Hyalomma truncatum and Rhipicephalus evertsi mimeticus (Ixodoidea,Ixodidae): reactions of adult ticks to vertically incident narrow- and wideband optical radiation with and without the influence of a CO2 gradient

Investigations concerning the reactions of unfed Hyalomma truncatum and Rhipicephalus evertsi mimeticus female and male ticks to vertically incident narrow- and wideband optical radiation at irradiances of 0.115 mWcm^-2 and 0.98 mWcm^-2, respectively, revealed that, independent of sex, adults of the two species are capable of perceiving a wide range of wavelengths. Considering the migration into the entry area of the optical radiation at the test chamber's ceiling as a positive phototaxis, H. truncatum always reacted with higher percentages than R. e. mimeticus. Compared to the controls, R. e. mimeticus ticks occupied the entry area of the optical radiation significantly more frequently only in the wavelength ranges of 415–474 nm, 529–628 nm, 611–707 nm and 190–2,600 nm, but always with low rates. High percentages of H. truncatum ticks, however, consistently reacted with a positive phototaxis in all the offered monochromatic sectors between 292–707 nm and also in the wide spectral range of 190–2,600 nm, particularly in the range of 470–520 nm. When ticks of both species were additionally confronted with a stable CO₂ gradient, continuously increasing from 0.18 vol% at the bottom to 0.90 vol% at a height of 40 cm in the test chamber, they moved less frequently to the entry area of the optical radiation, compared to ticks tested without CO₂, regardless of their exposure to darkness or to narrow- and wideband radiation. The percentages of ticks, however, that moved in a vertical direction, but did not reach the chamber's ceiling were always higher, with the exception of R. e. mimeticus at the wavelength range of 415–474 nm. A total of 30 unfed male and 30 unfed female adult ticks of both tick species were investigated at each combination of a narrow- and wideband spectral range, with and without the influence of a CO₂ gradient. Considering moving ticks only, the interval between exposure and first movement of ticks was shorter under the influence of an additional CO₂ gradient. The average delay in reaction with and without the stimulus of the CO₂ gradient was 156.2 s and 195.6 s, respectively, for H. truncatum and 126.6 and 226.3 s, respectively, for R. e. mimeticus.     

The identification of a shared immunogen present in the salivary glands and gut of ixodid and argasid ticks

The identification of a 70-kDa immunogen present in salivary gland extracts of several ixodid species, namelyHyalomma truncatum (sweating-sickness-inducing (SS+) and non-inducing (SS-) strains),Hyalomma marginatum. rufipes andRhipicephalus evertsi evertsi, is reported. The immunogen was identified by Western blots using a monoclonal antibody of the IgM isotype directed against a 70-kDa immunogen present in the salivary glands of (SS-) femaleH. truncatum ticks. Cross-reactivity with the gut of unfed adult ixodid ticks,Amblyomma hebraeum, Rhipicephalus simus simus, R. evertsi evertsi, Rhipicentor nuttali, H.m. rufipes, and salivary glands of adult argasid species,Ornithodoros savignyi andOrnithodoros moubata, was demonstrated using ELISA.     

Hyalomma truncatum (Acari; Ixodidae): investigations into the scototaxis of unfed adult ticks

Under controlled test conditions, unfed male and female Hyalomma truncatum ticks exhibited a positive scototaxis to stationary, two-dimensional targets. Upright-positioned rectangles were the most attractive targets. The attractiveness of these targets increased with their size. Significantly more ticks responded scototactically positively to the targets under a luminance contrast ratio of 5:1, as compared with other luminance contrast ratios. Targets with an elevation angle of 13° were occupied more frequently than objects with higher elevation angles. Scototaxis was the same towards a stationary and a sinusoid oscillating target. When an upright-positioned rectangle was combined with a CO₂ gradient, the number of ticks that migrated into the CO₂ gradient and contacted the target did not increase significantly. The interval between exposure and first locomotion of the ticks, however, was significantly shorter under the influence of a CO₂ gradient than in all other experiments without a CO₂ gradient. A temperature gradient simulating a natural host (cattle) did not alter the scototaxis. The results of these investigations suggest that the positive scototaxis exhibited by adult H. truncatum ticks is not likely to be part of their appetence behaviour but rather searching behaviour to find adequate protection from harsh climatic conditions.     

