If you thought it made sense to decrease disease-carrying ticks in your area by removing the deer that harbor ticks, Sarah Perkins has some news for you. Perkins, a postdoc in the Center for Infectious Disease Dynamics at Pennsylvania State University, recently looked at studies in which researchers removed deer from large areas, called deer exclosures, using deer-proof fencing of various kinds. The studies tended to find fewer ticks at the nymph stage, which is when they can transmit tick-borne encephalitis (TBE) virus and other diseases. But when they removed deer from smaller areas, they found more nymphs.
To try to understand that apparent paradox, she looked at transmission of the virus. Adult ticks lay thousands of eggs on blades of grass. The larvae and nymphs are lower to the ground, and tend to find mice to feed on, while adults typically feed on deer. Larvae, which are naïve to TBE, co-feed (see image). A nymph tick will feed in one spot, and a cluster larva finds a spot nearby. Langerhans cells are attracted to the original bite site and then carry the virus to the site where the larval tick is feeding. "It's absolutely bizarre," says Perkins. This unusual method of transmission was first demonstrated in Thogoto virus, another arbovirus, by Linda Jones and colleagues ( Science , 237:775-7, 1987).
The paradox, then, made sense, because deer are "a dead end for the pathogen," says Perkins. In large exclosures, the absence of deer reduces the overall population of ticks effectively. However, there seemed to be an inflection point at about 2.5 hectares. "As the size of the deer exclosures got smaller, there was a flip," she says. Excluding deer from smaller areas perhaps meant that ticks were transferred from deer to rodents outside the exclosure and then imported into it.
Perkins wanted to test the hypothesis that small exclosures somehow fostered nymph growth. She was able to make use of .64-hectare exclosures that had been created 16 years ago by the forest service in the province of Trentino, Italy. The exclosures are 2 meters high, so they exclude deer but not rodents and other small animals.
The researchers at the facility - a converted Austro-Hungarian hospital 1,500 meters up a mountain - get three waiter-served meals a day, and of course fine Italian wine. The province supports the research, she says, because of interest in anything that can infect tourists. The ticks like to bite mushroom hunters, who ply the same fields as the researchers. "They always ask us if we've seen any porcinis," says Perkins.
When they trapped rodents - in this case yellow-necked mice - in and out of the exclosures, Perkins' team found that the mice had more nymphs in the small exclosures. They also found that 8% of mice in the exclosures had antibodies to TBE, compared to none outside the exclosures. "We created a TBE hotspot by removing deer," she says, and even found a few percent of the rodents feeding adult ticks, which would ordinarily feed on deer.
The team published their meta-analysis and field results in the August 2006 issue of Ecology (87:1981-6). Their next step is to work with colleagues who have radio-tagged deer to track areas they naturally avoid, and try to determine whether such areas will become TBE hotspots. If the experimental results are replicated, there could be "a direct correlation to what we might expect to see with Lyme disease," says Perkins. TBE and Borrelia burgdorferi have the same life cycle, and Paramiscus species, the mice common in North America, are similar to yellow-necked mice.
Howard Ginsberg, an ecologist at the University of Rhode Island and the USGS Patuxent Research Center, agrees with the paper's conclusions on exclosures larger than 2.5 hectares, but he has some reservations about the data on smaller ones. The effects in small areas may be idiosyncratic, he says, because of magnified effects of weather, for example. Still, "it's an interesting paper and the difference in dynamics is definitely a worthwhile area of study," says Ginsberg, one of whose papers on the subject was included in the team's meta-analysis.
Perkins hasn't had much reaction to the study yet, although La Republica, the Italian newspaper, had Eva Orlowsky, an ex-porn star who campaigns for saving deer, agreeing with the study. Other than that, she had a few E-mails from people who say they're going to keep deer out of their gardens. She doesn't agree, but she just shrugs. "I say, OK then."
Correction (posted January 26, 2007): When originally posted, this article misidentified a rodent as yellow-tailed mouse instead of yellow-necked mouse, and described the deer now being studied as "radiolabeled" instead of ?radio-tagged.? The Scientist regrets the errors.