Adult black legged tick. USDA. Public Domain
(Click on the following link to listen to an audio version of this blog …. Lyme disease and vaccines
One of the things I don’t miss about Pennsylvania are the ticks and the tick-bourn diseases like Lyme disease. Colorado does not have the ticks that carry the bacterium that causes Lyme disease. We have other things to worry about, but that would be the topic of another blog.
For many years, Pennsylvania has led the nation in the number of Lyme disease cases. Between 1990 and 2021 Pennsylvania had, according to the CDC, 142,264 reported cases of Lyme disease. Only New York State came close to PA in terms of numbers of people affected by this extremely serious illness. New York reported 138,585 cases between 1990 and 2021. These numbers, as high as they seem, are probably gross underestimates, and each year the total number of Lyme disease cases exponentially increase. The CDC estimates that almost half a million people get Lyme disease each year and only a very small percentage seek medical help and get accurately diagnosed.
Distribution of Ixoides scapularis. Public Domain.
The vector for the transmission of the bacterium (Borrelia burgdorferi) that causes Lyme disease is the black-legged tick (Ixodes scapularis) (formerly called the “deer tick”). The black-legged tick is a small, common tick found throughout the northeastern and north-central United States. This tick is also the transmission vector for a number of other bacterial and viral pathogens.
The life cycle of the black-legged tick 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!
Black legged tick life stages. California Dept of Public Health. Flickr
Black-legged tick eggs are deposited in the fall in low, grassy or scrubby vegetation and 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 is often 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 (May and June in much of the eastern United States), the spring 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 typically attach to large mammals. The female adult ticks take a 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.
Winter, Upstate New York. Photo by Antepenultimate. Wikimedia Commons
So why have the number of Lyme disease cases increased in the past few decades? 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.
White-footed mouse. Photo by DGE Robertson. Wikimedia Commons
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 relative humidity, then, also favors survival of the tick!
Red fox (Vulpes vulpes). Photo by K. Billington. Wikimedia Commons
In a study published a few summers ago in the Proceedings of the Royal Society B the presence of active mice predators (foxes, martens, weasels etc.) did not significantly reduce local populations of mice. The presence of these predators, though, did reduce the number of ticks each mouse had on them (a 90 to 95% reduction!) and also reduced the number of ticks that were carrying the B. burgdorferi bacteria (a 96% reduction!). Speculating on these findings, the study’s lead researcher, Dr. Tim Hofmeester, felt that the presence of the predators curtailed the movements of the mice and acted to disrupt the proliferation and spread of the ticks and, possibly, their intra-specific exchanges of the bacteria!
Stop Lyme disease by adding foxes and weasels (and how about coyotes?) to our fields and woods and neighborhoods? Works for me!
Gedi and Heidi. Photo by M. Hamilton
Dogs are also affected by the bacterium that causes Lyme disease. Lyme infections in dogs can lead to debilitating limb paralysis and occasionally fatal kidney failure. The good news for dogs is that there is a vaccine that effectively generates immunological protection against B. burgdorferi infections and also reduces the severity of the Lyme syndrome if an infection does get established. The very puzzling part of any discussion about this dog vaccine, though, is the realization that there is not a human, anti-Lyme vaccine currently available to help control this growing epidemic of human Lyme disease.
Making this vaccine story even more opaque is the realization that there exists not just one but two human anti-Lyme vaccines! One of these was actually available to the public about thirty years ago and the other one passed through clinical trials but was never released for general distribution.
Both of these vaccines, released at a time when Lyme disease was a relatively limited, localized problem, encountered a public relations buzz saw driven by superstition and anti-science bias. A small group of people claimed that the vaccine caused paralysis (no clinical evidence supported this). This group threatened legal action against the pharmaceutical company that had released the vaccine and also mounted an extremely effective misinformation campaign against the vaccine. Under the weight of these attacks and under threat of skyrocketing legal costs, the small pharmaceutical company that made the vaccine stopped manufacturing it, and the second pharmaceutical company decided never to release their vaccine. In the 1990’s when the vaccine was initially released the number of Lyme cases in the United States were much lower than today, so there was little economic incentive for the pharmaceutical company to continue to make the vaccine..
Photo by Intropin. Wikimedia Commons
There are two new vaccines being developed to protect humans from Lyme disease: Penn Medicine has designed an mRNA vaccine (the same kind of vaccine that was developed against SARS-CoV2). This vaccine is in preclinical testing stages, and Pfizer and Valneva have designed a protein vaccine that is currently entering human testing. Pfizer hopes that this testing will be completed by 2025 and the FDA approval process will be able to begin in 2026.
Lyme disease has caused a great deal of human misery, and this suffering is increasing every year! We need these vaccines (or we all need to move to Colorado!).
Remember, your best defense against Lyme disease is a “tick check” after any potential tick exposure. 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!