| Plague |
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| Causative Agent |
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Bacterial
disease caused by infection with Yersinia pestis, causing an
acute
infection and high mortality rates in mammals.
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Plague is a flea-transmitted disease affecting and perpetuated by
rodents.
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This
same
bacteria caused three human
epidemics in recorded history; today,
wildlife act as
reservoirs for the
bacteria throughout the world in
semi-arid areas on every continent except Australia and Antarctica.
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| Images |
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Click on
image to
enlarge. |
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Plague cycles can occur both
in wild (sylvatic) and urban areas. |
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| Distribution |
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| Hosts, Transmission and Life
Cycle |
| Hosts: |
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Y. pestis
bacteria are maintained in a complex
cycle involving rodents and fleas.
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Infected fleas tend to
remain in burrows for prolonged periods. Hence, burrowing
rodents, their predators and animals that share similar
habitats as burrowing rodents will often have high rates of
infection when compared to other species.
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Spill-over from rodents to
other species often results in outbreaks or
epizootics.
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Rabbits, carnivores,
primates and birds are generally not involved in cycles,
although they may occasionally aid in spreading infectious
fleas or prey.
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Transmission: |
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Transmission of
bacteria usually occurs via a flea
bite; blood-borne
bacteria from the infected animal,
remaining in the
gastrointestinal tract of the flea, is transferred when the flea begins its blood meal from its
next uninfected host.
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Less commonly, a predator/scavenger can become infected upon ingestion of
tissues of an infected animal. Sharp objects, such as bones,
may puncture tissues of the mouth and throat, thereby
enabling entry of the
bacteria.
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Rarely, infection occurs
through inhalation of aerosolized
bacteria.
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Life Cycle: |
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See above diagram.
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Plague cycles can occur in both wild
(sylvatic) and urban areas.
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| Signs and Symptoms |
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Observation of
clinically affected wild mammals is unlikely. The discovery of dead
animals is more common.
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Lesions vary according to the mode of transmission and
susceptibility of the host.
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Accordingly, symptoms of animals that have contracted
plague will vary from:
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swollen lymph nodes and
abscess formation near site of inoculation (flea bites, oral
punctures);
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muscle soreness;
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loss of appetite;
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fever;
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depression;
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necrosis of
lymphoid
tissue;
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edema in the lungs;
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death may occur rapidly before the appearance of
clinical
symptoms.
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A history of rapid,
large declines of colonial rodents is suggestive of plague, but
confirmation requires submission of samples to an appropriate
diagnostic laboratory.
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Three types of plague are
possible in susceptible non-rodent mammal species:
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Bubonic:
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initially characterized by
swelling (from the Latin bubo = swelling) of tissues
around the flea bite;
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replication of the
bacteria occurs and, in this type of plague, is
restricted to the
lymph
nodes that drain the site of the flea bite, often
producing
hemorrhage and localized
necrosis of affected
lymph
nodes.
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Septicemic:
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defined by
bacteria in the blood without the
presence of buboes. Results from ingestion of infected prey
or through the bites from infected animals;
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lesions are typically first observed in the liver and
spleen;
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coagulation of blood within vessels, escape of
blood from vessels into surrounding tissues,
hemorrhage, and blood clotting may cause a dark,
reddish-black discoloration of tissues visible under the
skin - leading to the name
black death;
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bacteria in blood may spread to
lungs leading to the
pneumonic form of disease.
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Pneumonic:
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inhalation of aerosolized droplets (mist)
containing
bacteria;
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often fatal.
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| Meat Edible? |
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Human infection has
been reported from contact with recently dead animals (e.g.,
when animals are dressed skinned).
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If you suspect an
animal has been infected with plague DO
NOT CONSUME ANY MEAT and contact the nearest
Health Authority.
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| Human Health Concerns and
Risk Reduction |
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Plague is a
potentially deadly
zoonotic disease and precautions should be taken when an animal
suspected of having plague is encountered.
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Symptoms in humans include:
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In humans,
septicemia
and
pneumonic plague are the most serious. These are characterized
by fever, prostration, coughing, respiratory distress;
shock,
hemorrhage and death may follow.
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Viable Y. pestis
bacteria
have also been isolated from soft tissues of carcasses after
approximately 1 week and bone marrow of infected animals after
longer periods.
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Transfer of
bacteria to
humans has also been reported from bites/scratches/abscesses of infected domestic pets.
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Wild rodents are the natural
reservoir;
lagomorphs and carnivores may also be a source of infection to
humans.
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Since vaccination of free-ranging wildlife is not
possible, large-scale attempts to control plague both in humans and
endangered wildlife populations are directed mainly at eliminating
flea populations; however, removal of non-target insect species with
associated ramifications to ecosystems have made this control method
problematic.
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Removal of food sources and rodent habitats in areas
occupied by humans will help to reduce rodent infestation and,
subsequently, flea populations.
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Treatment of pets for fleas should also help to reduce
transmission to humans and other wildlife.
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No control measures have ever been required in British
Columbia, as the level of plague appears to be low.
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| Samples for Diagnosis |
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Appropriate personal
protection should be used:
eye protection, gloves, gowns, high-density surgical
masks or respirators.
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Plague suspects should be dusted with carbamate or
pyrethrin insecticides to kill fleas.
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Using appropriate personal protection, collect a
representative sample of fleas (mature, immature, male, female) from
fresh, affected mammals.
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DO NOT ATTEMPT TO COLLECT
TISSUE SAMPLES.
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Animals surviving infection with Y. pestis
develop serum antibodies that can be used for diagnosis of exposure.
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Detection of antibodies in the blood of carnivores has
been used to monitor plague activity in areas where plague is
normally found.
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| Similar Diseases |
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Acute
bacterial infections, such as
tularemia, can mimic signs associated with infection with Y. pestis.
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Both infections cause an
acute, feverish disease in certain species and can be
followed by
pneumonia or sudden death.
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Also found in colonial rodents,
tularemia does not appear to induce the high mortality typical
of plague.
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Infection with Pasteurella bacteria may cause
individual mortality in rodents and occasional localized die-offs.
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White spots or spotty
necrosis of
the liver and spleen, observed in Y. pestis infection, can
appear similar to infections by other species of Yersinia or
may result from migration tracts of parasites.
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Poisonings from rodenticides may also cause
acute
population declines in colonial rodents.
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| Further Reading |
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