Once bitten . . .

Not everyone bitten by a rabid animal contracts the disease but if they do, and once the clinical signs of rabies are visible, death is inevitable. No one knows exactly how many people die each year, nor how rabies deaths compare with other zoonotic diseases, such as bovine TB or brucellosis. What is known, however, is that the technical means to control rabies exist. So why does this fearful, fatal disease - aptly called by the French 'la rage' - not receive greater attention?

Official government statistics almost certainly under report the number of human deaths due to rabies. In Tanzania, for example, between 10 to 20 deaths per year are reported. These numbers are low and hardly justify high priority for action but actual numbers are almost certainly much higher. Research by Sarah Cleaveland of the Centre for Tropical Veterinary Medicine at the University of Edinburgh, working with partners in Tanzania, suggest perhaps 100 times higher than official figures. So why are official statistics unrealistic? It may be that people know that it is futile to seek help once clinical signs appear. Some may be discouraged from seeking help knowing that vaccine is often unavailable whereas others may put their faith in traditional healers. Furthermore, a person suffering from rabies may be misdiagnosed. Rabies affects people in the same way that it does animals; their behaviour changes. They may be diagnosed as suffering from hysteria or, where belief in witchcraft is widespread, the sufferer may be thought to have been bewitched. Such cases will go unreported as deaths from rabies.

Credit: Sarah Cleaveland, CTVM

A cycle of death

When a person, or animal, is bitten by a rabid animal, the rabies virus enters the peripheral nerve from where it travels to the central nervous system. It enters a massive replication phase in the brain and it is at this stage that clinical signs appear. From the brain, the virus travels to the salivary glands and is excreted in the saliva so that it can be transmitted in the bite. The rabies virus works very efficiently even though one might suppose that a virus that kills its host would be less effective than one that allows its host to survive. One reason for its success is that the virus stimulates restlessness and aggression in its host to facilitate its transmission through saliva. It also affects host species, such as dogs, foxes, raccoons or jackals, that live in high densities. This creates a high chance of an infected animal contacting another before its own death.

Children at risk

Children between the ages of 5 and 15 are at the greatest risk. Not only are they likely to come into contact with dogs, but they will be less familiar than adults of the signs that show a dog is infected. Also, being smaller than adults, they are more likely to be bitten on the face or neck. This is a crucial factor. The nearer the bite is to the head, the greater the risk that the bite will develop into a case of clinical rabies and the less time there is for administering a post exposure anti rabies vaccine.

Although there is no hard data, it does appear that the pattern of risk between rural and urban settings is different. In rural areas, although dogs may be ill cared for and apparently roaming freely, someone usually owns them and strays or feral dogs would not be tolerated by the population. Despite this, the number of rabies cases is surprisingly high. In urban areas the number of ownerless dogs is much higher. This is a particular problem in Asian cities, especially in societies where there is a taboo against killing animals. The result is high incidence of rabies. There may also be surprising ripple effects from apparently unrelated circumstances that increase the incidence of rabies. For example, over the last few years, a disease has caused a major decline in the number of vultures that once kept city streets in India clean of waste offal and rubbish. Feral dogs have moved in to take advantage of the unscavenged food and their numbers have increased dramatically. Dog rabies has now increased and many more people are dying from the disease.

Costs and benefits of control

In many situations, rabies can be brought under control relatively easily. Campaigns in rural Africa to encourage people to bring their dogs to a central point in a village for vaccination by injection usually work well. Admittedly, in societies where dogs are considered unclean, this is more difficult and more expensive to organize, but it is not impossible. Oral vaccination using bait is a further option. The major challenge is to demonstrate the value of such campaigns to government authorities. Livestock departments do not consider rabies a high priority and public health authorities often consider control the responsibility of veterinary services. So where should the responsibility lie? Government departments are notoriously disinterested in each other's sphere of activity. And yet the advantages are great. Bringing rabies under control not only saves human life, it reduces the public health cost of vaccine (Tanzania spends $0.5million annually on human rabies vaccine) and it reduces livestock losses as well. In South and Central America, public health authorities have taken the lead, even though the focus of control is on dogs, and the strategy is working well. With a better understanding of the true impact of the disease, and a more integrated approach to control, the rabies virus could be defeated thereby releasing its victims from a terrifying death.

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