Ticks and Tick-borne Diseases
Richard M. Houseman
Associate Professor of Entomology
Ticks are the most important vectors of disease in domestic and wild animals throughout the world and are second only to mosquitoes as transmitters of human disease. They harbor and transmit various pathogens including protozoa, viruses and bacteria to humans and their companion animals. This guide provides general information about ticks, tick-borne diseases and tick management practices for important biting ticks in Missouri.
Ticks are arthropods that are closely related to mites and spiders. The more than 800 species worldwide are divided into three families, the Ixodidae, Argasidae and Nuttallellidae. The hard ticks (Ixodidae) form the largest family with 14 genera worldwide. They are by far the most commonly encountered ticks feeding on humans or wild and domestic animals. Soft ticks (Argasidae) are less diverse with just four genera worldwide. The family Nuttalliellidae is very rare and consists of only a single African species. North America is home to all four genera of soft ticks and seven genera of hard ticks.
All ticks have a life cycle consisting of an egg, larva, nymph and adult. They eat only blood. All stages and both sexes require a blood meal for proper development. They locate and feed on a wide variety of vertebrate hosts and will normally attack any host they encounter in the environment. Tick-borne diseases result when pathogens acquired from one host infect the tick and are later transmitted to a second host during blood feeding. Management of tick populations is important to minimize the risks of tick-borne disease. Management consists of modifying tick habitats, minimizing host abundance and using personal protective measures such as tick removal, tick repellents and pesticide application when necessary.
Tick morphology consists of two primary regions, the mouthparts (capitulum) and the body (idiosoma). The mouthparts on hard ticks protrude in front of the body and are visible from above (Figure 1), but the body of soft ticks extends forward above the mouthparts so they are only visible from beneath (Figure 1). The body of ticks includes the eyes, legs, and respiratory, digestive and reproductive structures.
The mouthparts (capitulum) have three specialized structures called palps, chelicerae and a hypostome that are attached to a base called the basis capituli (Figure 1). These structures function together and allow ticks to penetrate the host skin and extract a blood meal from the tissues below (Figure 1). The palps are leglike structures that do not penetrate host skin but are sensory in function. The chelicerae are cutting organs whose cutting surfaces face outward. They cut by inserting the chelicerae into the skin then pushing them outward from the midline to open a hole that allows the hypostome to be inserted. The hypostome is the attachment organ, and the opening to the digestive tract is found on the tip of the hypostome. Small rows of backward-facing spines (denticles) on the outside of the hypostome help anchor the tick in the host skin tissues. In addition, many ticks secrete “cement” from the salivary glands around the bite site, which functionally glues the mouthparts to the host.
The body (idiosoma) of ticks is typically not hardened to a great extent. In hard ticks, most of the exterior cuticle is soft and has many internal folds that look like grooves on the surface of the body. The uniform, rectangular folds located on the rounded posterior end of hard ticks are called festoons (Figure 1). Unfolding and stretching of the soft cuticle along these grooves allows immature and adult female hard ticks to take enormous blood meals and swell to weigh 50 to 100 times their original weight. Soft ticks show a lot of variation in the structure of their cuticle, including various wrinkles, bumps or nipplelike appendages
Hard ticks possess a single upper plate near the front of the body, which provides protection like a shield (Figure 4). This plate also serves as an attachment point for the many muscles that are associated with the mouthparts. The scutum may show various patches of color, ranging from off-white to gold, green and red. Male and female hard ticks can be distinguished from one another by the size of the scutum. In males, the scutum is large and covers the entire upper surface of the body, whereas in females the scutum is much smaller and covers less than half of the upper body surface, thus allowing the body to greatly expand during engorgement with host blood. Soft ticks rarely possess a scutum, but they have small, smooth, disk-shaped structures that provide sites for internal muscle attachment. These smooth areas are arranged in various patterns. One pair of eyes is normally located near the front corners of the scutum in hard ticks and is found underneath in soft ticks.
The underside of the tick body is where legs and respiratory, digestive and reproductive structures are located. The legs normally have six segments. On the end of each leg is a claw; the tick uses these claws to cling to and move around on the host. The legs have several functions in addition to locomotion. The segment at the base of each leg often has spines that aid in attachment to the host (Figure 1). The segment at the tip of the first pair of legs has a specialized sensory organ called Haller’s organ. This organ is a small capsule that contains heat, moisture and chemical receptors. It functions in a similar way to the antennae of insects. These receptors are very important for tick survival and host location.
On each side of the body, just behind the last pair of legs, is a plate that contains openings to the respiratory system (spiracles). Breathing occurs through these openings and not through the mouthparts. The spiracles are controlled by the tick and can be closed as needed to limit water loss. The anus is located toward the rear of the body and may be bordered by distinctive grooves. The opening of the reproductive system is not next to the anus but is located about halfway up the body between the anus and mouthparts (Figure 1).
Key physical features of male and female hard ticks on the top (dorsum) and bottom (venter) of their bodies.
A typical soft tick. Soft ticks have no dorsal shield, are leathery and light-colored, and have small bumps on their bodies.
A typical hard tick (male). Hard ticks have a dorsal shield, are hardened and often dark-colored, and have folds on their bodies.
A brief discussion of tick morphology is a useful introduction to the biology of hard and soft ticks and is also necessary to successfully identify different kinds of ticks using illustrated guides. An illustrated identification key to the genera of hard and soft ticks in North America is shown in Figure 4.
Identification key to the genera of adult ticks in the United States (CDC, Atlanta, GA).
Common life cycle of hard ticks. Most hard ticks are called three-host ticks because they must locate three separate hosts during their life cycle — one for each stage of development. (Note that the on-host periods for each life stage are really very short compared to the total length of the life stage.)
Ticks have a general arachnid development, which includes a six-legged larval stage called a seed tick, followed by an eight-legged nymphal stage, followed by an eight-legged adult stage. Ticks shed their exoskeleton (they molt) between each of these stages. Hard ticks have a single larval and nymphal stage, but soft ticks have more than one larval stage and two or more nymphal stages before becoming adults. The total time for development from egg to adult may take 24 years for hard ticks and 10 years or more for soft ticks. Both sexes of ticks must parasitize and feed on the blood of a vertebrate host in all of their life stages.
Hard ticks all follow a very similar life cycle with little variation (Figure 5). Most are called three-host ticks because they must locate three separate hosts during their life cycle, one during each stage of development. Some hard ticks are called two-host ticks because the larva and nymph feed on the same host and then molt to the adult who finds a new host.
In a three-host tick, the larva must find a host, get on the host, attach to the host, engorge on the host’s blood, drop from the host, and molt to the nymph stage. The nymph must then find another host, get on that host, attach to that host, engorge on that host’s blood, drop from that host, and molt to the adult stage. Finally, the adult must find a third host, get on the host, attach to the host, engorge on the host’s blood, mate on the host, and drop from the host, after which females lay eggs. The three hosts in a tick life cycle may be of the same or different species. Many hard ticks feed on small animals during the larval and nymphal stages and on large animals during the adult stage. This kind of behavior facilitates the movement of pathogens from one host to another.
Soft ticks have life cycles that vary in the number of stages and feeding bouts. They are nest parasites and are likely to feed repeatedly on the same animal or the same family group of animals within the nest. Most soft ticks do not attach to the host or enlarge greatly while feeding. Those that do attach only do so during the larval stage. Nymphs and adults feed quickly on the host while it is in the nest and then return to their resting or hiding place within the nest.
Hard ticks spend most of their time on the ground. Larvae hatch from eggs on the ground; larvae and nymphs molt to the next developmental stage on the ground; and adult female ticks are on the ground as they produce and lay eggs. Ticks spend over 95 percent of their lives off their hosts. Most of this time off the host is spent in a state of suspended animation (diapause) both before and just after they molt to the next stage of development. Ticks are able to withstand long periods of starvation while off the host; some studies have shown that unfed ticks can survive more than a year. Interestingly, they may live even longer than some of their hosts. The main stress factor for ticks off the host is water loss from their bodies, which can lead to desiccation.
While on the ground, hard ticks must locate a humid, cool environment to avoid body water loss and desiccation, which would kill them. They typically find a suitable location in the relatively sheltered environment of the layer of soil and leaf litter at the base of the vegetation to enter diapause (Figure 6). In contrast to hard ticks, soft ticks are more specialized for living in dry habitats characteristic of their host’s nesting conditions, and the leathery exoskeleton of soft ticks is more effective for preventing water loss.
The beginning and end of diapause in hard ticks is regulated mostly by soil temperature and hours of daylight. Upon emerging from diapause, hungry ticks climb out of the sheltered soil-litter layer and begin to search for a host using a behavior called questing.
Questing is an ambush strategy in which ticks climb onto vegetation and cling head down to blades of grass or branches of shrubs or trees with their hind legs and hold their forelegs widely out in front of their bodies while waiting for a suitable host to pass by (Figure 7). During this time, they are subject to the warm, dry air currents and begin to lose body water. When body water drops to critical levels, they must stop questing and return to the litter layer where they replenish body water by absorbing water vapor through the exoskeleton.
Questing ticks do not jump, fly or drop from trees onto a host. They climb vegetation to a certain height, based on stage of development, and wait for a passing host to latch onto.
Adults climb higher and therefore are more likely to be found on large hosts, whereas larvae and nymphs, who are more susceptible to desiccation and stay closer to the soil, are more likely to come in contact with smaller hosts. Ticks detect host movement and other cues using Haller’s organ. They use the specialized claws on their front legs to snag onto the body covering (hair, fur, clothing) of passing hosts. In addition to questing, ticks may detect a stationery host using Haller’s organ; studies have shown they may travel up to 32.8 feet to climb onto these hosts. The primary limiting factor to the time spent questing or otherwise searching for hosts is water loss, so air temperature and solar radiation have the greatest influence on tick activity patterns.
Once on the host, ticks instinctively move upward to locate a protected site where they settle down to attach and feed (Figure 8). Ticks begin feeding by cutting a hole in the host skin using the chelicerae. The outer edges of the chelicerae cut the skin, so cutting is done as the chelicerae are pulled apart. The hypostome is inserted into this hole (Figure 9). The salivary glands of hard ticks secrete a cement substance that “glues” the mouthparts in place. If ticks are disturbed during this initial feeding phase, before the cement has locked them in place, they can pull out the hypostome and look for another feeding site. However, after the cement has locked them in place, they cannot detach immediately but rather remain in place until they are dislodged or until they have fully engorged (Figure 10). Inserting the mouthparts normally takes about 15 to 30 minutes but can take up to 1 or 2 hours. Only the slender mouthparts actually penetrate the skin.
All ticks secrete enzymes into the host’s blood to counteract the effects of clotting and maintain blood flow. Hard ticks may feed for three to seven days. Feeding is not continual. Engorgement is slow at first but becomes rapid later, with dramatic increases in size occurring within the last 24 to 48 hours of feeding(Figure 11). Studies have shown that engorged larvae may weigh 7 to 20 times their original weight, nymphs 9 to 80 times their original weight, and adult females 50 to 100 times their original weight. Soft ticks normally feed for less than one hour and do not engorge, so weight gain is not dramatic. The probability of disease transmission increases the longer a tick is attached and feeding.
Adult soft ticks mate while off of the host, but adult hard ticks mate on the host. Males mate with females while the females are feeding on the host. Blood feeding is required by males for their sperm to mature, but they do not engorge as much as females and immature stages do. Males take small quantities of blood as they wait for females to attach nearby where they are able to detect them. When a female is detected, males then detach and mount the engorging female to mate. Female ticks remain receptive following mating, so males normally remain on the engorged female and guard her until she detaches from the host following engorgement. After engorgement and mating, females drop to the ground to lay eggs (Figure 12). After an egg maturation period on the ground of about five to 20 days, the female lays a single large batch of 2,000 to 8,000 eggs, after which she dies.
Ticks find shelter at the surface of the soil, where the environment is humid and cool, to avoid water loss, which can lead to dehydration and easily kill them. They may also crawl along the ground toward hosts that are resting.
Ticks may use an ambush strategy called questing to find passing hosts. Questing ticks sit on vegetation with their forelegs outstretched waiting for their claws to hook onto a passing host. They do not jump, fly or drop onto the host. Photo by James Gathany, Centers for Disease Control and Prevention.
Hard ticks attach to skin of hosts by cutting a hole in the skin with their mouthparts and inserting a barbed feeding tube into the tissues.
Diagram of tick mouthparts attached to the skin of the host. Some ticks have a short hypostome that does not penetrate deep into the skin; other ticks have a long hypostome. Cement secreted by the hypostome glues the mouthparts into place, either at or under the skin surface.
Skin tissues attached to the hypostome of a tick removed from a host. The tissues are attached by cement secreted by the hypostome during feeding.
Engorged hard ticks grow significantly in size and weight while feeding on their hosts.
Female engorged hard ticks lay large batches of 2,000 to 8,000 eggs on the ground after dropping from their hosts. Photo by John Obermeyer, Purdue Extension Entomology.
Ticks are not equally common in all geographic regions. For example, soft ticks are most common in the western and southwestern regions of North America because of their ability to resist desiccation, or drying out. However, hard ticks are generally more common in areas where humidity is relatively high, such as the eastern and southeastern regions of North America. Within these broad geographic regions, the local distribution of favorable microhabitats and suitable hosts predicts where ticks are most abundant in a particular landscape.
Areas in the landscape where humidity is high and hosts are active typically support the highest densities of hard ticks in all stages of development. Some of the most favorable microhabitats occur along the border between a forest and a field. These transition zones, called ecotones, are the transition edges between woodlands and fields or lawns. Ecotones between forests and fields are characterized by a mix of vegetation types, including grasses, shrubs and trees, that grow together to create a “wall” of vegetation. This creates a unique habitat that isn’t found as you move outward toward either the field or the forest.
Ecotones along forest or field edges are important tick habitat for various reasons. Leaf litter that falls from nearby trees onto the ground of an ecotone helps insulate the soil and maintain soil moisture levels. The abundance of vegetation decreases air movement and direct sun, resulting in lower temperature and higher humidity than surrounding areas. It also creates good hiding places for many vertebrate animals that serve as hosts. Small mammals, birds, reptiles and amphibians are all found within ecotones, where the temperature is more moderate and they can hide from predators. All of these factors would seemingly influence the amount of time a tick spends questing, the likelihood of its encountering a host while questing, and its ability to rehydrate its body between questing periods.
Most species of hard ticks are not host specific as larvae, nymphs or adults. They feed on a wide variety of vertebrate hosts, including the majority of mammals, many different birds, lizards, snakes and turtles. They even feed on a few amphibians. Although ticks feed on a wide variety of hosts, one or a few hosts may be very important for a tick population to become abundant. The white-tailed deer (Odocoileus virginianus) is commonly considered to be the most important host for supporting tick populations in Missouri. Large vertebrates like deer are important as hosts for the adult stage of many ticks, and studies have shown that when deer are abundant, ticks are also abundant. Other large vertebrates in Missouri are mostly domestic animals that can be managed in ways to minimize encounters with ticks. No other large vertebrate is as widespread, abundant and available as deer in natural areas across Missouri, and therefore, the adults of many tick species rely on blood meals from this host for reproduction.
Ticks thrive in ecotones. These transition areas between fields or trails and adjacent forests provide habitat for their hosts and shelter ticks from the sun’s drying effects.
Common ticks in Missouri.
|Lone star tick||Amblyomma americanum||Wide range of mammals, birds|
|American dog tick||Dermacentor variabilis||Dogs, opossums, raccoons, foxes,coyotes, squirrels, cattle, sheep, horses, humans|
|Blacklegged tick||Ixodes scapularis||Birds, rodents, lizards|
|Brown dog tick||Rhipicephalus sanguineus||Dogs, other animals occasionally|
|Relapsing fever tick||Ornithodoros turicata||Snakes, birds, rodents, cattle, sheep, horses, pigs, humans|
|Bat tick||Ornithodoros kelleyi||Bats|
Although more than 850 species of ticks have been identified worldwide, most of them are not important as disease carriers. This may be related to host preference, geographic distribution or seasonality. The small proportion of tick species that are important in disease transmission have been identified and studied in greater detail. Only a handful of species are commonly associated with human tick bites and disease transmission in Missouri, and most of them are hard ticks. Only one soft tick in Missouri is of medical importance. This section explains the medical importance, identification, distribution and host associations of tick species in Missouri. [Note: Ticks are very small, and therefore are more easily expressed in millimeters (mm) than in inches. One millimeter is less than 4/100 of an inch (1 mm = 0.03937 inch).]
Adult female lone star tick. Photo by James Gathany, Centers for Disease Control and Prevention.
Identification: Hard tick
Adults are about 4 mm long. Females have scutum with distinct white spot. Males have inverted horseshoe markings at the rounded posterior edge of the scutum. Palps are much longer than basis capituli. Palps segment number 2 is at least twice as long as wide. Eyes and festoons are present.
Occurrence and activity
The lone star tick is primarily found in the southeastern and eastern United States. It is abundant throughout the entire state of Missouri. Adults and nymphs are generally active from early spring through midsummer. Larvae are active from late summer through early fall.
The mouthparts on the lone star tick are very large, and their bite may cause locally intense irritation. Massive infestations on animals may result in severe blood loss and debilitation. A toxin introduced during feeding can cause tick paralysis. This tick transmits southern tick-associated rash illness (STARI). It also transmits the pathogens that cause human ehrlichiosis and tularemia. It seems to be a poor vector for Rocky Mountain spotted fever and Lyme disease.
Lone star ticks are very aggressive, nonspecific feeders. They feed on a wide range of wild and domestic mammals, groundfeeding birds and humans during all of their life stages. Rodents do not appear to be very important hosts for immature stages. Whitetailed deer are the major host for this tick and an important reservoir for the human ehrlichiosis bacterium. In addition, heavy infestations on deer fawn have been known to result in blindness.
Adult male American dog tick.
Identification: Hard tick
Adults are about 6.5 mm long. Scutum of both males and females have distinct light coloration patterns. Basis capituli are rectangular. Palps are short, only about as long as basis capituli. Basis capituli and second segment of palps lack lateral projections. Seven festoons present. Anal groove is absent or indistinct.
Occurrence and activity
This tick is widely distributed east of the Rocky Mountains and also occurs in limited areas on the Pacific Coast. It is abundant across the entire state of Missouri. Adults are active from spring through early fall. Nymphs are active from summer through early fall. Larvae are active from early spring through summer. They are most numerous along roadsides, paths and trails in brushy woodlands and meadows with tall grass.
The American dog tick is of the most medically important ticks in the U.S. This is the species most responsible for transmitting Rocky Mountain spotted fever to humans. It also transmits tularemia and causes tick paralysis. One common mode of transmission of Rocky Mountain spotted fever is deticking dogs. Infected tick secretions that get on the hands while deticking may be transmitted through contact with the eyes, mucous membranes or skin cuts or scrapes.
Dogs and medium-sized mammals are the preferred hosts, although this tick feeds readily on other common mammals, such as opossums, raccoons, skunks, foxes, coyotes, bobcats, squirrels, cattle, sheep, horses, and humans. Larvae and nymphs feed primarily on small mammals, especially rodents. Adults prefer dogs but also bite humans.
Adult female blacklegged tick. Photo by James Gathany, Centers for Disease Control and Prevention.
Identification: Hard tick
This is a very small tick relative to other species. Adult females are 3 to 4 mm long and adult males 2 to3 mm long. It has an anal groove in front of its anus.
Occurrence and activity
This tick is widely distributed in the northeastern, southeastern and midwestern regions of the United States. In the Southeast, its distribution extends into southern Missouri. It is not common in northern Missouri.
This tick transmits the causat