Counting Ticks on Animals is More Complicated Than It Sounds
By Ed Ricciuti
Forget scary lions and tigers and—even though they seemingly pop up in every backyard nowadays—bears. Chances are the critters that give jitters to most Americans these days are ticks. As Lyme disease and other illnesses borne by ticks have spread, so has fear of the tiny arachnid, in some cases verging on paranoia.
Research on ticks has grown accordingly, expanding knowledge about their ecology, critical to controlling them. At the same time, according to a new review article published in June in the Journal of Medical Entomology, scientists have given short shrift to a process fundamental to understanding tick ecology, and by doing so, potentially skewed results of research.
After reviewing and comparing a bundle of research papers on tick ecology, scientists at Australia’s University of Sydney concluded that their fellow researchers (with apologies to the Captain in Cool Hand Luke) often are guilty of a failure to communicate, in this case the details of how they conduct sample counts of ticks on hosts. The nuts-and-bolts of a study can be lost while authors describing it focus on other, seemingly more pertinent aspects of the research—the methods used to identify the presence of a pathogen, for instance—rather than the tick counting itself. Yet, so many variables impact an accurate count, say the authors, “that it is important that we attempt to further understand how we count and what potential biases and limitations our methods have.”
“Methods to count ticks on hosts are not widely standardized, and in many cases we found that methods are vaguely described or justified,” according to the report by Henry Lydecker, Ph.D., Peter Banks, Ph.D., and Dieter Hochuli, Ph.D. “Unfortunately, there is no best method agreed upon by scientists to collect ticks from hosts.” Many studies described counting techniques used in terms as vague as “ticks were removed” or “tick burden recorded,” say the researchers, or there was no clarification at all.
The authors admit that, to put it mildly, “manipulating a host to count parasites is an unfortunately complex procedure,” while adding that its execution should be as rigorous as any other scientific procedure and described clearly enough to allow its use in other studies.
Anyone who has done a body check for ticks after a trek in forest and field knows how difficult they are to spot. Imagine trying it on a tiny white-footed mouse, primary reservoir of the bacterium that causes Lyme disease or, in Australia, a wiggly bandicoot, which has been implicated as a host for Ixodes holocyclus, known as the Australian paralysis tick. “Bandicoots are very jumpy and will struggle while handled, making it harder to search for ticks,” says Lydecker.
Many counts are performed on conscious animals, sometimes under restraint. It is easier, Lydecker and colleagues say, to count on the body of an animal that is sedated or euthanized, although some scientists suggest that ticks may fall of a dead animal, reducing accuracy of the count.
Importantly, the authors caution, when opting for postmortem examinations and, indeed, all handling of animals, animal welfare, conservation, and ethical implications come into play
The type of removal tool, physical traits such as thickness of fur, condition of the animal, and how it behaves during counting are among the myriad factors that can shape the results of a count, according to the paper. The ecological community in which the ticks live and relationship of tick to host also must be considered.
An important variable is also the life-cycle stages of the ticks on a particular animal. All stages are easy to see when engorged with blood, Lydecker says, but the tiny nymphal and larval stages often require extremely sensitive methods to detect. The authors stress the need for more precise and meticulous examination. Ticks may use different niches on an animal’s body or, conversely, a single tick species can use several host species of different size, affecting detectability. Conclusions based on counts also may have to recognize that some individuals within a host population may attract more ticks than others, the authors suggest.
Commenting on the findings of his research review, Lydecker suggests that a standard shortcut to getting a tick count—focusing on the head and neck, particularly the ears—might deserve a second look in many cases. However, he has also found that, with some host species, ticks may congregate on other parts of the body, leading to an estimate that is off target. As an example of the best-of-both-worlds approach, the paper cites research in North America by Lyme disease ecologist Rick Ostfeld, Ph.D., on the blacklegged tick (Ixodes scapularis), which vectors the bacterium that infects people with the disease. It feeds when young on white-footed mice and as adult on deer. In this case, the method of counting ticks was scrupulously detailed and referenced to other areas of research. “Ostfeld and his collaborators worked to first identify where most of the ticks are found on white-footed mice and after this have focused on sampling the areas [of the mouse] that have the most ticks,” Lydecker says. “This is the proper way to go about counting ticks.”
When checking ticks on animals such as white-footed mice and whitetail deer, it’s far quicker to check the head and neck than going over the entire animal, says Scott Williams, Ph.D., a wildlife biologist at the Connecticut Agricultural Experiment Station in New Haven, who has been counting blacklegged ticks for years and is among the scientists Lydecker credits with doing counts “the proper way.” It’s a matter of minutes compared to three or four hours, minimum, to do an entire deer.
“It would take a good 30–45 minutes to count all the ticks on a mouse,” says Williams, “and the anesthetic we use only lasts about two minutes. With a quick count of the ears and head you can compare differences in tick burdens on mice at different locations and over years, as well as with those that have received different experimental treatments.”
In the long run, the paper suggests, “the goal of any study involving ticks and their hosts is critically important in determining the applicability and efficacy of the methods that have been chosen.” Studies of tickborne disease ecology require scrupulous attention to detail while those for routine pest surveillance can be less rigorous.
Whatever the goal of research, says Lydecker, he and his colleagues hope that their survey “will encourage tick researchers to think more about how they sample ticks.”
“Counting Ticks (Acari: Ixodida) on Hosts Is Complex: A Review and Comparison of Methods”
Journal of Medical Entomology
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.
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