The Ugly World of Ticks.
Rocky Mountain spotted fever; Lyme disease; babesiosis; tularemia; relapsing fever--what is the common denominator for these illnesses? All of them are transmitted by ticks.
Of the roughly 850 species of ticks, less than 100 are known to transmit diseases. Yet this small number of species includes some of the most annoying and dangerous of all the world's creatures. While mosquito-borne sicknesses are more widespread and claim more human lives, ticks spread a greater variety of diseases than any other animal group. Tick-borne pathogens include bacteria, viruses, protozoa, and fungi.
Ticks are obligate external parasites that feed on the blood of vertebrates. Some species feed on specific amphibians, reptiles, birds, or mammals, but many have several hosts, including humans, their pets, and livestock. This feeding opportunism enables them to survive and thrive in practically every habitat in the world, as long as suitable hosts are available.
For many people, a tick bite can be as hazardous as a beesting. Reactions to tick saliva can include intense pain, headaches, nausea, muscle cramps, paralysis, and memory loss. Paralysis caused by a tick bite is sometimes fatal, but it can be reversed by removing the tick. In addition, untreated bites can lead to various secondary infections by microbes that gain entry via the open wound.
Populations of these tiny pests are high during spring, summer, and fall, with summer being the prime tick season. Then again, many adult ticks survive right through mild winters, much to the annoyance of winter hikers and wildlife enthusiasts. Given that all ticks use wildlife as primary or alternate hosts, the presence of large wildlife populations often translates into high tick populations, restricting the use of many recreational sites for at least part of the year.
Many ticks are an important food source for small mammals, birds, and insects, including ambush bugs and praying mantises. The oxpeckers (tickbirds) of Africa and brown-headed cowbirds of North America feed on ticks that they dig out of the hides of wild animals. In fact, cowbird numbers declined along with the buffalo population but have since recovered by feeding on the livestock that repopulated the Great Plains.
The tick tribe
Along with insects, spiders, mites, and crustaceans, ticks are invertebrates with jointed legs. They are therefore classified in the phylum Arthropoda. Their bodies, like those of other arthropods, have stiff outer coverings called exoskeletons. Ticks, however, are not insects. Every insect has six legs and a body subdivided into a head, thorax, and abdomen. By contrast, a tick has eight legs, and its body has either one or two segments. The closest relatives of ticks are mites [see "Minute Creatures Raise Mighty Concerns," The World & I, February 2001, p. 140].
Ticks have mouthparts that are specially modified to suck their victims' blood. The mouthparts consist of a pair of segmented palps (feelers), a pair of sharp-bladed chelicerae (pincers) that slice the skin, and a needlelike hypostome through which blood is sucked. In most cases, the hypostome is equipped with recurved hooks that serve as anchors, making it difficult to remove the tick.
Depending on the species, a tick may or may not have eyes, but every tick has a pair of sensors--called Haller's organs--on its first pair of legs. These sensors respond to enhanced carbon dioxide levels in the immediate environment, triggering feeding reactions.
Ticks have been categorized in three well-defined families: Argasidae (soft ticks), Ixodidae (hard ticks), and Nuttalliellidae. There are about 200 species in the first family and about 650 in the second. The third family has just a single, rare species, Nuttalliella namaqua, found only in southern Africa.
Soft ticks are covered by a leathery, highly folded skin, granulated in appearance. Males and females are about equal in size, and their mouthparts project downward. Most species are nidicolous--that is, they dwell in the nests, burrows, and dens of their hosts, which are usually small mammals and birds. Like fleas and bedbugs, soft ticks remain in these sheltered confines for weeks or months awaiting a host, which is recognized by its movement, heat, fur, feathers, and carbon dioxide emission.
The bodies of hard ticks are protected by a tough, shield-shaped plate called a scutum, and their mouthparts project forward rather than downward. Females are larger than males. While some species are nidicolous, the great majority actively roam through a variety of habitats in search of hosts.
The tick's life cycle, like that of certain insects, consists of four stages: egg, larva, nymph, and adult. Each of the latter three stages requires a blood meal for its growth and development.
The eggs, which represent the overwintering stage, hatch in the spring to produce minute, six-legged larvae called seed ticks. Scarcely more than one-sixteenth of an inch in size, the seed ticks immediately begin looking for an appropriate source for their first blood meal.