Investigation into the critical equilibrium humidity,active atmospheric water absorption and water content ofRhipicephalus evertsi mimeticus

The critical equilibrium humidity for fully engorged nymphs ofRhipicephalus evertsi mimeticus was shown to be between 91% and 93.5% r.h., and for adult male and female ticks to be between 82% and 84.5% r.h. Studies on gnathosoma and idiosoma by selective exposure to differing relative humidities have shown that dehydrated adult male and female ticks are capable of active uptake of atmospheric water vapor only through their mouthparts. The percentage of water content of unfed adult male and female ticks previously dehydrated was not influenced by subsequent exposure to different relative humidities, varying similarly in both sexes, male imagines between 59.6% and 63.7%, female imagines between 59.6% and 64.4%. Analogous values (62.3% and 61.1%) were obtained for the water content of male and female imagines not previously dehydrated, which were incubated at 30°C and 100% r.h. for 7 days.     

Hyalomma truncatum (Acari; Ixodidae): evidence for the inability of adult ticks to discriminate between colours

Behavioural investigations into the perception and differentiation of coloured objects by unfed adult Hyalomma truncatum ticks revealed that silhouettes of blue, green, red and yellow colour, under illumination by a sun-simulating waveband spectrum, are perceived by the ticks and responded to equally by a directed response. Two green or dark grey rectangles each with a luminance contrast ratio of 5 : 1 against the white wall of the test arena in combination with an overlapping, equally sized dark grey or green target were consistently reached by ticks in a ratio of 2 : 1. Since the outer targets were occupied by the double number of ticks compared with the central silhouette this shows that the response is independent of the colour of the object. Investigations into target perception under monochromatic radiation of different wavelength ranges which were evenly adjusted in their irradiances revealed that ticks responded equally to a black target irradiated by blue, green, yellow and red light of wavelengths 428–472, 517–563, 549–591 and 606–654 nm, respectively. These results indicate the lack of true colour vision in H. truncatum.     

Fine structure of the foveae dorsales and foveal glands in the female tick Amblyomma americanum (acari: Ixodoidea: Ixodidae)

The fine structure of the paired foveae dorsales and foveal glands in the unfed female Amblyomma americanum is described and compared with that in other tick species. Each fovea opens to the exterior via pores which lead internally into a single cuticular tube, the pore-tube. This is surrounded by 2–3 epithelial cells. The pore-tube enlarges basally forming a large cavity possessing a cup-shaped cytoplasmic protrusion. The pore-tube cuticular lining extends downward to form an electron-dense, flaplike protrusion bracketing narrow cytoplasmic extensions filled with microtubules. These extensions form a previously undescribed valvelike structure that seems to control the flow of pheromone secretion from the foveal gland to the pore-tube. The single foveal gland lying beneath each pore-tube is composed of 2–3 inner, large, storage cells surrounded by outer, spindle-shaped cells; both types of cells have a characteristic feature of epithelia involved in secretory activity and ion transport. The outer cells extend upward to join the base of the poretube cells by septate desmosomes. A nerve, the foveal nerve, containing axons with neurosecretory vesicles occurs in the vicinity of the foveal gland. The secretory activity of the pheromone glands seems to be partially, if not entirely, under a neural regulation.     

