Biologia dos Carrapatos

Biology of Ticks John F. Anderson, PhD * , Louis A. Magnarelli, PhD The Connecticut Agricultural Experiment Station, 123 Huntington Street, Post O\ufb03ce Box 1106, New Haven, CT 06504, USA Ticks are classi\ufb01ed in the subclass Acari [1] a nd are highly specialized obligate, bloodsucking, nonpermanent ectoparasitic arthropods that feed on mammals, birds, reptiles, and amphibians in all regions of the earth [2] . Although ticks are distribut ed from the tropics to subarctic areas, spe- cies diversity is greatest in tropi cal and subtropical regions [1] . Possibly, the earliest published account of their parasitic habits was documente d in 1550 BC [3] , but it was the seminal discovery of Smith and Kilborne in 1893 [4] that the tick Rhipicephalus (Boophilus) annulatus transmitted the protozoan parasite, Babesia bigemina , during feeding on cattle that pro- vided the key to the true importance of ticks. This discove ry established that ticks are not only ectoparasites that can damage human and anima l skin but also that ticks are parasites that transmit diseas e-causing patho- gens. Ticks are large-bo died arthrop ods that feed exclusively on vertebrate blood, lymph, or digested tissues. Molting and reproduction are regula ted by temperature, day-length, and ingestion of blood; thousands of eggs can be deposited by individual females. The external body regions a re comprised of the capitulum, body, and legs, and there is a specialized hold-fast organ called a hypostome, which is important in the feeding process [1,5,6] . Ticks harm pe ople indirectly by transmitting pathoge nic organisms, such as protozoa, viruses, and bacteria, during feeding. Addi- tionally, tick bites may cause tick paraly sis, toxicoses, and allergic reac- tions. This article reviews the biology of ticks, emphasizes important species that feed on people, includes infor mation on how these ectopara- sites survive on and o\ufb00 their hosts, a nd describes their feeding habits, which contribute to their ability to transmit etiologic agents. Further detai ls on tick biology are available in several publica tions [1,5\u201314] . We acknowledge the support of Hatch funds administered by the United States Department of Agriculture. * Corresponding author. E-mail address: john.f.anderson@po.state.ct.us (J.F. Anderson). 0891-5520/08/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.idc.2007.12.006 id.theclinics.c om Infect Dis Clin N Am 22 (2008) 195\u2013215 Morphology The structure of a tick is fused into two parts, consisting of the capitulum (gnathosoma) and the body (idiosoma), to which the legs are attached ( Fig. 1 ) [1,6] . Larval ticks ha ve six legs ( Fig. 2 ), while nymphs and adu lts have eight legs ( Fig. 3 ). A distinct he ad is absent. Unfed adult ticks range in length from 2 mm to 20 mm. Blood-engorged fema les may be 25 to 30 mm in length and wei gh up to 100 tim es their pre-engorgeme nt weights. The mouthparts are fou nd on the capitulum ( Figs. 4 and 5 ). They include the two four-segmented palps, each of which in ixodid (hard-bodied) ticks has numerous chemosens ory sensillae locat ed in the small dista l fourth seg- ment (see Fig. 5 ). The palps do not enter the wound; they are pressed late r- ally and horizontally against the skin during feeding. The pair of scler otized, two-segmented tubular chelicerae extends from the basis capitu li and is located medially to the palps. Two highly moveable and sha rp cutting digits are located at the extremities of the cheliceral shafts (see Figs. 2 and 4 ). The digits are situated laterally and are used to cut the skin during feeding. The relatively large medially positioned hypostom e with ventral, backward- pointing denticles (teeth) on its external surface has its intern al surface cov- ered dorsally by the chelicerae and is used as a holdfast organ and food canal (see Fig. 4 ). The size a nd shape of the hypostom e and arrangement of denticles vary among species and are important features used in identify- ing species. Blood passes from the host through this food canal, formed by the hypostome ventrally, and by the chelicerae dorsally. Saliva, containing proteolytic enzymes that digest and liquefy tissues, moves from the tick to the host through this channel. The basis capit uli, the basal portion of the capitulum, is attached to the tick body by a \ufb02exibl e membrane. Fig. 1. Scanning electron micrograph of a dorsal\u2013lateral view of an Ixodes scapularis female. B, body (idiosoma); C, capitulum (gnathosoma); L, leg; P, palp; S, scutum; SP, spiracle. Measurement bar ¼ 500 m m. 196 ANDERSON & MAGNARELLI The body is posterior to the cap itulum (see Figs. 1 and 3 ). The anterior part bears the legs and genital pore, and the posterior part possesses the spi- racles and anus. The genital apertur e is closed an d unapparent in larvae and nymphs and open in adults (compare Fig. 2 with Fig . 