The role of wildlife in the expansion of tick populations in eastern North America

Expansion in tick populations in eastern North America over the past several decades has changed veterinary medicine. Tick-transmitted diseases are emerging in locations where they previously did not exist, and in areas where they did exist, substantial increases in prevalence are occurring.^1-3 The proliferation of diagnostic tests for tick-vectored diseases and the development of topical and systemic acaricides have been unprecedented. The population growth of 2 native tick species (Amblyomma americanum and Ixodes scapularis) is responsible for the majority of these issues. Additionally, a recently introduced tick species (Haemaphysalis longicornis) has now generated concerns that tick populations and disease transmission might worsen. The story behind these expansions is remarkable.

Ticks and the diseases they transmit are an important concern in North America. Between 2013 and 2019 results from a nationwide survey, approximately 1 million, 1.9 million, and 950,000 dogs attained antibody-positive results for Anaplasma spp, Borrelia burgdorferi, and Ehrlichia spp, respectively.³ The Companion Animal Parasite Council parasite prevalence maps show a 1.6-fold increase (from 268,413 to 434,737) in positive test results for B burgdorferi from 2016 to 2021.⁴ Although positive antibody results do not necessarily equate to clinical disease, they do indicate just how many dogs have been parasitized by pathogen-infected ticks.

Over the past several decades, numerous factors have contributed to the increased range of A americanum (lone star tick) and I scapularis (black-legged tick), such as an increase in favorable habitat due to reforestation, climate fluctuations, and the remarkable increase in white-tailed deer populations. Substantial reforestation over the past century in urban and rural habitats has provided increased areas of suitable habitat for white-tailed deer and for survival and expansion of A americanum and I scapularis. This is critical because white-tailed deer are considered the preferred reproductive host for these tick species.^1,5-7

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It is well documented that before the early to mid-19th century, white-tailed deer populations were large and widespread throughout North America.⁵ As European settlers moved west across eastern North America during the 18th and 19th centuries, deer populations were decimated because of unregulated hunting, deforestation, and loss of natural predators. By the end of the 19th century, only an estimated 300,000 to 500,000 deer remained in North America.⁵ Concern for the survival of the species ultimately led to restrictions on deer hunting, and many states initiated restocking efforts. Combined with an increase in natural habitat and, by now, the virtual extirpation of natural predators (wolves, bears, and mountain lions) across much of their natural range, deer populations rebounded and then exploded. It was estimated that populations peaked in 2000 at 38.1 million.⁸ This remarkable, almost 100-fold increase in deer populations, along with increase habitat, has led to population increases of A americanum and I scapularis and associated tick-transmitted diseases of dogs and/or cats. I scapulars is responsible for the transmission of Lyme disease (B burgdorferi) and granulocytic ehrlichiosis (Anaplasma phagocytophilum).¹ A americanum is responsible for the transmission of cytauxzoonosis (Cytauxzoon felis), ehrlichiosis (Ehrlichia ewingii), tularemia (Francisella tularensis), and Rocky Mountain spotted fever (Rickettsia rickettsii).¹

But this is just part of the tick population story. The recent reevaluation
of original reports from entomologists and naturalists from the 18th and 19th centuries is of great importance.⁹ In 1754, Swedish naturalist Pehr Kalm traveled across what is now the northeastern United States and made some very interesting observations concerning ticks. A reexamination of his original report by a Swedish-speaking professional entomologist has left no doubt that Kalm described lone star ticks as far north as Saratoga Springs and Fort Ann in New York.⁹ This is much farther north of the assumed original natural range of this tick species. Similarly, entomologist Asa Fitch also reported that lone star ticks were formerly common in northern New York but were extirpated at the time of his writing in 1870.⁹ Additionally, Albert John Cook, professor of zoology and entomology at Michigan Agricultural College (now Michigan State University) in East Lansing, described the lone star tick as common in Michigan’s forests during his childhood, in the first half of the 19th century but that is gradually became rare and ultimately disappeared.⁹

Based on these combined historical reports, it has been determined that the lone star tick was much more widespread and abundant in Michigan, New York, and New England in the 18th to early 19th centuries than previously believed. Therefore, it has been proposed that the lone star tick’s current northward expansion is actually a recolonization of their former range and not an expansion into a completely new habitat.⁹ As previously discussed, the lone star tick became extirpated to rare in the northern parts of its range during the later 19th century because of extensive deforestation and extirpation of the white-tailed deer. Subsequent recolonization by second-growth forest and increases in white-tailed deer populations by the mid-20th century are now allowing A americanum to reclaim its former range.

