Ticks in Colorado

Nearly 30 species of tick are known to occur within Colorado. Many of these ticks pose potential risks to human, pet, livestock and wildlife health. Several tick species have been found outside of their previously known distribution ranges and Coloradans may encounter additional tick species within our state or when traveling. Small mammals are important host reservoirs for many of the diseases these tick vectors may transmit. Large mammals, like deer and elk, provide both blood meals and transport of ticks as they migrate between habitats, and sometimes into our own back yards. Birds are very efficient transporters of ticks as well as competent host reservoirs for many tick-borne diseases. The surveillance and study of ticks within Colorado is limited, and more research is needed.

In addition to the Rocky Mountain Wood tick, American Dog tickBrown Dog tick, Lone Star tick and the Relapsing Fever ticks, the following ticks have been reported to occur in Colorado:

Dermacentor albipictus  (the winter tick)

Dermacentor albipictus female (left) and male (right) adults, collected from cervids in Wisconsin. Image courtesy of Dr. Paskewitz, UW-Madison, Department of Entomology

Dermacentor albipictus is one of the most common North American Dermacentor species. This is a one-host tick, meaning that it attaches to a host individual as a larva, molts to the next stage while still attached, and normally does not drop from that host individual until after it becomes an engorged female; males do not engorge but do feed. It is the only one-host hard tick species in Colorado. This tick has large geographic distribution in North America. Its distribution includes most of the USA, southern Canada, and 26 states in Mexico (Yunker et 304 al. 1986, Guzmán-Cornejo et al. 2016).  The primary hosts for this species are larger domestic and wild ungulates, such as horses, cattle, elk, moose, and deer, mountain goats and sheep. Because winter ticks are a one host species of large mammals, they are not considered a threat to human health, they can however, pose a significant threat to wildlife, moose in particular. It is considered a potential vector of the agents of Colorado tick fever and bovine anaplasmosis, Anaplasma marginale Theiler, and Babesioisis duncani. In Colorado, there are numerous instances of high numbers of winter ticks infesting horses; elk; and mule deer.  Infestations of this species cause excessive grooming in elk (Mooring and Samuel 1998) and in moose, it causes tick worry, serious alopecia, and sometimes mortality from exsanguination via heavy infestations. Hosts include domestic and wild ungulates, such as horses, cattle, elk, moose, and deer. It has been found on dogs, cats and humans.

Dermacentor parumapertus

This western hard tick species may be found in parts of Texas (west of the 100th meridian), and throughout the states of Colorado, Kansas, New Mexico, Utah, Arizona, Nevada, and California. It is also found in southwestern Idaho and southeastern Oregon, and northwestern states of Mexico. Hosts are primarily rabbits, especially for immature stages, but adults have also been found on deer and coyote. Hunters and field biologists are most likely to encounter this species through contact with with wildlife, especially rabbits. Because of its nearly exclusive association with rabbits and hares, it is a likely vector of the agent of Tularemia, should it bite humans. A spotted fever group agent, Rickettsia parkeri,  was also isoloated from this tick in TX.

Haemaphysalis chordeilis ( bird tick or grouse tick )

Haemaphysalis chordeilis occurs in CA, CO, FL, GA, LA, MI, MT, ND, NE, NH, NY, OR, PA, SC, SD, TX, VT, WA, WI, and WY in the USA, and in Canada from British Columbia east into Ontario (Lindquist et al. 2016). One of the earliest recorded collections of this tick is a nymph from a killdeer (Charadrius vociferus L.), from Fort Collins, Colorado. Hosts include mainly ground-dwelling, especially gallinaceous birds and incidentally, mammals, including humans. Haemaphysalis chordeilis has infested quail in TX in excess of 80 ticks per bird. This tick may play a role in a sylvatic cycle of the agent of tularemia  and has been reported to kill turkeys in multiple locations in the USA.

