Jeff Hankey
BIOBD 495
Summer/Fall 2002
Immunology Research and the Availability
of Standard Observation Guidelines for the Rabies Virus in Domestic Animals and
Terrestrial Wildlife
The rabies virus belongs to the family Rhabdoviridae and causes acute encephalitis and myelitis in all warm-blooded animals (United States. CDC Rabies: The Virus). Without
treatment, rabies is almost always fatal in all animals (Rabies. Westport Weston Health District). The virus is bullet-shaped,
with an average size of 75nm by 180nm, and is composed of RNA enclosed by
nucleoprotein and a layer of glycoprotein on the exterior that is believed to
aid in the attachment to host cells (General Information Page). The
infected animal is only capable of transmitting the disease when it is in the
final stages of illness (Nadzam, Jill). Although
infection has been documented through aerosol transmission and eight corneal
transplants (in humans), the most common way for the disease to be spread is
through a bite of an infected animal (United States. Human Rabies
Prevention).
In theory, any warm-blooded animal can
contract and spread the virus. Four species of terrestrial mammals (known as
rabies-vector species), however, including foxes, coyotes, raccoons, and skunks
have become definitive reservoirs for the rabies virus. Even though the majority of recent human
rabies-related deaths were the result of an exposure to a rabid bat, bats will
not be discussed in this paper because the bat strain of rabies is fairly new
and not well understood (United States. CDC Rabies: Epidemiology). Among domestic animals, only dogs, cats, and
ferrets seem to be especially susceptible to the disease. These three domestic
animals are required by law to have an up-to-date vaccination against rabies
and, because of this law, the majority of rabies cases each year in the United
States (as well as most other developed countries) come from wildlife (Model
Rabies Control Ordinance). With an average of 92% of rabies cases reported
to the Centers for Disease Control and Prevention each year occurring in wild
animals, it is somewhat surprising to find out that there is much more known
about rabies in domestic animals than in wildlife (United States. CDC
Rabies: Epidemiology and Krebs, John).
Upon entering a new host, the rabies
virus sits in a seemingly dormant state (known as the incubation period), at
which time the animal is not capable of transmitting the virus. The incubation period is a key factor in the
survival of an infected animal because it can range from just a few days to as
long as nineteen years (Health: Rabies Information). Because rabies
vaccinations can be given after the initial exposure to the virus, the survival
rate among exposed patients is extremely high (United States. CDC Rabies: Questions and Answers). The incubation period, however, is never a
certain length of time because it is influenced by a number of factors
including the amount of the virus transmitted, the age of the victim, and the
location of the bite. To begin affecting
the host animal, the virus must go against the axioplasmic
flow of the peripheral nerves in order to invade the central nervous system and
begin its rapid reproduction. If the bite occurs in a largely muscular area of
the body, the incubation period is extended because there are fewer nerves, so
it takes the virus a considerably longer time to travel to the central nervous
system (United
States.
CDC Rabies: Natural History).
Some animals are far less likely to
contract rabies. The opossum, for example, rarely carries the disease because
its body temperature is too low to provide a stable environment for the virus (General
Information Page). Also, rodents (with the exception of the woodchuck)
almost never found to carry rabies because they are so small that are almost
always killed during an attack by an infected animal (Wildlife).
Once an animal has contracted the virus, there
are three phases of illness that precede the death of the victim. Each phase
follows a trend as to how long it persists, although there are exceptions to
every generalization. The first phase,
the prodromal phase, usually lasts from one to three
days after the onset of symptoms. The
second and third phases are referred to as either paralytic or excitative phases. Usually an animal will go through
periods of both phases, but some animals are more prone to demonstrate an
extreme occurrence of one phase and almost none of the other. By the time these two phases become obvious,
the host is already shedding large amounts of the virus and death is almost
certain within ten days of the onset of these symptoms (Merck Vet Edition
Rabies: Introduction).
In the United States and most other developed countries, less
than 10% of the reported rabies cases come from domestic animals (United States. CDC Rabies: Epidemiology). Even though the vast majority of rabies cases
occur in wildlife, there is a surprisingly great amount of information that is
known about the rabies epizootic in domestic animals (Krebs, John). For instance, if a domestic dog bites a
human, there is a set time period in which the dog is isolated and observed. If
it is not known whether the dog has rabies, it is kept in quarantine for ten
days (remember that animals are only able to transmit the disease after
symptoms first appear and they almost certainly die within ten days of the
first symptom). If the dog is currently vaccinated, it is kept under
observation for 45 days, and if the dog is known not to be vaccinated, it can
be kept in quarantine for 6 months (the quarantine time is lengthened due to
the incubation period of the virus) or it can be euthanized and submitted for
testing (United
States.
CDC Rabies: Questions and Answers).
Guidelines such as these exist because rabies is viewed as a threat to
human health. A human victim cannot be adequately treated if nothing is known
about the virus; therefore great lengths are taken to understand the risk to
human life.
The aforementioned guidelines, however, do
not exist for wild animals (Krebs, John).
There are at least three possible reasons as to why guidelines exist
only for domestic animals: 1) Human exposure to domestic animals vs. human
exposure to wildlife, 2) Availability of wild and domestic test specimens, and
3) The cost of research. Put together,
these factors dictate the priority of understanding the rabies virus in
domestic animals:
1.
