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John
Pape (Colorado Dept. of Health and the Environment) Whenever people talk about prairie dogs, everybody wants to talk about plague, and that’s appropriate. Ninety-nine percent of my talk is going to be involved with that, but there are other diseases that in fact can be involved. Tularemia is a disease that we have seen, and we’ve actually seen outbreaks of it in translocated prairie dogs before. We [also] reported on a couple of cases of a disease called Blastomycosis; it’s a fungus. This was related to a couple of prairie dog relocaters/researchers that occurred a couple of years ago in Boulder County. In augering through this colony, they apparently tapped into a pile of organic material that was supporting a growth of this particular mould. They inhaled it, and both ended up with a fungal pneumonia. But clearly, when we talk prairie dogs and disease, this [plague] is always the issue that comes up, both from conservation efforts and the impairment there, and also on those opposed to relocation and conservation efforts as the reason you shouldn’t do it. And we’ll go through and discuss a little bit of the myths and truths related to this particular disease.
In Colorado, our first plague case was reported in 1957. We have seen 48 cases from 1957 until today, and nine of those have been fatal. We’re probably going to see increasing number of plague cases, human cases, because our population is going up. And people are moving back to nature, which means they’re plopping their houses out in five acres up in the hills and that includes skunks under your porch and raccoons in your chimney and little plague-infested rodents crawling around in your basement.
Pets have been increasingly associated with human cases, and you’ll see some evidence, even with prairie dog involvement, that pet animals are increasingly linked. I think this will continue also. In the last 10 years, half of our plague cases, human cases, have had a pet-associated link–a dog or a cat tied into the person getting plague.
The plague was a disease primarily of Asian rodents and fleas. It was spread throughout the world in three major pandemics (worldwide outbreaks). The best known, of course, was the Black Death of Europe in the fourteenth century. But the last one started in the late 1800s/early 1900s and again was spread by traders throughout the world, including several ports in the U.S.A.–probably from San Francisco–and became established in the urban rat populations, and from there moved into the wild rodent populations. For some reason, at about the hundredth meridian, we have not seen a lot of eastern expansion. However, in the past 10 years that has changed, as there has been movement into Texas, Oklahoma and Nebraska. There’s actually been some reported [cases] in Kansas within the past six years.
The human cases are clustered around the country. New Mexico is clearly the number one state as far as human cases. Colorado follows close by. From 1947 up to 1989, Colorado was running a solid fourth in terms of the country. We now are the second in the country in terms of human cases, and I expect that trend to continue for several reasons. Basically, there are two human cases of plague per year. It is a very common disease out in the environment. We see lots of it in the rodent populations, but we see the jump into people fairly infrequently.
In looking at the distribution of plague, there is the Front Range focus, the Arkansas river drainage, the Four Corners area, and a cluster in northwest Colorado where nobody really lives, but they still had four cases of plague.
The plague cycle is a very complicated cycle. Most of it is the enzootic cycle–flea to rodent to flea to rodent, probably involving small mice, paramiscus and other species that are able to outbreed the die-offs that the plague causes. About every five-seven years, it amplifies and you get an epizootic cycle, which is kind of an outbreak cycle, and in fact, that involves prairie dogs, tree squirrels, rock squirrels and other larger rodents. And it’s when you get to this cycle, that you have the risk for plague. We very rarely see plague coming out of the enzootic cycle, and usually if it does, it’s from mouse to cat to person.
Fleas feed on rodents that have the bacteria in their blood. They will actually ingest some of the bacteria. The bacterium grows in the flea and over a period of several days, the entire digestive tract of this flea becomes filled with plague bacteria. So this thing is plugged–what we call a blocked flea. And they’re very hungry, so they keep trying to repeatedly feed. Every time they try to feed, their throats clog with this bacteria, they regurgitate into the bite wound, and that’s how the infection is transmitted.