Morphology and structural organization of gené's organ in Dermacentor reticulatus (Acari: Ixodidae)

Scanning and transmission electron microscopical investigations revealed that Gené's organ in unfed and ovipositing females of Dermacentor reticulatus is formed as a double-sac-structure consisting of an outer epithelial and an inner cuticular sac. In ovipositing ticks the latter emerges through the camerostomal aperture to the exterior. Gené's organ in unfed ticks consists of a corpus, two posterior horns and a pair of undeveloped glands at each side, which differentiate in ovipositing ticks to compound, branched tubular glands with a main efferent duct for each gland opening into the lumen between the epithelial and the cuticular sac. Gené's organ of egg-laying females corresponds basically in morphology and structural organization to that of unfed ticks. Compared with unfed ticks, however, in ovipositing ticks the corpus and horns are longer and broader, the glands are fully developed and the cuticular sac is evertable. The epithelial sac as the outermost part of Gené's organ is continuous with the hypodermis of the basis capituli and the scutum, arises at the camerostomal aperture, forms the corpus and the two blind-ending horns, passes into the epithelium of the main excretory ducts of the glands and envelops the cuticular sac. The cuticular sac passes into the cuticle of the basis capituli and the scutum, arises at the camerostomal aperture, is folded, expands into the horn tips and consists inwards of a smooth epicuticula and outwards of a fibrous endocuticula. Muscles originating from the scutum pass caudomedially through the epithelial sac and are inserted into the cuticular sac. The entire surface of the maximally everted cuticular sac is covered with an amorphous mass. In cleaned samples, ledge-like structures appear on the lateral surface. These ledges turn into balloon-like structures which extend over the medial and dorsal surface. The entire surface including the balloon-like structures and the ledges are provided with numerous cribrate pits.     

硬蜱盾窝与盾窝腺的细微结构

用扫描及透射电镜研究硬蜱盾窝与盾窝腺的结构.幼蜱只有横缝状盾窝原基,盾窝腺尚未发育;若蜱盾窝增大,其孔数相应增加;到成蜱阶段,不仅盾窝增大,而且盾窝腺完成发育.对9种雌蜱盾窝进行比较,在其大小和孔数方面有一定差别.亚洲璃眼蜱雄蜱盾窝腺不发达,几个叶瓣贴在一起,组成1—2个腺体组.雌蜱吸血前,叶瓣相互贴在一起.吸血后,球状叶瓣散开,连接叶瓣的导管清楚可见,分泌细胞中颗粒增多,这与其分泌性信息素有关.     

Structure and Role of the Renette Cell and Caudal Glands in the Nematode Sphaerolaimus gracilis (Monhysterida)

Ultrastructure of the renette cell and caudal glands was studied in the free-living aquatic nematode Sphaerolaimus gracilis. The renette cell occurred posterior to the esophageal-intestinal junction and opened through an ampulla to a ventral pore behind the nerve ring. The caudal gland system of the tail consisted of two gland cells opening through separate pores and 2 to 3 other gland cells of a different type opening through a common pore. The renette cell and the two caudal gland cells were similar and both contained secretory granules, 0.5-1.5 μm in diameter. The material released attached the nematode to the substrate. The renette ampulla was surrounded by a specialized cell, the ampulla cell, which had characteristics of myoepithelium. A plug or valve structure connected to the ampulla cell may regulate the output of the secretory material. The ampulla cell is able to contract and thus is probably under direct neuronal control. Other cells in the renette ampulla region of body cavity were termed supporting cells. Living, cold-relaxed nematodes were attached to sediment particles in the renette pore region and at the tail tip. Release from sediment particles was mechanical at the renette cell discharge site but appeared to be chemical at the caudal gland. In behavioral experiments, nematodes in a water current had the ability to release a thread from the caudal glands while maintaining contact with a sediment particle attached to the tail end. If the thread was strong enough, it also could be used to change location. Nematodes anchored by the thread from the caudal glands to a sediment particle could float in water currents until they attached themselves to another sediment particle with the help of secretions from the renette cells.     

Studies on survival and water balance of unfed adult Dermacentor marginatus and D. reticulatus ticks (Acari: Ixodidae)

