Download impacts of Yersinia pestis - Invasive Species Specialist Group

Impacts: Plague (Yersinia pestis)
IUCN SSC Invasive Species Specialist Group (ISSG)
Yersinia pestis is the causal agent of plague in humans and other mammals, although the overwhelming
proportion of attention and research has focused on its impacts on humans. Y. pestis is recognized as
causing three major disease pandemics in the 1st, 14th-17th and 19th centuries, resulting in around 200
million deaths. The second pandemic known as the Black Death caused the deaths of over 30% of the
population of Europe. While Y. pestis no longer causes problems of such magnitude, it is still a public
health concern in Africa, Asia and South America (Titball and Williamson 2001). There are at least 2000
cases of plague reported annually. In the United States it is a rare disease of humans, with only 112 cases
reported between 1988-2002, although fatality rates remain high (MNWR 2002 in Eisen et al. 2007-B).
Plague manifests itself as bubonic, septicemic, and pneumonic plagues. The most common mode of
transmission to humans is via bites of infected fleas, although the disease can also be transferred by direct
contact with infectious body fluids or tissues while handling an infected animal, or inhaling infectious
respiratory droplets of infectious materials (Gage et al. 2001).
Bubonic plague is the most common form of plague accounting for 80%-90% of cases in the United
States. Initial signs and symptoms of bubonic plague can be nonspecific, with fever, chills, malaise,
muscle pain or tenderness, nausea, extreme weakness, sore throat, and headache (Center for Disease
Control, 2002). According to Fix (1997), the signs of bubonic plague are seen within 2-6 days of being
bitten by an infected flea. Multiplication of the bacteria causes the characteristic "bubo" (meaning
swollen), painful lymph nodes. The case fatality rate for infected persons who are not treated is 50%60%. Outbreaks of the plague caused large percentages of the population to die in the past, but currently it
is only a serious problem in undeveloped countries where sanitary conditions are poor (Gage et al. 2001).
Septicemic plague occurs when Y. pestis invades and continues to multiply in the bloodstream; it can
occur secondarily to bubonic plague or can develop without detectable lymph node swelling and pain.
Patients who do not receive adequate treatment within 18 hours after onset of respiratory symptoms are
unlikely to survive. During 1947-1977 in the United States of the approximately 10% of plague patients
presented with septicemic plague approximately 50% of these persons died as a result of disease.
Complications of this form of plague include septic shock, consumptive coagulopathy, meningitis, and
coma (Gage et al. 2001).
Pneumonic plague is the least common but most dangerous form of Y. pestis. It can develop as a
secondary complication of septicemic plague or result from inhalation of infectious respiratory droplets
expelled from a human or animal that has plague pneumonia. Signs of pneumonic plague include severe
pneumonia accompanied by high fever, difficulty breathing and shortness of breath, and often coughing
up blood. This form is characterized by a shorter incubation period (1 to 3 days) and greater mortality
(90%) (Fix 1997).
Biologists are increasingly realizing that wild mammal species are highly susceptible to Y. pestis. In
North America more than half of rodent species of conservation concern occur within the range of Y.
pestis. The impacts of plague on these populations are not well understood, but certain features increase
the vulnerability of rodent species to plague. These include low natural resistance, high population
densities, coloniality and sociality, abundant flea vectors, and lack of ability to cope with high
demographic or environmental stochasticity.
Black-tailed prairie dogs (Cynomys ludovicianus) are particularly vulnerable to Y. pestis, as they are
major amplifying hosts for the bacterium. Plague epizootics can cause mortality exceeding 99%,
sometimes eradicating prairie dog colonies across much of their range. Epizootics in prairie dogs also
contribute to the spread of the disease among other small mammals, and in humans. Many black-tailed
prairie dog colonies are located near and within urban settings, and approximately 14% of North
American human plague cases are associated directly with epizootics of prairie dogs (Seery et al., 2003).
“Numerous plant and animal species of the western Great Plains exhibit adaptations to the resources and
conditions provided by prairie dog colonies (Kotliar et al. 1999; Lomolino and Smith 2003; Smith and
Lomolino 2004; Kotliar et al. 2006)” (Augustine et al. 2008). Prairie dogs are considered a high priority
species in this ecosystem as they provide a prey base for a variety of predators in the ecosystem as well as
habitat infrastructure to burrowing owls and mountain plovers through their burrowing activities. Losses
of prairie dogs threaten to disrupt these western North American prairie ecosystems (Biggins and Kosoy
2001).
The ‘Endangered (EN)’ black-footed ferret (see Mustela nigripes) is a predator whose diet consists
largely of prairie dogs. It has little alternative prey, and thus is greatly affected when prairie dog
populations are depleted or eliminated by Y. pestis. Black-footed ferrets are also directly susceptible to
plague. These features, combined with their relatively low reproductive rate have caused the total wild
population to drop below 250 individuals.
The ‘Nera Threatened (NT)’ Mountain plovers (see Charadrius montanus) are grassland birds that nest on
prairie dog colonies. “Plague occurs throughout most of the breeding range of the mountain plover, such
that plague effects on prairie dog colony dynamics, and the potential consequences for vegetative
conditions, may have significant implications for mountain plovers”. Augustine et al. (2008) measured
changes in plover nesting in response to plague driven dynamics of prairie dog colonies in the western
Great Plains. At two sites following plague epizootics it was found that mountain plover nest numbers
declined relatively rapidly (1-2 years) on plague affected colonies. This is hypothesized to be caused by
changes in vegetation by prairie dog presence, as mountain plovers prefer to nest in landscape with bare
soil, or changes in arthropod ability. If plague affects prairie dog colonies over a large area, it is unknown
to what degree plovers may respond by moving long distances or by nesting in areas not inhabited by
prairie dogs. Augustine et al. (2008) found that the large decline in nests in plague sites was not balanced
by an increase in nests in non-plague sites which indicates an overall negative effect to plovers caused by
declining prairie dog numbers.
Because plague outbreaks are associated with high precipitation (Stapp et al. 2004) which increases
vegetation production, the authors suggest that prescribed burning may be one way of providing
alternative plover nesting habitat in years where plague epizootics have recently occurred (Augustine et
al. 2008).