When plague comes up, prairie dogs tend to get the majority of the press. Why do they get most of the press? Because they’re a big, fat rodent that sits out in the middle of the ground, in the middle of the daytime, and people see them. When you have plague in an area where there are other rodents involved, they’re just not as visible. They’re nocturnal, or they’re a little more secretive. They’re not going to be out there. So these guys [prairie dogs] are the most visible source, and we’ll see how that comes into play. The rock squirrel actually has been involved with most of the human cases. The rock squirrel has a flea that is a very aggressive human biter and will feed on almost anything. There are two primary prairie dog fleas, and they like prairie dogs. They really won’t feed on other animals unless there are no prairie dogs left to feed upon. That’s kind of an important point. When you’re seeing the die-off in species like prairie dogs, you’re also getting die-off in the area in other species too. They just may not be quite as noticeable. In some areas, we can collect 200-300 fleas off of a rock squirrel in a plague die-off. The remaining fleas all congregate to the surviving animals. Packrats and Richardson ground squirrels are other species that have been involved. And increasingly, cats can both act as a carrier and bring fleas home after scavenging, but more often from ingesting rodents, contract plague themselves, and can directly transmit to people. And these guys are becoming an increasing point. When you look at the animals involved in plague, rock squirrels are by far the lead (undetermined) and prairie dogs are holding at two [cases per year], and cats have been moving up. Out of the 48 cases over this time period, seven of them have been tied into prairie dogs. Another four cases here involved multiple species that were generally prairie dogs and some other rodent species dying in the area, and we weren’t sure which one was probably responsible for the human cases. In regards to the human cases associated with plague (in Colorado and associated with prairie dogs), two were people who were exposed while skinning out the animal. One was a wildlife biologist and the other was a kid who had killed one with his spear. Two were involved with dogs that brought home fleas from the colony. They opened up the door in the morning and let Fido out for his run. He runs half a mile away, chases prairie dogs, picks up a few fleas, and brings them home. Three were residential exposures–people playing or working in colonies. And then the four others that involved prairie dogs and other species here (rock squirrels, packrats, and a couple of Richardson ground squirrels) were one child playing and a couple of occupational exposures.
The plague takes out prairie dogs quickly, [but] not usually a large colony in a few days. It may take several weeks to take out a colony, but 99.9 percent of them will die. And we actually use this as a surveillance tool by documenting [and] then mapping plague colonies around the state. And we have most of the colonies up and down the Front Range mapped, some in the Four Corners area and some in Fremont county. If an inspector or health department or animal control officer is going to a call up in an area and happens to be driving by a colony, we have them stop and spend five minutes looking at the colony. If they’re seeing them active and healthy and doing what they’re supposed to be doing, they can go about their business. If they notice that half of that colony is gone and two weeks ago it was there, we go out and take a little closer look at it.
In looking at outbreaks associated with plague and prairie dogs over the 1990s, there was a very small cluster out in eastern Colorado. 1992-94 was our last epizootic in Colorado–our last widespread epidemic. [Over the] last several years, there was very low activity. In fact, in the last two or three years, there was almost none.
In regards to dusting, long ago when I first started in this business, one of the things that was real popular then was to go out and start poisoning prairie dog colonies to prevent plague. What we found quickly was two things. One was that it is an enormously expensive, labor intensive endeavor and two, it was totally ineffective at doing anything as far as plague control goes in most situations. Applying insecticides as the animal goes in and out of the burrow, dusts itself and knocks the fleas off clearly was an easier, quicker, less expensive, and less intrusive way to go. There was a question asked–if you go out and dust a colony, does it eliminate the plague die-off? In my experience, no, it does not. It’s kind of like throwing a couple shovelfuls of dirt on a fire. You put out the flame, but it smolders under[neath]. And so the colony, in fact, will continue to experience a die-off, but it will be greatly reduced and that die-off is probably related to some of the cannibalism and transmission that way. So we’ve seen, when we’ve gone in and dusted colonies, that it really slows down the die-off, but a lot of times, you can lose the colony anyway. One of our recommendations is to keep your dogs at home and, if not, you’d better dust them because again that has been an increasingly important source of plague in this state.
Several years ago, we started talking with the [Colorado] Division of Wildlife about relocation efforts and these were the recommendations that we had made to them in terms of approving relocation permits: Monitor the
colony for any sign of die-off prior to trapping. I don’t have a real problem, or I’m not opposed to relocation within Colorado because we’re not introducing a disease that’s not already here and all throughout the state. And so I think if the other issues are covered, in fact, you can do relocation safely, provided an appropriate release site, which certainly is critical.
