With the exception of the vampire bat which tolerates chronic rabies infection with little disturbance, other mammals rapidly succumb to this infection once clinical signs develop. At first, there may be a change in the behaviour of the animal: restlessness, excitability, unusual aggressiveness or friendliness. Later, there are signs of difficulty in swallowing fluids and food. Paralysis of the lower jaw gives the 'dropped jaw' appearance in dogs. Even at the terminal stage the animal may be running around, attacking indiscriminately
('furious rabies'). Finally, it becomes comatose and paralysed ('dumb rabies').
The transmission of the infection is by the bite of the infected animal, the virus being present in the saliva. It can also presumably be transmitted by the infected animal licking open sores and wounds. Air-borne infection has been demonstrated in some special circumstances, notably in caves heavily populated by bats.
Various laboratory tests are used to establish the diagnosis in suspected animals or in human cases. The rabies virus may be demonstrated in the brain tissue, saliva, spinal fluid and urine, but brain tissue is most commonly examined. Microscopic examination of the brain may show characteristic cytoplasmic inclusion bodies (Negri bodies) in the nerve cells, especially those of the hippocampal gyrus. These may be demonstrated on microscopic sections of the brain or by staining smear impressions from fresh brain tissue. The organism can be demonstrated by inoculation of suspected material into mice (intracerebral) or into hamsters (intramuscular), infection being identified by the presence of Negri bodies in the brains of the animals which die, by the fluorescent antibody technique or by neutralization tests using specific antibody.
In urban areas, the problem is best tackled by the control of dogs; stray dogs should be impounded and destroyed if unclaimed. Pet dogs, and preferably also cats, should be vaccinated every 3 years. In rabies-free areas, the importing of dogs, cats and other mammalian pets should be strictly controlled, such animals being kept in quarantine for at least 6 months. Whenever a dog is found to be rabid, other animals that have been exposed to it should be traced so that they can be vaccinated, kept under observation or destroyed. Some w&f^rn'r.aiopeai. countries have used oral vaccination campaign» <o eliminate rabies in wildlife. This technique could eventually eliminate rabies from its terrestrial reservoirs in western Europe. Improved postexposure treatment of humans and the vaccination of dogs have resulted in dramatic decreases in human cases of rabies during recent years in China, Thailand, Sri Lanka and Latin America.
The wound should be cleaned thoroughly with soap or detergent; an antiseptic such as chlorine bleach should be applied.
Rabies can be prevented in persons who have been exposed to risk by the use of active immunization alone (rabies vaccine) or in combination with passive immunization (rabies immunoglobulin).
Immunoglobulin confers immediate protection while the patient responds to vaccination. The preparations available are listed in Table 5.9. The decision to use immunization should be based on a careful consideration of the risk in each case. Three points need to be carefully considered (see Table 5.9):
Severity of the bite: site and extent
This may be classified into severe or mild expo; ure. Severe exposure includes cases of multiple c deep puncture wounds; bites on the head, necl face, hands or fingers. After such a severe expo sure, the incubation period tends to be very short Mild exposure includes single bites, scratches anc lacerations away from the dangerous areas listec under severe exposure; also the licking of oper wounds.
Certain groups, such as veterinarians, dog catchers and hunters, who run a high risk of rabies can be protected by using HDCV (see Table 5.9). Three 1ml injections are given intramuscularly on days 0, 7 and 21. If HDCV is not available, DEV is also effective (e.g. two doses of 1ml given subcuta-neously 1 month apart, followed by a booster dose 67 months later).
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