A study by Igor Dumic and Edson Severnini entitled “Ticking Bomb: The Impact of Climate Change on the Incidence of Lyme Disease” published in Canadian Journal of Infectious Diseases and Medical Microbiology describes the relationship between climatic variables and the reported incidence of Lyme disease in 15 northeast and mid-west states that contribute to more than 95% of reported cases within the Unites States.
Researchers merged two types of data to conduct the analysis in this study: annual county-level epidemiological data on Lyme disease cases from the Centers for Disease Control and Prevention (CDC) and meteorological data from the National Oceanic and Atmospheric Administration (NOAA). For the period 2000–2016, researchers focused on 468 U.S. counties from states with a high incidence of Lyme disease cases and found sizable impacts of temperature on the incidence of Lyme disease. According to the authors “Assuming a 2°C increase in annual average temperature…we have predicted that the number of Lyme disease cases in the United States will increase by over 20 percent in the coming decades.”
According to Timothy J. Sellati, PhD, Chief Scientific Officer of Global Lyme Alliance “The solid foundation for the data presented and interpretation of the results derives from the robust empirical strategy outlined in the study.” Moreover, Sellati says “This is a beautiful ‘hard evidence-based science’ complement to Mary Beth Pfeiffer’s book Lyme: The First Epidemic of Climate Change, which offers an intriguing and informative historical perspective on climate change and its relationship to the increasing incidence of Lyme disease. Collectively, the findings presented in the published study and Ms. Pfeiffer’s book should improve preparedness and response by clinicians, public health professionals, and policy makers, as well as raise public awareness of the role climate change plays in increasing the risk of vector-borne human infection.
Lyme disease is spreading rapidly around the globe as ticks move into places they could not survive before. The first epidemic to emerge in the era of climate change, the disease infects half a million people in the US and Europe each year, and untold multitudes in Canada, China, Russia, and Australia.
Mary Beth Pfeiffer shows how we have contributed to this growing menace, and how modern medicine has underestimated its danger. She tells the heart-rending stories of families destroyed by a single tick bite, of children disabled, and of one woman’s tragic choice after an exhaustive search for a cure.
Pfeiffer also warns of the emergence of other tick-borne illnesses that make Lyme more difficult to treat and pose their own grave risks. Lyme is an impeccably researched account of an enigmatic disease, making a powerful case for action to fight ticks, heal patients, and recognize humanity’s role in a modern scourge.
Over the past few decades, warmer winters have been associated with major declines in the moose population. These massive and majestic animals are dying across the U.S.—from Maine, New Hampshire and Vermont to Michigan and Minnesota—as well as throughout Canada. Twenty years ago, for example, two separate moose populations roamed Minnesota, but since the 1990’s one of the moose populations has essentially vanished, plummeting from 4,000 to fewer than 100. The other moose population in Minnesota has declined from 8,000 to fewer than 3,000.
What are the reasons for this decline? Among other factors, warmer winters have caused spikes in the winter tick population. Many of the moose in the areas experiencing population decline are seen covered in Dermacentor ticks— sometimes as many as 100,000 ticks. The insidious pests attach themselves to the moose and feast on their blood, leading to anemia. The tick-infested animals are called “ghost moose” because in an attempt to dislodge the ticks they scratch, bite, and rub at their fur, exposing a pale undercoat and bare skin. If the moose loses too much of its fur, it can lead to hypothermia, reduced fat stores, and a compromised immune system.
Moose are not the only large mammals that ticks attach themselves to, so it is important to consider why tick infestations have such severe consequences on moose compared to other animals. For example, deer experience tick attachment to their fur, and yet they do not experience the same deadly symptoms as moose. It has been suggested that deer cope with ticks more efficiently because deer have lived among ticks for a longer period of time, and therefore have adapted grooming strategies that effectively remove ticks. On the other hand, moose habitats have only been exposed to ticks recently, leaving moose unprepared to handle ticks and their associated damage.
