Climate,satellite imagery and the seasonal abundance of the tick Rhipicephalus appendiculatus in southern Africa: a new perspective

Abstract. Recent predictive models for the distribution of the African tick Rhipicephalus appendiculatus Neumann, based on the computer packages CLIMEX and BIOCLIM and data derived from meteorological satellites, and for the seasonal dynamics of the same tick using the computer simulation models ECFXPERT and T3HOST, all have their limitations. Statistical analysis of the relationships between the seasonal abundance of all three life stages of this tick and climatic and satellite-derived data from five sites in southern Africa, taken from the literature, supports a new perspective that it is the timing of the questing activity of the desiccation-vulnerable larvae that determines the pattern of the tick's seasonal dynamics. The timing of the activity of nymphs and adults is determined by temperature-dependent development rates plus the delaying phenomenon of photoperiod-sensitive diapause, the timing and duration of which have evolved to achieve maximum generation survival by ensuring the occurrence of eggs and larvae during periods of optimal climatic conditions. The most important environmental factor appears to be night-time minimum temperature, determining condensation and saturation deficit and thus the tick's ability to replenish moisture lost during the daytime and so to survive while questing for hosts. It is the larvae whose numbers are correlated most closely with these factors, consistent with earlier experimental results showing larvae to be most susceptible to desiccating conditions. There is a statistical linkage between larval tick numbers and satellite imagery, arising from the correlation between larval numbers and minimum temperature and saturation deficit conditions, and in turn the relationship between these climatic conditions and the subsequent vegetational changes monitored by the satellites. Moisture availability to larvae is likely to be the critical factor throughout the geographical range of R.appendiculatus, but the precise combination of climatic conditions that optimize moisture availability and questing tick survival can be expected to vary geographically. The relationships between ticks, temperatures and satellite data in parts of equatorial Africa have yet to be established. These correlative patterns highlight both the critical life stage and environmental factors when trying to understand temporal, and ultimately spatial, variations in tick abundance.     

The effect of rainfall on tick challenge at Kyle Recreational Park,Zimbabwe

The effect of rainfall pattern on tick challenge was investigated at Kyle Recreational Park, Zimbabwe, from 1991 to 1992 using drag and removal plot methods to sample environmental tick density. The abundance of adults and nymphs of the brown ear-tick Rhipicephalus appendiculatus and larvae of the bont tick Amblyomma hebraeum was positively correlated with monthly rainfall, whereas no relationship with rainfall was revealed for larval R. appendiculatus, adults of the red-legged tick R. evertsi, or larvae of the blue tick Boophilus decoloratus. A comparison between 1991 (490 mm rainfall) and the drought year of 1992 (161 mm) revealed significant differences in the abundance of R. appendiculatus, A. hebraeum, and B. decoloratus. During the wet season, R. appendiculatus adults were 2–3 time more numerous in the environment during the higher rainfall year of 1991. A. hebraeum larval abundance exhibited a similar pattern to that of R. appendiculatus adults, but B. decoloratus larvae were more abundant in the drought year of 1992 during both the wet and dry seasons. Comparable tick abundance data collected at Kyle during the above-average rainfall years of 1975–1977 (mean = 1029 mm) were compared with tick challenge during the below-average rainfall years of 1991–1992 (mean = 326 mm). In grassland sand habitat and all habitats combined R. appendiculatus adults, nymphs, and larvae were much more abundant during the high rainfall years. In contrast, larvae of B. decoloratus were more numerous during the drier years. A. hebraeum larvae were also more abundant during the drier years. The strong positive correlation of adult R. appendiculatus abundance with rainfall and the coincidence of increased adult tick challenge with increased rainfall indicates that adult R. appendiculatus tick burden on hosts would be heaviest during the wet season and high rainfall years.     

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.     

Survival of six species of African ticks in relation to saturation deficits

The survival of unfed males and females of six species of African ticks was monitored at five different saturation deficits at constant temperature (25°C). The survivorship curves for each species comprised a pre-mortality period, prior to when ticks started to die and a mortality period corresponding to a rapid increase in the mortality rate. Longevity was defined as pre-mortality plus mortality. A negative correlation between the longevity of the ticks and the saturation deficits was found with ticks surviving longer at lower deficits. The survival of males and females was similar. At low saturation deficits (2–4 mmHg) Amblyomma hebraeum survived the longest periods (74 weeks). Some correlation was found between the tick survival under dehydrating conditions and habitat associations. Rhipicephalus appendiculatus and Haemaphysalis leachii, the most mesic in distribution, had the shortest longevity (21 and 13 weeks, respectively) at high saturation deficits (7–21 mmHg). Hyalomma marginatum rufipes, the most xerophilic in distribution, had the longest survival (39.3±10.5 weeks) at high saturation deficits. Other factors apart from the adult survival should be taken into account when accounting for the tick distribution, in particular the tolerance of earlier developmental stages to desiccation.     

