Count me in

In order to be counted as a Lyme disease case by public health, very specific parameters must be met. These vary between countries and over time, but in general, the parameters are strict. Public health agencies don’t want to risk counting cases that might not be true cases. 

This also means that there are cases of Lyme disease that aren’t counted. The question is, how many? 

Reported cases don’t tell the whole story

In the US there were 63,000 reported cases in 2022. The CDC also relies on other reporting methods, and estimates that 479,000 people are diagnosed with Lyme disease per year.¹ That’s a big difference.

In Canada, there were 2,525 reported cases in 2022.² Taking into account cross border US estimates, testing considerations and dog and tick surveillance, Lloyd and Hawkins estimated a minimum 10 fold under-estimation of cases in Canada.³

In response, a team of Public Health researchers reviewed the accuracy of case reporting and concluded that “as for other reportable disease surveillance programs, under-reporting of LD occurs in Canada, and in part the surveillance is designed to discover this. However, tick surveillance data, combined surveillance data, combined with evidence from serological studies in dogs, do not suggest high levels of under-reporting in Canada”.⁴

What isn’t seen can’t be measured

Why is this a problem? Case reporting is a measuring tool.⁵ It relies on risk maps, tests, and multiple human variables which limit its accuracy. Some of those variables are listed below. 

There are many reasons that cases are not recognized, diagnosed, reported and counted.

The other problem with this tool though, is how it is used; how the numbers are interpreted and applied, not only by policy makers, but also by patients, healthcare providers and all Canadians who want to know if they should be concerned. 

What are the odds?

When a patient says to their doctor “I think I might have Lyme disease,” their doctor may look at publications that report those numbers, and deduct that the likelihood of their patient having the disease is quite low. In Canada for example, 2,525 in 33 million is a very low probability. A small fraction of 1%. Unfortunately, those numbers don’t tell the whole story. Many people are not counted.

How cases are missed

There are likely a lot of people in Canada and worldwide with unrecognized, undiagnosed or unreported Lyme disease that are excluded from case reports. Here are some of the reasons why.

People may:

• Not know the symptoms of Lyme disease.
• Assume they will notice a tick bite or have a “bullseye” rash.
• Under-estimate tick attachment time. Bites are often painless.
• Mistake the symptoms for other health problems. It can be confusing.
• Believe the risk is very low, and that they’re not likely to have it.
• Have symptoms but don’t go to the doctor.
• Not have a doctor.
• Go to their doctor, but do not receive a diagnosis.
• Receive a negative test result but may still have the disease. It does happen. 
• Receive a clinical diagnosis of Lyme disease which doesn’t count as a case, unless the patient had a specific rash and lives in a specific area.

Their doctor may:

• Not recognize the symptoms of Lyme disease. There are many.
• Assume their patient would have noticed a tick bite or had a “bullseye” rash.
• Attribute symptoms to other diseases. It can be confusing.
• Believe the risk is very low, and discount a diagnosis of Lyme disease.
• Rule out Lyme disease if estimated tick attachment is less than 24-36 hours.
• Not have examined their patient for signs of a rash.
• Not order a Lyme test when it might have been appropriate.
• See a negative test result and rule out Lyme disease altogether.
• Have concerns about conflicting information and the implications of diagnosing and treating Lyme disease.
• Make a clinical diagnosis of Lyme disease which won’t count as a case, unless the patient had a specific rash and lives in a specific area.

Lyme disease can be challenging to manage for patients and for their healthcare providers.

Before a Lyme disease case is reported, it first has to be recognized and considered by a patient and their doctor. It must then be diagnosed. Even then, not all diagnosed cases meet strict case reporting criteria.

Considering all of the barriers along this pathway, it’s neither realistic nor helpful to rely on case reporting numbers as an accurate indicator of the risk of contracting Lyme disease. It could reduce a patient’s chance of receiving a diagnosis and minimize the perception of risk for everyone else. 

Lyme disease can be complicated

Looking back at the quote above, surveillance for Lyme disease is equated with “other reportable disease surveillance programs”. Lyme disease is not like other reportable diseases. That’s becoming very clear thanks to emerging research and reports from around the globe.⁶ If most people were easily diagnosed and cured, we wouldn’t be hearing about it in the news. 

Case reporting is an important public health tool but it can be counterproductive when it’s misunderstood, and even harmful if clinicians use case reporting criteria to dismiss a diagnosis of Lyme disease. The Public Health Agency of Canada points out that “these are definitions for surveillance and epidemiologic purposes only, they are not clinical case definitions.” 

Unfortunately, a lot of people are likely still being “counted out” by not being recognised, diagnosed and reported as cases of Lyme disease in Canada. The question remains: how many?

1. “Human case reports and case estimates in the US in 2022” retrieved June 12, 2024.
2. “Human case reports of Lyme disease in Canada in 2022” retrieved June 12, 2024.
3. “Under-Detection of Lyme Disease in Canada” retrieved June 12, 2024.
4. “What is the real number of Lyme disease cases in Canada?” retrieved June 12, 2024.
5. “Canadian Lyme Disease Case Definition 2016” retrieved June 12, 2024.
6. Provincial, federal, US and international working groups have been established to address the challenges in diagnosing, treating and understanding Lyme and other tick-borne diseases. Reports confirm that these challenges are significant, complex and ongoing.