Rabies
Author: Prof Darryn Knobel
Licensed under a Creative Commons Attribution license.
Introduction
there are no reliable tests for rabies infection in live
Rabies is an acute, typically fatal, progressive disease
animals. Confirmation of a diagnosis therefore relies on
of the nervous system of humans and other mammals,
demonstrating the presence of the virus in the brain
caused by infection with virus species in the Lyssavirus
tissue after the animal’s death.
genus of the Rhabdoviridae family. The genus
Lyssavirus includes rabies virus (RABV) and the so-
Where does rabies occur?
called rabies-related viruses. The disease is most
Rabies virus is widely distributed around the world. The
commonly caused by infection with RABV. Bats are the
territories reported to be free of the virus are mainly
principal reservoir hosts for the majority of lyssaviruses,
islands and peninsulas. The virus is associated with
but circulation of RABV outside of the Americas is
dogs and other carnivores in Africa, much of Asia and
maintained in terrestrial carnivores, and the vast
Latin America. Several countries in western Europe
majority of human cases of rabies result from contact
have eradicated RABV as a result of successful oral
with infected domestic dogs. Virus present in saliva late
vaccination campaigns in fox populations, but have
in infection is generally transmitted to susceptible hosts
nevertheless reported the presence of bat-associated
by the bite of diseased animals.
lyssaviruses. In 1996, European bat lyssavirus 2 was
also found in a bat in Britain, hitherto considered to be
Salient features of rabies
free of lyssaviruses. Apart from rare imported cases of
Rabies is a progressive neurological disease. In dogs,
the disease, Australia has always been free of RABV
this may manifest as a ‘furious’ form, characterised by
but Australian bat lyssavirus has been recognized in
restlessness, aggression, disorientation, salivation and
fruit bats (flying foxes) as well as in insectivorous bats,
an increased tendency to bite animate and inanimate
and there have been several human infections with
objects, or as a ‘dumb’ form, characterised by paresis
fatal disease resembling rabies. Dog-associated RABV
and paralysis. A drooping jaw and dysphagia (difficulty
has been eliminated through vaccination of dogs in the
in swallowing, with the animal sometimes appearing to
U.S.A., but strains of the virus are still maintained in
have a bone stuck in its throat) are other common
wildlife hosts, including bats.
signs. In all cases the diseases progresses, terminating
in death. Cattle may also be affected, and commonly
What triggers an outbreak of rabies?
exhibit salivation, persistent bellowing and paralysis.
Outbreaks can occur when the virus is introduced into
Rabies should be suspected in cases of abnormal
susceptible populations of maintenance host species.
behaviour in animals, particularly when the affected
Although all mammals are susceptible to infection with
animal comes from an area where the disease is
RABV, certain species are capable of sustained
known to be active, or when there is a history that
intraspecies maintenance of particular viral variants
suggests possible exposure to infection. Currently
adapted to those species. Hosts of a particular virus
‘biotype’ tend to be more susceptible to lethal infection
achieving
with that biotype, and tend to excrete virus of the
populations
adapted biotype more readily than other biotypes. This
controlling and even eliminating rabies in the long term.
leads to improved maintenance of the virus biotype in
Thus 70% vaccination coverage in annual campaigns
populations of that host species (hence the designation
should remain a universal target for programmes that
as ‘reservoir hosts’); however, spillover of host-adapted
aim to eliminate dog-maintained rabies.
70%
at
vaccination
each
coverage
campaign
are
in
effective
dog
in
strains to other species occurs frequently, with
occasional
adaptation
to
those
species.
Rabies
Find out more
outbreaks also occur in dog populations when the
The CPD module on rabies provides an in-depth
proportion of vaccinated dogs in the population drops
account of this disease, including its historical and
below a critical threshold (see next section).
current distribution and epidemiology in Africa and
elsewhere, the clinical signs seen and laboratory tests
Prevention and control
needed to confirm a diagnosis, other diseases that may
The mainstay of successful rabies control programmes
be confused with rabies, and the control measures to
in animals is the vaccination of a sufficient proportion of
prevent outbreaks of rabies in the most important
the main reservoir host population to achieve herd
reservoir host species, the domestic dog.
immunity and thus to prevent outbreaks of the disease.
(Herd immunity is the phenomenon that occurs when
vaccination of a significant proportion of the population
provides a measure of protection for those individuals
who are not vaccinated.) Domestic dogs are an
important reservoir host of RABV across most of its
range, and are the principal source of human
infections.
Rabies
control
programmes
in
these
regions, that include Africa, Asia, and Latin America,
must focus on vaccination to attain herd immunity in
this species. It is estimated that a vaccination coverage
of around 40% of a dog population will achieve herd
immunity against rabies (i.e. 40% of the dog population
should be immune at any point in time in order to
prevent sustained outbreaks of rabies). Although low
levels
of
vaccination
coverage
are
theoretically
required to control dog rabies, the sustained control of
the disease in these situations is hampered by the
decline in the proportion of the vaccinated population
that occurs following a mass vaccination campaign, as
new susceptible dogs are born into the population and
vaccinated dogs die. In most dog rabies-endemic
areas, dog birth rates are high and therefore coverage
levels
decline
rapidly.
Empirical
evidence
and
theoretical studies show that annual campaigns