What's causing the surge in Lyme disease this year?

Pittsburgh Post-Gazette

PITTSBURGH If you become infected with Lyme disease this year, blame the acorn the bitter-tasting nut that falls from oak trees and looks to be wearing a knitted beret.

Don't blame deer. They now may be off the hook as the main vectors of the infectious disease.

Instead, most scientists and publications point to the expected overpopulation of white-footed mice this year as the reason why Lyme disease incidence is likely to surge in southwestern Pennsylvania and throughout the Northeast. So you might be tempted to blame mice that is, until you realize the full story.

That's where red oak acorns drop onto the scene.

Late last summer and early fall, red oaks, all in coordination, produced a mast production of acorns, which means six to 10 times more than they typically produce. The result was an early Thanksgiving feast for such "seed predators" as mice, chipmunks, shrews and deer, all well-documented Lyme-disease "reservoirs."

Mice, however, represent the most available target for the black-legged or deer tick and routinely serve as major carriers of the Lyme bacteria, Borrelia burgdorferi. A 2004 University of California, Irvine study, for example, showed that almost all mice become infected with Lyme bacteria "during a single transmission season" throughout the Northeast and Midwest.

Gary Witmer, a rodent expert with the U.S. Department of Wildlife Services, said the abundant food supply along with the warm winter weather, especially in February, will equate to more mice this spring. Their access to large quantities of high-quality food means they can breed earlier and have larger litters.

"One thing about deer mice (out West) and white-footed mice (here), they do well under human-disturbed environments cleared areas in urban and suburban settings," he said. "In a good reproduction year, young animals have to disperse and find new places to live, so there is a lot of expansion."

Unfortunately, he said, they like being around humans.

More mice mean more ticks, and the two exchange Lyme bacteria. The problem for people is the infected tick's propensity to crawl into the body's warm, moist places armpits, groin area or back often to remain undiscovered for the 24-hour infection cycle.

All of which circles back to acorns.

"When a mast year happens, a much greater than normal number of acorns are produced by oak trees. Other nut producing tree species such as hickory, also have mast years. This is an important food source for large and small mammals including mice and deer, " said Marc D. Abrams, a Penn State University professor of forestry. Research indicates that the number of mice and deer increase the year following abundant mast production."

Given the mast year for red oak acorns last year, he said, "You probably will see more ticks and a higher incidence of Lyme disease, as the theory goes."

This year, the warm February weather, more mice and the epidemic continuing to move westward through Western Pennsylvania, all portend bad days for Lyme disease.

"We already are seeing cases of Lyme disease, which is a bit unusual. Usually we see more in the late spring and summer," said Neel Shah, a UPMC infectious disease physician and clinical assistant professor at the University of Pittsburgh School of Medicine. "It's a little bit early."

Michelle Paulson, an infectious disease physician in the Allegheny Health Network, said she hasn't yet seen any Lyme cases this year. Typically, she said, primary care physicians are first to see such cases.

The classic symptom for infection is a bull's-eye rash but more commonly it is a round, red area expanding outward, Shah said. If the person is unaware of the rash, within a few weeks of the bite, he or she can develop fatigue and a fever. One round of antibiotics typically resolves the infection.

But if the infection goes undetected, the bacteria can infect the neurological system, the heart, or joints, respectively resulting in bacterial meningitis, a heart infection potentially causing an abnormal heartbeat or arthritis, Shah said. Once treated, the person can have symptoms for months, due to a persistent immune response.

"Fifty percent of people don't notice the rash," Shah said. "More and more patients are becoming savvy about what to look for, but a good number of patients are not fully aware that Lyme exists in this area of the country."

About 55 of every 100,000 Pennsylvanians become infected with Lyme disease, with infections having burgeoned by about 75 percent each year, from 2012 to 2015, according to the U.S. Centers for Disease Control and Prevention.

Forty-nine of 50 states, with Hawaii the exception, have reported Lyme infections in recent years. While the CDC reported about 30,000 confirmed infections nationwide in 2015, it also said cases are under-reported and potentially could be 10 times higher.

The real villain, of course, is the black-legged or deer tick, but acorns help assure that more carry the infection.

Energy in oak trees likely builds up over a period of years, leading to a massive release of acorns. It's random and unpredictable but an important strategy in helping oak trees to survive.

By putting out fewer acorns in most years, the trees keep acorn-consuming animal populations in check, including mice, said Abrams, who holds a Ph.D. in forestry. When oaks finally have a mast production year every three to five years the acorns stand a better chance of germinating with fewer "seed predators," to eat them.

In a mast year, acorn-eaters engorge themselves with enough acorns left over to germinate and produce seedlings. The oak succeeds in reproduction. "Think of a normal dinner vs. a Thanksgiving feast," Abrams said. "You eat all you want at Thanksgiving and there's always some left over. In a mast year, there are leftover acorns."

Other common species of oak trees white, black, scarlet and chestnut oak trees also live in the state, he said. So an oak species is likely to have a mast acorn production on any given year to feed both mice and the Lyme epidemic.

"This year it might be a mast year for the chestnut oak or white oak or one of the many hickory species," he said, noting that red oak are well represented in the northern and western tier of Pennsylvania, including the Pittsburgh region.

"There's a very high threat of Lyme disease pretty much every year now," he said. "There are several reasons why ticks have increased over the last 35 years climate change, the lack of fires, less spraying of pesticides such as DDT, and the increase in the number of deer and small mammals."

The percentage of ticks carrying Lyme disease and other diseases also seem to be increasing every year, creating the biggest danger for people working or recreating in the woods. "Everyone needs to be on the lookout for ticks and tick bites," Abrams said.

Another factor boosting the mouse population is the steady decline in animals that eat mice.

A noted 2012 research study by Taal Levi of the University of California, Santa Cruz found a decline in small-mammal (mouse) predators, which "can sharply increase Lyme disease risk."

"We then show that increases in Lyme disease in the northeastern and midwestern United States over the past three decades are frequently uncorrelated with deer abundance and instead coincide with a range-wide decline of a key small mammal predator, the red fox, likely due to expansion of coyote populations."

The Pennsylvania Game Commission says coyotes now could number 250,000 statewide, although the total is uncertain. But every county including Philadelphia has coyotes. In 2013 alone, 41,000 coyotes were killed statewide. While their impact on the deer population is unclear, coyotes do feed on or force out mouse predators, even if their own diets include mice and just about any other animal with four legs.

So, when all is considered, you can blame both acorn and coyote.

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