Visiting Veterinarian

Tracking the pesky tick

By Michele Gerhard Jasny V.M.D. - April 13, 2006

If I had a cracker barrel and a checkerboard in my waiting room, this is the conversation you would hear repeatedly every spring. "It was an awful mild (cold, wet, dry, snowy) winter," remarks checkers player Number One. "That means we'll have more (less) ticks this year." "Naw," drawls player Number Two. "I heard that there are less (more) deer (mice, rabbits) because of the cold (snow, hunters, traffic) so it should be a light (horrible) tick season.... King me."

The annual speculation about the impending tick season highlights Vineyarders' understandable preoccupation with those ubiquitous little bloodsuckers with whom we share our Island. How does the weather affect our tick population? It's a complex story that depends on many factors, starting with the tick's life cycle. Let's focus on Ixodes scapularis, the critter we call the "deer tick," the main transmitter of Lyme disease in our neck of the woods.

In the northeastern United States, the life cycle of the deer tick takes two years to complete. It begins with adult ticks laying eggs in the springtime. These eggs hatch into larvae in the spring/summer. The larvae feed on small mammals, especially the white-footed mouse, although they also find squirrels, voles and shrews tasty. If Mr. White-footed Mouse is carrying Borrelia burgdorferi, the agent that causes Lyme disease, then Mr. Tick Larva gets infected with Lyme while dining on rodent blood. After eating, Mr. Tick Larva eventually goes dormant for the winter. In the spring of the second year, the larvae wake up and molt into nymphs. Hungry nymphs. These guys need another blood meal and have a more eclectic palate, feeding on mice, squirrels, chipmunks, raccoons, opossums, shrews, cat, dogs...and people. The majority of human Lyme infections in our area are thought to be transmitted by the nymph stage. After feeding, the nymphs molt to adult ticks in the fall, then seek out another host to feed on, preferably a big mammal like a deer. Adults that survive the winter and succeed in finding another blood meal lay their eggs in the springtime, starting the cycle all over again. If Mr. Tick becomes infected with Lyme disease at any stage of his life cycle, the infection will persist from molt to molt. In other words, an infected larva becomes an infected nymph becomes an infected adult.

Cause and effect?
What environmental factors impact the annual variation in the number of ticks that leap from the beach grass onto our ankles and our dogs? And how do we determine cause and effect? Many different correlations between climate, tick numbers, habitat, and incidence of Lyme disease in people have been proposed. For example, in one study it was found that a high June moisture index resulted in a higher number of early summer cases of human Lyme disease... two years later. The theory is that Mr. Tick likes a nice moist cozy place to spend his time, especially in the dormant phases. He likes soggy soil and decomposing plant matter. Some studies have shown a correlation between cumulative rainfall and tick density. Nice wet weather makes for many happy ticks. But why the two-year time lag? Dry spells apparently affect nymphs more profoundly than other stages. A drought coinciding with that stage of the tick's life cycle will result in fewer egg-laying adults the following year, and fewer nymphs a year after that. Drought may also indirectly lower tick population by adversely affecting the food supply of Mr. Mouse. Less rain, less mouse food. Less mouse food, fewer mice. Fewer mice, fewer ticks.

How about the cold? White-footed mice apparently don't handle cold as well as some other mice. A hard winter can knock down the spring mouse population significantly. Although the mice numbers usually bounce back by the end of the year, that temporary drop may be enough to reduce the likelihood of spring tick larvae finding a mouse blood meal. Without this feeding, the larvae will not live to molt to nymphs the following year. Cold winter, fewer spring mice. Fewer spring mice, less food for summer larvae. Fewer larvae survive, fewer nymphs a year later. Nymphs are thought to be responsible for two-thirds of human Lyme infections in the Northeast. So a cold winter may cause a drop in Lyme incidence, and a warmer winter correlate with a high incidence of disease, a year and a half later.

Acorns, mice, and ticks
Another hypothesis (I love this one) links the incidence of Lyme disease with periodic variations in acorn production. Apparently there is a phenomenon known as "masting" in which every now and then there is a superabundance of acorns. The theory is that lots of acorns leads to hordes of happy mice the following spring. More acorns, more mice. More mice, more ticks, More ticks... you get the picture. Believe it or not, there are actually several studies looking at numbers of acorns in particular regions of the Northeast over specific years, primarily in relationship to white-footed mice and gypsy moth populations, but let's just stick to our ticks today. Those small studies, when correlated with Lyme disease incidence in the same regions at the same time, failed to show a significant effect of acorn masting with Lyme. We'd have to spend a lot more time counting acorns to know for sure if they are relevant to our topic. So now the checkers players' conversation goes more like this "Hmmm, remember last year we had a helluva lot of acorns?" Player Number Two responds, "But wasn't it particularly dry two years ago in June?"

In one study, researchers were surprised that the amount of rain in the spring and summer did not appear to cause an increase in Lyme disease in people during the same year. After all, if nymphs like wet weather, and nymphs are mostly responsible for transmitting the disease, wouldn't we expect a same-year effect? Apparently not. Hhmmm again. One proposed explanation was that nymphs may like the rain, but people don't, so we all stay inside more when it's a wetter season. Less time outside, fewer opportunities for ticks to bite us. Fewer tick bites, fewer cases of Lyme disease. On the other hand, if it's too hot outside, that may drive people inside to sit by the fan with an iced tea.

The bottom line is that there are a huge number of variables that play into these relationships, but the overall data supports the fact that wetter springs and summers, and warmer winters may increase tick populations and the risk of tick-borne disease in people. We can easily extrapolate and assume that this would hold true for our pets also. Add global warming to this scenario, and we may be looking at not only an ever-increasing number of hurricanes, but a typhoon of ticks as well. Spring is here. Break out those flea and tick products, the Frontline-Plus, the K-9 Advantix, the Preventic collars. Read your labels carefully as some products are great for dogs but lethal to cats. Get advice from your veterinarian. Take the usual tick precautions for yourselves. And while you're at it, fight ticks by fighting global warming. Take the bus. Walk. Buy a hybrid car. Vote. Remember: it is all interrelated in one way or another.