Examples of nymph (top), female (bottom left) and male (bottom right) Haemaphysalis leporispalustris ticks. Image courtesy of Dr. Paskewitz, UW-Madison, Department of Entomology

Haemaphysalis leporispalustris (the rabbit tick)

This widespread tick species can transmit the agent of Rocky Mountain spotted fever and has been shown to be naturally infected with the agents of Q fever and tularemia. Silverwater virus was also isolated from all life stages of this tick collected in WI and in Alberta, Canada. It occurs almost anywhere its hosts reside in North America  and in parts of northern and central South America. Hosts include lagomorphs, for all stages; birds, for immature stages; and rarely, humans. During years that Sylvilagus and Lepus spp. become abundant, this tick may also build large populations in Colorado.

Ixodes angustus

Nymph of Ixodes angustus, collected from rodent in Wisconsin. Photo Couresy of Dr. Paskewitz, UW-Madison, Department of Entomology

This tick is widely distributed throughout North America, it ranges from Alaska and Canada to the southwestern United States,including Colorado. Its primary hosts are rodents (mice and voles), but has also been found on shrews, birds, cats and dogs. It is considered a “nidicolous” species, meaning that it does not “quest” widely in ground vegetation, but rather in cool, moist micro-habitats in or around the nests or burrows of preferred hosts. This rodent-associated ixodid is a proven competent vector of the agent of Lyme disease, Borrelia burgdorferi s.l., in the USA  and a sylvatic vector of the agent of human babesiosis, Babesia microti. Recently, DNA of the agent of human granulocytic anaplasmosis has been detected in I. angustus ticks from California. Distribution in the USA includes AK, AZ, CA, CO, GA, ID, IN, 387 MA, ME, MI, MN, MT, NC, ND, NH, NM, NV, NY, OR, PA, RI, SD, TN, UT, VA, VT, WA, WV, WI, and WY; also found in northern Mexico, as well as all of the southern Canadian provinces, from British Columbia east to Newfoundland. Hosts are primarily small rodents, but also shrews, domestic cats, domestic dogs, and humans. This tick has suggested transmission of Lyme disease to a child in a Washington State case study.  In a recent study by Xu et al. 2018 I.angustus was identified as a human biter predominantly from submissions from Washington State.

Ixodes baergi

Ixodes baergi occurs in AR, CO, IL, MI, OK, TX and Ontario, Canada, but it may be present undetected elsewhere in association with its typical hosts, cliff swallows. Larvae and nymphs feed on adult birds, whereas adult females feed on nestlings and have a specific drop-off rhythm that occurs between 2200 (10 p.m.) and 0400 hrs. (4 a.m.). Males were not collected from hosts; they rested nearby (on culvert walls), suggesting that they may not feed as adults. In TX, this species appears to overwinter as larvae. An uncharacterized, transovarially transmitted arbovirus (Bunyaviridae), was isolated from I. baergi in OK.

Ixodes (Trichotoixodes) brunneus Koch, 1844

Durden and Keirans (1996) stated that I. brunneus is an ectoparasite of avians, migratory passerines in particular, and, because of this strong host association, it can be found in any of the United States. This tick’s known distribution in the USA includes AL, AR, CA, CT, FL, GA, IN, KS, MA, MD, MO, MS, NC, NJ (New Jersey), NY, OH, OK, OR, PA, SC, TN, TX, VA, WI (Durden and Keirans 1996), and MI (Keith et al. 2015); in Canada it occurs in Manitoba (MB), New Brunswick, and ON (Lindquist et al. 2016). We now add CO to the USA distribution, based on a recent collection: 17 May 2018; collection number CHAT-002; one larva ex American yellow warbler, Setophaga petechia (L.) (Parulidae); Jefferson Co.; deposited in the C. P. Gillette Museum of Arthropod Diversity, Colorado State University, Fort Collins. This tick is well known for causing paralysis, primarily in passerine birds (Luttrell et al. 1996), but it also has been associated with the agent of RMSF (Clifford et al. 1969), though its vector potential for this pathogen is unknown. Recently, Candidatus Rickettsia mendelii Hajduskova et al. (possibly non-pathogenic, also found in Palearctic Ixodes [Ixodes] ricinus [L.]) was detected in all active stages of I. brunneus collected in southeastern VA (Cumbie et al. 2020).