The last thing I will close with is that, as I said, I have not advocated nor have we advocated for probably 15 years now, extermination of prairie dogs as a method of plague control. This again often comes up as an issue to say, “Well, we’re going to get rid of the prairie dogs because they all have plague and our children are at risk, etc., etc.” If, in fact, you’ve got a colony living across the street from you that’s dying from plague and you, your kids and your pets never cross the street; your risk of getting plague is zero. Because the fleas aren’t coming to you; they’re sitting in the burrows. So you’ve got to go to them. You’ve got to get into the colony or handle the animals. So because of that we really have not felt that extermination is an appropriate method. If you want to get rid of prairie dogs, have plague introduced into the colony. It will get rid of them quicker than anything else. And in fact, when we do die-off investigations, we’ll often have ranchers call and ask us where the die-off is, so they can come and collect a few fleas for their colonies–which we don’t provide them for various public health, bioterrorism reasons, and other things. But the final point is that there may in fact be some situations where prairie dog control, i.e. terminal or extermination of them, is appropriate from a public health point of view. Where you have prairie dogs pushing into a playground, into people’s backyards (one of the human cases was acquired this way), into areas where they’re really inappropriate (and since you don’t have in these situations a natural predator control), humans end up having to do it. Now whether that means you go in and trap them out and relocate them to keep them out of the school playground, or you do terminal control, kind of depends on the situation.
Question-and-Answer
Period Response to question regarding the black death in medieval Europe and the large mortality that was suffered by the European population and why that was the case and we don’t see the same thing now:
There have been some discussions that there are some differences in virulence of the strain that was circulating then vs. now or that the human populations over the centuries have developed some resistance. But I think the real reason is that you have to look at the social and cultural structure at that time vs. now. For instance, it was not unusual to have rodents, rats, etc. living in your house in town with you. It was not unusual for people to be carrying a load of head lice and other ectoparasites, including fleas. You also would have seen pneumonic transmission; that is, where the bacteria gets into the lungs and you actually get person-to-person transmission. And back then, of course, there was no way to treat it. They didn’t even know what was causing it, although the associations with rat die-offs was known at that point in time, but the cause wasn’t known. These days, with our medical system, if we see a case of plague, it gets treated pretty quickly. We may put family members and close contacts on antibiotics as a preventative measure. We have not seen the kind of person-to-person transmission in this country since 1924, where there were some cases in Los Angeles. The other reason is there are a lot of cities, primarily eastern and coastal cities, that have very extensive rat and rodent control programs, and those were all put in as a result of plague.
Response to question regarding weather patterns and the effect that weather has on plague and plague die-off:
Weather does have an effect. We know that, with most rodent populations, the food supply is the limiting factor. So if you have a couple years with lots of rain [and] lots of moisture, you get lots of grass growth and heavy seed production. Your rodent population booms right after it. And we actually saw that about three years ago and saw an increase in Hantavirus, another rodent-associated disease. I talked about some of the mapping techniques that we use. Clearly, we see most of our human plague cases occur in pinyon juniper type of habitat, where the rodent populations and mixes are fairly large and fairly dense. And so, as we overlay with some of the new mapping techniques, vegetation type overlaying with where our human cases are occurring, some very distinct patterns are emerging from that kind of work. The other thing we’ll see is you could have a plague die-off going through a summer or even through a colony or in an area and, if you suddenly get a long heat wave where you get temperatures above 90 degrees for several days or a week or two in a row, it absolutely shuts off the die-off. The fleas just don’t survive. They dry up. They blow away. One of the advantages of an arid climate is you don’t have a lot of those flea problems and it literally shuts the die-off off.
Response to question of when you have a plague die-off, you see a 99.9 percent mortality rate but not 100 percent, so why do some prairie dogs survive:
Could be a variety of factors. Could be in relation to weather. Could be the plague starts moving through a large colony and then the temperature shuts it off. I think a lot of times what I see is you’ll have a 10-acre colony and then across a creek or across a road or something else, there is a little pocket and plague takes out the main colony but misses the pocket. And then from that pocket, you get reseeding back into those areas. Even though the plague moves through an area in kind of a wave-like fashion, it does miss areas. We investigated a case a couple of years ago down in Fremont County where I think dogs were involved and brought home rock squirrel fleas from a nearby area, a drainage. When we went through that area, the rock squirrels were all gone and the burrows were empty. We were pulling fleas out of them but up on the mesa where the house was, there was quite an extensive prairie dog colony there and the colony was untouched. They monitored it throughout the summer and never saw any die-off, so this probably moved in that case through the rock squirrels and, for no more than just sheer luck, didn’t make the jump and get into the prairie dog colony.
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