An introduction to Rabies (Hydrophobia)
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Dr Asem Suresh Kumar Meitei
Contd from Wednesday
In nature, blood borne transmission does not play any important role but blood borne transmission is possible when large amount of virus is inoculated experimentally into susceptible animals.
Traditionally, the dog, and to a minor extent the cat, have been considered to be the main source animals. However, native fauna, including foxes, skunks, wolves, vampire, insectivorous and fruits eating bats, raccoons, mongoose, and squirrels may provide the major source of infection in countries where domestic carnivores are well controlled.
After peripheral infection the virus remains viable locally for a number of days, invades nerve fibres within 24 to 48 hours, and then travels centripetally to the central nervous system at the rate of 2 to 3 millimeters per hour. There is no evidence that the blood stream is concerned with dissemination of the virus to any significant extent after peripheral infection. When the virus reaches the central nervous system it first affects the walls of the vessels, causing small-cells infiltration of the nerve tissue, but finally it attacks the nerve cells themselves. As a result of the irritation of these cells nervous disturbances like over excitement, increase reflex irritability and loss of consciousness are produced. Finally degeneration of the nerve cells ensues, leading to paralysis of several groups of muscles, the paralysis of the respiratory muscles being the direct cause of death. In some animals, such as the rabbit, the stage of irritation may be so short and so inconspicuous that the generation of the nerve cells, with its associated paralysis, apparently sets in from the commencement. The virus multiplies in the central nervous system, from where it may pass centrifugally at a faster rate along certain nerve tracts until it reaches structures like the salivary gland where conditions are favourable for further propagation. The virus is the excreted with saliva.
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.
(To be contd)
Contd from Wednesday
In nature, blood borne transmission does not play any important role but blood borne transmission is possible when large amount of virus is inoculated experimentally into susceptible animals.
Traditionally, the dog, and to a minor extent the cat, have been considered to be the main source animals. However, native fauna, including foxes, skunks, wolves, vampire, insectivorous and fruits eating bats, raccoons, mongoose, and squirrels may provide the major source of infection in countries where domestic carnivores are well controlled.
After peripheral infection the virus remains viable locally for a number of days, invades nerve fibres within 24 to 48 hours, and then travels centripetally to the central nervous system at the rate of 2 to 3 millimeters per hour. There is no evidence that the blood stream is concerned with dissemination of the virus to any significant extent after peripheral infection. When the virus reaches the central nervous system it first affects the walls of the vessels, causing small-cells infiltration of the nerve tissue, but finally it attacks the nerve cells themselves. As a result of the irritation of these cells nervous disturbances like over excitement, increase reflex irritability and loss of consciousness are produced. Finally degeneration of the nerve cells ensues, leading to paralysis of several groups of muscles, the paralysis of the respiratory muscles being the direct cause of death. In some animals, such as the rabbit, the stage of irritation may be so short and so inconspicuous that the generation of the nerve cells, with its associated paralysis, apparently sets in from the commencement. The virus multiplies in the central nervous system, from where it may pass centrifugally at a faster rate along certain nerve tracts until it reaches structures like the salivary gland where conditions are favourable for further propagation. The virus is the excreted with saliva.
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.
(To be contd)