Rabies

Organism Rabies is caused by a rhabdovirus in the genus Lyssavirus. There are seven related viruses, including Mokola and Duvenhage (found in Africa), which produce rabies-like illness. The virus withstands freezing temperatures for considerable periods of time, but is killed by boiling, sunlight and drying. It is not easily destroyed by disinfectants.

Clinical features The disease starts quietly with malaise, fever, sore throat and lack of appetite; paraesthesia develops and abnormal muscle movements occur. The patient then enters the excitable stage when they become anxious, there is difficulty in swallowing and frank hydrophobia and generalized convulsions may take place. The patient either dies in the convulsive stage or enters progressive paralysis as the terminal symptom. In the bat-transmitted

© R. Webber 2005. Communicable Disease Epidemiology and Control, 2nd edition (Roger Webber)

Table 17.1. Infections transmitted to humans from dogs or in which the dog is a reservoir.

Viruses

Arboviruses

Rabies

Rickettsiae

R. rickettsii, R. conorii, R. africae, R. australis, R. siberica

Bacteria

Anthrax

Brucella canis

Campylobacter jejuni

Capnocytophaga

Escherichia coli

Leptospirosis

Mycobacteria

Pasteurellosis

Salmonella

Spirillum minus

Tularaemia

Fungi

Ringworm

Protozoa

Chagas' disease

Cryptosporidiosis

Isospora belli

Leishmaniasis

Helminths

Ancylostoma (larva migrans)

Brugia malayi, B. pahangi, B. patei

Diphyllobothium latum,

Dipylidium caninum

Dirofilaria immitis

Dracunculus medinensis

Echinococcus granulosus, E. multilocularis, E. vogeli

Echinostoma

Gnathostoma spinigerum

Heterophyes heterophyes

Metagonimus yokagawai

Multiceps multiceps

Opisthorchis sinensis, O. felineus

Paragonimus westermani

Strongyloides

Schistosoma japonicum

Toxocara canis

Arthropods

Fleas

Ticks

Pentastomids (Linguatula)

form of the disease, there is no excitable stage and the patient dies from respiratory paralysis.

Diagnosis The clinical picture following a history of an animal bite is usually sufficient to make the diagnosis, but the virus may be isolated from saliva, tears, CSF, urine and many other tissues if facilities exist to culture it. Immunofluorescence antibody staining of tissue smears (e.g. skin biopsy) is of value.

Transmission Virus enters the body through a bite or abrasion of the skin. Classically, it is a dog bite, but if an infective dog, cat or cow licks the abraded skin, then transmission can occur in this manner. The vampire bat also transmits rabies, but mainly to cattle, with humans only occasionally infected this way. People have contracted rabies by entering bat-infested caves where it is thought that fine particles of bat faeces contaminate the conjunctiva or enter the respiratory mucosa.

The virus has a special affinity for brain and mucous-secreting tissue, travelling along peripheral nerves to the CNS and salivary glands. Large quantities of virus particles are present in the saliva from 1 to 10 days before the development of symptoms in the animal, right up until it dies.

The disease in the dog occurs in two forms, the furious and dumb. In furious rabies, the animal becomes restless, wanders away from home and bites anybody or anything that comes in its way. It is unable to bark, may attempt to eat sticks and stones, but is foiled in the attempt by a difficulty in swallowing. It foams at the mouth and suffers from the progressive paralysis of dumb rabies and is dead within a few days. Sometimes the furious course is not followed and dumb rabies only is manifest.

While the disease is invariably fatal in domestic dogs, cats and cows, it would appear to have a more variable effect in wild dogs, such as foxes and wolves. Certainly, rabies controls fox populations, but individuals do recover from the disease. There is little evidence to support the finding of a reservoir in such canines, but this may not be the case in rodents and bats. Rabies virus has been found in mongooses and the multimammate rat, Mastomys nata-lensis. These animals suffer from rabies, but sub-clinical infections may occur. When canines feed on small mammals they can acquire rabies. Vampire bats have been shown to recover from the disease and rabies virus has also been isolated from insectivorous bats, which do not take blood meals. This suggests that rabies may exist in a mild and asymptomatic form for most of the time in these mammals, but when they bite or people enter their virus-contaminated habitat, they are at risk of losing their life (see further Section 18.3).

