Many readers will recall the powerful scene in the film To Kill a Mockingbird where Gregory Peck kills the rabid dog with a single rifle shot while his children look on in fascinated horror.
Rabies is a name that instills fear - a disease transmitted by a bite from a crazed animal, producing a horrific derangement and certain death unless vaccine is quickly administered. Ireland remains rabies-free, but the disease persists in many parts of the world.
Rabies is an acute, contagious infection of the central nervous system caused by a virus that enters the body from the bite of an animal. The disease is most common, in Europe and America, in foxes, bats, cats and dogs, and most cases of human rabies result from a bite of one of these animals. The average incubation period of rabies in humans is four to six weeks, although in rare cases it can last for several years.
At the end of incubation, the bite wound becomes painfully irritated, and depression and anxiety set in for about two days. This is followed by a period of excitation. The victim feels terror which is intensified by difficulty in swallowing and breathing, and feelings of strangulation.
When the victim tries to drink, the throat closes and inflamed nerves cause painful spasms. The mere sight of water terrifies the patient and one of the oldest names for rabies is hydrophobia (fear of water). A thick mucus collects in the mouth and throat. Vomiting and fever are common, and after about five days the patient dies, usually from cardiac or respiratory failure.
The disease in animals can take two forms: furious rabies, and dumb or paralytic rabies. Furious rabies is similar to the course outlined for humans. During the period of excitation the animal runs riot, biting at anything in its path. Dumb rabies is less common, has a short excitement stage and paralysis sets in early.
Rabies is an ancient disease, but it only began to cause human problems when people started to live in cities and to keep dogs as pets. Stray dogs gathered in packs and bred quickly. When numbers reached a critical level, epidemics of rabies arose and spread amongst people. Laws were written as early as 3000 BC in Mesopotamia prescribing punishment for dog-owners who allowed mad dogs to bite someone. Rabies epidemics have spread on every continent except Australia and Antarctica.
Louis Pasteur developed a protective vaccine against rabies in 1884. Pasteur discovered he could protect dogs against rabies by injecting them with minced spinal cords from rabies-infected rabbits. Pasteur's first use of the vaccine on a human is a dramatic story.
Joseph Meister, a nine-year-old boy, came to Pasteur, having received 14 bites from a rabid dog. Joseph had been declared doomed by two other doctors. Pasteur agonised over whether to try out his new vaccine - which worked with dogs - on the boy. He went ahead, Joseph survived and worked for the rest of his life as a guard at the Pasteur Institute.
Treatment for rabies now is completely effective. The animal bites are washed and human antiserum is injected into the wounds and into the buttocks. Five doses of vaccine are administered over the next four weeks. The vaccine contains inactive rabies virus grown in human tissue cultured cells. Fewer than two people per year now die of rabies in Europe or in America. Few of the cases come from dog bites because of animal control policies and vaccination. In many areas, immunisation of cats and dogs against rabies is required by law.
But rabies remains a threat. Between 20,000 and 100,000 people die each year world-wide from rabies, mostly in developing counties. A series of rabies epidemics erupted in wildlife since the second World War. An oral vaccine was developed and administered by spreading edible bait. This greatly reduced rabies amongst European foxes. However, in America more wild animals have rabies now than ever before.
The rabies virus is a parasite. It makes little sense for a parasite to kill the host on which it depends, and most parasites live in harmony with their hosts. Rabies is an exception. It is naturally fatal in most animal species. Viral diseases usually induce symptoms that help transmission of the virus to new victims, e.g. a cold causes sneezing. The rabies virus accumulates in saliva, but it can only be transmitted efficiently through a break (bite) in the skin of the new victim. In order to effect this bite, the rabies must fatally take over the host's brain. The individual host is expendable.
Because rabies always kills its wild victims, it can only work with a species that is thriving. Foxes make ideal natural hosts. They reproduce well and are only social to a certain extent. Parent foxes drive off adolescent young who become shy of contacting other foxes. This protects them during a rabies epidemic amongst foxes and they survive to be available for future infections.
Rabies is common in American bats and no vaccines have been developed for bats. Some bats are ideally suited for spreading rabies. They live in caves in huge numbers and they pass the virus among themselves. When bats vocalise they atomise saliva and spread the virus in a fine mist through the air. Animals kept in bat caves in bat-proof cages have contracted rabies through airborne transmission.
It is almost certain that rabies will never be eliminated from animal hosts. The disease is currently under control in the developed world, but it is not defeated and health authorities must remain wary. Long may Ireland retain its rabies-free status.
William Reville is a senior lecturer in biochemistry at UCC.