July 30, 2019

expert reaction to study estimating the annual number of Lyme disease cases in the UK

Research, published in BMJ Open, reports that the number of tick borne Lyme disease cases in the UK may be three times higher than expected.

Prof Sally Cutler, Professor of Medical Microbiology, University of East London (UEL), said:

“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, this established method reports some 3000 cases per year. It is well recognised that these are demanding criteria and will result in an under-estimation of Lyme disease cases, this method is particular likely to miss early stage cases where patients have erythema migrans (an expanding rash often seen in the early stage of Lyme disease). It is believed that more than 60% of cases of Lyme disease will have this characteristic rash, and these patients will be managed promptly with appropriate antimicrobial therapy by GPs, and subsequently recover. Estimates of missed cases vary but they might amount to some 2000 cases per year.

“The report of Cairns et al. 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. 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. 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. 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. The authors do discuss these limitations, but the discussion is more angled to support the last patient category of treated possible cases that had ambiguous test results. There was no mention as to whether these patients responded favourably after their treatment. If the diagnosis was correct, the patients should have reported clearing up of their symptoms, but this information was not provided.

“Given this method, I would advise caution when interpreting the number of Lyme disease cases provided in this study as they are likely to be an overestimation. Interestingly, the numbers of patients in the clinically diagnosed category is similar to the official figures for Lyme disease in the UK.

“The authors also compare 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. Even if the tick is infected, it would need to remain attached for some days to successfully transmit the infection to a person. Furthermore, Lyme disease risk is further reduced in the UK by about half of the infected ticks carrying a relatively poor strain for causing disease in humans. These facts have been known for some time and play a role in the reduced cases seen in the UK (see missing Lyme disease patients, Lancet 1991).

“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. The authors 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. Alternatively, climate change might enable ticks to remain active for longer, lengthening the time available for a tick to infect a human. For such reasons, continued observation is a priority, though personally I favour use of more rigorous data than that used in this study for such purposes, despite its limitations and potential for underestimation. This study was reassuring in that suspect cases were receiving benefit of doubt from their GPs and were being treated.”

‘Incidence of Lyme disease in the UK: a population-based cohort study’ by Cairns et al. was published in BMJ Open at 23:30 UK time on Tuesday 30 July. 

DOI: 10.1136/bmjopen2018-025916

Declared interests

Prof Sally Cutler: “I do not have any conflicts of interest to declare.”