The water content, the survival time at various relative humidities(r.h.) and the critical equilibrium activity of unfed adultDermacentor marginatus and D.reticulatus ticks were investigated at a constant temperature of20 °C. It was also examined whether these ticks use liquidwaterto compensate water loss. Both Dermacentor spp. showed nosignificant differences in water content in relation to body mass. The meanwater content of D. marginatus and D.reticulatus was 54.6% and 54.7%, respectively, in females and 56.3%and 57.0%, respectively, in males. The survival time of unfed adults prolongedwith decreasing saturation deficits. On average, males survived longer thanfemales and D. marginatus ticks survived mostly longerthanD. reticulatus ticks. The 50% mortality period rangedbetween 40 d at 33% r.h. and 420 d at 95% r.h. in D.marginatus, and between 43 d at 33 r.h. and 366 d at 95% r.h. inD. reticulatus. The critical equilibrium activity of unfedadults was estimated to be 0.84 for both species and was independent of sex.When dehydrated adult D. marginatus and D.reticulatus ticks were offered liquid water, only a few slightlygained weight while most further lost weight. Liquid water was not attractivefor dehydrated or non-dehydrated ticks and drinking was not observed. Aftersubmerging in water for 2 d, most of the dehydrated ticks had gained weight.     

Acquisition of Borrelia burgdorferi by Ixodes ricinus ticks fed on the European hedgehog,Erinaceus europaeus L

A hedgehog, Erinaceus europaeus, was found to be heavily infested with larval and nymphal Ixodes ricinus in a forest park in Co. Galway, Ireland. A large proportion of the ticks that engorged and detached were infected with the spirochacte, Borrelia burgdorferi, the causative agent of human Lyme borreliosis. The identity of these spirochaetes was confirmed by immunofluorescent assay with B. burgdorferi-specific monoclonal antibody and by polymerase chain reaction test and they were transmitted from the hedgehog to laboratory-reared ticks and from the ticks obtained from the hedgehog to gerbils (Meriones unguiculatus). The high infection rate of the larvae that fed on the hedgehog in comparison with unfed larvae from the same habitat was interpreted as strong evidence that this host species is reservoir competent. Since hedgehogs can evidently feed adult ticks as well as many immature stages, they may well have an important role in the ecology of Lyme borreliosis in some habitats.     

Morphological identification of ticks (Acari: Ixodidae) infesting donkeys (Equus asinus) in Maputo Province,Mozambique

Donkeys (Equus asinus) are rustic animals, but in Africa’s poorest regions, they can present multiple health problems, including tick infestation. The study’s objective was to determine the species composition of ticks that infest donkeys in Maputo Province (Mozambique). Ticks were collected in five conveniently selected southern districts of Maputo Province (Moamba, Matutuíne, Marracuene, Boane, and Matola) and were identified to species level using a stereoscopic microscope with the aid of dichotomous identification keys. In total, 500 ticks were collected from all 88 selected donkeys. Three genera of ticks were identified, namely Rhipicephalus (97.2%; 486/500), Amblyomma (2.2%; 11/500), and Hyalomma (0.6%; 3/500). Seven species were identified, of which Rhipicephalus evertsi evertsi with 50.4% (252/500) was the most prevalent, followed by Rhipicephalus appendiculatus (27.4%; 137/500), Rhipicephalus turanicus (11.6; 10/500), Rhipicephalus (boophilus) microplus (6.8; 20/500), Amblyomma hebraeum (2.2%; 11/500), Rhipicephalus sanguineus (1%; 5/500) and Hyalomma truncatum (0.6%; 3/500). Rhipicephalus evertsi evertsi occurred in all locations, whereas Hyalomma truncatum occurred only in the Boane district. Males were the most prevalent (67.2%; 336/500). The study revealed that donkeys in Maputo Province were infested with seven tick species of which R. evertsi evertsi was the main species.

     

Salivary gland degeneration and vitellogenesis in the ixodid tick Amblyomma hebraeum: Surpassing a critical weight is the prerequisite and detachment from the host is the trigger

