Gary Mullen and Renee Anderson

Ticks are a natural part of the outdoors, feeding on a wide range of wild animals, livestock, household pets, and humans. In an effort to better understand the tick species involved, particularly in relation to people and the five tick-borne diseases documented in Alabama, AAES entomologists are conducting an on-going study of the occurrence of these blood-feeding parasites on Alabamians.

From 1989-99, the first 11 years of the study, researchers examined more than 900 ticks removed from Alabama residents. Most specimens were received from physicians or from individuals referred to the study by their county health departments. Specimens were received from 57 of Alabama's 67 counties (see map), providing a representative, statewide perspective of the ticks that attack humans. These ticks were identified according to species, and specific information was recorded about each tick, including the collection date; location by county and nearest town or city; developmental stage; gender, in the case of adult ticks; and attachment site on host.

These data provide a general picture of the ticks that attack humans in Alabama, including valuable information on the exposure of residents to different tick species, the developmental stages most frequently encountered, geographic and seasonal occurrence of individual species, and potential exposure risks to ticks that transmit human disease agents in Alabama.

For ticks, the four developmental stages are egg, larva, nymph, and adult. Eggs typically are deposited in large masses (Figure 1), from which tiny larvae, called “seed ticks,” emerge. The larvae attach to and feed on animal hosts, then drop off the hosts to develop into nymphs. The nymphs, in turn, feed on other hosts, then drop off and develop into adults to begin the life cycle once again. Often, within the same species, animals that serve as hosts of the larvae and nymphs are different from those of the adult ticks.

Figure 1. Egg mass produced by lone star tick. A single female typically produces 3000-4000 eggs after feeding on a host.

Figure 2. Lone star tick, adult female, distinguished by conspicuous white spot on back.

During the study’s first 11 years, five of the 17 species of ticks that are known to occur in Alabama were recovered from humans. Those five included the lone star tick (Amblyomma americanum) (Figure 2) the Gulf Coast tick (Amblyomma maculatum), the blacklegged tick (Ixodes scapularis), the American dog tick (Dermacentor variabilis), and the brown dog tick (Rhipicephalus sanguineus). With the exception of the brown dog tick, which largely is limited to feeding on dogs, these species parasitize a wide range of animals. Generally, as adults, they attach to large mammals other than humans; however, they can and do readily attach to people who enter tick-infested areas.

The percentages of ticks recovered from human hosts in this study are presented in Figure 3. The developmental stages of the ticks involved and their relative numbers are shown in Figure 4.

Figure 3. Ticks recovered from human hosts in Alabama, based on 934 specimens examined.

Figure 4. Developmental stages of ticks removed from human hosts in Alabama, 1989-1999.

The tick most commonly found on humans in Alabama during the study was the lone star tick, the adult female of which is easily recognized by a conspicuous white spot on the back (see Figure 2). It represented more than two-thirds of all ticks submitted for identification. Nymphs were the developmental stage most frequently recovered from human hosts, comprising 57% of the lone star ticks submitted. Larvae were involved in 12% of the cases, with the remaining more or less equally divided between adult males (19%) and females (12%). Adults are particularly common on white-tailed deer but also are found on raccoons, rabbits, dogs, cats, and other medium-sized mammals. The immature stages are typically parasitic on rodents, other small mammals, and a variety of birds (e.g., wild turkey, bobwhite quail, owls). Altogether, more than 26 mammals and 10 varieties of birds have been recorded as hosts for the lone star tick.

The second most common species was the American dog tick, accounting for 18% of the ticks found on humans during the study period. With the exception of a small number of nymphs (3% to 4%), all the ticks of this species were adults, represented by approximately the same number of males (45%) and females (52%). It parasitizes a wide range of small to medium-sized mammals. Adults are particularly common in Alabama on opossums, raccoons, dogs and foxes, in addition to squirrels, woodchucks, skunks, bobcats, and domestic cats. Hosts of the immature stages occur primarily on rodents, such as the cotton mouse, cotton rat, and rice rat.

The other three of the five ticks were found less frequently on humans. The brown dog tick, Gulf Coast tick, and blacklegged tick comprised only 7%, 4%, and 3%, respectively, of the total ticks examined. The majority of brown dog ticks found on humans were nymphs (72%), followed by adults (22%), and only a few larvae (6%). The Gulf Coast tick was recovered from people primarily as adults (86%), with the remainder being nymphs. The blacklegged tick was the least commonly submitted species. Two-thirds of the specimens of this tick were adult females, followed by males (31%), a few nymphs (3%), and no larvae.

The brown dog tick occurs almost exclusively as both immatures and adults on dogs. Those that are found on humans often are crawling on the clothing or skin and are reluctant to attach. Only nine of the 67 brown dog ticks recovered from humans in this study were confirmed as attached, although in the majority of these cases, it was not specified whether they were attached or crawling.

The Gulf Coast tick, at the adult stage, prefers large mammals. It was a significant pest of domestic animals such as cattle, horses, mules, and sheep in Alabama at one time, before effective pesticides became available and before the tick was eliminated from many pastures by predatory fire ants. Still, under certain circumstances, adult Gulf Coast ticks can become abundant in specific local areas and can attach to humans and dogs that pass through the infested sites. The larvae and nymphs are primarily parasitic on ground-inhabiting birds and rodents.

The blacklegged tick exhibits a wide host range, with white-tailed deer being the preferred host of the adults. Adults also feed on other mammals, such as raccoons, bobcats, dogs, and cats, whereas the immature stages feed notably on reptiles (e.g., skinks, lizards), a variety of birds, and rodents.

Of the five ticks recovered from Alabamians during this study, all except the brown dog tick serve as vectors of diseases in humans. For this reason, identification of ticks removed from human hosts, together with an appreciation of the role that each tick plays in the transmission of human pathogens, can be very helpful in assessing the potential health risks posed in individual cases of tick infestations.

The AAES entomologists are continuing to record data on all ticks received from human hosts so they can continue to build a database on the geographic and seasonal occurrence of ticks in the state.

Anyone who removes a tick from a human host in Alabama can contribute it to the researchers’ continuing study (Mail Those Ticks to Us!). Researchers are particularly interested in receiving ticks from Barbour, Bullock, Cleburne, Crenshaw, Dale, Franklin, Geneva, Henry, Lamar, and Washington counties.