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RABIES (HYDROPHOBIA, LYSSA)
Known also as hydrophobia, rabies is an acute infective disease, nearly
always fatal in animals and usually transmitted by bite and saliva from an
affected animal. Humans are susceptible and usually are infected by a dog
bite (other way of infection is rare: oral, aerogen). It is a notifiable disease.
The disease is seen particularly among carnivores, in which the main
method of offence and defense is biting.
Rabies is worldwide in distribution with the exception of Australia and New
Zealand, being endemic in North and South America, Asia and Africa,
where the domestic or semi-domestic dog is the commonest source of
human infection, and wild animals (racoons, skunks, jackals, vampire bats,
mongooses) are the main reservoir of the virus. The sequential
(descending) order of susceptibility is fox, wolves, coyote, cat, jackal, bat
cattle, dog sheep, goat, horse opossum and man. Birds are also
susceptible but in a much less degree.
The urban rabies has been disappeared from developed countries (from
1930 in Hungary) owing to the official registration and regular vaccination of
dogs and the capture of stray dogs and cats. In recent years, however,
rabies has spread alarmingly westward across Europe, mainly in foxes
(sylvatic origin, from 1954 in Hungary) but also in dogs, cats, cattle and
deer. It has reached the north cost of France, only Britain, Spain, Portugal,
Greece, Italy, Scandinavia and Finland are free.
Rabies is caused by a Lyssavirus of different serotypes (1-5) most
concentrated in the brain and spinal cord of affected animals, but also in
the saliva, salivary glands, and occasionally in the blood, but rarely in milk.
The resistance to environment of the virus is low, it is killed at 56 °C and in
excreted saliva, it survives only a few days. In brain of dead animals,
however, it can survive for 3 weeks. Usual disinfecting agents rapidly
destroy the virus.
The presence of the virus, particularly in brain tissue, is associated with
typical cell inclusions known as Negri bodies that are round, oval or
triangular in shape. The presence of Negri bodies in the brain tissue is
conclusive evidence of rabies and, where the disease is suspected, the
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head of the animal is submitted to an approved laboratory. In recent years,
diagnosis has been based on an immune-fluorescence test of brain frozen
sections and also inoculation experiments on mice and also mouseneuroblastoma cell line inoculated with brain-suspension and virus
detection by immune-fluorescent test. Serological tests (VN and ELISA) are
for checking of the efficiency of vaccination.
Pathogenesis
The disease is commonest in the dog, from which 80% of cases originate
and can be transmitted to practically all mammals. Infection of food animals
is nearly always from the bite of a rabid animal. As the virus possesses an
affinity for nerve tissue, especially the brain and spinal cord, wounds on the
lips, cheeks and nose are particularly dangerous. From the entry wound
(following local neural multiplication) the virus is conveyed via the nerves to
the CNS (spinal cord and brain) where the nerve cell damage produces the
typical disease symptoms. From the brain, the virus is transported to the
salivary gland. It is multiplicating also in the cornea and in the brown
adipose tissue. The incubation period is a long one, in dogs usually
between 2 and 6 weeks, but sometimes 6 months or more.
Sheep and cattle are most likely to be infected by bites around the face, for
the thick wool and hide protect the other parts of the body from injury. It
should be noted, however, that every animal bitten by a rabid dog does not
necessarily contract rabies.
Symptoms
Routine ante-mortem inspection of the food animals, particularly where the
disease is endemic, is great value, for these animals may show typical
symptoms of the affection though macroscopic post-mortem findings may
be completely lacking.
In carnivores the disease occurs in two stages of furious rabies and dumb
rabies. Furious rabies is preceded by a period of melancholy that last 1 or 2
days, this being followed by a period of rage or excitement in which the dog
commences to wander or run, and may cover considerable distances
without resting. The animal will attempt to eat indigestible articles such as
stones, rags or sticks and, should it encounter a flock of sheep or herd of
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pigs, may bite and injure many animals in a short time. Rabid dogs usually
avoid human beings, however, and will attack them only when threatened.
The furious period is followed by the so called dumb rabies, in which the
animal seeks a secluded spot and develops paralysis of the lower jaw that
soon becomes generalized. Death occurs within 8 days of the onset of the
symptoms.
Rabid cattle, sheep and goats, exhibit sexual excitement and aggressive
behaviour.
In pigs there is pronounced irritation and aimless wandering, followed by
aggressiveness, paralysis and death.
Lesions
Post-mortem examination of rabid food animals that have died or been
slaughtered reveals no diagnostic lesions. The stomach contains little food
and the gastrointestinal mucosa may be inflamed. The characterisitc
changes in the brain substance are only evident in microscopical
examination.
Rabies in humans
The disease in humans is nearly always acquired from bite of rabid dog (or
from vampire bats in South America). An incubation period of 14-90 days is
succeeded by symptoms of both the dumb and furious type. There may be
itching in the region of the bite, with cramp, salivation and avoidance of
fluids, delirium and convulsions, death being due to respiratory paralysis.
Control
In addition to the measures already outlined that are aimed at preventing
exposure to the virus, vaccination of wildlife using attenuated live and
inactivated vaccines are being used in North America and Europe. The
vaccines (in EU containing mainly SAD-B19 strain from Tübingen) being
contained in baits that are eaten by foxes, etc.
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Judgement
Rabies belongs to the former OIE List “B” diseases: carcase is totally
condemned for human consumption.
In certain third countries:
Though rabies is transmisible to humans by inoculation rather than by
ingestion, carcases from animals that showed symptoms of the disease
should be condemned.
The danger from such an animal is to those handling such meat rather than
to those who consume it.
The Codex Alimentarius Commision (FAO/WHO) recommend the total
condemnation of cases of rabies except where the animal has been bitten
within the preceding 48 hours when approval for human consumption is
given with local condemnation of bite area, special precautions to be taken
to prevent occupational hazards.
(Observation: 90 days and in case of positive, killing without bleeding).