The Lone Star Tick (Amblyomma americanum) was the first tick to be described in the United States, in 1754 (Childs and Paddock 2003). Female Lone Star Ticks have a conspicuous whitish spot (usually just one) on the back near the posterior end of the scutum (the scutum is the hard shield extending over roughly the anterior third of a female hard tick's dorsal surface and may be obscured in an engorged tick) (Cooley and Kohls 1944). Males lack this conspicuous white spot and, as in other male hard ticks, the scutum extends over most of the dorsal surface.
Because of its aggressive and mostly non-specific feeding habits and its high population densities, the Lone Star Tick is one of the most annoying and economically important ticks in the United States. Adults parasitize medium and large mammals (including cattle), and the larvae and nymphs feed on a wide variety of small to large mammals and ground-feeding birds (a large number of known hosts are listed in Cooley and Kohls 1944 and Bishopp and Trembley 1945), although availability of large mammalian hosts such as White-tailed Deer is likely essential to maintain large populations (Childs and Paddock 2003). Bishopp and Trembley (1945) counted around 4800 ticks, mainly Lone Star nymphs, on a single ear of a deer.
The Lone Star Tick is a three-host species, and is a general feeder in all its active stages (Bishopp and Trembley 1945). All three motile life stages will bite people (Cooley and Kohls 1944; Bishopp and Trembley 1945; Goddard and Varela-Stokes 2009), but it was not until the early 1990s that this tick was shown to be the principal vector for any human disease. Since the late 1980s, researchers have come to view this tick as more than just a nuisance (from a human perspective), but in fact an important vector of several diseases affecting humans (Childs and Paddock 2003). Although these diseases--human monocytic (or monocytotropic) ehrlichiosis (HME), Ehrlichia ewingii ehrlichiosis, and southern tick-associated rash illness (STARI)--were presumably present in the United States prior to their recognition, it is very likely that the prevalence of both these three diseases and the Lone Star Tick itself increased as a result of exploding populations of their keystone host, the White-tailed Deer, during the 20th century (Paddock and Yabsley 2007).
An innovative effort to control populations of ticks relying heavily on deer hosts has involved the "4-poster" topical treatment device, which passively applies acaricide (i.e., mite-killing chemicals) to the head, neck, and ears of deer as they feed from the device. Pound et al. (2000), for example, applied the acaricide amitraz to White-tailed Deer through free-choice interaction with a "4-poster" device and observed a significantly reduced abundance of free-living Lone Star Ticks.
Lone Star Ticks are active from early spring until fall; adults appear in February, and population numbers peak in May or June and diminish by the end of July. Nymphs may appear as early as March after overwintering as flat nymphs and are active until August or September with two peaks of activity, one in May or June and one again in August representing the in-year population. Larvae are generally active from June through October, with numbers peaking in August. In the southern part of its range, the activity of the Lone Star Tick begins in February and declines by July, while in the northern part of the range, activity may begin later with peak activity in June and July. (Goddard and Varela-Stokes 2009 and references therein) Lonestar Ticks overwinter as replete larvae, unfed or replete nymphs, or unfed adults.
The lone star tick is endemic to the United States, occurring from west-central Texas north to the lower Midwest, east to the Atlantic Coast and northward all the way to Maine. This range represents a significant range expansion for this species, which was regarded as an essentially southern species well into the latter half of the 20th century (Good 1972; Cooley and Kohls 1944; Goddard and Varela-Stokes 2009 and references therein).
Although the time it spends feeding on a host is a critical period in a tick's life cycle, ticks spend most of their lives off-host. Between blood meals, nutrient reserves must be used economically and body water content must be maintained or desiccation and ultimately death results. Body water homeostasis is among the most important processes that influences off-host survival. Ticks as a group can survive longer without food or drinking water than any other arthropod. Needham and Teel (1991) review the physiological challenges and coping mechanisms associated with the "gorging-fasting" life history characteristic of ticks.
The Lone Star Tick inhabits meadows, woodlands, and hardwood forests. Its primary hosts are diverse wild and domestic mammals, although deer are considered to be definitive hosts (hosts upon which the reproductive stage depends) (Parola et al. 2005).
The Lone Star Tick is a known or suspected vector for several diseases affecting humans. Transmission of a number of diseases caused by rickettsiae has been attributed to the Lone Star Tick at one time or another. Although in the past this tick was believed to carry and transmit the bacterium Rickettsia rickettsii, the cause of Rocky Mountain Spotted Fever, much evidence now suggests that this rarely, if ever occurs, although a normally non-pathogenic rickettsia that has been referred to as Rickettsia amblyommii may cause mild disease in some individuals (Goddard and Varela-Stokes 2009 and references therein). In the early 1940s, a mysterious disease characterized by low white blood cell counts, fever, and severe occipital headache affected many (>1000) soldiers at Camp Bullis, Texas, near San Antonio. Investigations indicated that this disease was caused by an unidentified rickettsia transmitted by Lone Star Ticks, but Bullis Fever has not been reported since the late 1940s. (Childs and Paddock 2003; Goddard and Varela-Stokes 2009 and references therein) Based on current knowledge, the likelihood of Lone Star Ticks being important vectors for any rickettsial diseases of humans appears to be low.
Since first being recognized in 1986, a disease known as human monocytotropic ehrlichiosis (HME) has become an important public health issue in the southeastern and south-central United States. HME may cause morbidity and can result in severe illness, and even death, if left untreated or if treatment is delayed. In contrast to Rocky Mountain Spotted Fever, an associated rash is relatively uncommon in HME patients. Hundreds of cases a year are now reported and it is possible that many cases go unreported. At least one author has suggested that Bullis Fever (see above) may actually have been HME. The causative agent of HME, Ehrlichia chaffeensis, was not isolated until 1991. Considerable evidence indicates that the Lone Star Tick is an important vector for E. chaffeensis and that White-tailed Deer and other wild and domestic mammals (including dogs) are important hosts. The greatest influence on the emergence of ehrlichioses associated with Lone Star Ticks has been the explosive growth of White-tailed Deer populations in the United States. Other pathogenic Ehrlichia may also be transmitted by the Lone Star Tick. (Childs and Paddock 2003; Goddard and Varela-Stokes 2009 and references therein)
Several diseases caused by non-rickettsial bacteria can be transmitted by Lone Star Ticks. Tularemia is caused by infection with Francisella tularensis, which can be transmitted by Lone Star Ticks (as well as by other routes), often via cottontail rabbits. Lyme Disease, caused by the spirochete bacterium Borrelia burgdorferi, is apparently not transmitted by the Lone Star Tick, but only by the Ixodes ticks I. scapularis in the eastern United States and I. pacificus in the western United States. However, a Lyme Disease-like illness has been reported in the southern United States that has been referred to as ‘‘southern tick-associated rash illness’’ (STARI) or Master’s disease and this illness appears to be associated with the Lone Star Tick. Inconclusive data suggest the possibility that STARI may be caused by a recently discovered Borrelia, B. lonestari, isolated from the Lone Star Tick. (Goddard and Varela-Stokes 2009 and references therein)
There is evidence that Lone Star Ticks are capable of transmitting pathogenic viruses to humans, but so far no indication that this is a common or widespread phenomenon. The same can be said regarding transmission by Lone Star Ticks of pathogenic protozoans. Although much has been learned about the role of the Lone Star Tick (and ticks in general) in transmitting disease organisms to humans, many questions and uncertainties remain, making investigation of the ecology of tick-borne diseases an exciting and dynamic area of research (Goddard and Varela-Stokes 2009 and references therein)