Incubation period depends upon the proximity of the point of introduction of the virus to the brain and the size of the infective dose. It is usually 2-12 weeks, but can be years in young children with a minor bite.

Period of communicability Theoretical transfer from person-to-person is possible, so barrier nursing should be instigated. Animals are infectious from 4 to 14 days before clinical signs start until they die or are killed, but their saliva remains infectious. Sixty per cent of persons bitten by a known rabid animal do not contract the disease.

Occurrence and distribution A disease that strikes terror into people, it is widely spread throughout the world, as important in temperate as tropical countries. It is common in Russia, Africa, Asia and South America. India reports 30,000 deaths per annum, more than half the total cases in the world, with other countries in Asia accounting for most of the rest. It is particularly dangerous to children aged 5-15 years who are the main victims.

It is mainly transmitted by dogs, including wolves, foxes, jackals and hyenas, but the cat and cow have also been responsible. Other infected wild animals are mongooses, skunks and raccoons. In South America, the vampire bat transmits rabies particularly to cattle, but insectivorous and fruit-eating bats have also been found infected.

Control and prevention Control measures can be aimed against the domestic dog, the reservoir in wild animals, and the protection of humans.

Domestic dogs should be licensed and vaccinated, destroying all the stray ones. Vaccination of all domestic animals with an approved vaccine should be mandatory in all endemic areas.

Control of the wild animal reservoir is a massive undertaking, but alteration of habitat and local destruction around dwellings or place of work can be practised. As rabies follows a natural cycle in many wild animals, their total destruction over large areas may upset this balance and produce a rebound increase, so it is preferable to try and maintain this balance by using vaccination. This has been effectively used in Europe and Canada by leaving vaccine baits for wild animals to take. With bat rabies, control of bats is largely unsuccessful and it is preferable to immunize cattle, which are the main victims.

People who are at special risk, such as veterinarians, animal handlers and those working with bats, can be vaccinated with human diploid cell vaccine (HDCV), Vero cell vaccine (VCV) or purified chick embryo cell vaccine (PCEC). Adverse reactions do occur so the vaccine should only be administered to those at risk of exposure to rabies. The vaccine is given on days 0, 7 and 28 with a booster after 1 year. If the risk continues, then boosters every 5 years are recommended, although protection probably is maintained for 10 years with HDCV and VCV.

Because children in the 5-15-year age group are at the greatest risk of dying from rabies, some countries may consider vaccinating children between 2 and 4 months of age. To save costs, three doses of 0.1 ml HDCV by the intradermal route at 2, 3 and 4 months of age may be more feasible.

Treatment Fortunately, rabies virus can be inactivated on its passage along the peripheral nerves and this is the main method of protecting the individual bitten by a rabid dog. The first procedure is to wash out the wound thoroughly with soap or detergent under running water, followed by a quaternary ammonium compound or 0.1% iodine. Any alcohol, such as whisky or gin, can be used if there is nothing else available. If there is a high suspicion of infection, then rabies antiserum should also be injected locally around the wound. Tetanus toxoid and penicillin should be administered, as tetanus is often a greater danger from a bite than rabies.

If the biting animal can be caught, then it should be tied up and observed for 10 days. After this time, it would either have died from the disease or remained well. If it has died or was killed, then the head is severed with aseptic precautions (as the saliva is highly infectious), packed in ice and sent to a laboratory for viral antigen testing or histological studies. Section of the brain will show characteristic Negri bodies.

Post-exposure vaccination can be given with HDCV, VCV or PCEC immediately and on days 3, 7, 14, 28 and 90. If pre-exposure immunization was given, then give one dose immediately and a second in 3 days. The normal dose is 1 ml intramuscularly, but because of the high cost, money can be saved by giving much smaller doses (0.1 ml of HDCV) intradermally.

Hyper-immune anti-rabies serum is given as soon as possible, half around the wound and the rest intramuscularly. Human immune globulin is preferable, but if horse serum only is available, then a test dose must first be given. The dose is:

• human immune globulin, 20IU/kg body weight;

• animal immune globulin, 40IU/kg body weight.

Table 17.2 summarizes the procedure to be followed in treating a person who has been attacked by a rabid animal.

Surveillance Cases of rabies should be reported to WHO. Countries should work towards a system of vaccine certification of dogs using microchip implants or permanent collars containing vaccination details.

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