Rabies is an extremely dangerous disease of the nervous system, killing approximately 50,000 people a year, and is endemic among wild animals in most parts of the world. It is caused by a type of RNA virus shaped like bullets, called a Rhabdovirus, and is transmitted very easily due to the presence of the virus in the saliva of the affected animal.

The Symptoms

Rabies is deadly and by the time the symptoms appear it is too late to treat the disease - it will be fatal. It has an incubation period of roughly 20 days, although on rare occasions this can be significantly longer - up to several years. During this incubation period the disease cannot be passed to others, which means there are no cases of carriers infecting whole populations without realising it. At the end of the incubation period, the first symptoms appear - although, as with most diseases, not all people get all of the symptoms. They are generally vague, flu-like symptoms, such as fatigue, headaches and nausea. Some of the symptoms will echo those of meningitis, such as an aversion to light, developing very stiff muscles and high fever, but they are all non-specific and do not, at this stage, point to rabies. The period over which these symptoms occur is roughly a fortnight. The next stage of the disease is a lot less comfortable for the patient. More severe symptoms start to appear, with the brain becoming extremely inflamed, causing violent delirium and prompting convulsions.

The strangest symptoms of all are the ones that gave the disease its alternative name of hydrophobia. The patient experiences dramatic, and very painful, spasms of the throat when they come into contact with water. This makes the sufferer avoid water and so appears to cause a fear of water - which is what 'hydrophobia' means in Latin. The patient deteriorates quickly and either dies of a heart attack during this stage, or, if they survive, they are afflicted with a paralysis spreading upwards from the legs, eventually reaching the heart and chest.

Transmission

In most parts of the world rabies is present in some form. It only affects mammals and, although it can affect any of them, some animals seem more prone to it than others. Small rodents are very rarely found with the disease, while, along with bats, larger animals such as dogs are the most frequent carriers. Humans can catch the disease from any infected animal, as long as the animal is actually rabid - showing the symptoms of rabies. It is not always easy to tell when an animal is rabid. Although many animals go wild and show classic signs of rabies - including an unfortunate tendency to bite people, thus spreading the disease - some appear unusually calm, while others appear ill and weak.

Since the rabies virus is found in saliva, it is nearly always spread by bites. Apart from this, there are other ways of being contaminated with the saliva of a rabid animal, namely getting saliva into wounds or into the eyes, nose or mouth. Cases of rabies being inhaled are extremely rare, although this can occur either in laboratory conditions, or in an enclosed space filled with a large number of bats, notorious carriers of the disease.

You can't catch rabies from coming into contact with fur, faeces or urine.

Protection Against Rabies

If a pet is bitten by a suspected rabid animal, it should be taken to a veterinary surgeon as soon as possible. Usually the animal will be destroyed, although sometimes they can be kept in isolation and tested for signs of the virus.

If a person is bitten by a suspected infected animal, they should clean the wound and seek medical attention. The doctor will take samples from skin, blood, saliva and spinal fluid and examine them carefully, but there is no single test. If there is any doubt then medication will be administered - Human Rabies Immune Globulin (HRIG) will be given to the patient instantly. This provides strong protection, but only temporarily. In addition to this, a course of the rabies vaccine will be started.

The quickest way to confirm rabies is to do a post-mortem examination of the animal that gave the bite. Because rabies is found in the nervous system a piece of the brain is usually chosen for testing. A fluorescent dye is added to a small piece of the tissue, and then rinsed away. The dye binds to rabies virus, so although in a normal case there would be no dye left in the sample, in a rabies victim dye would still be present. This dye can be detected under ultra-violet light.

Prevention

If you are in an area that is at risk from rabies, it is wise to stay away from animals you don't know, especially wild animals. If there are strays in the area then it is best to report them to your local animal sanctuary, and if you have pets, then having them neutered will make sure they don't reproduce, adding to the stray problem. In some countries people routinely vaccinate their pets against rabies - if this is the case, make sure the vaccinations are up to date. Old vaccinations can provide more protection than no vaccine at all, but shouldn't be relied on. This is because they decrease the chances of survival significantly compared to a recently vaccinated animal.

Keeping animals under control is essential - letting them wander unchecked will increase the likelihood of them coming into contact with wild animals and strays. Making your home secure from wild animals is advisable, since some like to enter houses - especially bats.

It is wise to know whether you are at risk, and what from:

• The United Kingdom is probably a rabies-free area, but there is a possibility that it exists in bat colonies, and so they should be treated with caution.

• When Western Europe has cases of rabies they are often caused by foxes, although attempts to control them has limited the spread of the disease.

• In Africa, Asia and South America the largest group of rabies carriers is dogs. This is where rabies is the biggest risk.

• Bats are the biggest threat in Australia, although their ground mammals are probably rabies free.

• Rabies in North America is most frequently caused by raccoons and skunks, but also by foxes and coyotes in the more southern regions, although bats also pose a substantial risk.

Remember though, that all mammals are capable of contracting rabies, and so treat them with caution, especially if they are unknown to you, or their behaviour seems odd. If rabies is not present in your native country, don't forget to take precautions against it when you go on holiday.

Also remember that some countries have very strict rabies laws which could get you into trouble. For example, the United Kingdom will not allow any pets into the country that have not got certificates of vaccination and which have been tested for immunity to rabies. Animals which do not meet these criteria are impounded for six months, in quarantine, which is not only inconvenient for the owner - it is also very expensive.

The Vaccines

The first person to develop a rabies vaccine was Louis Pasteur, the French scientist, in 1884. His version was a weakened strain of the rabies virus, which did work, but could have been dangerous.

The first commercial vaccine was made of dead virus' grown inside duck or chicken eggs, which was injected either before or immediately after exposure to rabies. The body would attack the virus, and produce antibodies against it, which could be reproduced when a real infection occurred.

The normal vaccine today, the Human Diploid Cell Vaccine (HDCV) is made of human diploid cells¹. These are infected with the rabies virus and inactivated so they pose no risk of infection. The cells are then freeze-dried until use.

There is an oral vaccine for rabies, which was developed especially for animals - it is not used on humans. It uses the vaccinia, or cowpox, virus. The portion of the rabies genome which codes for surface proteins is inserted into the vaccinia virus. This causes it to function normally, except that its surface proteins now match that of the rabies virus. The body fights the virus, and when it meets the real rabies virus it recognises the proteins and produces the correct antibodies.

The oral vaccine is used for wildlife immunisation projects, where bait containing the vaccine is distributed over wide areas, especially in the United States. This is proving effective in lowering the rabies in that country.

There are two instances where the rabies vaccine would be administered in humans. Pre-exposure means before the patient comes into contact with the virus. This would apply to people who work in the veterinary profession, laboratories which handle the virus, and people who handle animals which might have the virus. Pre-exposure vaccination consists of three doses of the HDCV, on day 0, 7 and 21.

Post-exposure vaccine is administered to a patient who is suspected of having come into contact with the virus. The vaccine is given on day 0, 3, 7, 14 and 28.

The vaccine is used more for animals than people because it often has unpleasant side effects. Aside from headache, nausea and pain, there are cases where it has caused severe allergic reactions. No one has ever received permanent damage, or come close to dying, as a result of the vaccine.