University of East London (UEL) Professor Sally Cutler has refuted new figures just published in BMJ Open, which show an increase in Lyme disease in the UK as “likely to be an overestimation.”
Sally Cutler, who is Professor of Medical Microbiology at (UEL) said that the study, which says that the disease is more common in the UK than at first thought, uses methodology and inclusion of patients who were only “suspected” and “possible” Lyme disease cases. This meant that the numbers in the study were likely to be overestimated.
The BMJ Open study estimating the annual number of Lyme disease cases in the UK suggests that the infection may be three times more common than the current annual estimate.
Lyme disease has become the most common tick-borne infection in many parts of Europe and the US. The bacterial infection is spread to people via bites from infected ticks, and symptoms can include a circular red rash resembling a dartboard bullseye.
Researchers in the new report looked at a database of 8.4 million anonymised patient records, covering about eight per cent of the population of the UK. Among these, 4083 cases of Lyme disease were detected between 2001 and 2012.
They saw a steep rise in cases during this time, from 60 in 2001 up to 595 in 2012. Extrapolating this to the wider population would suggest that there were more than 7738 cases of Lyme disease across the country in 2012, which is far higher than the usual estimate of 2000 to 3000 cases a year.
“The report of Victoria Cairns and other experts claims there were 7738 cases of Lyme disease in the UK in 2012 (12.1/100,000) compared to the 1040 cases (1.84/100,000) which were previously recorded for England and Wales,” Professor Cutler explained. “This might cause alarm; however, it is important to note that the algorithm used to collect data in this new study is distinctly different to previous methods.
“This study reports a different way to calculate the number of cases of Lyme disease in the UK. Their results differ to previously reported rates that have been based on officially reported cases of Lyme disease that fulfill a rigid set of criteria and require positive blood tests for the disease to be counted, and this established method reports some 3000 cases per year.”
Lyme disease includes many non-specific signs that can often overlap with other conditions making diagnosis particularly challenging. In this study, three categories of patients are included: clinically diagnosed; treated suspect cases with a positive diagnostic test; and treated possible cases.
“Including patients who were only ‘suspect’ and ‘possible of Lyme disease cases’ in this study will result in over-estimation of cases,” Professor Cutler added. “This is emphasised by the particularly low percentage of cases with the characteristic Lyme disease rash (892/4083 or 21.8%).
“There is no mention in the study of whether clinical guidelines were used to establish cases of Lyme disease and no case definition is mentioned. The treated suspected category used support of diagnostic tests, however the detail of types of tests was not given. This is a major limitation as these tests are hugely variable with some being rigorous, whereas others are particularly error prone. They can be further complicated by the time during infection that they are used and whether the patient has received antibiotics.”
Professor Cutler also pointed out that the authors have also compared UK case numbers with Europe, but they do not support this by comparing how many ticks are infected in the UK with those infected in Europe. The infection rate of ticks in the UK is considerably less than that reported in much of mainland Europe, with an average of 1.9% of ticks waiting for a host being infected.
“In the UK we do seem to be seeing an increase in Lyme disease cases reported over time; this is not reflected by an increase in number of infected ticks, but the density of ticks (the amount of ticks in one place) over time has not been studied,” she said.
Professor Cutler added that the authors of the report discuss various factors that could account for the rise in Lyme disease cases, such as increasing public awareness of the disease and therefore more people visiting a health professional and being diagnosed. “If this accounts for these increases, we should start to see a plateau, but neither this study or reportable cases have shown this to date.”
Another alternative professor Cutler has suggested is climate change, which might enable ticks to remain active for longer, lengthening the time available for a tick to infect a human.
Professor Cutler concluded that this study was “reassuring in that suspect cases were receiving the benefit of doubt from their GPs and were being treated.”