An introduction to Rabies (Hydrophobia)
- Part 3 -

Dr Asem Suresh Kumar Meitei *

Latent infection

Bats are the only known species in which symptomless carriers exist. The virus can multiply in fatty tissue without invasion of nervous system in this species which might be the reservoiring mechanisms. The violent behaviours of rabies rarely occur in rabid bat. They represent a serious threat of spread of rabies because of their migratory habits.

Although rodents can be infected with the rabies virus they are not thought to play any part in epidemiology of rabies, either as multipliers or simply as physical carriers of the virus. Many of the viruses they carry are rabies-like rather than classical rabies.

Domestic livestock are rarely a source of infection although chance transmission to man may occur if the mouth of a rabid animal is manipulated during examination or treatment. The virus may present in the saliva for periods up to 5 days before signs are evident.

Spread of the disease is quite often seasonal with highest incidence in the late summer and autumn because of large scale movements of wild animals at mating time and in pursuit of food. In general, foxes are less dangerous than dogs, foxes tending to bite only one or two animals in a group, while dogs will often bite a large proportion of a herd or flock.

Not all bites from rabid animals result in infection because the virus is not always present in saliva is wiped from the teeth by clothing or the coat of the animal. The virus may appear in the milk of affected animals but spread spread by this means is unlikely as infection thorough ingestion is not known to occur.

Transmission by vampire bats was first demonstration in Brazil (1961) and is the most important transmission of rabies in Central and South America and Caribbean region (Trinidad).

The latent infected carriers in animals need to be investigated thoroughly. There are reports of a number of fatal cases in human rabies from bites of apparently healthy dogs, (Yukovsky, 1962). It is mentioned that rabies infected dogs may survive and may remain as transmitting factor of the disease (Bell, 1966).

Veerarghavan (1969) reported a case of apparently healthy dog bitting of a person who later died of rabies. He observed the dog for three years and collected 1044 saliva samples from this dog and isolated virus in 14 occasions. Again, his interesting finding was that he could not detect the antibody from the serum of that dog.

Signs of Rabies in animals

Clinical signs of rabies are suggestive but rarely definitive. Rabid animals of all species usually exhibit typical signs of CNS disturbance, with minor variations among species. The most reliable clinical signs, regardless of species, are acute behavioral changes and unexplained progressive paralysis.

Affected animals may seek solitude. Ataxia, altered phonation, and changes in temperament are apparent. Uncharacteristic aggressiveness may develop—a normally docile animal may suddenly become vicious. Commonly, rabid wild animals may lose their fear of humans, and normally nocturnal species may be observed wandering about during the daytime.

The clinical course of rabies may be divided into three general phases—prodromal, acute excitative, and paralytic (end stage). However, this division is of limited practical value because of the variability of clinical signs and the irregular lengths of the phases.

During the prodromal period, which lasts 1 to 3 days, animals show only vague nonspecific signs, which intensify quickly. The disease is fatal once clinical signs appear. The disease progresses swiftly after the onset of paralysis, and death is virtually certain a few days thereafter. Some animals die rapidly without marked clinical signs.

The signs of rabies are divided into three stages according to the degree of advancement attained by the disease as:

The first stage is the incubation stage or melancholic stage or Stadium prodromus. This stage depend on such variables as virulence and amount of virus injected, the depth and severity of the wound, the amount of nervous tissue near the site of the wound and the distance of the bite from the brain or cord. In man bites about the head and on the hands are the most dangerous.

The bite is contaminated with the virus-laden-saliva. Once the virus is injected under the skin or mucous membrane it becomes established in the nervous tissue and travels by way of the sensory nerves to the spinal cord and brain. The virus may be excreted in the saliva of a dog seven days before clinical symptoms are manifested.

The incubation period may be as short as 10 days, or the animal may not show any symptoms until several months have elapsed. In most cases rabies will develop from 21 to 60 days after exposure and in rare instances 12 months after even later. There is report of rabies in man where the symptoms developed after three years of exposure. The incubation period in man is rarely under three weeks and usually ranges from 30 to 90 days.

Towards the end of the incubation period and a few days before the appearance of characteristics symptoms, a number of prodromal or premonitory symptoms set in. In case of dog, it changes its disposition, becomes strange in its behavior and capricious in its habits. It frequently hides in dark places avoid light and noises, and when is is called it obeys unwillingly as if it has been doing something wrong.

Sometimes, it crouches towards its master and jumps about in an unusual way. The dog often appears to be uneasy, lying down and getting up alternatively at frequent intervals. It walks in a peculiar manner and often snaps in the air or towards it body at imaginary flies or fleas. At times it suddenly becomes alarmed and barks at nothing.

The second stage is the stage of furious rabies or true rabies. This stage is known as stage of irritation, stage of mania or Stadium irritans. After a few days, the reflex irritability of the affected dog becomes considerably increased. It may snap anybody or anything within its reach on the slightest disturbance and without any provocation. I may appear quite friendly, but is easily irritated, especially if restrained.

If held, it will attempt to break away and if not released at once will bite savagely. At first, it becomes annoyed with strangers only, but in course of a day or two, it shows resentment at the friendly attention of its regular associates. Such external stimuli as strong light, noise, and touch may easily startle the animal and cause it to jump up.

In the majority of cases, the appetite becomes depraved and the patient refuses its normal food but readily chews foreign objects of any kind within its reach, including its own faeces. Swallowing becomes more and more difficult and finally impossible.

Sometimes frequent vomition occurs and salivation becomes progressively more and more profuse. In the majority of cases, the patient develops a severe thirst, but it always finds great difficulty in drinking. Some animals have an abnormal sexual desire.

In the course of one to three days, a violent rage sets in. The dog fiercely chews any object it can hold of and furiously tears it to pieces, if possible. It becomes more and more restless and manifests an insuperable desire to get away from its surroundings. If it is tied up, it tries to break loose and if it is shut up in a cage, it jumps viciously at the bars, often snapping with such violence at the bars that it breaks its teeth.

The nature of the object that it bites is immaterial and it will grab a red-hot iron or a burning coal as readily as a piece of rag. An attack of rage is usually interrupted by a arable intervals of depression and at times a raging dog may drop down exhausted and unconscious. Soon after consciousness returns, the dog rises again, but remains standing with terror stricken and furious expression on its face until a fresh attack of mania sets in.

If the mad dog breaks loose, it is inclined to wander about aimlessly and seldom returns home, or it may return in an extremely exhausted condition with its hairs ruffle and its skin often covered with bite wounds.

A rabid dog is extremely aggressive and if it should encounter another dog during its wanderings it furiously attacks the other dogs without any cause. Although, a mad dog is always more aggressive towards other dogs, it may attack any species.

To be continued...