Tick-killing chemicals may not be enough to reduce rates of Lyme disease and other tick-borne diseases on their own. The largest study of its kind on using tick control to prevent disease found killing ticks reduced the number and proportion of ticks carrying the main bacterium that causes Lyme disease (Borrelia burgdorferi), but did not lead to fewer reports of people getting sick.
Rates of Lyme disease in the US have increased several-fold in the past four decades to at least each year. The rise has spurred research on better treatments, including a Lyme disease vaccine close to completing clinical trials, as well as better ways to prevent infection from the ticks that transmit the disease.
Black-legged or deer ticks (Ixodes scapularis) can transmit bacteria that cause Lyme disease to humans through bites. The ticks become infected when they feed on the blood of infected animals, such as mice or small birds.
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A previous of an anti-tick spray on thousands of residential properties found the method reduced tick abundance by around 60 per cent but did not lower rates of disease.
at the Cary Institute of Ecosystem Studies in New York and his colleagues wanted to test if the methods might work better when applied together across entire neighbourhoods rather individual houses. 鈥淓cologically that makes a lot of sense,鈥� says at Western Connecticut State University, who wasn鈥檛 involved with the study. 鈥淎 mouse doesn鈥檛 stay in your backyard.鈥�
Over four years, the researchers tested two different tick control methods both separately and together across 24 neighbourhoods in Dutchess County, New York, with more than 2000 people and 849 pets participating in the trial.
They tested an anti-tick fungal spray called Met52, spraying all vegetation on properties and several metres inside surrounding forested areas. They also tested boxes that brush mice and other small mammals with an anti-tick chemical, setting the boxes 10 metres apart in places frequented by the animals.
Although the researchers found , and the spray reduced the proportion of ticks infected with the Lyme bacterium, there was no effect on the rate of Lyme disease or other tick-borne disease reported in people, compared with placebo controls. Rates of tick-borne disease reported in pets were lower by around half, however.
The findings add to the evidence that such tick-control methods, at least as currently used, don鈥檛 reliably protect people from disease, says Connally. Prevention might require reducing tick abundance even more, or for longer periods, for instance. It may also be that people are often infected by ticks outside their yards or neighbourhoods, which might help explain the differing results reported in people and pets.
Ostfeld says the results make him less confident in tick control but says it is premature to conclude such methods don鈥檛 work. 鈥淲e need better diagnostic tools and better therapies,鈥� he says. 鈥淵et think of how wonderful it would be if we could prevent those people from getting sick in the first place.鈥�
Connally says researchers are pursuing other ways to reduce disease, such as vaccinating deer against ticks or vaccinating other animal hosts of the Lyme bacterium.
Pathogens
