Babesiosis Rising, 2011–2019: Maine, New Hampshire and Vermont Join the Endemic States

Babesiosis is a tickborne disease caused by intraerythrocytic Babesia parasites. In the United States most cases come from Babesia microti, spread by blacklegged ticks (Ixodes scapularis) in the Northeast and Midwest. It can also spread through blood transfusions, organ transplants and from mother to child.

• Illness ranges from none or mild to severe and fatal, especially in people who are immunocompromised or asplenic. Common symptoms are fever, muscle and joint pain and headache; severe cases can bring thrombocytopenia, kidney failure and acute respiratory distress syndrome.
• Treatment: a combination of antimicrobial drugs, such as azithromycin and atovaquone.
• History: the first U.S.-acquired human case was found on Nantucket Island, Massachusetts, in 1969; babesiosis became nationally notifiable in 2011.

U.S. tickborne disease cases overall rose 25%, from 40,795 in 2011 to 50,856 in 2019.

The analysis

CDC looked at 2011–2019 data from the 10 states that reported throughout and had at least 10 cases a year for two or more consecutive years: Connecticut, Maine, Massachusetts, Minnesota, New Hampshire, New Jersey, New York, Rhode Island, Vermont and Wisconsin.

• 16,456 cases were reported by 37 states — 16,174 (98.2%) from these ten.
• Most cases: New York (4,738; an average of 526.4 a year), Massachusetts (4,136) and Connecticut (2,200). Fewest: Vermont (114) and New Hampshire (340).
• Highest incidence: Rhode Island, 18.0 per 100,000 in 2015, and Maine, 10.3 in 2019.

Babesiosis cases in the ten states analyzed, 2011–2019. Image from the Centers for Disease Control and Prevention

Where it grew

Incidence rose significantly in eight states — Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island and Vermont — and held steady in Minnesota and Wisconsin.

Three new endemic states

Until now, CDC counted babesiosis as endemic in seven states: Connecticut, Massachusetts, Minnesota, New Jersey, New York, Rhode Island and Wisconsin. Maine, New Hampshire and Vermont have found new cases every year, report counts similar to or higher than the endemic states, show significantly rising incidence, and have B. microti in their ticks — so CDC now considers babesiosis endemic there too. Small numbers of cases have also turned up where it was rare or unknown, including Manitoba and Ontario in Canada and Delaware, Illinois, Iowa, Maryland, North Dakota, Ohio, South Dakota, Virginia and West Virginia.

The blood supply

People infected through transfusion fare much worse and are more likely to die than those infected by ticks. Since 2019, the Food and Drug Administration has recommended screening blood donations in 14 states and the District of Columbia where residents are at high risk — including Maine, New Hampshire and Vermont. Watching risk in states bordering endemic areas will keep that policy current.

Limitations

1. Babesiosis isn't reportable everywhere — for example, not in Pennsylvania, despite documented transmission there.
2. Many cases go uncounted; people with vague symptoms may not be tested.
3. Cases are counted by home state, not necessarily where infection happened.

What to do

• Clinicians in endemic and neighboring states should know babesiosis's signs and risk factors — especially since other tickborne diseases look similar and share the same places and risks.
• Travelers to these states should know the risk.
• Anyone outdoors in endemic states should prevent tick bites: long pants, avoiding underbrush and long grass, and tick repellent.

Sources

Based on "Trends in Reported Babesiosis Cases — United States, 2011–2019," by Megan Swanson, Amy Pickrel, John Williamson and Susan Montgomery, Morbidity and Mortality Weekly Report 72(11), corrected version, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The source gives New Hampshire's 2019 cases as both 78 and 63, and Massachusetts's peak incidence as both 9.1 and 9.2, so those figures are left out.