Zoogeography of the Gulf Coast tick,
Amblyomma maculatum Koch
H.R. Williams, P.D. Teel, O.F. Strey, R.E. Wright1, D.E. Mock2
Texas A&M University, Department of Entomology
1Oklahoma State University, Department of Entomology
2Kansas State University, Department of Entomology
The Gulf Coast tick Amblyomma maculatum Koch is a 3-host tick. Larvae and nymphs feed on small rodents and ground dwelling birds. Adults primarily feed on the ears of large mammals and are considered an economic pest of cattle.
The original distribution of this tick follows the Atlantic Coast to the Gulf Coast with a 100 mile penetration from the coastline (Bishop and Hixson 1936). Periodically, adult ticks are collected at remote inland locations without establishing permanent populations (Harrison et al., 1997). First recognized to have established permanent populations in south-central and northern Oklahoma and southeastern Kansas in the 1960's, A. maculatum became an economic pest by 1970 (Semtner and Hair 1973, Goddard and Norment 1983).
The Gulf Coast tick was discovered to be a putative vector of Cowdria ruminantium Cowdry, the causative agent of heartwater (Uilenburg 1982). Heartwater is an exotic disease of ruminants discovered to be in the West Indies with its principal African vector Amblyomma variegatum (Uilenburg 1982). Amblyomma variegatum has been on two of the Caribbean islands since 1830, and began spreading in the 1970's and 80's. This expansion was coincident with the spread of cattle egrets Bubulcus ibis (L.) in the western hemisphere. Corn et al. (1993) confirmed that cattle egrets are responsible for inter-island dissemination of A. variegatum in the Caribbean and has raised concerns regarding bird migration to North or South America. This expansion of A. variegatum and potential for native Amblyomma to serve as vectors of heartwater increases the risk of a foreign animal disease outbreak in North America. Furthermore, there is currently no clinical lab test available which can be performed to determine the presence or absence of the heartwater disease among a suspected infected animal population. Emerging differences in zoogeography of coastal and inland populations of Gulf Coast ticks could prove important to United States agency's emergency planning to contain a foreign animal disease outbreak.
Compare historic and current distribution records outlining permanent populations of the Gulf Coast tick and highlighting the expanding range of this ectoparasite.
Compare the seasonal phenologies of Texas and Oklahoma-Kansas Gulf Coast tick populations.
Discuss the potential influence of bird migration as a mechanism of tick dispersal.
Examine the influence of precipitation on the geographic distribution of the Gulf Coast tick populations in Texas.
Collection records were compiled from the following sources:
Texas Animal Health Commission (TAHC)
USDA, APHIS, National Veterinary Services Laboratory
Kansas State University, Department of Entomology
Oklahoma State University, Department of Entomology
Oklahoma Cooperative Economic Insect Survey and Detection Reports 1954 - 1993
Texas A&M University, Department of Entomology
National Geographic Society, 1983 Bird Migration Map
NOAA Annual Climatological Summary, College Station, Texas 1978 - 1998
In the 1960's and 1970's, the Gulf Coast tick established an inland population in 27 and five Oklahoma and Kansas counties, respectively (Figure 1). In recent years, the Gulf Coast tick has expanded its range to include 65% and 18% of the counties in Oklahoma and Kansas; respectively.
In contrast to the ancestral coastal populations, established populations in Oklahoma and Kansas differ in their seasonal phenology (Figure 2). Adults, larvae, and nymphs of the Texas population are more active in August, January, and February; respectively, (Teel et al., 1988,1998) whereas April, June, and July are peak months of Gulf Coast tick activity for adults, larvae, and nymphs in Oklahoma and Kansas (Semtner and Hair 1973).
The distribution of the Gulf Coast tick in the U.S. can be influenced by a variety of factors. Migratory birds may pick up immature Gulf Coast ticks in one locale and drop them in another (Figure 3). It has been demonstrated that Gulf Coast ticks readily feed on meadowlarks Sturnella spp., a migratory species. Cattle egrets, another migratory species (Telfair 1983), are commonly seen foraging with livestock and would provide an additional mechanism for dispersal. The impact of tick dispersal by this host remains unstudied.
Inland populations may have been established by the transportation of livestock. During periods of drought in the southern U.S., livestock are commonly shipped north to more temperate interior regions where seasonal precipitation patterns promote adequate pasture forages. Collection data from the state-federal tick surveillance program in Texas suggest that precipitation cycles may influence trends in Gulf Coast tick distribution (Figure 4). The 1978 distribution map of Gulf Coast ticks followed eight consecutive years of above average rainfall. Similarly, the 1998 distribution map was preceded by an extremely wet cycle. In contrast, 1987-88 constituted an acute dry period and reflects a diminished distribution of the Gulf Coast tick. The Gulf Coast tick is subject to the effects of desiccation (Needham and Teel 1991) and the dynamics of a univoltine biology. Acknowledging that surveillance data can be influenced by a multitude of factors, these data provide a valuable tool in assessing population dynamics for presence/absence and frequency distributions in relation to other factors such as weather trends, land use, and livestock transportation.
Populations of the Gulf Coast tick in Oklahoma and Kansas have doubled and tripled, respectively their distributions between 1960-70 and 1998.
The seasonal phenology of Gulf Coast ticks in Oklahoma and Kansas is approximately five months earlier than coastal populations, making this a spring-summer versus late summer-winter problem for livestock and wildlife.
The impact of immature Gulf Coast tick dispersal by migratory birds and the subsequent establishment of adult populations outside its range remains unclear.
Trends in precipitation alter land use patterns by wildlife and domestic livestock, influencing the survival and spatial distribution of the Gulf Coast tick.
Knowledge of the Gulf Coast tick's zoogeography and seasonal phenology would influence the design of an emergency preparedness program were this tick to become involved in a foreign animal disease outbreak, such as heartwater.
Dr. Terry Bealls and Mr. Rick Nabors, TAHC, Austin,TX
Dr. Jack Schlater and Dr. J. Mertins, NVSL, Ames, IA
Dr. Don Arnold, Oklahoma State University
Dr. Bob Barker, Oklahoma State University
This project is part of Regional Project S-274.
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