Emergency Animal Diseases Bulletin No. 110 - Rabies virus

​​​​​Introduction

Rabies is a zoonotic viral infection that causes a fatal encephalitic disease. It is a significant animal and public health issue in affected parts of the world.

Rabies in animals is notifiable under legislation in all states and territories in Australia. All suspected cases should be reported to the relevant state or territory government animal health authority.

Aetiology

Rabies is caused by viruses in the genus Lyssavirus within the family Rhabdoviridae. The rabies virus is classified as genotype 1 of the genus. Other viruses within the same genus (such as Lagos bat virus and European bat lyssaviruses 1 and 2) may also cause rabies disease but are differentiated from the rabies virus on the basis of genotype (Table 1).

Host susceptibility and transmission

The rabies virus can infect any warm-blooded animal. Hosts are typically categorised as either maintenance hosts (species in which the virus life cycle is sustained) or spill-over hosts (species which may be infected but do not normally maintain the cycle of the virus biotype in question). Maintenance hosts for the rabies virus are most commonly bats and canines (wild or domestic). Spillover hosts typically include people, other primates, domestic livestock species and some wildlife species.

The rabies virus can be classified into biotypes which are adapted to a single maintenance-host species in which infection and transmission are highly efficient.

Rabies is sometimes also categorised as either sylvatic or urban rabies. Sylvatic rabies involves one or more wildlife vectors including bats and foxes (among others), whereas urban rabies involves domestic dogs as the primary host.
The most common route of transmission is by a bite from a rabid animal but it may also be transmitted by the transfer of infected saliva across mucous membranes, eating parts of a rabid animal or by inhaling an aerosol of rabies virus in infected bat caves.

Geographic distribution

Rabies virus infection occurs in animals on all continents except for Australia and Antarctica; however, the disease in animals is effectively managed in some areas. Urban rabies occurs primarily in parts of Africa, Central and South America, the Indian subcontinent and South-East Asia and mostly occurs in communities with a large number of unvaccinated or free-ranging dogs.

Clinical signs

The incubation period for rabies is typically 1–3 months, though this can vary from less than a week to more than a year. The clinical signs of rabies are very variable, may be intermittent, and may change as the disease progresses. Common clinical signs include excitation (such as aggression and restlessness), changes in behaviour, unusual vocalisation and pica. In contrast, some animals may be depressed and lethargic (sometimes referred to as ‘dumb’ rabies).

The common clinical presentation of rabies may also vary with the species of animal affected. Dilation of the pupils and increased restlessness is often seen in dogs and cats; sexual excitement, salivation, teeth grinding and self-mutilation may feature in affected livestock species; and, affected wildlife often lose their fear of people. As the disease progresses, ataxia, paralysis and coma typically precede death.

A more detailed description of the clinical signs of rabies in domestic animals is available in the Australian Veterinary Emergency Plan (AUSVETPLAN) disease strategy for rabies.¹

Diagnosis

The only reliable diagnosis of rabies virus infection is to identify the presence of virus in animal tissues using laboratory tests, usually performed post-mortem. The most commonly used test to diagnose rabies virus infection is the fluorescent antibody test (FAT), which is recommended by the World Organisation for Animal Health (OIE).² The FAT can confirm the presence of the rabies viral antigen in brain tissue. This test is also applicable to cell culture based virus isolation procedures or to the laboratory animal inoculation test. The advantages of FAT include that it is sensitive, specific and cheap. The challenges of FAT are that it is reliant on a fresh specimen tested within a few hours of collection, the types of specimen collected, the lyssavirus involved and the proficiency of the diagnostic staff. Alternative tests used to diagnose rabies virus infection include immunochemistry, enzyme-linked immunosorbent assay (ELISA), serum neutralisation tests, virus isolation and molecular tests (such as polymerase chain reaction and sequencing).

Treatment and prevention

There is no specific treatment for rabies and death is almost inevitable once clinical signs appear.

