Abstract
The first appearance of this unwanted visitor to the United States was in New Jersey in 2017, but the longhorned tick, Haemaphysalis longicornis, is now known to have entered the United States as early as 2010. This menace has spread to several eastern states as well as Arkansas. Should we be alarmed?
H. longicornis is a native of Eastern Asia that has become invasive in Australia and New Zealand (Hoogstraal et al. 1968). This tick has three life stages, each of which seek a host, take a bloodmeal, and drop off. A generalist, H. longicornis has a broad host range but prefers large mammals such as sheep and cattle. The longhorned tick is aggressive and is considered a biting nuisance (Heath 2016) that can cause damage to livestock, as well as companion animals and humans. In Asia, field-collected longhorned ticks can harbor pathogens related to those found in the United States, including Anaplasma phagocytophilum, Ehrlichia chaffeensis (Kim et al. 2003), and Babesia species. Whether this tick can act as a vector for these pathogens in the United States is unknown.
Worryingly, the longhorned tick is considered a potential vector severe fever with thrombocytopenia syndrome virus (SFTS), which has a reported human mortality rate of up to 12.0% (Luo et al. 2015). Indeed, several recent publications in Vector-Borne and Zoonotic Diseases have highlighted the emergence of SFTS in Asia. SFTS RNA was isolated from H. longicornis in syndrome outbreak areas in the Republic of Korea in 2016, with a minimum infection rate of 0.11% per 100 ticks (Yun et al. 2016). The same study also found SFTS RNA in one pool of Haemaphysalis flava ticks, and in H. longicornis larvae, raising the specter of transovarial transmission. Subsequently, SFTS nucleic acids as well as antibodies to the virus were detected in wild animals, including Korean water deer and wild boar in the Republic of Korea (Oh et al. 2016).
SFTS has continued to expand in the Republic of Korea, with a more recent study finding an infection rate of 3.61% in H. longicornis nymphal and adult ticks (Jo et al. 2018). The number of SFTS cases had been increasing annually, with a peak of 172 cases in 2017 (Im et al. 2018). As information on infection in wild animals is very limited, additional studies have examined small mammals as potential amplifiers for SFTS, including hedgehogs and mink in China (Sun et al. 2017, Wang et al. 2017). Worryingly, 8/11 mink farms examined had mink positive for SFTS.
Since its arrival in the United States, H. longicornis has been found feeding on a variety of animals, including horses, dogs, deer, and a sheep. The sheep infestation was particularly dramatic. A woman who had been shearing her pet Icelandic sheep came to an entomologist, Tadhgh Rainey, at the Public Health Department of Hunterdon County New Jersey with ticks on her hands and wrists in November of 2017 (Rainey et al. 2018). The sheep was penned by itself and had never traveled, so where the ticks came from was a mystery. Her pants alone were covered in ticks. Mr. Rainey drove out to visit the sheep, and upon entering the paddock, before even touching the sheep, he was also covered in ticks (McNeil 2018).
The idea of instantly being swarmed by ticks is terrifying, but beyond the “ick” factor, how worried should we be? There are several aspects of the longhorned ticks that are concerning. Invasive populations of this species are capable of reproducing without fertilization (no males needed), meaning a single female could rapidly colonize an area with her offspring. The tick is reasonably cold tolerant, and easily overwintered successfully in New Jersey. In their native range, these ticks can survive temperatures as low as −10°C (14°F) (Yu et al. 2014). Scientists at Rutgers and the CDC have tested numerous specimens and so far have not identified the presence of known tick-borne pathogens endemic to the United States (e.g., Borrelia species, Babesia species, Anaplasma, Ehrlichia, or Powassan, Heartland, and Bourbon viruses) (McNeil 2018); however, H. longicornis is a known transmitter of the parasitic disease bovine theileriosis in Australia and New Zealand, and a suspected vector of SFTS in its native China (Burtis et al. 2017).
How the tick arrived in the United States to begin with is unknown, although Dr. Andrea Egizi of Rutgers has found at least three mitochondrial lineages, suggesting at least three females arrived (McNeil 2018). Multiple invasion events are concerning and raise larger issue about prevention of the establishment of other disease vectors in the United States. A recent publication examined the North American distribution potential of H. longicornis using occurrence data from its range in other areas of the world, combined with climate data (Raghavan et al. 2019). The best-fitting models indicated potential broad distribution for H. longicornis, mainly in the southeastern United States, the Pacific Northwest, and parts of Mexico (Raghavan et al. 2019). The authors of this study pointed out the potential for transport of the tick through movement of livestock and domestic animals in North America.
There are, however, good reasons not to panic. SFTS is not found in the H. longicornis that invaded Australia or New Zealand. In those countries, the ticks are a significant problem for livestock due to babesiosis and bovine theileriosis but remain only a nuisance to humans. These ticks have the potential to become infected with pathogens such as Babesia species, yet they will be competing with well-established populations of much more common ticks such as Ixodes and Dermacentor. As an invasive species, these ticks have not coevolved with either pathogens or reservoir host species native to the United States, which may limit the ability of H. longicornis to establish. Although cold tolerant, H. longicornis is not as hardy as Ixodes, which handily survives winters on the high prairie [e.g., North Dakota and Manitoba (Galloway 1989, Russart et al. 2014, Stone et al. 2015, Gabriele-Rivet et al. 2017)]. The aggressive spread northward of ticks such as the lone star tick, Amblyomma americanum, or the westward and northward expansion of Ixodes scapularis, is likely a much bigger threat to human health.
And yet, ticks are tough… a different Haemaphysalis species (H. flava) has been demonstrated to survive vacuum conditions during scanning electron microscopy (