Maine experienced a dramatic drop-off in reported Lyme disease cases in 2018, and the dry weather this past summer may have played a role.

The state recorded 1,310 Lyme cases through Dec. 27, according to the Maine Center for Disease Control and Prevention, down from 1,852 in 2017. That represents a 29 percent decrease, although the final number of cases may rise slightly because of a lag in reporting.

Lyme is an infectious disease caused by the deer tick, which has expanded its range in Maine over the past 20 years. Lyme disease cases had climbed every year in Maine since 2011 – except for 2015 – before falling off in 2018.

The 1,852 Lyme cases in 2017 represented an all-time high, and the tick-borne illness has persisted in Maine as a public health threat, as there have been more than 1,000 reported cases per year since 2011. Lyme disease can cause flu-like symptoms, swelling, neurological problems like Bell’s palsy and joint pain.

Some, but not all, people get a bull’s-eye rash where they were bitten by a tick. If caught early, Lyme disease can be treated with antibiotics, but untreated it can cause long-term health problems, especially in the joints and nervous system, according to the U.S. Centers for Disease Control and Prevention.

Anaplasmosis, another disease caused by the deer tick, also experienced a sharp decline in 2018, from 663 cases in 2017 to 476 cases through Dec. 27 this year.

Researchers are still learning about what causes fluctuations in deer tick populations and interactions with humans. In addition to Lyme cases being down, there were fewer dead tick submissions to the University of Maine Cooperative Extension, and fewer ticks showing up in field surveys conducted by Maine Medical Center Research Institute staff.

“The big challenge for tick survival may not be the winter, but may be the summer,” said Susan Elias, a research associate at the Maine Medical Center Research Institute in Scarborough, which studies the arachnids and Lyme disease.

Rainfall was below average this summer, while high temperatures in July and August were higher than average, according to the National Weather Service in Gray.

Elias said researchers have been examining how the deer tick survives in its southern range, such as Georgia or South Carolina. In the South, the ticks can survive in the hot weather, but they stay below leaf litter and do not seek humans as hosts as frequently, instead preferring low-traveling animals like lizards, she said.

“Ticks are like Goldilocks. It can’t be too hot or too cold or too dry. It has to be just right,” Elias said. “But they are experts at finding the right microclimate for them, so if it’s too hot and dry, for instance, they will retreat under the leaf litter.”

Still, if the weather is hot and dry enough for an extended period, that could lead to reduced tick populations, researchers say.

Researchers will compare Maine’s dry summer of 2016 with this year’s summer, looking at humidity and temperature patterns, to see if there’s any correlation with tick activity. In 2016, Lyme cases were down in the summer but came back stronger in the fall, and Maine ended up with 1,464 cases that year.

This year, Lyme cases started strong in the spring and early summer but fell off in the late summer, compared to previous years, and did not rebound in the fall, as they did in 2016.

Extreme cold temperatures without snow can also kill ticks, but the temperatures have to be consistently below or near 0 degrees Fahrenheit for weeks before researchers believe tick populations would be significantly affected. Snow acts as an insulator and helps ticks survive the winter.

Griffin Dill, integrated pest management professional at the University of Maine Cooperative Extension, said it’s difficult to tell whether 2017 was an aberration or whether 2018 represents a one-year drop-off before Lyme cases climb again.

Dill said there were 20 percent fewer tick identification submissions to the University of Maine’s “tick lab” this year, with a steeper decline in the fall. People who are bitten by ticks can submit them to the lab for identification.

“Fall is usually a very active time for ticks, but not this year,” Dill said. “There also may not have been as many opportunities for tick-human interactions.”

Dill said the snow and colder temperatures that arrived in November probably reduced tick activity at a time when people were still outside. For instance, hunters may have come across fewer active ticks.

“In those conditions, there’s not going to be a lot of tick activity,” he said.

Researchers are also studying how Lyme may correlate with the rodent population. Maine had an abundance of squirrels in 2018, probably caused by a higher-than-normal acorn crop in 2017. The theory is that the increased rodent population this year will result in more ticks next year, but Dill said he’s skeptical of that, and more research is needed.