3 ). In the hard-bodied Fig. 2. Scanning electron micrograph of the ventral morphology of an Ixodes scapularis larva. A, anus; BC, basis capituli; D, digits of a chelicera; H, external surface of the hypostome; L, leg; P, palp. Measurement bar ¼ 500 m m. Fig. 3. Scanning electron micrograph of the ventral morpho logy of an Ixodes scapularis female. A, anus; BC, basis capituli; GA, genital aperture; H, external surface of the hypostome; L, leg; P, palp; SP, spiracle. Measurement bar ¼ 500 m m. 197 BIOLOGY OF TICKS ticks, a hardened shiel d, the scutum, covers the dorsal anterior por tion of the body of female and juvenile ticks (see Fig. 1 ). In males, the scutum covers the entire body. During feeding, the cuticle, with the exception of the scutum, expands to acco mmodat e the ingested blood meal. The body of argasid ticks lacks a scutum and is characterize d by a leathery and \ufb02ex ible cuticle ( Figs. 6 and 7 ). The jointed six-segmen ted legs are used for locomotion. Haller\u2019 s organ on the tarsus of the \ufb01rst leg detects tempe rature, air currents, odor s, and chem- icals [1,5] . Systematics Ticks are classi\ufb01ed in the class Arachnida, subclas s Acari, order Parasit- iformes, and suborder Ixodida [5] . There are approximately 878 species [15,16] , divided into four families, speci\ufb01cally the Argasidae (soft-bodied ticks), Ixodidae (hard-bodied ticks), Nutalliellidae, and Laelaptidae ( Ta- ble 1 ). The latter two families are of minor importance, and each has but one species. Fig. 4. Scanning electron micrograph of the dorsal morphology of the capitulum of an adult female Ixodes scapularis . BC, basis capituli; D, digits of a chelicera; DEN, denticle of the external surface of the hypostome; H, internal surface of the hypostome; O, opening to the food channel; P, palp; PA, porose area; SC, shaft of a chelicera. Measurement bar ¼ 500 m m. 198 ANDERSON & MAGNARELLI The soft-bodied ticks are ch aracterize d by having the capitulum posi- tioned subterminally ( Fig. 8 ), except in larvae, which have the capitulum in an anterior position, a high ly sculptured leathery integ ument, and do not have a dorsal shield [5] (see Figs. 6 and 7 ). All feeding stages of ha rd- bodied ticks have an anterior-positioned capitulum, striate integ ument, and a scutum (see Figs. 1\u20133 ). At least 222 species of approximatel y 878 known species have been reported to feed on people, but relative ly few commonl y feed on people [2,17,18] . This article identi\ufb01es 33 species that the authors judged to com- monly feed on people ( Tabl e 2 ). Of these, 28 specie s harbor and transmit pathogens known to cause human disease. Excellent taxonomic keys to these and other ticks by faunal regions of the world are available [2,19] . Geographic distribut ion maps of several important species have been pub- lished [6,7,20\u201322] . Water balance Ticks can have a relative ly long life [1,6] . Ixodid ticks can live 2 to 6 years, whereas soft-bodied ticks can live as long as 20 years. Long-term survival Fig. 5. Scanning electron micrograph of the ventral morphology of the capitul um of a nymphal Ixodes scapularis . BC, basis capituli; F, fourth segment of the palp; H, external surface of the hypostome; P, palp. Measurement bar ¼ 500 m m. 199 BIOLOGY OF TICKS without blood meals is accomplished by preventing desicca tion and by rely- ing on stored reserves obtained from a prior feeding. Aside from feeding on vertebrate hosts, ticks do not acquire nutrients from plants. It is not unusual for ticks to live for a year or more, but relative ly short periods of time are normally spent on the host. Ixodid ticks such as Ixodes scapul aris (black- legged tick) and Dermace ntor variabilis (Am erican dog tick) may spend 98% or more of their lives o\ufb00 their host animals [23] . Dermacentor albipictus (winter tick) spends comparatively more time on its large mammalian hosts (eg, deer, moo se, and cattle). Argasid ticks may spend less tim e on their hosts. Whether ticks are feeding or fasting, however, maintenance of water balance is crucial