Monitoring of naturally acquired and artificially induced immunity toAmblyomma variegatum andRhipicephalus appendiculatus ticks under field and laboratory conditions

The ability of rabbits, goats and cattle to acquire immunity to the ixodid ticksAmblyomma variegatum andRhipicephalus appendiculatus was studied under laboratory and field conditions. Rabbits were successfully immunized with crude salivary gland extract (SGE) and midgut extract (ME) obtained from flat or partly fed femaleR. appendiculatus ticks. The lowest numbers of larvae were produced by females fed on rabbits immunized with unfed midgut extract. Similar reductions in larval production could be induced after three infestations of rabbits with adultR. appendiculatus. Also, successive feedings of nymphs ofR. appendiculatus on rabbits resulted in significantly reduced engorgement weights. Skin testing with SGE induced delayed-type hypersensitivity reactions, which could be correlated with immunity toR. appendiculatus in rabbits. Moreover, circulating antibodies were detected in rabbits with an ELISA using SGE ofR. appendiculatus.Immunity toA. variegatum nymphs could be induced in rabbits by repeated infestations, but this failed in goats. Immunization of goats with midgut extract from adultA. variegatum did not protect against subsequent nymphal challenge, but strong skin reactions were noticed when adults ticks fed on immunized goats. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of SGE and ME fromA. variegatum revealed the presence of 48 protein bands in SGE and 29 bands in midgut extract. Western blotting employing serum from a rabbit immune toR. appendiculatus recognized a number of bands in SGE fromR. appendiculatus, but also in SGE ofA. variegatum.Immunity acquired by cattle to ixodid tick infestations under field conditions was monitored by skin testing with SGE and western blot analysis. In general, cattle with the lowest tick numbers manifested the strongest delayed-type hypersensitivity responses. Finally, western blot analysis employing sera from tick-infested and tick-naive cattle could not be related to actual immune status.     

Artificial-membrane feeding of the ixodid tick,Rhipicephalus appendiculatus,to repletion

An artificial feeding device was constructed with a Baudruche, membrane bearing olfactory and tactile stimuli, to induce voluntary attachment and feeding of the brown ear tick,Rhipicephalus appendiculatus. Preparation of the membrane and experimental conditions used are described. Unfed adults ofR. appendiculatus, 2–3 weeks after moulting, were confined on the membrane, which was treated with cattle ear-wash and other stimuli. They attached and commenced sucking blood within 12–48 h. Fully engorged ticks started detaching from the membrane on the 6th day. After a 5–6-day incubation period, they started laying eggs which hatched into normal larvae.     

Immunisation of rabbits against the brown ear tick,Rhipicephalus appendiculatus using tick haemolymph

Tick naive rabbits were immunised with haemolymph components from partially fed Rhipicephalus appendiculatus adult ticks and subsequently challenged with all the developmental instars of the tick. The results obtained showed that the rabbits were rendered resistant to all the instars of the tick. However, the resistance was more pronounced in adult ticks than in the immature stages. The resistance was manifested as a reduction in the number of ticks that fed successfully to engorgement, reduced engorgement weights and reduced fecundities. Re challenging the resistant experimental rabbits with all the developmental instar stages of Rhipicephalus appendiculatus showed that resistance was maintained in subsequent infestations.     

Resistance in cattle againstRhipicephalus appendiculatus with an assessment of cross-resistance toR. pulchellus (Acari: Ixodidae)

A comparison of the yield of engorged nymphs ofRhipicephalus appendiculatus andR. pulchellus was carried out between cattle highly resistant toR. appendiculatus but susceptible toR. pulchellus, and cattle and rabbits susceptible to both species.Resistant cattle yielded fewer and smallerR. appendiculatus nymphs than either susceptible cattle or rabbits. More nymphs of both tick species dropped from susceptible cattle but no differences were detected in the mean weight of engorgedR. pulchellus nymphs fed onR. appendiculatus-resistant and susceptible cattle, although resistant animals yielded more nymphs ofR. pulchellus than ofR. appendiculatus.An acute inflammatory reaction, accompanied by intense pruritus and grooming, was observed in the ears of the resistant cattle infested withR. appendiculatus, but no reaction was detected in the opposite ears whereR. pulchellus was feeding.The low cross-resistance between these two closely related tick species suggests that different antigens may be involved in the development of resistance in cattle to different tick species.     

