Rabies is caused by lyssavirus. Most infections are reported in wild animals, but domestic animals can be infected. The virus affects the central nervous system, causing brain disease and death.
Infection occurs when saliva from an infected animal is passed to the uninfected through bites or contamination of a wound.
Dogs are the source of the vast majority of human infections. When humans are infected, it takes between one and three months for symptoms to develop (incubation period), although it can be as short as less than a week, or more than a year.
The nearer the infected site is to the brain, the shorter is the incubation period. For example, an infected site in the head or neck has a shorter incubation period than one in the arm or leg.
Treatment can be successful only if it is carried out during the incubation period.
Previously, the last case of rabies in Malaysia was in 1999, and the country was declared rabies-free by the World Organization for Animal Health in 2012.
The initial symptoms of rabies are flu-like in nature, i.e. fever, headache and feeling unwell, which lasts two to 10 days. There may be pain and a tingling sensation at the infected site in about half of those infected.
The infection then can take one of two forms. The symptoms of “furious” rabies, which occur in 70-80%, include abnormal behaviour, hallucinations, agitation, delirium, irrational fear of water (hydrophobia), excessive saliva production, difficulty swallowing, and sometimes, an irrational fear of air (aerophobia).
Once these symptoms appear, it is almost always fatal, with death due to cardiorespiratory arrest, and treatment is only supportive.
However, about 20-30% of infected humans have paralytic rabies, in which there is gradual muscle paralysis starting at the infection site. Coma develops slowly, with eventual death.
Clinical diagnosis may be difficult unless there is hydrophobia or aerophobia. There is no single diagnostic test in humans. The various tests detect the whole virus, viral antigens or nucleic acids in infected tissues i.e. brain, skin, blood, urine or saliva.
Medical attention should be sought as soon as possible after a bite or scratch from an animal, especially a dog, or an open wound that has been contaminated by animal secretions. The term used for the measures taken is rabies post-exposure prophylaxis (PEP).
The wound has to be washed immediately under a running tap for at least 15 minutes and cleaned with an antiseptic or alcohol.
The wound should be left open. If animal secretions got into the eyes, it should be washed thoroughly with clean water. This measure should preferably be done at the nearest healthcare facility.
An injection of rabies immunoglobulin, which are ready-made antibodies designed to neutralise the rabies virus and prevent its spread, will be given to those at increased risk of rabies – i.e. the biting animal is a known rabies reservoir; the animal appears sick or behaves abnormally; a wound or mucous membrane was contaminated by the animal’s saliva; the bite was unprovoked; and the animal has not been vaccinated.
The immunoglobulin buys time for the body to produce antibodies after vaccination.
The rabies vaccine is given to everyone suspected to have been exposed to rabies. If a person has never been vaccinated before, there will be five doses given – the first at the start of treatment, and subsequent doses at three, seven, 14 and 30 days later.
A person who has been previously vaccinated will be given two doses – at the start of treatment and three to seven days later.
There are three types of rabies vaccine: human diploid cell vaccine (HDCV), purified chick embryo cell rabies vaccine (PCEC) and nerve tissue vaccine produced from samples of nerves taken from animal brains. The World Health Organization recommends only the HDCV or PCEC vaccines because there are safety concerns about the nerve tissue vaccine, which has a one in 650 chance of causing permanent disability like muscle paralysis.
A common side effect of the rabies vaccine is redness, swelling and pain at the injection site (the upper arm) about 24-48 hours after the injection. Other side effects include a mild fever, rash, headache, muscle aches and vomiting.
Severe side effects are rare.
In general, the rabies vaccine is not given to the pregnant unless the potential risk of rabies is high or there is limited access to healthcare.
The infection is usually fatal once symptoms develop.
Treatment is supportive in such circumstance – treating complications and keeping the person comfortable.
Although there are no reported cases of human-to-human transmission, it is theoretically possible. As such, anyone who has been in close contact with an infected person may be advised PEP.
Domestic pets should be kept healthy with regular visits to the veterinarian and rabies vaccination kept current.
Neutering of pets helps to reduce the number of unwanted animals which do not have proper care or vaccination. When outdoors, pets should be kept under supervision. Garbage should be securely covered to avoid access to stray or wild animals. Stray animals should be kept away from the neighbourhood.
Anyone exposed to any animal bites or secretions is strongly advised to undergo PEP. Measures can also be taken to reduce exposure to wild animals.
Rabies elimination strategies previously included culling based on the rationale that rabies transmission is density-dependent, i.e. disease density is proportional to host density.
However, studies have shown that culling is ineffective, and mass vaccination is most efficacious to reduce the incidence of rabies in all species.
Mass dog vaccination campaigns, in which at least 70% of the dog population are vaccinated, will break the cycle of transmission in dogs, and to humans.
Last, but not least, it must be remembered that rabies is a 100% vaccine preventable disease.
Dr Milton Lum is a member of the board of Medical Defence Malaysia. This article is not intended to replace, dictate or define evaluation by a qualified doctor. The views expressed do not represent that of any organisation the writer is associated with.