The normal engorged body weight of female ixodid ticks (Acari: Ixodidae) is about 100× the unfed weight. Virgin female Amblyomma hebraeum normally do not feed beyond 10× the unfed weight. However, about 10–20% of a population of virgins will feed to perhaps 20× the unfed weight, but not much beyond that. In A. hebraeum, when females surpass about 10× the unfed weight, the following changes in physiology occur if they are removed from the host: (a) they will not reattach if given the opportunity, (b) their salivary glands (SGs) will undergo autolysis within 4 days if they are mated or 8 days if they are virgin, and (c) egg maturation and oviposition will occur in due course. Mated or virgin female ticks removed from the host below about 10× the unfed weight do not experience the latter changes (Kaufman, W.R., Lomas, L., 1996. ‘Male Factors’ in ticks: their role in feeding and egg development. Invertebrate Reproduction and Development 30, 191–198). In 1984 we named this transitional weight, the ‘critical weight’ (CW). Its absolute value is probably a species-specific characteristic (Kaufman, W.R., 2007. Gluttony and sex in female ixodid ticks: how do they compare to other blood-sucking arthropods? Journal of Insect Physiology 53, 264–273). Although mated females tend to engorge within a day of surpassing the CW, virgin females surpassing the CW can remain attached to the host for at least several weeks more. It is not known whether the physiological changes in the SGs and ovaries listed above occur in those large virgins that remain attached, although we suppose that this would be maladaptive. Instead, we hypothesize in this study that surpassing the CW is only a prerequisite for inducing these changes, and that detachment is the actual trigger. We support our hypothesis by demonstrating that large virgins, remaining attached to a host for 8 days, did not undergo SG degeneration nor complete egg maturation during the attachment period. Those changes occurred only within 8 days following detachment. So some type of sensory information associated with attachment to the host, and still undefined, inhibits expression of the physiological changes hitherto associated merely with surpassing the CW.     

Borrelia, Coxiella, and Rickettsia in Carios capensis (Acari: Argasidae) from a brown pelican (Pelecanus occidentalis) rookery in South Carolina, USA

Argasid ticks are vectors of viral and bacterial agents of humans and animals. Carios capensis, a tick of seabirds, infests the nests of brown pelicans, Pelecanus occidentalis, and other ground nesting birds along the coast of South Carolina. This tick is associated with pelican nest abandonment and could pose a threat to humans visiting pelican rookeries if visitors are exposed to ticks harboring infectious agents. We collected ticks from a pelican rookery on Deveaux Bank, South Carolina and screened 64 individual ticks, six pools of larvae, and an egg mass for DNA from Bartonella, Borrelia, Coxiella, and Rickettsia by polymerase chain reaction amplification and sequencing. Ticks harbored DNA from “Borrelia lonestari”, a novel Coxiella sp., and three species of Rickettsia, including Rickettsia felis and two undescribed Rickettsia spp. DNA from the Coxiella and two undescribed Rickettsia were detected in unfed larvae that emerged in the laboratory, which implies these agents are transmitted vertically by female ticks. We partially characterize the novel Coxiella by molecular means.     

Postembryonic Hematopoiesis in Drosophila

We have investigated the blood cell types present in Drosophila at postembryonic stages and have analysed their modifications during development and under immune conditions. The anterior lobes of the larval hematopoietic organ or lymph gland contain numerous active secretory cells, plasmatocytes, few crystal cells, and a number of undifferentiated prohemocytes. The posterior lobes contain essentially prohemocytes. The blood cell population in larval hemolymph differs and consists mainly of plasmatocytes which are phagocytes, and of a low percentage of crystal cells which reportedly play a role in humoral melanisation. We show that the cells in the lymph gland can differentiate into a given blood cell lineage when solicited. Under normal nonimmune conditions, we observe a massive differentiation into active macrophages at the onset of metamorphosis in all lobes. Simultaneously, circulating plasmatocytes modify their adhesion and phagocytic properties to become pupal macrophages. All phagocytic cells participate in metamorphosis by ingesting doomed larval tissues. The most dramatic effect on larval hematopoiesis was observed following infestation by a parasitoid wasp. Cells within all lymph gland lobes, including prohemocytes from posterior lobes, massively differentiate into a new cell type specifically devoted to encapsulation, the lamellocyte.     

