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Connecticut has traditionally been among the states with the most reported Lyme disease cases, despite having only about 1% of the United States’ population. But in recent years, Connecticut’s case rate has not increased as quickly as in other Northeast states.

The number of cases per 100,000 Connecticut residents declined fairly steadily from 2010 through 2021. But a big increase in 2022 and 2023 was still not as large as the increase in case rates in other Northeast states.

Connecticut’s Lyme disease rate has been below the Northeast average since 2016. In recent years, only New Jersey has had a lower rate.

The number of Lyme disease cases overall has remained somewhat steady in Connecticut compared to other Northeast states, with an average of 2,718 cases per year since 1995, according to the Department of Public Health.

The Northeast — all the New England states, along with New York, New Jersey and Pennsylvania — has remained the epicenter for Lyme disease. Every year, the Northeast accounts for about 70% of the country’s cases, according to a CT Mirror analysis of CDC data.

On an average year, almost 9% of Northeast cases came from CT.

Lyme disease is the most common tick-borne illness in both Connecticut and the United States, according to the state Department of Public Health. The disease was first reported in 1975. It is caused by a bacterial infection spread through the bite of infected blacklegged ticks, also known as deer ticks. Early symptoms include fever, fatigue, headache, and a distinct rash. If untreated, the disease can lead to more serious complications affecting joints, the heart and the nervous system, according to the CDC.

A recent study from Dartmouth found that in the Northeast, including Connecticut, New York, New Hampshire, Vermont, and Maine, half of all adult blacklegged or deer ticks carry the bacteria that cause Lyme disease. Up to 25% of nymph-stage ticks, which are harder to detect, also carry the bacteria.

Recent tick surveillance shows that deer ticks are by far the most common species collected in the state. New London County recorded the highest density of adult and nymphal blacklegged ticks. Meanwhile, Litchfield County showed the highest rate of adult female tick infection, and Fairfield County had the highest nymph infection rate.

Renata is the data reporter for CT Mirror. She recently graduated from Columbia University with a master’s degree in data journalism. For her undergraduate studies, she graduated cum laude from Pennsylvania State University with dual bachelor’s degrees in international politics and broadcast journalism, and minors in global security and Middle East studies. Renata has a background in data analysis and programming, with proficiency in Python, QGIS, and HTML, among other tools. She previously interned at the Malala Fund and has reported stories from Scotland, Ireland, Northern Ireland, and Brazil. She speaks four languages and is currently learning a fifth.