August 10, 2017
Few diseases over human history have struck more fear in the hearts of people than plague. Also known as bubonic or septicemic plague, this highly contagious bacterial disease caused by Yersinia pestis is the “Black Death” responsible for killing one-third to one-half of the world’s population in Europe and Asia in the late 1340s.
Plague is spread primarily through the bites of infected ticks. Often thought of as an ancient disease, the World Health Organization notes that plague continues to sicken and kill people and animals in many parts of the world. Approximately 600 people die from plague each year in Malaysia alone. Though most Americans don’t spend much time today worrying about plague, it still exists in the United States in certain regions (on average, the United States has seven plague cases a year in humans and several dozen pet cases).
Plague was first introduced to the United States around 1900 by ships carrying infected rat stowaways. As the rats (and their fleas) snuck off the ships into the port cities, they spread the disease to native rodent species and into the countryside. While plague cases in people and pets are minimal in the United States, the real toll of this disease is seen in wildlife, in which it is referred to as sylvatic plague.
Sylvatic plague is regularly found in prairie habitats in six western states: New Mexico, Arizona, Colorado, California, Oregon and Nevada. Epidemics in these areas usually are associated with prairie dogs and their fleas. Because prairie dogs are highly social animals living in tightly knit colonies, they suffer particularly high mortality rates from plague; upwards of 90 percent of a colony can die off during an outbreak. Over time, new prairie dogs may move in and recolonize these areas, but it can take many years for a colony to return to pre-plague levels. The endangered black-footed ferret, which preys nearly exclusively on prairie dogs, is struggling to recover as a species due primarily to the plague-related loss of their favorite food source.
When a prairie dog colony is decimated by plague, the effects reverberate through the entire local ecosystem, from animals to plants and even the soil. Prairie dogs are considered a keystone species; more than 100 species rely on prairie dog towns for their livelihood. They are a key food source for many animals, including coyotes, foxes, hawks, eagles, snakes and ferrets. As prairie dogs graze and burrow, they act like giant earthworms, helping to fertilize and aerate hard-packed soils and prevent water erosion, important for maintaining a robust prairie habitat. Holes created by the prairie dogs also provide shelter for rabbits, toads and burrowing owls.
Tackling plague epidemics
In recent years, researchers have tried a variety of strategies to minimize the effect of plague on wildlife. Addressing infectious diseases in wildlife poses unique challenges: wildlife managers and researchers have to approach infectious agents as both a “herd health” and an environmental problem. Treatment must put the population ahead of the individual, and must consider the health and welfare of other species sharing the same environment.
A variety of strategies currently are used to control outbreaks of plague, including dusting insecticides in prairie dog holes to control fleas.
“Dusting does work to reduce flea loads on animals, but it has been known to fail to prevent plague in some locations,” said Dr. Toni Rocke of the USGS National Wildlife Health Center and Morris Animal Foundation-funded researcher. “It is a good tool to use as an immediate response to plague; however, fleas have begun to develop resistance to the most commonly used pesticide.”
Another proposed strategy has been to develop a vaccine that could be administered to an entire colony. An experimental vaccine for prairie dogs, developed jointly by Dr. Jorge Osorio, University of Wisconsin, and Dr. Rocke, recently was field tested in several key states.
“The vaccine protected prairie dogs in our study, but not all of them” said Dr. Rocke. “However, we think that was largely due to the small size of the plots and large numbers of unvaccinated animals nearby. It will not work as an immediate response to plague but may be applied as a preventive measure.”
While the new vaccine shows promise as a possible way to help control sylvatic plague, there remain many unanswered questions. How will the vaccine affect the deer mice, grasshopper mice and ground squirrels that share common areas with prairie dogs, and likely will consume the vaccine baits? What factors in the ecology of rodents, fleas and plague bacteria drive the plague cycle? What other environmental factors are at play that may lead to more robust prevention strategies?
With Morris Animal Foundation funding, Dr. Osorio’s and Dr. Rocke’s teams are working to find answers to these questions. They assessed the effects of the vaccine bait in small rodents other than prairie dogs, and confirmed that the vaccine baits are safe for consumption in these “non-target” animals. In another Foundation-funded project, the team is using samples from the field vaccine study to help understand plague ecology and overlap among rodent communities and prairie dogs in grassland ecosystems. Discovering which species of rodents and fleas maintain the bacteria in nature will help researchers design better plague control strategies.
Other factors at play
Y. pestis is extraordinarily virulent, even when compared with closely related bacteria. It can persist silently for many years and then return with a vengeance. However, it’s a mystery as to how and where plague hides between outbreaks after a mass die-off of host animals. The length between outbreaks is highly variable. Researchers are trying to understand how the bacteria “hibernates,” especially in the winter when fleas die off.
Morris Animal Foundation-funded researcher Dr. Viveka Vadyvaloo, at Washington State University, is investigating the possibility that the plague bacteria Y. pestis is kept alive during non-disease cycles by very small organisms, single-celled animals called amoebae, living in the soil in plague-endemic areas.
“Bacteria need nutrients to survive. Once the rodent dies and decays, the availability of nutrients for the bacteria are gone,” said Dr. Vadyvaloo. “To live, the bacteria need to find a new way to sustain themselves until the next plague outbreak. Amoebae readily consume bacteria, and research has shown that a large number of bacteria are able to persist within the amoebae.”
Dr. Vadyvaloo and her team are studying if Y. pestis can live in amoebae for a length of time sufficient to be a potential player in the next plague cycle. Her team currently is analyzing soil samples collected from plague-endemic areas.
“If amoebae turn out to be the ‘Trojan horse’ of plague bacteria by keeping Y. pestis under cover until conditions are favorable for an outbreak, control measures can target these organisms,” said Dr. Vadyvaloo.
Pets (and people) are at risk from plague, too
While researchers continue to work toward understanding the sylvatic plague cycle and finding better strategies to combat the disease in the wild, it’s important to know that all mammals, including pets and people, potentially can contract plague from infected animals in the natural areas around them. If left untreated, the disease can be fatal.
According to the American Veterinary Medical Association, the fatality rate in untreated cats is as high as 60 percent. Dogs tend to fare better when contracting plague, exhibiting far fewer symptoms than cats, but still need treatment. Seek immediate veterinary help if your pet exhibits signs of plague including lethargy, poor appetite, vomiting, diarrhea, fever, and enlarged or abscessed lymph nodes. Because infected pets are a source of infection for their owners, pets need to be quarantined during treatment. Prompt diagnosis and treatment are key to dealing with this virulent disease.
Both hikers and pet owners should be diligent in plague-endemic areas, especially after a wet spring season when the disease is likely to emerge. To decrease the risk of contracting plague in endemic western states, keep pets from roaming and hunting to limit their contact with rodent and rabbit carcasses. Using appropriate flea control can help reduce the risk of plague as well.
“The good news,” said Dr. Vadyvaloo, “is plague is treatable with antibiotics in both humans and pets.”
Morris Animal Foundation is committed to finding solutions to plague and other wildlife diseases that impact a wide range of animals. Visit Morris Animal Foundation, to learn more about the wildlife projects we fund and how you can support this important work.
For more information about the plague, check out these publications by our featured researchers: