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RABIES
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Description
Rabies is an important zoonotic viral disease that is invariably fatal and can infect all mammals.
Over 27,000 cases of rabies are reported in animals every year, and in 1993 over 31,000 cases were reported in humans. It has been estimated that as many as 25,000 human deaths due to rabies may occur annually in India alone. Dogs are the main vector for human rabies.
Australasia and other countries which have enforced strict quarantine restrictions on animal movements. Within Europe these restrictions on movement are being gradually removed. Dogs which originate from licensed premises in Europe and which have been vaccinated with rabies vaccine, and identified by a microchip implant, are now being allowed to pass into the UK without the need for a period in quarantine.
In most countries Rabies is a notifiable disease and suspect animals must be kept in isolation and barrier nursed.
Cause
Rabies virus is a member of the Rhabdoviridae - RNA-viruses. It exists worldwide
and there are a variety of antigenic variants amongst wild "field"
virus from different Continents. Nucleoprotein gene studies suggest that the
disease probably spread from Europe to America, Asia and Africa.
Humans usually contract the infection by one of the following routes :
- Bites - most common
- Inhalation - bat caves
Breed Occurrence
Human workers at greatest risk of contracting rabies include :
- Animal handlers
- Veterinarians
Human infection usually is transmitted from :
- Dogs
- Cats
- Rodents
- Wild animals - bats, foxes, skunks
- Rarely - horses, cattle and other mammalian species
Signs
Members of the Canidae family are some of the most susceptible species to infection ie coyotes, jackals and wolves. Other highly susceptible species include foxes, rodents, skunks, racoons, bats, rabbits, and cattle.
Cats and Dogs : The incubation period of the disease varies from 9 days to more than a year. Indeed one of the last cases reported in the UK occurred after an imported dog had been released from it's mandatory 6 month quarantine period. The delay in some individuals is because the rabies virus has to migrate from the site of initial entry into the body to the spinal cord or brain along peripheral nerves. On average clinical signs are seen within 4 weeks of infection, they are similar in cats and dogs and there are 3 different presentations or stages :
Stage 1 . Local irritation often with self-trauma at the entry site - usually a bite wound, fever, mild changes in demeanour, behaviour and temperament. Dilated pupils and slow eye reflexes. In dogs and cats the sound or pitch of the animal's bark or meow may alter. Lasts 2-3 days.
Stage 2 . Aggression, hide in corners, incoordination, disorientation, hyperaesthesia, seizures, increased salivation, pica, photophobia. Lasts up to 1 week.
Stage 3. Paralysis, laryngeal paralysis (uncommon in cats) creating difficulty swallowing, excessive salivation, respiratory failure, coma and death. Lasts 2-4 days.
Humans : Similar to cats and dogs. Hydrophobia is due to painful pharyngeal spasms when attempting to drink fluids.
Transmission
Rabies is almost always transmitted by the bite of an infected animal with high virus concentrations in it's saliva. Less common mechanisms of transmission have included aerosol spread by inhalation in bat-colonised caves, and transplacental infection in skunks, bats and cattle.
In humans, rabies has been transmitted by corneal grafts, and for a large number of cases no obvious source has been identified
Diagnosis
The head or carefully removed brain of the suspect animal should be cooled and
chilled with wet ice during immediate transportation to an approved laboratory.
The brain should not be frozen.
At postmortem immunofluorescence tests are used to identify viral antigen - and this technique does not require live particles to be present.
Other findings are the presence of intracellular inclusion bodies in large neurones of the brain - called negri bodies on histological examination of brain tissue.
Treatment
There are a few instances of recovery by people who have been given intensive
support treatment during the disease process, but intensive care and supportive
treatment is not recommended for animals because they can become carriers and
shed virus after recovery.
Prevention
Rabies vaccines are available for cats, dogs, cattle and horses.
Dogs and cats are vaccinated from 4 weeks of age and the dose is repeated at 12 weeks of age. Only one dose is needed if they are over 12 weeks of age at the time of first vaccination. Booster vaccinations are advised every 1-3 years.
Horses and cattle are usually vaccinated at 6 months of age, though they can be done from 2 months of age with a second booster at 6 months. Annual booster vaccinations are needed in these species to maintain protection.
Animals (or humans) that have come into contact with saliva from suspected rabies carriers should have the wound vigorously cleansed and flushed out with ethanol and benzalkonium chloride under pressure to remove the saliva.
For humans there are a rabies immunoglobulin vaccine and a human diploid cell vaccine which are used routinely when
Further Reading
Updated October 2013
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