Medical Research: What are the main findings of the study that were just published in Ticks and Tick-borne Diseases?
Dr. Salkeld: The primary findings of this new study show that western black-legged ticks, which can transmit Lyme disease, are active throughout the year in Northwest California, making the threat of Lyme disease year-round phenomenon.
More specifically, my colleagues from California Department of Public Health Vector-borne Disease Section and University of California, Berkeley and I found that the activity of Western Black-legged ticks (Ixodes pacificus), which are the ticks most commonly known to carry Lyme disease (caused by Borrelia burgdorferi) in Northwest California, is largely predictable and year-round. In general, tick larvae (newly hatched immature ticks) are active April to June, and sometimes activity extends into October, while adult ticks are active from October to May. Nymphal ticks (the tick stage following larvae and preceding adults) are active from January to October but peak from April-June. This is important because nymphs are responsible for most Lyme disease infections.
Medical Research: Were any of the findings unexpected?
Dr. Salkeld: Yes. Based on these results, tick season in Northwestern California is longer than even we expected and quite different from patterns in the Northeast USA. While tick activity in the Northeast peaks in the warmer months, Northwest California sees tick activity year-round.
Furthermore, the host animals that most commonly carry Lyme disease in northwestern California – the western gray squirrel and dusky-footed wood rat – are also active throughout the year and often live longer than one year, so they are able to pass the infection to different tick life-stages. In contrast, in the Northeast USA, the main hosts (e.g., white-footed mice) often do not survive the winter, so transmission of the Lyme disease bacteria may be more reliant on tick survival.
Interestingly, the highest reported incidence of Lyme disease in humans in Northwest California correlate to the times when the nymphal I. pacificus (which are smaller than a poppy seed) are most active.
Medical Research: What should clinicians and patients take away from your report?
Dr. Salkeld: Clinicians and residents in the Bay Area need to be vigilant year-round about the threat of Lyme disease. Residents should do tick checks whenever they spent time outdoors – no matter what season it is. And, clinicians should always consider the possibility of Lyme disease if patients present with the appropriate symptoms. Symptoms of the first stage of Lyme disease include headaches, flu-like symptoms, joint pain, fatigue and sometimes a rash that has many different shapes, one of which may look like a bull’s-eye centered on the tick bite.
Medical Research: What recommendations do you have for future research as a result of this study?
Dr. Salkeld: We are continuing research into the ecology of ticks and their pathogens in California, supported by funding from the Bay Area Lyme Foundation. There is still a great deal that we do not yet know about tick activity and the diseases that ticks carry despite the fact that Lyme disease is one of the most common infectious diseases in the US: approximately 300,000 new cases of Lyme disease are diagnosed each year in the U.S., according to statistics released just last year by the Centers for Disease Control and Prevention (CDC).
Salkeld DJ1, Castro MB2, Bonilla D2, Kjemtrup A2, Kramer VL2, Lane RS3, Padgett KA2.
Ticks Tick Borne Dis. 2014 Aug 8. pii: S1877-959X(14)00127-7. doi: 10.1016/j.ttbdis.2014.05.002. [Epub ahead of print]