Moose habitats are being exposed to an increasing number of ticks due to the effects of climate change. Climate change is allowing more ticks to survive the warmer winters, and expand their range into more northern territories. Ticks attach to moose in the fall, remain dormant until January or February, feed on the moose until March or April, and then drop off. With warmer and shorter winters, more ticks are able to remain attached to the moose throughout the winter, increasing blood and hair loss. Moose are also most likely to be found dead in March or April, after the most tick–related damage has been done.
Concomitantly, Canada, a country that was never previously vulnerable to ticks and their diseases, is seeing a rising number of Lyme disease cases in humans and animals. Regions like Canada that are facing their first exposure to ticks are both less aware of, and less equipped to handle the health hazards that ticks bring to a habitat, and therefore are more heavily affected.
In addition, moose are often co-infected by brainworm, a worm parasite that is associated with severe neurological illness. The cumulative stress imposed by brainworm and tick infestation, especially for calves, may be overwhelming.
It is important that we remain aware of the tick impact on moose. Hopefully, with more research and information, climate change mitigation, and conservation efforts, we will preserve and perhaps restore populations of this iconic symbol of the American wilderness.
For more on Lyme disease and climate change, click here.
Lyme disease, which is endemic in the United States, is caused by the bacterium Borrelia burgdorferi . In Africa, however, there are different tick-borne diseases affecting communities.
Tick-borne relapsing fever (TBRF) is one of the most frequent bacterial diseases in Africa, and is transmitted by bites of the Ornithodoros genus of tick, which has a soft body instead of the hard body of ticks that are common in the US. These ticks also carry different species of the Borrelia bacterium, including B. hispanica and B. crocidurae, which are known to cause TBRF in northern and western Africa. In addition to harboring different pathogens, these species of ticks also have extremely varied preferences when it comes to their chosen habitat and climatic range.
In Africa, ticks colonize burrows to find shelter and hosts for blood meals. These burrows are typically dug by small mammals in dry ground, and are located near houses, agricultural fields, or in undeveloped areas. Since many of the burrows are close to where humans live and work, many people are at risk of getting bitten and becoming ill with TBRF.
A groundbreaking study conducted by Jean-Francois Trape et al. in 1996 revealed how quickly TBRF was spreading, and how climate could enhance the success of the TBRF bacterium. This research revealed that 10% of the study population in Senegal became infected with TBRF during a two-year prospective study period, and that the range of this disease was almost an entire degree of latitude further south than the previous southernmost boundary of the disease.
To find the underlying cause behind the increased infection rate and southward range expansion of TBRF, Trape et al. (1996) investigated the possible connection with climate, specifically, a drought in Senegal and other areas of sub Saharan Africa in 1970. To see if dramatic changes in precipitation could be related to the spread of ticks, they mapped out the average annual precipitation in Senegal from 1947 to 1969 and from 1970 to 1992, the periods before and after the drought. They then compared precipitation maps to the maps of the tick’s range in these two time periods, and found that the ticks preferred habitats with annual rainfall below 750mm. Areas receiving 750mm of rainfall were at the southernmost border of the tick’s range. The maps also revealed that the drought caused annual precipitation to drop in all areas. Regions that had average annual precipitation of 850mm to1,000mm from 1947 to 1969 had less that 750mm rainfall in the period during the drought. The shift in annual precipitation allowed areas that were previously unsuitable for ticks to become welcoming habitats, and with the spread of ticks across Senegal, came the increasing prevalence of TBRF.
So why do the ticks in Africa prefer drier conditions, while the ticks in America thrive in humidity and high precipitation? The answer links back to the habitat choice of the ticks. In Africa, small mammals often dig the burrows that the ticks inhabit, so that they can evade the hot and dry climate that they face above ground. Environmental stressors usually invoke an appropriate behavioral response in pants and animals, and for small mammals in Africa, the response to severe drought is to dig burrows in the ground. This could be the reason why the persistent drought in sub Saharan Senegal that started in the 1970’s allowed ticks to expand their range, and why ticks are further extending their ranges now, as Africa is undergoing the impact of climate change.