Chemical composition of lipophylic compounds from the body surface of unfed adult Ixodes persulcatus ticks (Acari: Ixodidae)

Chemical compositions of ethereal extracts of the body surface of unfed male and female Ixodes persulcatus ticks (Acari: Ixodidae) were studied by gas chromatography using mass-spectrometric detection. More than 100 different organic compounds were detected. The predominant components were saturated fatty hydrocarbons, saturated and unsaturated fatty acids, aldehydes, squalene, cholesterol and cholesterol derivatives. A number of compounds found in I. persulcatus are known as components of pheromones or constituents of dermal gland secretions in tick species of the genus Amblyomma: nonanoic acid, saturated fatty acids having from 14 to 16 carbons, and squalene. Saturated fatty aldehydes have not been reported previously as body surface components of hard ticks. Substituted phenols were not found in the extracts, although they are known as common components of sex and attraction–aggregation–attachment pheromones in Amblyomma ticks. With a few exceptions (henicosanal, 2,4-holestadiene and two unidentified cholesterol derivatives), there was no marked difference in composition of surface components between male and female I. persulcatus. The possible role of the different chemical groups in communication between I. persulcatus ticks is discussed.     

Feeding patterns of immature stages of Hyalomma truncatum and Hyalomma marginatum rufipes on different hosts

In this study we examine the feeding patterns of immature stages of Hyalomma truncatum and Hyalomma marginatum rufipes ticks on different hosts. Larvae of H. truncatum developed through a three-host pattern on two species of field mice, Rhabdomys pumilio and Lemniscomys rosalia. On guinea-pigs, both Hyalomma species followed a mixed two-host and three-host pattern, with the latter route being preferred, since more than 70% of the fully fed larvae dropped off from their hosts. H. truncatum was a two-host tick on rabbits. Larvae of H. marginatum rufipes did not prefer R. pumilio and L. rosalia as hosts. On guinea-pigs, H. marginatum rufipes immatures showed a mixed two-host and three-host pattern with a bias towards the three-host life cycle, since approximately 58% of the fully fed larvae dropped off. On rabbits, H. marginatum rufipes was exclusively a two-host tick. Mean engorgement weights and blood quantities ingested by H. truncatum nymphs that developed through a three-host pattern on mice were significantly higher (p<0.0001) than for those that developed through a two-host pattern on guinea-pigs and rabbits. For H.marginatum rufipes, there were no significant differences (p>0.05) between engorgement weights of nymphs that developed through two-host and three-host patterns. However, there were significant differences (p<0.0001) in blood quantities ingested by nymphs of this tick species following feeding on different hosts.     

Physiological Age of Field-collected Female Taiga Ticks, Ixodes persulcatus (Acari: Ixodidae), and their Infection with Borrelia burgdorferi Sensu Lato

In some studies the prevalence of tick infection (infection rate) and the intensity of infection are negatively correlated with unfed tick age (in the broad sense of this term). However, no special research has been carried out to consider the phenomenon thoroughly. The infection indices of the female taiga ticks, Ixodes persulcatus, infected with Borrelia burgdorferi s.l. were related to tick physiological age, an index that more precisely reflects tick physiological state than the time of tick collection in the field or the duration of tick survival under laboratory conditions. A novel quantitative technique of physiological age determination based on the evaluation of the ratios between sizes of the stable (scutum) and the changing (alloscutum) structures of the tick body was used. The age was estimated in accordance with the classical age-grade scale introduced by Balashov and a more fractional scale determined by the new technique. In total, 131 female ticks were examined for their infection and physiological age, 46 of which were infected with B. burgdorferi s.l. (mean infection rate 35.1%). The minimal intensity of infection was 0.4 bacterial cells per 100 fields of view whereas the maximal infection was 172 cells. There was no difference between the prevalence of infection in ticks of different physiological age. The intensity of infection obviously differed between ticks of different age groups in the scale introduced by Balashov but did not significantly differ between ticks of different age groups according to the fractional age-grade scale. The data concerning the relationships between Borrelia burgdorferi and unfed Ixodes ticks are considered.     