for survival, distribut ion, and activity [24,25] . Durin g the process of ingesting blood, ticks need to preven t overhydration. Conver sely, ticks need to prevent dehydrat ion during fasting periods. Water is expell ed or retained as needed. Duri ng active engorgement, salivary glands of ixodid ticks play an important role in excretion of \ufb02uids as large volumes of blood are ingested. Excess ions and water from the blood meal are trans ported through the gut ep ithelium into the hemo coel and secret ed back to the host during feeding by means of the sali vary glands [23] . By contras t, arga- sid nymphs and ad ults take several small blood meals inter mittent ly from multiple hosts and remove most of their ex cess water through the coxal glands during or afte r the blood meal. Fig. 6. Scanning electron micrograph of the dorsal view of a female soft-bodied tick ( Argas brumpti ). ( Courtesy of Rocky Mountain Laboratories, US Public Health Service, NIH.) 200 ANDERSON & MAGNARELLI Fig. 7. Scanning electron micrograph of the ventral view of a female soft-bodied tick ( Argas brumpti ). A, anus; C, capitulum; GA, genital aperture. ( Courtesy of Rocky Mountain Labora- tories, US Public Health Service, NIH.) Table 1 Number of tick species by genus Family Genus Approximate number of species Ixodidae Ixodes 245 Amblyomma 131 Bothriocroton 5 Haemaphysalis 164 Hyalomma 24 Nosomma 1 Anomalohimalaya 3 Cosmiomma 1 Dermacentor 34 Margaropus 3 Rhipicentor 2 Rhipicephalus (Boophilus) 5 Rhipicephalus 74 Nuttalliellidae Nuttalliella 1 Argasidae Argas 58 Carios 88 Ornithodoros 35 Otobius 3 Laelaptidae Gammaridacarus 1 Total 878 201 BIOLOGY OF TICKS When fasting, the tick must conserve water and maintain water balance. They accompl ish this when salivary glands secrete and subsequently imbibe hygroscopic substances that actively absorb water from unsaturated air [26] , by having an integument that restricts transpirat ional water loss, by infre- quently opening spiracular valves to prevent water loss from the tracheal system, and by actively moving to nearby microhabitats, which have higher humidity. Feeding Blood meals are needed for survival, grow th to the next stage of develop- ment, an d for reproduction. Feeding involves a complex array of behavioral events that begin with hunger and end in sati ation [27] . It is by this means that a tick can harm its host by acqu iring pathogenic organisms from an infected hos t and by subsequently transmitting these agents to other hosts in subsequent blood feedi ngs. The sequence of events leading to successful feeding has been divided into nine major steps 1. Appetence (hunting or seeking a host) 2. Engagement (adherence to the skin or fur of the host) 3. Exploration (searching on the skin for a suitable attachment sit e) 4. Penetration (insertion of the mouthparts into the host\u2019s epidermis an d dermis) 5. Attachment (feeding site established) 6. Ingestion (uptake of blood and other \ufb02uids) 7. Engorgement (partial or complete meal s of blood taken) 8. Detachment (withdrawal of the mouthparts) 9. Disengagement (tick drops o\ufb00 of the hos t) Fig. 8. Scanning electron micrograph of the ventral morphology of the capitulum of an adult Argas persicus (soft-bodied tick). C, capitulum; H, external surface of the hypostome; P, palp. Measurement bar ¼ 500 m m. 202 ANDERSON & MAGNARELLI Compared with mosquitoes, the relatively slow feeding process in ticks enhances dispersal of these ectopa rasites as the host moves ab out in the environment. For example, larvae and nymphs of Ixodes scapularis often feed on birds and can travel several miles while feeding. Some ticks are car- ried between continents by birds. Stimuli that induce appetence include odor, vibration, shado wing, and visual appearance. In some sp ecies, sim ple eyes may be locat ed along the lat- eral or submarginal areas of the scutum. Many tick species lack eyes. Touch, skin and radiant temperatures, and the pr eviously mentioned stimuli lead to engagement. Tactile stimuli, received from the hair-like projections (setae) on the palps or other body parts , are used to select attachment sites on the host\u2019s skin. These sites a re often in sheltered locat ions on the body of the host. Upon penetration, receptors on the chelicerae are used to \u2018\u2018taste\u2019\u2019 the host. The tick grasps the skin surface with its tarsal claws, posi- tions its body at an angle to the skin surface, and begins cutting the epider- mis and the