Ticks and Lyme Disease in Contra Costa County 2006

2006 Annual Report, Contra Costa County Mosquito and Vector Control District

Lyme disease is a bacterial infection transmitted by the Western black-legged tick (Ixodes pacificus), also known as the deer tick. While Lyme disease is rare in Contra Costa County (on average there are two to four human cases reported per year), it can cause serious complications if not treated promptly. The District monitors the risk of Lyme disease by collecting and testing black-legged ticks from several locations that we have been monitoring for as long as 12 years. These include Bollinger Canyon Rd., San Ramon; Springhill Rd., Lafayette; and the Bear Creek area of Briones Regional Park. The adult Western black-legged tick population was below average for most of the season, except during the last half of March where counts slightly exceeded the average. An unusually high number of days with rainfall during the collecting season limited the number of possible collection days; thus reducing the numbers of ticks collected. The graph below shows the average number of ticks that were collected from these locations.

On average, only one or two in a hundred black-legged ticks test positive for Lyme disease, although we have found a few locations where the rate is higher, and these tend to change over time. In 2006, we collected and tested 132 ticks from three locations; the highest infection rate was in the Bollinger Canyon area in San Ramon where three out of 58 ticks tested positive (just over 5 percent).

We also identify and test ticks brought in by members of the public who have been bitten. If the ticks are reasonably intact and not dried out, we can test them in our own laboratory free of charge. If they are in poor condition or dead, a more sensitive test is required and county residents have the option of allowing us to send the tick to a private laboratory for a fee of $60 (our cost). Out of 54 ticks tested for residents in 2006, only one was positive for Lyme bacteria (just below 2 percent).