Ticks are a recurring topic on the pages of this blog. We are exploring the ecology of Western Pennsylvania, after all, and Pennsylvania has been experiencing a population explosion of black legged ticks and is also the epicenter of the national Lyme disease epidemic. So ticks and their symbionts have to be a part of our discussions!
A quick review of what we know about black legged ticks and the bacterium that causes Lyme disease:
The black-legged tick (Ixodes scapularis) (formerly called the “deer tick”) is a small, common tick found throughout the northeastern and north-central parts of the United States. It is the transmission vector for a number of bacterial and viral pathogens including the bacterium that causes Lyme disease, Borrelia burgdoferi. For the last five years, Pennsylvania has led the nation in the number of human cases of Lyme disease and the number of cases is growing each year (From the Center for Disease Control, human Lyme disease cases in Pennsylvania: preliminary 2016 (12,092 cases), 2015 (9000 cases), 2014 (7400 cases), 2013 (5900 cases)).
The life cycle of the black-legged can involve combinations of over one hundred different potential hosts (fifty-two different species of mammals, sixty species of birds, and eight species of reptiles) and can stretch out over a two or even a three year period with staggered emergences of different instar stages during different months of the year. Also, early instars of this tick are extremely small and difficult to see! Below is an idealized version of the life cycle that I described in a blog a few years ago:
Eggs deposited in the fall in low, grassy or scrubby vegetation hatch the next summer into the very small, six-legged larva life forms. These tiny ticks typically seek out small hosts (white-footed mice (Peromyscus leucopus) seem to be the preferred host for this life stage) but are able opportunistically to attach to larger mammals, too, including humans. These larva, though, are not born with any of the pathogens associated with Ioxdes scapularis and are, thus, unable to transmit any of its diseases (a small piece of good news!). If these larvae feed on a host that is carrying one of I. scapularis’ bacterial or viral pathogens, though, that tick will become infected with that disease causing agent and will carry it and be able to transmit it throughout the rest of its life cycle.
After the larva has taken its blood meal it molts into the larger, eight-legged nymph life form. This molt often is delayed until the following spring. These nymphs, then, seek a host for their blood meal. These hosts are usually mammals ranging in size from white-footed mice to dogs to cats to deer to humans. Because of the timing of this nymph emergence the spring (May and June here in Western Pennsylvania) is a time of great risk for ticks bites (and disease transmission) for humans!
After the nymphs have taken their blood meals they molt into adults. These adults are especially abundant in the fall. These much larger ticks (like the one in the picture to the left) typically attach to large mammals. The female adult ticks take a large blood meal from their hosts and then use the energy from this feeding to make eggs. The adult male ticks attach to the same hosts, but do not feed (and, therefore, do not transmit pathogens at this stage). They are there to find a female and to mate! The males die shortly after mating and the females die after dropping off of their hosts to lay their eggs in the grassy and scrubby vegetation. Those eggs then overwinter and hatch in the summer to start the life cycle all over again.
So why have the number of Lyme disease cases increased in recent years? Media reports stress the “common sense” inference that our increasingly warm winters (possibly due to climate change) are leading to increased survival of the ticks and increased spring and summer populations. Unfortunately, scientific research does not support this logical connection. A study published in 2012 in the Journal of Medical Entomology clearly showed that in spite of “common knowledge” to the contrary, cold winters (and they used Upstate New York as their cold winter site!) do not reduce the numbers of overwintering black-legged ticks. The ticks just have too many adaptations for cold tolerance and too many protected microhabitats available for even the brutal winter temperatures of New York State to have any effect on them at all.
Most researchers looking at these ticks attribute their increases to increases in the most critical host in the black-legged tick’s life cycle: the white-footed mouse. Fragmentation of forest habitats and the optimal conditions of suburban ecosystems for these mice along with significant declines in their natural predators have led to great increases in their numbers. Black legged ticks, then, in their larval and nymphal life stages are increasingly likely to find a white-footed mouse on which to feed and are, therefore, increasingly likely to survive to the next instar level. White-footed mice are also significant reservoirs for the Lyme disease bacterium, so the ticks have a higher probability of assimilating and then passing on these bacteria.
Weather and climate factors can have an impact on populations of white-footed mice, but it is not temperature that is the most important weather/climate feature but precipitation. Wet and humid conditions favor the growth of the plants upon which white-footed mice feed and thus can lead to increased population sizes. More white-footed mice means that nymphal black-legged ticks have an increased chance of finding its ideal “blood meal” host thus increasing the numbers of later instar stages. Further, right after a black-legged tick has taken its blood meal its ability to control its body water concentration is greatly impaired. A tick, then, right after a blood meal is very likely to die if it encounters a dry environment. Increased precipitation and higher relative humidity, then, also favors survival of the tick!
An article in the Wall Street Journal this past April (sent to me by my WSJ watcher, Larry in California!) warned of a “bad summer for ticks” because of the mild winter (probably not) and because of the 2015 “bumper crop” of acorns in the Northeast! These acorns fed more white-footed mice which in turn sustained (and infected) more black-legged ticks! The adults of the tick explosion are out there this year searching for their last blood meal!
Your best defense against Lyme disease is a “tick check” after any potential tick exposure. Remember, the ticks may be anywhere that white-footed mice might live (yards, fields, woods, etc.). The ticks have to be attached to you for 36 hours before they can begin to transfer the Lyme bacterium. Use a tick puller and dispose of the tick in a creative manner (drown them in alcohol or flush them down the toilet). Don’t let the threat of ticks keep you from the woods or hiking trails!
The story of the Lyme disease vaccines is very interesting, by the way! I will talk about that in a future blog!