Eighty thousand reported cases, and an estimated 476,000 treated

More than 80,000 Lyme disease cases are reported to the CDC each year by
states and jurisdictions. **An estimate from insurance claims suggests 476,000
people are diagnosed and treated for Lyme disease each year.**

Both numbers are real, and neither is wrong. Understanding why they differ by
almost six times is the point of this page.

What the reported number is

Lyme disease became a nationally notifiable condition in 1991. State and
local health departments collect and verify case reports under their own legal
mandates and surveillance practices, classify them against the national
surveillance case definition developed by the **Council of State and
Territorial Epidemiologists**, and — with identifying information removed —
share selected case information with the CDC through the **National Notifiable
Diseases Surveillance System**.

That is a count of cases meeting a specific definition, reported through a
specific chain. It was never intended to be a count of everybody who has Lyme
disease.

Four things that limit it

• Underreporting and misclassification happen in every disease
  surveillance system. Surveillance does not capture every case, and some
  reported cases have another cause.
• Data is recorded by where people live, not where they caught it. A case
  in a city resident who was hiking elsewhere is counted at home.
• States and the CDC finalise yearly data at different times, so CDC
  counts may differ slightly from a state's own figures for the same year.
• The case definition has changed five times — in 1996, 2008, 2011, 2017
  and 2022 — and some of those changes affected how cases were counted. **Any
  trend line across those years is partly a record of the definition
  changing.**

Where the larger number comes from

Insurance claims data shows how the number of Lyme diagnoses changes over
time, and the estimate built from it is **476,000 people diagnosed and treated
each year**. The CDC notes this may be an overestimate, because billing
codes do not always reflect a confirmed diagnosis — a patient tested for Lyme
and treated presumptively can appear in claims without ever meeting the
surveillance definition.

So one number undercounts and the other probably overcounts, and the truth is
between them.

Two newer sources

• SubLyme — the Surveillance Based Lyme Disease Network, established in
  2023, strengthening surveillance and research using **electronic health
  record data**, in partnership with large healthcare systems in Maine,
  Massachusetts, Pennsylvania and Wisconsin.
• The Tick Bite Data Tracker — emergency department visits for tick bites
  by week, region, age and sex, drawn from the National Syndromic Surveillance
  Program. It measures exposure rather than disease, and can tell people in a
  given part of the country when their risk is rising, which a yearly case
  count cannot.

Source: Centers for Disease Control and Prevention.