An example of this is the Gharb region in northwestern Morocco, which has recently documented a high incidence of TBRF in humans. A study by Souidi et al. (2014) investigated the reasons behind the increase of TBRF by looking at how many small mammal burrows are colonized by ticks in the area, how many of those ticks are carrying infectious bacteria, and how the changing climate might tie in to these variables. The data that this study collected suggests that the increasing aridity of the northwestern Morocco could be a significant factor in the rising incidence of ticks and TBRF in the Gharb region.
Climate change will impact each ecosystem differently, with each species and community reacting differently to the series of severe effects that it brings. Ticks around the world are facing different outcomes of climate change, such as increasing temperatures, higher rates of humidity in some areas, as well as increasing aridity and dryness in others. It seems that in all cases, however, natural selection drives the ticks to exploit these changes, and they are spreading faster than ever.
For more on Lyme disease and climate change, click here.
Even though climate change is believed to be a major influence on tick abundance and Lyme infections around the world, there are other human practices that have the potential to contribute to the spread of ticks and Lyme disease. A study by Simon et al. (2014) discussed the reasons that ticks and white-footed mice might be spreading from the northeastern US into southern Canada. In addition to increasing global temperatures, land fragmentation could be a huge factor in changing tick abundance.
Woodlands are being destroyed to build cities and agricultural fields, breaking forests into smaller and smaller units of land. Deforestation and land fragmentation are suspected to be why Lyme might be spreading so rapidly, but this idea is highly contested and there have been studies pointing in both directions even as research continues.
Studies conducted across various European regions investigated this hypothesis and found a significant effect of certain elements of land fragmentation on infected tick abundance. Research done in central France and Belgium has revealed that land with more forest edge has a higher prevalence of infected ticks. Other studies identified the transition areas between woodlands and developed areas as another factor in tick abundance in the Latvian countryside as well as in northern Spain. Li et al. (2012) conducted an experiment that followed up on these studies and supported their findings, discovering the importance of adjacent land cover, or “transition area”, in particular.
Li et al. used computer models to test a diverse range of land use scenarios. The land parcels varied in size and adjacent land cover, or “transition area” between woodlands. Using the models, they predicted the tick nymphal infection prevalence (NIP) and density of infectious nymphs (DIN) at the various sites. Results suggested that decreasing the size of the blocks of land led to an increase of NIP and DIN in woodlands adjacent to non-vegetated areas. However, when the adjacent land cover was grass instead of a non-vegetated area, the NIP and DIN measures decreased, showing that adjacent land cover can make a significant impact on the Lyme risk in an area.
The presence of the grasslands beside woodlands led to a lower risk of Lyme in the woodlands because of lowered tick survival rate. This is because the ticks would spread out into nearby grasslands, where many would not survive, since available hosts like mice and deer are scarcer, rather than concentrated into small areas. This larger land area dilutes the amount of infected ticks in the woodlands, and lowers the risk of being infected with Lyme. According to this study, land fragmentation does have an effect on the prevalence of infected ticks, and having grasslands adjacent to woodlands instead of non-vegetated areas is an effective way to reduce the risk of Lyme in a habitat.
A study by Zolnik et al. (2015) voiced an opposing perspective, arguing that land fragmentation does not contribute to the prevalence of infected ticks in wooded areas. The authors instead proposed that, it is high biodiversity in a community that reduces the risk of Lyme and tick-borne illnesses, by diminishing the abundance of reservoir species like the white-footed mouse. However, they found that levels of biodiversity had no significant effect on the amount of infected ticks. Thus, the effect of land fragmentation on variables other than host biodiversity may impact the abundance of infected ticks.
Even though this topic is still highly debated, many studies expose the likelihood that human interventions in woodlands and nearby areas have the potential to put people at a higher risk for Lyme disease. Common human practices such as deforestation, clearing land for agriculture or to create urban areas, mowing down grasslands, and fencing in forests, all have consequences when it comes to infected tick prevalence. As we continue deforestation, we are creating smaller patches of land where, many studies have shown, that the density of infected nymphs increases. More research is necessary to know the full effects of land fragmentation on Lyme disease risk. But while we are making future plans or thinking about how to manage our land, the potential impact on tick populations should definitely be taken into consideration.