Human Exposure to Domestic Animals vs. Human Exposure to Wildlife
Wildlife, for the most part, is very
elusive. Most wild animals avoid human contact at all costs and human-animal
interaction is minimized because the majority of mammals are at least
semi-nocturnal. When this as well as
other mammalian behavior is taken into account, it comes as no surprise that
the average person taking a walk in the woods for a few hours on the weekend
rarely see mammals of any kind especially rabies-vector species (Stokes and
Stokes 3).
On the other hand, one estimate suggests
that there are 60 million dogs living in homes in the United States alone (Its Estimated). Clearly
there is a far greater interaction between humans and domestic animals than
there is between humans and wildlife. Because most dogs (as well as cats and
ferrets) live in close quarters with their owners, there is a far greater risk
of a person contracting the rabies virus from a bite from someones pet than
from a wild animal. In most
underdeveloped countries, dogs are by far the most common carriers of rabies
and, as a result, there are a high number of rabies-related human deaths each
year in those areas (Merck Vet Edition Rabies: Introduction).
To combat this problem in the United States, rabies in dogs was studied, and in 1885
Lewis Pasteur developed the first post-exposure prophylaxis treatment, which
consisted of rabies virus immune globulin (United States. Historical
Perspectives). This method of
post-exposure treatment has proven more effective with every year because there
has been an overall downward trend of domestic rabies cases since the 1940s
when organized vaccination programs were initiated (United States. CDC Rabies: Epidemiology). As research continued, the observation
guidelines were formed for domestic animals as another means to ensure that any
exposed humans would get the required treatment. For example, if the biting animal survives
the 10-day observation period, then it can be assumed it wasnt infectious when
the bite occurred, and the victim no longer needs to complete the 28-day post-exposure
prophylaxis treatment (United States. CDC Rabies: Questions and Answers).
2.
Availability of Wild and Domestic Test Specimens
Much is known about the fundamentals
of how the rabies virus infects the central nervous system of its victims.
This, however, is not enough to fully understand the intricate course of the
virus on a species-by-species basis.
Rabies affects different species in slightly different ways (Krebs,
John). This number of ways is exponentially magnified by the fairly recent
discovery of different genetic strains of the virus. To completely understand a disease, research
must be conducted on it, but to conduct the research, there must be an ample supply
of test specimens.
According to one estimate by the Humane
Society of the United States, around 9 million dogs and cats are put into
shelters each year, and 4 to 5 million of those animals are euthanized because
homes are not found for them (The Crisis of Pet Overpopulation). With this many healthy animals being
euthanized every year, a reservoir of domestic animals is created that could
potentially be used for research of the rabies virus. The only reservoir of test specimens for
rabies-vector wildlife is the natural population, which is inherently far less
than the population of domestic animals that are susceptible to rabies. The Pennsylvania coyote population, for example, is
estimated to be about 25,000 individuals (More About
the Eastern Coyote). Assuming that coyotes had an even distribution
throughout all 50 states, the overall coyote population would only add up to
1.25 million individuals; a far less number than their domestic dog
counterpart.
In addition to the relatively low
population of rabies-vector species, a large-scale rabies study would have the
potential to severely alter the balance of the local ecosystem even if only two
or three individuals were taken from any given location. Mammals are at the top
of the food chain; they keep the populations of prey animals at a reasonable
level and are a necessity for a stable environment.
3)
The cost of research
The cost of rabies prevention in this
country is enormous. Prevention includes pet vaccinations, pre- and
post-exposure prophylaxis treatments, research, and (within the past year) a
plan to air drop vaccinations to cut down on the incidence of the virus in wildlife. Annually, approximately $300 million is spent
on rabies prevention in this country alone, with a majority of that money used
to fund vaccination programs for domestic animals (United States. CDC Rabies: Introduction). With such a large amount of money already
being spent on rabies-related endeavors each year, it makes more financial
sense to spend the money on the main source of the problem. As previously
discussed, there is a far greater chance of contracting rabies from a domestic
animal due to the sheer number of them and the amount of human interaction with
pets, so it only makes sense to begin to solve the problem of rabies by
researching the virus in domestic animals before researching it in wildlife.
For now, a fairly inexpensive to researching
the finer points of the virus in rabies-vector species has been proposed. Over
the past 10 years, an oral rabies vaccine has been tested on wildlife with
promising results (Rabies Vaccines). It is very cheap to make, averaging
less than $1 per vaccine and, when compared to the $40 million post-exposure
prophylaxis tab in the United States, it seems to be an effective means to hold
off the spread of the rabies epizootic until more research is carried out on
the virus in wildlife (Rabies and Feral Cats).
Rabies is not inherently a human
disease per-se because humans are not a reservoir of the virus. However,
virtually everything that is known about rabies is known because it must be
understood to preserve human health. Rabies
in domestic animals is the primary way it is transmitted to people, so the
course of the disease in these animals is far better known than it is in
wildlife. Frequency of human exposure, availability of wild specimens, and cost
of research all have a direct effect on which animals are studied and to what
degree. But until there is a major
impact on human health by rabies in wildlife, there is not likely to be any
in-depth research on the course of the virus in rabies-vector species.
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