Reporting Tick Bites to Public Health Agencies Helps ID Risk Areas for Lyme Disease
By Andrew Porterfield
Lyme disease is the most common vector-borne illness in the United States, amounting to between 20,000 and 35,000 reported cases every year. But not every state reports the disease; instead, nearly all cases come from 14 states, largely in the Northeast or Upper Midwest.
And now, Canada has started to report more cases. In the Province of Quebec alone, human cases have risen from two in 2008 to 174 in 2016. Public health officials are often challenged to find early signs of tick and disease infestation before the number of cases start increasing. The “gold standard” method of surveillance is “active,” which involves collecting tickets through dragging tick habitat with white flannel sampling equipment or capturing rodents and testing for tick presence. This method, however, is expensive, especially if it involves multiple sites and combining drag sampling and rodent capture.
Researchers at the University of Montreal have studied active surveillance as well as two other methods: “passive” methods that involve collecting ticks that are submitted voluntarily by veterinary clinics from pets and documenting reported human cases of Lyme disease that are submitted from medical clinics.
Lyme disease, named after the Connecticut town where it was first discovered, is spread by a bacterial spirochete, Borrelia burgdorferi, which is carried by ticks and some mammals. Typically, a blacklegged tick (Ixodes scapularis) will attach and feed on a mammal, often a rodent or deer. If that mammal is already carrying the pathogen, it can be transmitted to the tick. Or, conversely, an infected tick can transmit it to the host animal, which can then transmit it to other ticks. Then, when an infected tick bites a human, the human can become infected and contract the disease.
Marion Ripoche, a Ph.D. student at the University of Montreal, and her team discovered that passive surveillance was the most accurate method for predicting an emerging risk of Lyme disease. The team published its results today in the Journal of Medical Entomology.
The researchers used data collected in Quebec from 2009 to 2014 on I. scapularis collected through passive surveillance and tick nymphs collected through active methods (nymphs are far more prevalent and active, so make a better active collection target), as well as human disease reports.
For passive surveillance testing, the team used census subdivision data that had at least one passive tick report or human case (totaling 870 cases). To test active surveillance, they looked at census data and density of nymphs or infected nymphs, totaling 217 cases. They also collected human cases and compared active surveillance as a possible predictor of passive tick submissions.
Between 2009 and 2014, 6,261 I. scapularis larvae, nymphs, or adults were gathered through active surveillance. Of those, 13.4 percent were positive for B. burgdorferi. During the same period, passive surveillance data showed 1,702 people submitted a tick, mostly adults. About 12.9 percent were positive for B. burgdorferi. For human cases, 145 diseases were reported, 92 from local sites.
The study found that all three methods could signal some risk of a Lyme disease outbreak but that passive surveillance had the strongest relationship, comparing number of people bitten by a tick and the eventual number of human cases reported. The passive surveillance method could detect areas with at least three human cases and showed its usefulness as an early-detection system for public health officials.
“Public health authorities need an early signal for the timely detection of new at-risk areas,” the authors write. “This finding is significant in that it shows that the volume of passive tick submissions received from a municipality, which is the most geographically continuous signal of risk currently available, is able to successfully predict the emergence of human cases in that municipality.”
The researchers emphasize, however, that passive surveillance is good at early detection, but it should work in concert with the other two methods, not as a replacement. Both dragging and capture (active) and reported humn cases are essential for further determination of the extent of an infection, and successful public health agencies (such as in the Province of Quebec) will employ all three methods.
Journal of Medical Entomology
Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.