An update on the ecological distribution of Ixodid ticks infesting cattle in Rwanda: countrywide cross-sectional survey in the wet and the dry season

As part of the epidemiological studies aimed at developing an East Coast fever (ECF) immunisation control strategy, which combines an infection and treatment method with strategic tick control, a countrywide tick survey was carried out in both the dry and the wet season to determine the abundance and the dynamics of the tick populations infesting cattle in Rwanda. Six Ixodid tick species where identified from a total of 12,814 tick specimens collected. Rhipicephalus appendiculatus, the main vector of ECF was the most abundant (91.8%) followed by Boophilus decoloratus (6.1%) and Ambyomma variegatum (1.2%). Few ticks from the three other less economically important Ixodid species (Rhipicephalus compositus, R. evertsi evertsi and Ixodes cavipalpus) were recovered. Both adult and immature stages of the most dominant tick species were found to be widespread with a year round presence. The numbers of ticks were high in low land and medium zones and declined markedly in the higher regions of Rwanda. The geographical distribution of various tick species throughout the country and their epidemiological implications are discussed.     

High-resolution predictive mapping for Rhipicephalus appendiculatus (Acari: Ixodidae) in the Horn of Africa

The brown ear tick Rhipicephalus appendiculatus, vector of East Coast fever (ECF) and related cattle diseases caused by Theileria parva has never been reported from the Horn of Africa. Habitat suitability for this tick species was predicted using Maxent modelling technique based on R. appendiculatus records in Sub-Saharan Africa. Two models were developed: the first is based on the tropical R. appendiculatus distribution and the one is based on the distribution records in the temperate region of Sub-Saharan Africa. The tropical model shows favourable habitat in much of the Ethiopian highlands. The whole Djibouti, the south eastern Ethiopian lowlands, majority of Somalia and Eritrea were found to be not suitable for the survival and development of this tick species. Highly suitable areas occur in areas which have moderate temperature and high precipitation. Introductions of R. appendiculatus into the Horn of Africa probably have been prevented by the natural barrier between the known R. appendiculatus distribution range in East Africa and the Horn of Africa. The effect of an introduction of R. appendiculatus and thereby ECF into the Horn of Africa could be catastrophic since the cattle in this area have no immunity against ECF, and mortality might be considerable in all age groups of cattle.     

Ecology and phenology of cattle ticks in Zambia: Development and survival of free-living stages

A study on development and survival of free-living stages of three important cattle ticks in Zambia,Amblyomma variegatum Fabricus,Boophilus decoloratus Koch, andRhipicephalus appendiculatus Neumann, was carried out to complement studies on seasonal dynamics of parasitic stages.Different instars of engorged ticks were exposed under quasi-natural conditions according to the season in which they occur naturally. Generally, development rates of all stages of the three species were related to temperature, whilst the duration of survival was influenced mainly by rainfall and consequent relative humidity.Observations on the effect of age and climate on the behaviour of ticks on pastures were also made. BothA. variegatum andR. appendiculatus completed only one generation per year. InA. variegatum, engorged females detaching early in the adult season (August to October) undergo morphogenetic diapause. Adults ofR. appendiculatus emerging between August and October enter a period of behavioural diapause before becoming active in December. These mechanisms effectively synchronize the life-cycles of these two univoltine species. The one-host tick,B. decoloratus, is able to complete three to five generations each year with no indication of seasonal synchronization.     

Acquired resistance to ixodid ticks induced by tick cement antigen

Antisera from guinea pigs made resistant to infestation with an ixodid tick of east and central Africa,Rhipicephalus appendiculatus, were used to identify the tick antigens they recognized by immunoblotting. Most of the antigens were found in tick salivary glands and in tick attachment cement. Antisera fromR. appendiculatus-resistant guinea pigs also recognized some salivarygland antigens in ticks of other species (R. pulchellus, R. evertsi, Amblyomma variegatum andA. gemma). Antibodies against the most strongly recognizedR. appendiculatus antigen, a 20-kDa molecule, were only poorly reactive with similar-sized molecules in the other ticks. A 94-kDa antigen, which appeared to have broader cross-reactivity, was purified fromR. appendiculatus attachment cement, and a monospecific rabbit serum was raised against it. This antiserum clearly recognized a molecule of similar molecular weight inR. pulchellus andR. evertsi. Intravenous inoculation of rabbits with the purified molecule elicited delayed-type hypersensitivity to the antigen. The hypersensitive rabbits demonstrated resistance to feeding ofR. appendiculatus ticks but slight enhanced feeding ofR. pulchellus ticks. These results are discussed with respect to their relevance for artificial induction of tick-feeding resistance.     