Rabies vaccines for individual animals are effective and safe when used for pre-exposure to rabies virus. Vaccination against the rabies virus may be a requirement for animals (especially dogs and cats) travelling overseas or entering Australia.³

Vaccination is also used to control the rabies virus in animal populations. In the case where the rabies virus is circulating in wild animals, oral administration of vaccine in baits may be used to control or reduce infection in the wild reservoir. Overseas experience suggests that vaccination rates of more than 70% are needed to achieve population protection in wildlife. In the case of urban rabies, vaccination of domesticated carnivores such as dogs, will protect animals against infection and reduce the exposure of humans to rabies. The World Health Organisation (WHO) recommends that at least 80% of target canine populations need to be vaccinated to control the rabies virus.

Pre-exposure immunisation is also available for people and has been used to address concerns relating to both rabies and Australian bat lyssavirus.4 As such, it is recommended for people in Australia likely to receive bites or scratches from bats (such as bat handlers, veterinarians or wildlife officers), people travelling for extended periods to rabies-affected areas, those working with mammals in rabies-endemic areas, and those working in certain occupations (such as laboratory workers dealing with live lyssaviruses). The Australian Government Department of Health and Ageing provides guidance on the immunisation of people against rabies.⁴

When people are bitten by a suspected rabid animal they can obtain post-exposure treatment. Post-exposure treatment consists of treatment of the wound after exposure, a course of rabies vaccine and dispensing of the human rabies immunoglobulin (HRIG). For people previously vaccinated, HRIG may not be administered.³

Rabies virus and Australia

The rabies virus is not present in Australia. A related virus, the Australian bat lyssavirus, is carried by bats in Australia and rarely infects humans. It has not spread from bats to other terrestrial species despite close contact between infected bats and dogs and cats.

A significant risk for rabies virus introduction into Australia is a dog in the incubative stage of rabies entering northern Australia (for example, through visiting boats or smuggling) and coming into contact with wild dogs or dingoes. The possibility of this happening is likely to increase should the rabies virus spread to our nearest neighbour, Papua New Guinea, given the high boating traffic across the Torres Strait to Australia.

The rabies virus is spreading in Indonesia into areas which were previously free from infection, including Bali.⁵ The continued spread in Indonesia is of particular concern to Australia as it may lead to the introduction of the rabies virus into Indonesia’s Papua Province and so, into Papua New Guinea – increasing the risk of rabies virus introduction into Australia. The spread of the rabies virus in Indonesia might be due to increased inter-island trade, with people now taking their dogs with them to islands which were previously inaccessible.

If the rabies virus does enter Australia, it may become established here because there are sufficient numbers, density and distribution of susceptible species, such as wild dogs, dingoes, foxes and bats. Australia’s policy if the rabies virus enters the country is articulated in the AUSVETPLAN. The policy is to eradicate rabies virus in order to protect the health of domestic and wild animals, and humans. Strategies include quarantine and movement controls, destruction of infected animals, vaccination of domestic carnivores, monitoring and consideration of vaccination programs for wild animals, surveillance and a public awareness campaign.

Clare Hamilton​
Animal Health Policy
Australian Government Department of Agriculture, Fisheries and Forestry

References and further reading

1. Animal Health Australia (2011). Disease strategy: Rabies (Version 3.0). Australian Veterinary Emergency Plan (AUSVETPLAN), Edition 3, Primary Industries Ministerial Council, Canberra, ACT, Accessed May 2013.
2. Manual of Diagnostic Tests and Vaccines for Terrestrial Animals 2012. Chapter 2.1.13: Rabies. World Organisation for Animal Health. Accessed May 2013.
3. Bringing cats and dogs (and other pets) to Australia. Australian Government Department of Agriculture Fisheries and Forestry. . Accessed May 2013.
4. Australian bat lyssavirus infection and rabies. The Australian immunisation handbook, Edition 9, Vaccines listed by disease. Department of Health and Ageing 2008;110–119. Accessed May 2013.
5. World Health Organisation. Rabies country profile: Indonesia. Accessed May 2013.