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Lyme disease incidence three times higher in UK than previously thought, study suggests
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The incidence of Lyme disease in the United Kingdom is around three times higher than previously estimated, findings from a retrospective cohort study published in BMJ Open suggest.
“Lyme disease is not as widely known in the U.K. as it is in the United States, but it is far more common in the U.K. than previously thought,” Victoria Cairns, PhD, a consultant statistician from Oxford, England, told Infectious Disease News.
Cairns and colleagues note that Lyme disease has become the most common tick-borne infection in many parts of Europe and in the U.S. According to the CDC, study findings suggest that there are approximately 300,000 cases of Lyme disease diagnosed in the U.S. each year, and WHO estimates that there are 85,000 annual cases in Europe — an estimate that varies widely between and within countries, the researchers wrote.
The official estimate for the U.K. according to the British Infection Association is between 2,000 and 3,000 news cases per year, Cairns and colleagues wrote. They noted that, as in the U.S., “numbers based on centralized reporting are likely to be considerable underestimates” in the U.K. Further, incomplete recording of antibiotic use by specialists and in-hospital may also exacerbate underestimated cases, the researchers wrote.
For their study, they extracted data from the Clinical Practice Research Datalink, a primary care database that covers approximately 8% of the U.K. population across 658 primary care practices. The cohort of 8.4 million patients was registered with general practitioners between January 2001 and December 2012. The researchers identified patients with Lyme disease using an algorithm based on medical codes for Lyme disease, erythema chronicum migrans, laboratory tests and anonymized medical notes to reflect the difficulty in diagnosing the disease. They divided patients into three diagnostic categories — clinically diagnosed Lyme disease, treated suspected Lyme disease and treated possible Lyme disease. They calculated annual incidence rates and the estimated total number of Lyme disease cases separately for each U.K. region.
According to the study, the number of cases increased rapidly between 2001 and 2012. The trend culminated in an estimated incidence rate of 12.1 (95% CI, 11.1-13.2) per 100,000 individuals per year, with a U.K. total of 7,738 Lyme disease cases in 2012. The disease was detected in every region of the U.K., with Scotland reporting the highest incidence rates and most cases, followed by South West and South England. The researchers projected that if the trend continues, the number of cases may surpass 8,000 by the end of the year.
“The general public should become aware of the magnitude of Lyme disease so that they are ready to take preventive measures,” Cairns said. – by Joe Gramigna
Disclosures: Cairns reports no relevant financial disclosures. Please see the study for all other authors’ relevant financial disclosures.
Perspective
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Eugene Shapiro, MD
It is well-recognized that "reportable" diseases in the U.S. are underreported because it is a passive surveillance system that depends on voluntary reports by health care providers. This system is particularly useful for monitoring trends in the frequency of disease over time because it is presumed that the proportion of underreporting is relatively constant. In the U.K., surveillance for Lyme disease is laboratory based. This is a poor method for estimating incidence. In up to 90% of cases, Lyme disease manifests as either single or multiple erythema migrans, and most of these cases have negative antibody results for Lyme disease. In addition, the positive predictive value of an antibody test result is highly dependent on the prior probability of Lyme disease. If the prior probability is low, the vast majority of positive antibody results will be falsely positive. In the U.S., the number of Lyme antibody tests ordered exceeds the number of cases of disease by a very large magnitude (by 100-fold or more if one considers that most patients with erythema migrans are not tested), and numerous studies have documented high rates of false-positive antibody results in patients with no clinical evidence of Lyme disease.
The authors of this study concluded that the incidence of Lyme disease in the U.K. was three times higher than had previously been estimated. I have a problem with the case definition of Lyme disease used by the authors of this study. Anyone who had a laboratory test for Lyme disease and was treated with antibiotics on the same day (regardless of whether the result was positive) was classified as having Lyme disease. Many more people are treated for Lyme disease than actually have Lyme disease. Indeed, only a minority (about a third) of the patients classified as having Lyme disease in this study had what the authors defined as "clinically diagnosed Lyme disease." Thus, although it is likely that government estimates of the incidence of Lyme disease in the U.K. are underestimates, the methods used in this study to determine the incidence of Lyme disease are highly suspect. If there is interest in determining the true incidence of Lyme disease in the U.K., better methods of surveillance should be implemented.
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