dermis wi th its chelicerae. The hypostome, with its relatively large backward projecting teeth, is inserted into the wound . The addition of a proteinaceous cement-like substance that hard ens is secreted by ixodid ticks around the inserted mouthpar ts, thereby helping to secure atta chment. The inner surface of the hypostome forms the v entral portion of the feeding channel, which is covered by the cheliceral sheaths. The feeding channel is used for the intake of tissue \ufb02uids from the host and for the injec tion of saliva into the host from the tick. Ingestion of blood and host tissues by ixodid ticks begins relat ively slowly [1,6,28] . In most cases, tick bites in people are painless. Larvae an d nymphs feed on average 2.5 to 8 days, whereas adults feed 5 to 12 days. Upon engorgement, most ticks dro p o\ufb00 their host. Ther e is little if any increa se in weight of the tick during the \ufb01rst 24 hours of feeding. Ther eafter, weight increases slowly for several days, followe d by a rapid increa se in weight on the last day. It is believed that high dosages of the Lyme disease spiroc hete ( Borrelia burgdorferi ) are transferred during the later period of feeding when relatively large volumes of blood are ingested [29] . Initial salivary secret ions prevent blood clott ing, dilate skin capillaries, digest host tissues, cause extensive hemorrhage, and suppress in\ufb02ammatory responses of the host. Saliva also stimulates the host\u2019s immune system and interacts with the immune e\ufb00ector components [28,30] . There are periods of sali vation alter- nating with ingestion of blood, tissue \ufb02ui d, or lymph. The relatively slow feeding process in ixodid ticks is associated with the need to produce new cuticle to accommodate the ever increa sing volume of blood. Upo n comple- tion of feeding, the fertilized fully-fed female may weigh 100 to 120 tim es its original weight, but because so much water is secreted back into the host, the total volume of blood ingest ed may be two to three times the amoun t calculated from the postf eeding weight. Production of antibodi es, comp lement, antigen-presen ting cells, and T lymphocytes are host responses to tick feedi ngs [31,32] . Host anima ls are 203 BIOLOGY OF TICKS Table 2 Species of ticks reported to feed relatively frequently on people, many with high public health signi\ufb01cance Species Geographic distribution Etiologic agents causing disease in people Amblyomma americanum Eastern United States E cha\ufb00eensis, B lonestari, F tularensis Amblyomma cajennense Texas, Mexico, Central and South America R rickettsii Amblyomma hebraeum Southern Africa R africae Amblyomma variegatum Africa, Caribbean Islands CCHFV Argas re\ufb02exus SE Europe, Middle East None reported Carios erraticus North and East Africa, Near East, southeastern Europe B crocidurae, B hispanica Carios rudis Panama, South America B venezuelensis Dermacentor andersoni Western North America R rickettsii , CTFV, F tularensis Dermacentor auratus India, Malaysia, Vietnam, Burma, China None reported Dermacentor marginatus Eurasia C burnetti, R slovaca, R sibirica , TBEV, OHFV Dermacentor nuttalli Russia, Mongolia, China R sibirica, F tularensis Dermacentor occidentalis California, Oregon, Baja Mexico R rickettsii, C burnetti, F tularensis , CTFV Dermacentor reticulatus Eurasia R conorii, R sibirica, F tularensis , OHFV Dermacentor silvarum Russia, Eastern Europe, Mongolia TBEV Dermacentor variabilis Eastern and western North America R rickettsii, F tularensis Haemaphysalis concinna Central Europe, Russia, Far East TBEV Hyalomma anatolicum Russia, southern Europe, Near East CCHFV Hyalomma marginatum Africa, Turkey, Yemen, Israel, Russia CCHFV Ixodes cookei North America POWV Ixodes holocyclus Australia R australis Ixodes ovatus Japan, China, Tibet, Burma, Nepal None reported Ixodes paci\ufb01cus Western North America B burgdorferi, An phagocytophilum, Babesia spp Ixodes persulcatus Eurasia B spp, TBEV Ixodes ricinus Europe B spp, R conori, An phagocytophilum, Babesia divergens , TBEV, CCHFV Ixodes scapularis Eastern, midwestern North America B burgdorferi, An phagocytophilum, Babesia microti Ornithodoros asperus Caucasus, Iraq B caucasica Ornithodoros coriaceus California, Mexico None reported Ornithodoros hermsi Western North America B hermsii 204 ANDERSON & MAGNARELLI

Biologia dos Carrapatos

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Parasitologia

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