The larvae of nidicolous species remain within the confines of the den or nest, where a host is likely to appear. On the other hand, the larvae of most hard ticks exhibit a behavior called questing, by which they actively seek out suitable hosts. The latter larvae may climb tall grasses or crawl to the ends of leaves or twigs, awaiting the passing of a host. The life history of a questing tick differs from that of a nidicolous tick in certain respects.
Questing ticks. The larva of a questing tick waves its front legs back and forth, checking the level of carbon dioxide in the air with its Haller's organs. It also relies on pressure, heat, and movement sensors to identify a passing host. Contrary to popular belief, the tick does not "jump" onto an animal but uses its front legs to grab hold of a host that brushes by. After a successful transfer, the larva may take several hours to find an appropriate spot at which it attaches itself.
Feeding is stimulated by the combination of a warm skin and butyric acid--a common oil on the skins of birds and mammals. The bladelike chelicerae cut into the skin, tearing the underlying blood vessels. As blood and tissue fluids pool into the wound, they are sucked up by the hypostome, which also anchors the tick into the skin by means of the recurved hooks. The tick's oversized salivary glands secrete anticoagulants and various other chemicals that prevent blood clotting and suppress the host's natural defense mechanisms.
After the blood meal, the larva detaches from the host and seeks a sheltered habitat, such as leaf litter, rotting vegetation, or natural crevices in rocks and soil. Here, the larva undergoes a molting process. Several days later, an eight-legged nymph emerges from the molt. The nymph immediately goes through the cycle of questing, attachment to a suitable host, and feeding. It then detaches from the host, undergoes a final molt, and produces a sexually mature adult, which repeats the steps of questing, attaching, and feeding.
In the course of a blood meal, a hard tick may remain attached to its host for days or even weeks. Its integument grows slowly and may ultimately expand to several hundred times its original size.
While feeding, the mature female exudes chemicals called pheromones that attract a male. Following mating, the female detaches from the host and lays several thousand eggs--either as a single mass or in several smaller batches--and dies. The male may remain on its host for several more days, feeding and mating with other females.
In most cases, the larva, nymph, and adult feed on three separate animals--which may correspond to three host species. These ticks are therefore known as three-host ticks. The Rocky Mountain wood tick (Dermacentor andersoni), a carrier of the rickettsial disease Rocky Mountain spotted fever, is an example of a three-host tick. The brown dog tick (Rhipicephalus sanguineus), which feeds mainly on dogs but occasionally attaches to humans, is another example.
Some species--including the winter tick (D. albipictus) and cattle tick (Boophilus microplus)--feed on a single host during the three stages and are therefore known as one-host ticks. Others, such as the red- legged tick (R. evertsi), are called two-host ticks because they feed on one animal during the larval and nymphal stages, while the adult seeks a second host.
Nidicolous ticks. In the case of most soft ticks, the larva feeds on its host in the den or nest, then detaches and molts to produce an eight-legged nymph. Depending on the species, the tick then goes through two to seven nymphal stages, gradually increasing in size. Eventually, the nymph molts into an adult.
The soft tick is capable of waiting for months or even years for the arrival of a suitable host in the den. It survives in the meantime by digesting its last blood meal extremely slowly. Once a host becomes available, the tick feeds quickly, for a period ranging from several minutes to days, before dropping off. It goes through a cycle of detaching from and reattaching to its host several times during each life stage. To accommodate the blood meal, the tick expands its skin, accordionlike, to 5--10 times the original size.
Of the enormous variety of diseases transmitted by hard and soft ticks, just a handful can be mentioned here. A number of them, such as Lyme disease, are currently reaching epidemic proportions in many areas. The economic losses and the cost of programs needed to protect livestock and humans reach into the billions of dollars annually.
Lyme disease, which was discovered in 1977 near Lyme, Connecticut, now occurs in parts of the United States, Europe, Russia, and Australia. It is caused by a spirochete bacterium (Borrelia burgdorferi) transmitted by deer ticks (Ixodes scapularis and I. pacificus). The disease is especially difficult to control because the ticks have abundant wild hosts, including deer, mice, and chipmunks. Symptoms include a rash that often looks like a bull's-eye, headache, fever, muscle pain, fatigue, and general malaise. Although Lyme disease is treatable with a cocktail of antibiotics, many patients suffer long-lasting cardiac and arthritic problems.
Babesiosis is a relatively rare disease that occurs in all regions of North America but especially in New York, Connecticut, and Massachusetts. It is caused by protozoans (of the genus Babesia) carried mainly by deer ticks and brown dog ticks. Symptoms of this sometimes lethal disease include chills alternating with high fever, profuse sweating, fatigue, and headache.
Rocky Mountain spotted fever (caused by Rickettsia rickettsii) is transmitted by the Rocky Mountain wood tick and the American dog tick (D. variabilis). First recognized in Idaho's Snake River Valley, the disease is broadly distributed in the United States, southern Canada, and Central America. Early signs include fever, headache, and muscle pain, followed by a reddish-purple rash. If left untreated, it can result in death.
The causative agent of tularemia, or rabbit fever, is a bacterium (Francisella tularensis) carried by the Rocky Mountain wood tick, American dog tick, and lone star tick (Amblyomma americanum). The disease, which affects both animals and humans, has been identified in almost every state of the United States. Symptoms include an ulcerlike formation at the wound site, fever, chills, swollen lymph glands, and appetite loss.
Relapsing fever is a worldwide disease caused by several species of spirochetes (genus Borrelia) that are carried by soft ticks of the genus Ornithodoros. The illness is so named because the patient experiences a fever every few days. In this group of ticks, one species--O. coriaceus of California and Mexico--has a highly venomous bite, producing intense irritation to humans.
Other soft ticks are important parasites of wild and domestic animals. The fowl tick (Argas persicus) is a serious pest of chickens and other poultry around the world, transmitting a bacterial spirochete that inflicts a high mortality rate on infected birds. Humans, especially poultry farmers who routinely handle chickens, can also be infected by this pathogen.
Cattle in the African sub-Sahara and other semiarid regions of the world are attacked by the spinose ear tick (Otobius megnini), which feeds deep in the ear canal of livestock and wild animals, causing a condition known as canker ear. The tick has protruding spines, making it difficult to dislodge, and heavy infestations produce irritation and sometimes death of infected cattle.
Managing tick populations
Despite millennia of study, dating from Aristotle and Pliny to the present, our knowledge of ticks and the diseases they transmit is far from complete. Nonetheless, we have learned that tick numbers can be sharply reduced by programs that combine vegetation management with animal control.
Given that ticks thrive in thick, brushy vegetation, one approach to limiting their populations is to burn or remove shrubs and other undergrowth and to cut tall grasses. These steps reduce the likely hiding places for ticks, decrease humidity, and raise soil temperatures, thereby increasing the mortality of ticks and their eggs.
Hunting programs, installation of deer-proof fences, and trapping of rodents around campsites and other recreational areas can further lower tick populations by eliminating animals that constitute their alternate hosts. In addition, in areas where ticks are particularly abundant, special pesticides (called acaricides) may be sprayed.
Various fish and wildlife agencies have experimented with localized releases of parasitic wasps (Hunterellus hookeri) to control tick populations on several islands off the New England coast. The efficacy of this biological control program, which is still in its initial stages, needs to be further assessed.
Ticks that attack dogs and cats can be controlled by the use of flea and tick control powders, shampoos, special collars, and pet dips. Infestation of livestock is controlled by periodically applying approved dusts and dips and providing feed that contains insecticides.
Homeowners should note that ticks tend to avoid hotter and drier lawns but will often be found within a few feet of the property edge, where yards and gardens transition to brush and woods. Garden expert Pamela Weil suggests that "a single application of low-toxicity pesticide around the perimeter of a yard can reduce 80--90 percent of tick nymphs when applied early in the year." Those who dislike synthetic pesticides may use natural toxins such as pyrethrins, produced by chrysanthemum flowers. They have proven effective against ticks when applied early in the season.
It should be noted, however, that none of these techniques eliminates 100 percent of ticks. Indeed, these tiny parasites have continued to withstand all eradication attempts, and we can expect that tick-borne diseases will plague humans, their pets, domestic livestock, and wildlife for a long time.n
Dwight G. Smith is professor and chairman of the biology department at Southern Connecticut State University in New Haven.
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|Author:||Smith, Dwight G.|
|Publication:||World and I|
|Date:||Sep 1, 2002|
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