Effects of combining the fungi <Emphasis Type="Italic">Beauveria bassiana</Emphasis> and <Emphasis Type="Italic">Metarhiziumanisopliae</Emphasis> on the mortality of the tick <Emphasis Type="Italic">Amblyomma variegatum</Emphasis> (ixodidae) in relation to seasonal changes

The effect of formulation, fungal concentration, type and seasonal changes on the mortality of the tick Amblyomma variegatum was investigated. A previous study demonstrated high pathogenicity of strains of the fungi Beauveria bassiana and Metarhizium anisopliae against the ticks Rhipicephalus appendiculatus and Amblyomma variegatum (Kaaya et al. J Invertebr Pathol 1996; 67: 15–20). The present study was undertaken to explore possible additive or synergistic effects of the two fungi on A. variegatum. The effects of oil and water formulations at different concentrations of each fungus and combination of the two on the mortality of A. variegatum in the laboratory and in the field during the wet and dry seasons were determined and compared. The oil formulation performed better in all assays, with highest tick mortality of 92% occurring during the wet season at conidia concentration of 1 × 10¹⁰ conidia/ml of the mixed fungi compared to 49% for the water formulation at similar conidia concentration. However, at the same conidial concentration during the dry season, mortalities in the field were relatively low with the mixture of the fungi recording 24% and 17% tick mortality for the oil and water formulation respectively. The effect of infecting the ticks with a cocktail of the two fungi was inconclusive under more controlled conditions in the laboratory but field results under both wet and dry seasons indicated significant differences between the separate and mixed fungi infections. The results demonstrate a potential of cocktail formulations in the control of ticks and possibly of other arthropod pests.     

Survival and behaviour of unfed stages of the ticksRhipicephalus appendiculatus,Boophilus decoloratus andB. microplus under field conditions in Zimbabwe

The survival and behaviour of the unfed stages ofRhipicephalus appendiculatus, Boophilus decoloratus andB. microplus in gauze columns were observed in long and short grass in the highveld of Zimbabwe. Ticks were exposed in the cool, hot and rainy seasons of 1980 and 1981. All species and stages survived longer in long grass than in short grass. Larvae from engorged female ticks released in the cool season hatched much later than incubator-reared controls. They were consequently not present during the cold weather and survived longer than larvae subjected to the low temperatures, in which the shortest survival-times were recorded. The survival of nymphs was insensitive to season. The longest survival-times were recorded in adults. Median survival-times of incubator-reared adults ranged from 165 to 375 days in short grass and from 333 to 493 days in long grass. These times were usually longer than those for adults which moulted in the field. Larvae of the three species and nymphs ofR. appendiculatus were active soon after hatching or moulting, irrespective of the season. In contrast, adults ofR. appendiculatus showed different patterns of activity in different seasons. Adults first appeared at the base of the columns in October/November and then gradually ascended to reach a maximum height in December/January. They remained high up in the columns until May/June when the weather became increasingly cold and dry. Larvae ofB. decoloratus climbed higher up in the columns in the long grass than did the larvae of the other two species.Larvae and nymphs ofR. appendiculatus and larvae ofB. microplus migrated up and down the columns daily, but larvae ofB. decoloratus and adults ofR. appendiculatus did not migrate.     

