It affects about 250 to 1,200 people a year in the United States and usually occurs from April until September, but it can occur anytime during the year where weather is warm. The mid-Atlantic and southeastern states are most affected. According to the Centers for Disease Control and Prevention, the people most at risk include:
- Children 15 and under, particularly children between 5 and 9 years old
- Children who are frequently around dogs
- Children who live near wooded areas or areas with high grass