Kinetics of ingested host immunoglobulin G in hemolymph and whole body homogenates during nymphal development of Dermacentor variabilis and Ixodes scapularis ticks (Acari: Ixodidae)

Patterns in the utilization of host immunoglobulin G (IgG) during nymphal development differed between Dermacentor varibilis (Say) and Ixodes scapularis Say ticks. In unfed nymphs of D. variabilis, host IgG was readily detectable in both hemolymph and whole body homogenates. In unfed nymphs of I. scapularis, host IgG was absent in hemolymph and at very low concentrations in whole body homogenates. Host IgG in unfed nymphs was undoubtedly the remnants of IgG acquired during the larval bloodmeal that persisted through metamorphosis to the nymphal stage. In both tick species, host IgG crossed the midgut into the hemocoel during the latter phases of engorgement. Concentrations of host IgG in I. scapularis declined considerably after replete nymphs molted to the adult stage. In contrast, concentrations of host IgG in D. variabilis remained elevated throughout metamorphosis to the adult stage. When larval D. variabilis were fed on a rat, then 2 months later as nymphs on a rabbit, the rat IgG (“old IgG”) present in unfed nymphs was totally replaced by rabbit IgG (“new IgG”) within 2 d of nymphs attaching to the rabbit. Presumably, the old IgG acquired from a previous bloodmeal was secreted via saliva into the new host. This revised version was published online in July 2006 with corrections to the Cover Date.     

Factors affecting the distributions of the ticks Amblyomma hebraeum and A. variegatum in Zimbabwe: implications of reduced acaricide usage

The ticks Amblyomma hebraeum and A. variegatum are the main vectors of heartwater, a disease of ruminants caused by Cowdria ruminantium, in the agricultural areas of Zimbabwe. At present, A. hebraeum is widely distributed in the dry southern lowveld, and occurs in at least seven foci in the higher rainfall highveld. Amblyomma variegatum occurs in the Zambezi valley and surrounding dry lowveld areas in the northwest. The distribution of A. hebraeum has changed considerably over the past 70 years, while that of A. variegatum appears to have remained fairly static. The distribution patterns of both species in Zimbabwe display anomalous features; the ticks occur in areas of lowest predicted climatic suitability for survival and development and in areas where the densities of cattle, the most important domestic host, are lowest. The only factor favouring the survival of the species in the lowveld habitats in which they occur is the presence of alternative wildlife hosts for the adult stage. Their absence from more climatically favourable highveld habitats appears to have been the result of intensive acaricide treatment of cattle over a long period and a historic absence of significant numbers of wildlife hosts. Eradication of A. hebraeum and A. variegatum by intensive acaricide treatment of cattle can be achieved in the absence of significant numbers of alternative hosts, because of the long attachment and feeding periods of the adults of these tick species. However, eradication becomes impossible when alternative hosts for the adult stage are present, because a pheromone emitted by attached males attracts the unfed nymphal and adult stages to infested hosts. The unfed ticks are not attracted to uninfested hosts, such as acaricide-treated cattle.Regular acaricide treatment of cattle is expensive and so, for economic reasons, the Government of Zimbabwe is no longer enforcing a policy of strict tick control. It is likely that reduced tick control will result in the spread of Amblyomma ticks to previously uninfested areas. Added to this, recent introductions of various wildlife species to highveld commercial farming areas have created conditions in which the ticks could become established in higher rainfall areas. Amblyomma hebraeum is more likely to spread than A. variegatum, because its adults parasitize a wider range of wildlife hosts (warthogs, medium to large-sized antelope, giraffe, buffalo and rhinoceros), whereas adults of A. variegatum appear to be largely restricted to one wildlife species (buffalo) in Zimbabwe, the distribution of which is now confined to very limited areas of the country, as part of foot and mouth disease control measures. A model to predict the rate of spread of A. hebraeum through the highveld is described.Possible control options for dealing with the spread of Amblyomma ticks and heartwater to previous unaffected highveld areas, include (1) continuation of intensive acaricide treatment of cattle to prevent the spread, (2) establishment of a buffer zone of intensive tick control around affected areas to contain the spread and (3) allow the spread to occur and control heartwater by means of immunization. An economic analysis to determine the costs and benefits of the control options, which takes into account the development of Amblyomma-specific tick control technologies and improved heartwater vaccines, is recommended.Deceased.