Rabies

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Introduction

Close-up of a dog's face during late-stage "dumb" paralytic rabies

Source: Wikimedia Commons; Author: Centres for Disease Control and Prevention's Public Health Image Library (1963)

Rabies is very severe and almost always fatal notifiable, zoonotic disease seen to affect any mammal in the world. The UK, Australia and New Zealand are free of rabies. All animals are susceptible, but canines, especially jackals, foxes and wolves are particularly so. In many countries where the disease is endemic, there are certain mammals which can act as reservoir species for the virus. Below is a table of reservoir species in certain regions:

Region
Reservoir species
Europe
Red Fox
Russia, Turkey, Middle East
Raccoon
USA
Raccoon, Fox, Skunk
Africa
Dog, Jackal, Mongoose
Asia
Dog, Mongoose
South America
Dog, Vampire Bat
Worldwide
Insectivorous Bats

The current position of DEFRA holds bat lyssaviruses to be present in the UK. Human infection and death can occur after infection with bat rabies; nocturnal bat bites account for 50% of human cases in the USA. Humans develop the disease but excrete little virus.

The virus is moderately resistant and can survive well in dark places at low temperatures for several days. Infection occurs through a penetrating bite in which virus is transmitted via saliva. Incubation period is variable but can be long (10 days to 12 months), and increases with distance of the bite to the CNS (80% of cases show signs within 4 months). Primary replication of the virus occurs in the muscle, then the virus gains access to nervous tissue and travels toward the brain, where it continues to replicate. It then migrates down cranial nerves to infect salivary glands and the cornea. Virus is shed in saliva and tears and can be shed BEFORE the onset of classical signs.

Clinical Signs

The onset of clinical signs in dogs and cats includes behavioral changes, pyrexia, salivation and dilation of pupils.

Then, the virus will either take the furious form or the dumb form. The furious form is mostly seen in dogs and cats, however, prior to popular belief, the dumb form is actually the most common form in dogs. Signs of the furious form include abnormal aggression, salivation, and attack without provocation. There may also be disordered wandering, "copulatory" movements, incoordination, convulsions, coma and death within 3-10 days. The dumb form may or may not follow the furious form, or it may appear alone. This form is most common in ruminants or horses affected with the disease. Signs include ruminal tympany, tenesmus, diarrhoea, paralysis of lower jaw, drooling saliva, tremors and progressive paralysis. In dogs, a change in voice may be observed (bellowing).

Diagnosis

Immediately isolate any animal suspected of having rabies. If the animal has died or been put to sleep, then remove brain core sample via straw through occipital foramen in the direction of an eye and place contents in a 50% glycerol/PBS for FAT or virus isolation, or in 10% formaldehyde for histology. Or, the whole head can also be removed and sent to the laboratory.

In the lab, specialist teams will perform a Fluorescent Antibody Test (FAT) on smears or frozen sections of hippocampus or cerebellum. They should show presence of Negri bodies. Histological detection of Negri bodies using Mann's or silver stains can also be performed, as can the intracerebral inoculation of mice followed by FAT testing of brain smears of affected mice. RT-PCR on brain or saliva can be performed to determine genotype.

Rabies Control

For countries where rabies is considered exotic, 6 month quarantine of carnivores and ruminants is required (Norway, Sweden, Portugal, Australia, New Zealand). In the UK, this has been replaced by the Pet Travel Scheme (PETS).

For virus-endemic countries, control in pets is achieved by annual vaccination. Antigenicity allows a single antigenic type to vaccinate. The vaccine is either inactivated virus or canarypox recombinant viruses. Control is also achieved by control of the stray population, enforcing all dogs to be muzzled in public and attempts to control wildlife hosts by feeding vaccinated bait.

In humans that may be working with animals and be at risk of contracting the disease then prevention is by vaccination with a single booster if bitten. The current vaccine is grown in human diploid cells (HDCV) and is BPL-inactivated, and is therefore costly. Developing countries utilize older vaccines passaged from a 1939 case ("Flury" vaccines). If bitten and unvaccinated, one should clean and disinfect the wound and human anti-rabies immunoglobulin should be administered around the bite. Then, an intramuscular vaccination program: 2 dose - 1 dose - 1 dose at 0, 7, and 14 days post-bite should be started.

For more on Rabies control in Europe, see here

References

Blood, D.C. and Studdert, V. P. (1999) Saunders Comprehensive Veterinary Dictionary (2nd Edition) Elsevier Science

Bridger, J (2007) Virology Study Guide, Royal Veterinary College




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