Figure 1. How tick abundance varies based on land block size and percentage of woodland area in those units of land. This is shown in two situations. Situation I being when the blocks of land are adjacent to non-vegetated area, and Situation II where the blocks of land are adjacent to grassland. The color scale on the right shows the number of infectious ticks per hectare of land, red signifying more ticks in an area, and blue signifying less. This shows that the smallest patches with least woodland cover have the most infected ticks when it is surrounded by non-vegetated area and have the least amount of infected ticks when it is surrounded by grassland. Adapted from Li et al. (2012) PLoS ONE Vol. 7(6) page 1-12.
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The Centers for Disease Control and Prevention (CDC) recently announced that diseases spread by mosquitoes, ticks and fleas tripled between 2004 and 2016. More than 640,000 of such illnesses were reported during these years, including Zika, dengue, chikungunya, Lyme disease and plague. These diseases are known as vector-borne diseases, because infection of humans requires transmission of the microbe by an intermediate species. The CDC acknowledges that the US is inadequately prepared to diagnose and treat these expanding epidemics, which are fueled in part by climate change and its effects.
For example, migratory birds have been linked to the spread of West Nile virus, avian influenza virus, Lyme disease, and many other illnesses. Etc. etc.
Migratory birds have been linked to the spread of West Nile virus, avian influenza virus, Lyme disease, and many other illnesses. Climate change is altering the amount of suitable habitats for disease-carrying animals such as ticks. Even though many new habitats now provide ideal living temperatures for ticks, it would be difficult for them to physically move to these new areas without some help. Migratory birds facilitate the movement of ticks to new territories. Avian migration has opened the door for many diseases to spread over vast distances each year by carrying disease vectors such as ticks, or by the birds being themselves infected by the disease and spreading it to others as they migrate.
To see the effect of bird migration on the spread of ticks across America, Cohen et al. (2015) conducted a study that measured how many ticks are brought into America by migratory birds. They also screened the ticks for various microbes, including pathogens that cause spotted fever, and the bacterium that causes Lyme disease. In the spring of 2013 and 2014 they captured 3,844 birds, of 85 different bird species that were returning north for the summer. Out of these 137, about 3.56%, were infected with ticks. All of the ticks collected were either in the larva or nymph stage of development, and 67% of the ticks they collected were neotropical ticks, meaning they were from Central or South America. After screening the ticks for diseases, they found that 38 of the ticks were infected with some variation of the spotted fever bacteria, Rickettsiae, and that none of the ticks harbored Borrelia burgdorferi, the bacterium that causes Lyme.
The authors estimated that about 4 to 39 million neotropical ticks are brought to the United States each year. Although the ticks found in this study did not have Lyme, migratory birds are still introducing ticks to new habitats thus increasing tick populations across the country. Since the ticks they found were all larvae or nymphs, there was still a chance for them to become infected with Lyme, and spread it to future hosts. Ticks have three blood meals in their lifetime, one to help their development from larvae to nymphs, another for their transition from nymphs to adults, and finally, as adults before laying eggs. The Lyme bacterium or other pathogens are transmitted to ticks if one of their blood meal hosts is infected. Thus, the ticks that are transported to America on birds can still put humans at danger for Lyme disease, if they acquire their next blood meal from an infected host.
Ogden et al. (2015) conducted a somewhat different study examining the relationship between migratory birds and ticks. The study tracked how far north the birds were taking the ticks. They found that many birds went further north than their previous breeding sites when returning to Canada in the spring, and even went into territories beyond the tick’s climatic range (Figure 1). Even though ticks cannot currently survive in some of these areas, the increasing temperatures will make more and more of these habitats suitable over time. Climate change will also impact the migration routes of birds, and perhaps cause them to continue flying further north each year. Therefore birds will continue to bring ticks into new habitats, and the ticks will be more likely to survive in these northern areas as the temperatures continue to rise. Moreover, with rising temperatures, mice will also expand further north, possibly bringing the Lyme disease bacterium with them.
Ticks are very small, slow moving creatures; alone, it would be impossible for them to spread into new territories as far as northern Canada. However, ticks receive a lot of help from the small animals they latch onto, with organisms like migratory birds helping them cover vast distances. The actions of migratory birds amplify the effects of climate change on tick populations, and together they will help spread ticks into new territories. There can be little doubt that Lyme disease and other tick-borne diseases will follow.