As if these native tick species in North America were not bad enough, the recently introduced Asian longhorned tick (Haemaphysalis longicornis) is now a widespread foreign invader and can reproduce without males (parthenogenesis). This 3-host tick, which is native to southeast Asia, was first discovered in the United States in summer 2017 in Hunterdon County, New Jersey, where ticks were found attached to sheep and humans. The sheep flock was treated with permethrin in hopes of wiping out the population over the winter. However, in April 2018, surveillance at the site found that the species had successfully overwintered.^10,11 Interestingly, based on retrospective examination of tick collections, populations could have been present in West Virginia since 2010, either misidentified or not examined until 2018.¹² It is undetermined exactly when this tick was introduced and whether there was more than 1 introduction. There do appear to be at least 3 tick variants in the United States, which means that at least 3 individual different ticks were introduced.¹²

Since its introduction, this tick species has spread very rapidly across the eastern United States, with specimens being collected in at least 19 states as of July 2023.13 It is currently found from the East Coast to as far west as Missouri and Arkansas.¹¹

This rapid geographic spread has been facilitated by its host range, unique reproductive strategy, and ability to survive in wide temperature ranges (−2° C to 40° C). The Asian longhorned tick has a remarkably wide host range that currently includes more than 35 mammals in the United States,¹¹ such as black bears, cats, cattle, dogs, foxes, goats, horses, opossums, sheep, rabbits, raccoons, and white-tailed deer. Availability of hosts is clearly not a limiting factor in its ability to expand its range.¹¹ H longicornis is now considered established in North America and, based on computer modeling, will continue to spread westward to the eastern Great Plains, north into southern Canada, and south to the Gulf Coast.¹² Additionally, temperate zones in Northern California, Oregon, and Washington may also offer suitable conditions for its survival.¹⁴

This tick raises concerns for livestock production and human and small animal medicine. Populations on livestock can reach high levels that can cause production losses, and in its native range in Asia, it has been shown to transmit Theileria, Babesia, Anaplasma, Bartonella, Borrelia, and several viruses.^12,15 A recent report from North Carolina provides evidence of the dangers to livestock in North America, where 5 cows died from acute anemia due to massive longhorned tick populations.¹⁶ Another report also provides an example of just how rapidly Asian longhorned tick populations can increase. Medium- to large-sized mammals were trapped in 6 city parks throughout Staten Island in New York between June and September 2019. Five different tick species were collected, with 12,081 ticks in total. H longicornis accounted for 61.6% of all ticks collected.¹⁷ Most commonly infected hosts were raccoons, opossums, and white-tailed deer.

Data are currently sparse in the United States, but the Asian longhorned tick has been shown in a laboratory setting to be capable of acquiring and transmitting R rickettsii.¹⁸ In Pennsylvania, A phagocytophilum has been found in field-collected Asian longhorned ticks, although at a very low prevalence.19 Although B burgdorferi sensu stricto has been recovered from a single adult female Asian longhorned tick in Pennsylvania,²⁰ findings from another study demonstrated that this tick species failed to transmit the Lyme disease spirochete.²¹ As this tick continues to expand
its range and feed on a variety of pathogen-infected hosts, which diseases it will be able to transmit in North America remain to be seen.

The Asian longhorned tick is a common tick species recovered from dogs in other countries and is the most common species recovered from dogs in SouthKorea.¹² It is also frequently recovered from cats. Companion animals (particularly dogs) may be sources of exotic ticks entering the United States, and improved biosecurity measures are needed to minimize future tick introductions. Currently, there are no importation regulations concerning inspection or management of external parasites on dogs and cats.¹²

Regardless of how many dogs and cats are ever placed on acaricides, ticks will be ever present because of their presence in/on native wildlife species. Therefore, continued prevention and vigilance will always be necessary.

Michael W. Dryden, DVM, MS, PhD, DACVM is a university distinguished professor of veterinary parasitology (emeritus) at Kansas State University College of Veterinary Medicine in Manhattan, and received his veterinary degree from the university in 1984.

References

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