Where Lyme Disease is Rare, Can It Still Be Diagnosed Accurately?
By Andrew Porterfield
An old saying in medical schools goes like this: “When you hear hoofbeats, don’t assume zebras.”
But what if the hooves do belong to an unusual species? For Lyme disease, caused by the bacterium Borrelia burgdorferi and transmitted by blacklegged ticks (Ixodes scapularis) and western blacklegged ticks (Ixodes pacificus), this question can be important, because as the disease spreads it will likely become more prevalent in areas where doctors aren’t used to seeing it.
That, in turn, presents another question: How well can doctors in low-incidence areas diagnose Lyme disease? To find out, a team of researchers from the University of California, Davis, and the California Department of Public Health surveyed physicians in that low-incidence state to determine their knowledge, attitudes, and practices regarding Lyme disease. Their results, published in September in the Journal of Medical Entomology, show that physicians in California are knowledgeable about the disease and its diagnosis and treatment. However, physicians in the state fall a little short on test-ordering practices, interpreting test results, and awareness of California’s changing disease landscape.
Lyme disease in California is unusual. About 0.2 cases per 100,000 people are confirmed each year, which is very low compared to the highest incidence rate in the U.S., which is Maine, at 121.2 cases per 100,000, according to the Centers for Disease Control and Prevention. The California number, however, is not zero.
And California actually has a wide variation in risk among different parts of the state. The incidence rates in high-risk areas range from 1.1 to 6.2 cases per 100,000. In fact, B. burgdorferei has been recovered from western blacklegged ticks in 44 of California’s 58 counties.
Lyme disease rates are generally low in California because while the western fence lizard (Scleloporous occidentalis) may act as a host for the tick’s immature stages, its blood contains a protein that kills B. burgdorferi in ticks that feed on the lizard, the researchers explain. Moreover, in southern California, the risk of being bitten by infected ticks is very low.
To find out what doctors know about Lyme disease and how they respond to it, the researchers sent an electronic questionnaire to 3,488 practicing physicians in 16 low- and high-risk counties, as well as physicians practicing at the UC Davis Medical Center, the UC San Francisco Medical Center, Stanford Medical School, Palo Alto Medical Foundation, and Dominican Hospital. Sixty-two physicians responded.
In counties where Lyme is endemic, 18 physicians (29 percent) reported that it was not, and two (3.2 percent) responded that they did not know. Forty-nine (79 percent) reported no increases in Lyme disease patients in the past year, while 42 (67.7 percent) reported patients who asked for treatment even though the physician did not think their symptoms were from Lyme disease.
On the survey questions, scores ranged between 20 and 100 percent. Four physicians answered all 10 questions correctly. The average score was 71.9 percent. On the testing questions, 60 percent correctly chose the laboratory serological tests for diagnosis of Lyme disease. Meanwhile, 75.8 percent correctly responded that patients from non-endemic areas with ongoing symptoms for more than two years were not likely to have Lyme disease. Current recommended serological testing is a conventional two-tiered testing enzyme immunoassay (first tier) followed by a specific western blot, or a modified two-tier test involving two enzyme immunoassays in sequence. Overall, the researchers note that “physicians in this study deviated from national guidelines on diagnostic testing for Lyme disease when patients sought care for symptomatic disease and asymptomatic tick bites.”
“The challenge for physicians is determining appropriate testing when a patient has symptoms compatible with Lyme disease to minimize potential for false positive or false negative test results,” the authors write. “These testing complexities, coupled with low overall exposure risk in a low-incidence state, can make diagnosis of Lyme disease more complicated.”
“Physicians in California could benefit from targeted education to improve test-ordering practices and test result interpretation,” the researchers add. “Additionally, increased education on Lyme disease geographic and seasonal patterns, as well as on characteristics of the diverse disease ecology across California, could be beneficial to physicians for patient evaluation.”
Journal of Medical Entomology
Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.