Adult female Ixodes cookei. Photo courtesy of Dr. Paskewitz, UW-Madison, Department of Entomology

Ixodes cookei  (the groundhog or woodchuck tick)

The status of Ixodes cookei as a Colorado resident is based on a single record in Denver, reported by Banks (1908). This specimen probably was actually I. marmotae, a similar western species that was not recognized and described until three decades later. The recent mention of I. cookei in CO (Cranshaw and Peairs 2014) probably originates similarly and should also refer to I. marmotae. This hard tick species is very similar in appearance to Ixodes scapularis. It is common and widespread on American wildlife, primarily east of the Rocky Mountains. The natural transmission cycle of Powassan virus includes the enzootic or sylvatic transmission between Ixodes cookei and small and medium-sized rodents (mice and voles) and carnivores. The primary hosts for this tick are a variety of carnivores (including domestic cats and dogs) and rodents and can be especially abundant on groundhogs (Marmota monax). It is considered a nidicolous species, meaning that it does not typically “quest” widely in ground vegetation, but inhabits cool, moist microhabitats in or around the nests or burrows of preferred hosts. It is known as an occasional human biter.

Ixodes howelli

The geographic distribution for this little-known species includes at least AK, CA, CO, MT, and TX  but, like I. baergi, it may be more widespread. Its presence in Colorado is based on a single nymph collected in Weld County in 1943 from a prairie falcon and identified by Kohls and Ryckman (1962). Hosts are primarily cliff swallows; but also other birds that may nest nearby, including the gray-crowned rosy finch, the prairie falcon; and the gyrfalcon. This tick has no known medical or veterinary importance.

Ixodes kingi  (the rotund tick)

This hard tick occurs throughout western North America and occasionally in the east. Its distribution in the USA includes AZ, CA, CO, IA, ID, KS, MI, MS, MT, NC, ND, NE, NM, NV, OK, OR, SD, TX, UT, and WY; collections in the eastern USA (GA, MD, and OH) are dubious. In Canada, it occurs in Alberta, British Columbia, Manitoba, and Saskatchewan.  It is commonly found in prairie habitats, but may also occur in shrubland and forested habitats. It has been recorded from domestic animals, including cats and dogs, several species of carnivores and rodents, and a few lagomorphs. Hosts include pocket gophers, kangaroo rats, and sigmodontine mice in areas west of the Rocky Mountains; and carnivores, ground squirrels, and prairie dogs in areas east of the Rockies. Overall, several spp. of carnivores, especially weasels and American badgers, domestic dogs. Observations from Colorado show I. kingi commonly infesting rodents (such as grasshopper mice). The agents of Q fever, tularemia, and Colorado tick fever have been isolated from this tick species  It is known to bite humans rarely.

Ixodes marmotae

Distribution for this species includes, in the USA, CO, ID, MT, OR, UT, WA, and WY; in Canada, British Columbia. Hosts include marmots, porcupines, woodrats, and ground squirrels. We have been unable to find any reported medical or veterinary importance for this tick species.

Nymph of Ixodes marxi. Image courtesy of Dr. Paskewitz, UW-Madison, Department of Entomology

Ixodes marxi

Banks (1908), in his description, reported a single female of I. marxi, collected from a fox, also from Denver, but we found no Colorado collection records since then. Cooley and Kohls (1945) questioned this record, citing the anomalous host and primarily eastern distribution of the tick species; a population in Colorado would be remotely disjunct and the host record would be unique. Conversely, fox squirrels were introduced to Denver and 5 other urban areas of CO in the early 1900s (Armstrong et al. 2011), and they are hosts for I. marxi in the eastern USA. This introduction suggests a possible misstatement of the host by Banks (i.e., “fox” instead of “fox squirrel”), and perhaps an ephemeral presence of I. marxi on a recently introduced host. This hard tick is most commonly associated with squirrels, but chipmunks, fox and raccoons are also hosts. The natural transmission cycle of Powassan virus includes the enzootic or sylvatic transmission between Ixodes marxi and small and medium-sized rodents (mice and voles) and carnivores.

