A study on the incidence of new cases and the demographics of Lyme disease patients in the UK has just been published.
Like the paper published by Cairns et al in the Summer of 2019, it provides evidence of increasing annual incidence of Lyme disease, and that the number of cases diagnosed in primary care is notably higher than the those officially confirmed via laboratory testing.
Though it does not suggest annual case numbers are as high as the study published by BMJ Open last year, there are differing factors of how the study was carried out which may explain this.
The study was funded by the NIHR’s (National Institute of Health Research) Health Protection Unit for Emerging and Zoonotic Infections.
What the study looked at
This epidemiological study used data from The Health Improvement Network (THIN), sourced from a database used by around 9% of UK GP surgeries, which represents around 11 million patients (around 6% of the population).
They looked at the number of Read codes – a coded thesaurus of clinical terms – that were specifically associated to Lyme disease related to Lyme disease that were recorded by primary care in the period between 1998 and 2016.
The codes they included formed a strict case definition for Lyme disease. These didn’t include conditions that can have other multiple causes, such as Bell’s Palsy, so as to remove the possibility of identifying false cases. The study authors acknowledge that choosing such strict definitions “will likely underestimate the number of cases and sensitivity may be lost, as cases of mixed non-specific clinical signs could be missed.”
The study also looked at factors such as sex, age, nation (England, Scotland, Wales, Nr. Ireland), and rural-urban status, though information on ethic minority was somewhat unreliable due to missing data.
Also, because there is a lack of evidence around how to record/agree/identify re-infection or long term infection of Lyme disease, the study didn’t include multiple coded mentions on one patient record. For this reason, they would have been counted as one incidence.
Results of the study
According to the data, cases of Lyme disease diagnosed in UK primary care rose from 1.77 per 100,000 people, to 4.89 per 100,000 people between 1st January 1998 and 31st December 2016.
In total, 3,725 unique patients were identified with a Read code for Lyme, suspected Lyme disease, or related conditions in this period.
As with most other studies, the highest incidence of new diagnoses were in the summer, peaking in July and August.
The results also indicated that as with previous studies, use and access to the countryside seems to be a driver of Lyme disease risk, and the socio-economic and rural-urban status of patients plays a factor. It’s thought that areas of low deprivation, which tend to be in more rural areas, is where residents have easier access to the outdoors. This leaves these people more at risk of tick bites and consequently, more at risk of developing Lyme disease.
Contrary to some other recent studies, there were no particular differences found in the number of cases between sexes.
Estimated UK case numbers versus lab confirmed case numbers
The authors of the study write that: “the mean annual ratio between primary care and laboratory confirmed incidence suggest that for every laboratory-confirmed case there are about two cases potentially identified within UK primary care practices.“
With lab confirmed cases in 2016 at around 1,136, this ratio could be used could put an estimate of around 3,408 cases for that year, to include those diagnosed in primary care, without testing.
Whilst this is an increase in number on the 2,000-3,000 total cases that Public Health England (PHE) estimates each year, it is not as large an increase in estimate for all UK annual cases as many (including Caudwell LymeCo) would argue.
Limitations of the study and points of interest
The study lists limitations that may have affected the data, with the largest being an “absence of knowledge about GP coding practices and changes in their coding behaviour“. They don’t know what symptoms are being attributed to Lyme disease by these GPs. Also, analysis of the free text narrative used by GPs in which they may write down their Lyme diagnosis, (but not use the code) was not used in the study, because gaining access to these is ethically challenging because of data anonymisation.
There is also the aforementioned issue around the strict case definitions/list of read codes that were used in this study, which may have led to some cases being missed.
Interestingly, the introduction of “suspected Lyme disease” and “suspected erythema migrans” read codes in 2014 saw an increase in their use, eventually overtaking confirmed codes of “Lyme disease” and “erythema migrans”. The ‘suspected’ codes were the most in 2016. This could suggest that GPs are still not confident, or still lack knowledge when it comes diagnosing Lyme disease.
When discussing the higher prevalence of cases in mainland Europe, the study stated that: “One possible explanation is lower awareness about Lyme disease in the general population and primary care in the UK, compared to the rest of Europe. This would result in fewer presentations to primary care, the potential for mis-diagnosis and a resultant underreporting of cases.“
It’s worth noting that this study only looks at cases diagnosed in primary care, and does not include diagnoses that may be made in secondary care (though these will be significantly lower).
It also doesn’t take into account those who may receive a false negative test (unless a GP agrees to treat as suspected), or those who may have Lyme disease and not even be aware of the cause of their symptoms. Therefore it’s our opinion that the true number of cases per annum will be a lot higher.
How this fits with research priorities
Though this research is in the area of epidemiology and we feel there are areas of research in greater need when it comes to Lyme disease, it does focus on one of the research recommendations within the NICE Guideline, which called for more robust epidemiological data.
With the study contributing towards this need, we hope that the focus for future funding can now be directed towards the research recommendations concerning better diagnostic testing and effective treatment, which will make a greater impact on patient suffering.
What the study does support is that cases of Lyme disease in the UK are on the rise, and are higher than government body estimates, which gives greater weight and importance to calls for more research into the disease.
Findings around how certain demographics and geographical locations are more heavily affected could also help to inform the need for targeted public health awareness campaigns and the need for increased training of health professionals in those areas.