) phases. Antemortem diagnostic tests for rabies are not generally used. The brains of animals which develop clinical signs should be examined for the presence of virus using the direct fluorescent antibody test. Other methods include demonstration of intracytoplasmic inclusions (Negri bodies) histologically, virus isolation or RT-PCR. Rapid laboratory confirmation is essential for the implementation of appropriate treatment of human patients.

Most countries which are free of rabies rely on rigorous quarantine measures to prevent the introduction of disease. In countries where rabies is endemic, control methods are aimed mainly at reservoir species. Urban rabies can be effectively controlled by vaccination and restriction of movement of dogs and cats and by the elimination of stray animals. Control of sylvatic rabies requires special measures. Vaccination of red foxes with live oral vaccines delivered in baits has eliminated sylvatic rabies from several regions of western Europe. Although attenuated virus vaccines were used initially, there was uncertainty about their ultimate safety. A vaccinia–rabies virus glycoprotein (VRG) vaccine was developed and has proved effective for vaccinating foxes, coyotes and racoons.