Ixodes ochotonae

The geographic distribution of I. ochotonae in the USA includes CA, CO, ID, MT, NV, UT, WA, and WY; in Canada, British Columbia and Saskatchewan.Foley and Piovia-Scott (2014) detected Anaplasma phagocytophilum DNA in Ixodes ochotonae ticks from California.  Hosts include primarily pikas and woodrats, but also chipmunks, pocket gophers, voles, and native mice.

Adult female Ixodes sculptus. Photo courtesy of Dr. Paskewitz, UW-Madison, Department of Entomology

Ixodes sculptus

This tick occurs throughout western North America; occasionally in the east. Distribution of I. sculputus in the USA includes AZ, CA, CO, IA, ID, IL, IN, KS, LA, MI, MN, MT, ND, NE, NM, NV, OK, OR, 443 SD, TX, UT, WA, WI, and WY; in Canada, Alberta, British Columbia, Saskatchewan and Manitoba. It is commonly found in prairie habitats, but may also occur in shrubland and forested habitats. Agents of tularemia, Q fever, and  Colorado tick fever have been isolated from this species, suggesting it could be an enzootic vector. It is most commonly associated with squirrels, usually burrowing mammals, and their predators.A study in Colorado found that hosts include several species of burrowing mammals, especially thirteen-lined ground squirrels (Spermophilus tridecemlineatus). Several species of carnivores, including domestic dogs, cats, as well as rodents, lagomorphs (rabbits) and goats have also been found to host this tick. It is a rare human biter.

Ixodes spinipalpis

Ixodes spinipalpis, Image is courtesy of Joel Hutcheson, USDA APHIS National Veterinary Laboratory

This hard tick is a wide ranging species throughout the western US. It verifiably occurs only in the Pacific Coastal and Mountain States and Provinces of the USA and Canada, including CA, CO, ID, MT, NV, OR, SD, UT, WA, Alberta, and British Columbia. Claims of the occurrence of this tick in NM, TX, and Mexico appear to be questionable. Adult and immature I. spinipalpis feed on numerous species of rodents, lagomorphs (most recently in Boulder County, CO 2018), and sometimes humans; immature stages also feed on birds, as we have also seen in recent collections from migratory birds sampled in two counties in Colorado. It is a sylvatic vector of the agents of Lyme disease-like illness (Borrelia bissetti), human granulocytic ehrlichiosis/anaplasmosis, and human babesiosis. Canadian researcher, John Scott isolated Borrelia genospecies 2 (Scott et al. 2017), another member of the B. burgdorferi species complex (i.e., s.l.) from it; Keirans and Clifford (1983) isolated Powassan virus from Ixodes spinipalpis collected in SD; and Dr. Heather Szerlong, CEO of Ticknology Lab has detected a potentially novel species of Borrelia Burgdoferi s.l. as well as Anaplasma phagocytophilum  in a specimen collected from a Colorado wild rabbit in 2018.

Adults and immature feed on numerous species of rodent hosts that have been found to harbor these pathogens in Colorado; the Mexican pack rat, prairie vole and deer mouse have been documented in a Colorado study. Lagomorphs (rabbits) are also hosts. A recent study in Canada documented this tick on an eastern cottontail.  Because this tick species has been conventionally considered nidicolous, it would be expected to spend most of its time on a host or in nesting material; however, immature I. spinipalpis quest for hosts outside of actual nest sites, making them a potential threat to incidental hosts as competent  vectors of multiple pathogens of public health importance. Studies in California and Colorado have documented questing behavior with these ticks in areas of wooded habitat.  This tick has been recorded during bird banding studies in Colorado and Canada. Studies suggest that these ticks avoid arid open grassy habitats, but questing behavior in shaded wooded habitats may increase human exposure potential to diseases, including Lyme, Anaplasmosis and Babesia in Colorado and in other regions of the US and Canada where this tick occurs.  In a recent study by Xu et al. 2018 on the west coast, I.spinipalpis was identified as a human biter predominantly from submissions from California and Washington State. More surveillance and research is needed regarding this tick’s potential risk to human health.