Figure 1. Regions used to assign birds to latitudinal bands in the Ogden et al. (2015) study. The results indicated that up to 70% of birds carrying ticks could bring them north of their capture locations (into bands C, D, and E), and up to 17% could transport ticks further into the boreal region of eastern Canada (bands D and E). This map shows how far north these ticks are being transported. Adapted from Cohen et al., 2016, Journal of Environmental Microbiology Vol. 3 page 322-350.
For more on Lyme disease and climate change, click here.
*This blog post has been updated in June 2018 to include the CDC announcement of May 2018.
Climate change has the potential to expand the incidence of Lyme disease. For example, shifting environmental conditions in ecosystems in the northeastern United States may contribute to the rising prevalence of Lyme disease in Canada. Lyme disease is spread by bites of black-legged ticks, and is caused by Borrelia burgdorferi, bacteria that live in the tick midgut. As the geographic range of ticks and the bacteria expands northward due to regional warming, new human and animal hosts are now at risk of infection.
Ticks thrive in a specific temperature range, and if the environment is too cold, their metabolism cannot function at a high enough level to reproduce or look for a host to provide their next blood meal. Therefore, increasing global temperatures allow ticks to proliferate in areas that would normally be too cold to support their growth.
Not only are warmer temperatures providing ticks with new habitats, they also help the ticks in one of their most important pursuits, finding a host. In order to mature and reproduce, ticks require three separate blood meals from vertebrate hosts in their lifetime. To find the source of their meal, ticks engage in “questing behavior”, in which they climb to the top of blades of grass or vegetation to get a better chance of clinging onto a passing host. However, the tick’s ability to make this great climb is dependent on two factors, temperature and humidity. Studies have shown that ticks are more likely to quest in higher temperatures, due to the boost that temperature has on metabolism.
One study by Tomkins et al. (2014) sampled ticks from three areas in the UK, including northeastern Scotland, northern Wales, and southern England. They also studied ticks from France, one site at low, and another at high altitude. The climates in each of those locations vary from cool to warm, with annual mean maximum temperatures ranging from 9.9°C to 16°C (49.8°F to 60.8°F). Ticks from cooler habitats started questing at lower temperatures, and had a lower maximum tolerated temperature, showing their adaptation to their specific environments. The study also examined how the ticks from different regions responded to increases in temperature, and discovered that ticks from colder climates were more sensitive to changes in temperature than ticks from warmer areas. It’s possible then, that ticks in northern regions will start questing at even slight temperature increases. Therefore, a warmer climate would mean that ticks could have the opportunity to quest more frequently, and increase their chances of latching onto a host.
Global warming will lead to shorter and more favorable winters for ticks, increasing both the time that they are able to quest, and the frequency that they will find a host. A study by Simon et al. (2014) revealed that warmer temperatures are associated with northern expansion of the Peromyscusleucopus, the mouse species that serves as blood meals for tick nymphs, providing them with the resources to molt into adults, which then potentially infect larger mammals, including humans. Lyme disease has been emerging in Canada, and this study suggests that it is most prevalent in those areas where both mice and ticks have successfully expanded their range. This study predicts that by the year 2050, the black-legged tick will have expanded about 186 miles, the white-footed mouse will expand about 155 miles, and the range of the Borrelia burgdorferi bacterium will move about 93 miles further north in Canada. During the last 15 years, Lyme disease has been spreading at a rapid rate in the US. With projected changes in temperature, there will be even more factors favoring Lyme proliferation, making it even more important for us to act now.
To tackle the coming wave of Lyme disease prompted by climate change, we can mitigate our impact on the environment by being conscious of our energy consumption, reducing our carbon footprints, generating less waste, eating locally, and conscientiously managing land use to reduce human exposure to areas with a high tick presence. However, temperatures are already on the rise, and Lyme disease is already expanding into new regions. Therefore, in addition to fighting climate change, we must redouble our efforts on Lyme disease research, so that we can be better prepared to care for the rising number of patients suffering from this debilitating disease.
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