Researchers find increase in Lyme disease mirrors drop in red fox numbers

A continued increase of Lyme disease in the United States, once linked to a recovering deer population, may instead be explained by a decline of the red fox, UC Santa Cruz researchers suggest in a new study. 

The team's findings, published in the Proceedings of the National Academy of Sciences, reveal that although deer populations have stabilized, Lyme disease has increased across the northeastern and midwestern United States over the past three decades. The increase
coincides with shrinking populations of the red fox, which feeds on small mammals, such as white-footed mice, short-tailed shrews, and Eastern chipmunks, all of which transmit Lyme disease bacteria to ticks.

Dwindling numbers of red foxes, the authors suggest, might be attributed to growing populations of coyotes, now top predators in some eastern regions where wolves and mountain lions are extinct.

"A new top predator has entered the northeast and has strong impact on the ecosystem," said Taal Levi, a recent UCSC Ph.D. graduate in environmental studies. Levi is the lead author of Deer, Predators, and the Emergence of Lyme Disease, published this week online.  Coyotes can and will kill foxes and more significantly, Levi said, "foxes often don't build dens when coyotes are around."

More significantly is that fewer coyotes will inhabit an area once populated by more foxes, Levi said. The greater number of foxes would have consumed a larger number of small, tick-bearing animals than the coyotes that replace them.

Levi and his UCSC co-authors, A. Marm Kilpatrick, assistant professor of ecology and evolutionary biology; Marc Mangel, distinguished professor in applied mathematics and statistics; and Chris Wilmers, assistant professor of environmental studies, used an extensive dataset from five states as well as mathematical models to determine why Lyme disease continues to rise despite stabilized numbers of deer, long known to act as reproductive hosts for adult ticks that carry Lyme disease bacteria.

The loss of red foxes can result in an increase in the abundance of the smaller animals that serve as hosts for bacteria-carrying ticks. Red foxes may have once kept those populations under control.

"We found that where there once was an abundance of red foxes there is now an abundance of coyotes," said Levi, who has just begun a position as a researcher at the Carey Institute for Ecosystems Studies in the hotspot for Lyme disease, Duchess County, north of New York City. There he works as a postdoctoral research fellow with Lyme disease expert Rick Ostfeld, who literally wrote the book on Lyme disease, Lyme Disease Ecology of a Complex System.

Lyme disease was first reported in Old Lyme, Conn. in 1975. Ticks pick up the bacteria when they bite infected mice and later infect other animals including humans. Levi said tick nymphs, about the size of a sesame seed, carry the bacteria and are so small that many people who contract Lyme disease never knew they were bitten.

Lyme disease can be treated with antibiotics if discovered early, however long term patients may suffer muscle and joint pain.