template31

Rabies

Rabies virus

Scientific classification

Domain:	Virus
(Unranked)	(-) ssRNA viruses
Order:	Mononegavirales
Family:	Rhabdoviridae
Genus:	Lyssavirus
Species:	Rabies virus

Rabies (from a Latin word meaning rage) is a viral disease that causes acute encephalitis in people. In unvaccinated humans, rabies is almost invariably fatal once full-blown symptoms have developed, but post-exposure vaccination can prevent symptoms developing.

Transmission and symptoms

The stereotypical image of an infected ("rabid") animal is a "mad dog" foaming at the mouth, but cats, ferrets, raccoons, chipmunks, skunks, foxes and bats also become rabid. Squirrels, other rodents and rabbits are very seldom infected, perhaps because they would not usually survive an attack by a rabid animal. Rabies may also present in a so-called 'paralytic' form, rendering the infected animal unnaturally quiet and withdrawn.

The virus is usually present in the saliva of a symptomatic rabid animal; the route of infection is nearly always by a bite. By causing the infected animal to be exceptionally aggressive, the virus ensures its transmission to the next host.

After a typical human infection by animal bite, the virus directly or indirectly enters the peripheral nervous system. It then travels along the nerves towards the central nervous system. During this phase, the virus cannot be easily detected within the host, and vaccination may still confer cell-mediated immunity to pre-empt symptomatic rabies. Once the virus reaches the brain, it rapidly causes an encephalitis and symptoms appear. It may also inflame the spinal cord producing myelitis.

The period between infection and the first flu-like symptoms is normally 3-12 weeks, but can be as long as two years. Soon after, the symptoms expand to cerebral dysfunction, anxiety, confusion, and agitation, progressing to delirium, abnormal behavior, hallucinations, and insomnia. The production of large quantities of saliva and tears coupled with an inability to speak or swallow are typical during the later stages of the disease; this is known as "hydrophobia". Death almost invariably results 2-10 days after the first symptoms; the handful of people who are known to have survived the disease were all left with severe brain damage.

The virus

A Lyssavirus causes rabies. This group of RNA viruses includes the Rabies virus as well as bat lyssaviruses Duvenhage viruses and others. Human-infecting viruses may commonly have cubic symmetry and take shapes of regular polyhedra.

The Lyssaviruses are the only viruses known to travel along the nerves after infection. Biopsy shows typical "Negri bodies" in the infected neurons.

Prevention

There is no known cure for symptomatic rabies, but it can be prevented by vaccination, either of humans or of animals. Rabies originally doomed almost everyone who got it to die, until Louis Pasteur developed the first rabies vaccination in 1886 and used it to save the life of Joseph Meister, who had been bitten by a mad dog. Current vaccines are relatively painless and are given in one's arm, like a flu or tetanus vaccine.

Treatment after exposure (known as post-exposure prophylaxis or "PEP") is highly successful in preventing the disease if administered promptly. In the United States, the treatment consists of a regimen of one dose of immunoglobulin and five doses of rabies vaccine over a 28-day period. Rabies immunoglobulin and the first dose of rabies vaccine should be given as soon as possible after exposure, with additional doses on days 3, 7, 14, and 28 after the first. In case of animal bites it is also helpful to remove, by thorough washing, as much infectious material as soon as possible.

PEP is possible in rabies because the virus must travel from the site of infection through the peripheral nervous system (nerves in the body) before infecting the central nervous system (brain and spinal cord) and glands and causing lethal damage. This travel along the nerves is usually slow enough that vaccine and immunoglobulin can be administered to protect the brain and glands from infection. The time of travel is dependent on how far the infected area is from the brain by nerve: if bitten in the face, for example, the time until the brain is infected is very short and PEP may not be successful.

Prevalence

40,000 to 70,000 people die annually from rabies, mostly in Africa and Asia, where rabies is endemic. About 10 million people receive treatment annually after suspected exposure to rabies.

Dog licensing, destruction of stray dogs, muzzling and other measures contributed to the eradication of rabies from Great Britain in the early 20th century. More recently, large-scale vaccination of cats, dogs and ferrets has been successful in combating rabies in some developed countries.

Rabies virus survives in widespread, varied, rural wildlife reservoirs. Mandatory vaccination of animals is less effective in rural areas. Especially in developing countries, animals may not be privately owned and their destruction may be unacceptable. Oral vaccines can be safely distributed in baits, and this has successfully impacted rabies in rural areas of France, Ontario, Texas, Florida and elsewhere. Vaccination campaigns may be expensive, and a cost-benefit analysis can lead those responsible to opt for policies of containment rather than elimination of the disease.

Since the development of effective human vaccines and immunoglobulin treatments the US death rate from rabies has dropped from 100 or more per year early in the 20th century, to 1-2 per year, mostly caused by bat bites, which may go unnoticed by the patient.

Australia is one of the few parts of the world where rabies has never been introduced. However, the Australian Bat Lyssavirus occurs naturally in both insectivorous and fruit eating bats (flying foxes) from most mainland states. Scientists believe it is present in bat populations throughout the range of flying foxes in Australia.

Many territories, such as the United Kingdom, Ireland and Guam, are free of rabies (although there may be a very low prevalence of rabies among bats in the UK; see below).

Transport of pet animals between countries

Rabies is endemic to many parts of the world, and one of the reasons given for quarantine periods in international animal transport has been to try to keep the disease out of uninfected regions. However, most developed countries, pioneered by Sweden, now allow unencumbered travel between their territories for pet animals that have demonstrated an adequate immune response to rabies vaccination.

Such countries may limit movement of animals from countries where rabies is considered to be under control in pet animals. There are various lists of such countries. The European Union has a harmonized list.