Adult female Ixodes texanus, Image courtesy of Dr. Paskewitz, UW-Madison, Department of Entomology

Ixodes texanus

Distribution in the USA includes AL, AR, AZ, CA, CO, CT, DE, FL GA, IA, ID, IL, IN, KS, LA, MD, MI, MO, MT, NC, NM, NY, OH, OK, OR, SC, TN, TX, UT, VA, WA, WI, and WV; in Canada, British Columbia, Quebec, and Ontario. It is also thought to occur in Mexico, specifically the states of Guerrero and Nuevo Leon. The agents of raccoon babesiosis, ehrlichiosis  and Rocky Mountain spotted fever have been detected from Ixodes texanus. Hosts include weasels, badgers, skunks, and other mustelid carnivores in western states of the USA, and primarily raccoons in the East. Occasional hosts include Virginia opossums,rodents, lagomorphs; domestic dogs and humans.

Ixodes woodi

The distribution of I. woodi in the USA includes AL, AZ, CA, CO, ID, IN, KS, NC, NM, NV, OK, 495 OR, SC, TX, UT, and WY; in Mexico,  the states of Coahuila, Morelos, and Tamaulipas. DNA of the agent of human granulocytic anaplasmosis was isolated in Ixodes woodi ticks in California; although no vector status was demonstrated, it seems likely that this tick is involved in a zoonotic cycle of the agent among woodrats.  Hosts are primarily woodrats, but also other rodents, birds, shrews, and incidentally, humans.

Soft Ticks

Argas cooleyi

Hosts include birds, especially cliff swallows.

Carios kelleyi

Carios kelleyi (Bat Tick), Image by James Gathany-CDC

This soft tick is widely distributed in the US, collected from at least 24 states (California to New York, and from New Mexico and Texas to Minnesota). Hosts for this tick species are bats. Unlike many species of argasid ticks, C. kelleyi larvae are slow feeders requiring days (9-20) to engorge, which may explain the widespread distribution of this species as it may be easily transported by its host. A Borrelia bacterium has been identified in this tick a as a relapsing fever spirochete most closely related to, but distinct from Borrelia turicatae. The name Borrelia johnsonii is proposed for this novel spirochete in honor of Dr. Russell C. Johnson. Human encounters may occur when bats vacate roosts or hibernacula, especially if roosting in homes. It is a known human biter.

Otobius megnini  (the spinose ear tick)

This one-host tick attaches most often deep within the host ear canal as larvae and drop from that host as engorged nymphs; adults do not feed.  Hosts include wild and domestic ungulates, especially pronghorns, and occasionally humans. Spinose ear ticks are generally associated with semiarid or arid environments such as those found in the southwestern US, however they can also be found in other climate areas due to transportation of livestock. Larvae and nymphs usually remain within the ears of their host. Fully grown nymphs and adults live off of the host, but still within the host’s general environment. They usually prefer dry, protected places such as in cracks and crevices or under logs and fence posts.

Otobius nymph-Image by CDC

Otobius lagophilus

This tick is rarely encountered in Colorado. Otobius spp. are one-host ticks that attach as larvae and feed for weeks before dropping from the host as engorged nymphs. The adults do not feed. Unlike O. megnini, it usually attaches elsewhere on the head of its host, rather than in the ear canal like the Spinose Ear tick. Hosts of this species are lagomorphs (rabbit, hare, and pika).


Awaiting publication: Ticks and Tick-borne Diseases of Colorado, USA, Including New State Records for Argas radiatus (Ixodida: Argasidae) and Ixodes brunneus (Ixodida: Ixodidae)

Joel Hutcheson,1,4,5 James W. Mertins,1 Boris C. Kondratieff,2 and Monica M. White3