Saliva-activated transmission (SAT) of Thogoto virus: dynamics of SAT factor activity in the salivary glands ofRhipicephalus appendiculatus,Amblyomma variegatum,andBoophilus microplus ticks

Thogoto (THO) virus is transmitted from infected to uninfected ticks when co-feeding on uninfected guinea-pigs, even though the guinea-pigs do not develop a detectable viraemia. This form of non-viraemic transmission is potentiated by a factor (s) secreted by the saliva of ticks and hence has been termed saliva-activated transmission (SAT). The synthesis of the SAT factor by the salivary glands of three ixodid tick species was determined by placing uninfected nymphal ticks on guineapigs that were subsequently inoculated with a mixture of THO virus and salivary gland extract (SGE) derived from one of the tick species. SAT factor activity was measured by determining the number of nymphs that acquired THO virus. For the three-host ixodid species,Rhipicephalus appendiculatus andAmblyomma variegatum, maximum enhancement of THO virus transmission was observed when salivary glands were derived from uninfected, female ticks that had fed for a period of 6 or 8 days, respectively. In contrast, when salivary glands were derived form uninfected femaleBoophilus microplus, a one-host ixodid tick species, enhancement of THO virus transmission was observed throughout the tick feeding period. Thus, the natural feeding behaviour of ticks appears to be an important factor in determining the relative importance of these vectors in mediating SAT.     

Cattle ticks of the genera <Emphasis Type="Italic">Rhipicephalus</Emphasis> and <Emphasis Type="Italic">Amblyomma</Emphasis> of economic importance in Tanzania: distribution assessed with GIS based on an extensive field survey

In order to implement a robust integrated tick and tick-borne disease control programme in Tanzania, based on ecological and epidemiological knowledge of ticks and their associated diseases, a national tick and sero-surveillance study was carried out in all 21 regions of the mainland, as well as on Mafia Island, between 1998 and 2001. The current distributions of Rhipicephalus appendiculatus, R. pravus, Amblyomma variegatum, A. gemma, and A. lepidum are illustrated and discussed. Tick distribution maps were assessed using the Weights-of-Evidence method (WofE), and employing temperature, humidity, NDVI, rainfall, and land-cover predictive data. Ground-truthing was done to check correspondence both of the data employed in prediction with land-cover characteristics discerned in the field as well as of the surveyed and predicted tick distributions. Statistical methods were used to analyse associations of the tick species with their environment, cattle density, and other ticks. Except for R. appendiculatus, no appreciable changes were demonstrated in the predicted and observed tick distributions compared to the existing maps that originated in the 1950–1960s. Cattle density influenced the distribution of A. variegatum and, to a certain extent, of A. lepidum, but had no appreciable influence on the distribution of any of the other ticks discussed in this paper, neither did livestock movement. Distinct differences for environmental requirements where observed between different tick species within the same genus. The predictive maps of R. appendiculatus and R. pravus suggest their mutually exclusive distribution in Tanzania, and simultaneous statistical analysis showed R. pravus as a greater specialist. Of the three Amblyomma species, A. variegatum is the most catholic tick species in Tanzania, while both A. gemma and A. lepidum belong to the more specialized species. Despite dissimilar habitat preferences, all three Amblyomma spp. co-exist in central Tanzania, where very heterogeneous habitats may simultaneously satisfy the environmental requirements of all three species. The current study, conducted about 4 decades after the last major survey activities, has shown that changing livestock policies, unrestricted livestock movement and a continuous change in climatic/environmental conditions in Tanzania have brought about only limited changes in the distribution patterns of R. appendiculatus, R. pravus and the three Amblyomma species investigated. Whether this observation indicates a relative indifference of these ticks to environmental and/or climate changes allows room for speculation.     

Development rates,fecundity and survival of developmental stages of the ticksRhipicephalus appendiculatus,Boophilus decoloratus andB. microplus under field conditions in Zimbabwe

To determine development rates, fecundity and survival ofRhipicephalus appendiculatus, Boophilus decoloratus andB. microplus, a study was carried out in long and short grass in the highveld of Zimbabwe. Engorged adult females of the three species and engorged larvae and nymphs ofR. appendiculatus were buried beneath the soil in small cages in the rainy, cool and hot seasons in 1980 and 1981. Half the number of cages were examined regularly to determine development rates and half were left undisturbed to determine survival rates and the fecundity of engorged females. Development was most rapid during warm conditions and slowest during cool conditions, but high temperatures appeared to prolong the preoviposition periods of all species. The relationship between fluctuating temperatures and rate of development in the field was defined using a least-squares procedure. Survival of engorged females was usually high, but was reduced by predation when they were not protected. Fecundity was reduced in long grass during the cool season and in short grass during the hot season. A higher percentage of eggs hatched in the rainy season than in the cool or dry seasons. The survival of engorged larvae and nymphs was usually high in all seasons. Engorged nymphs were the hardiest stage and eggs the most suceptible stage to adverse microclimatic conditions.     

<Emphasis Type="Italic">In vitro</Emphasis> splenocyte proliferation responses of BALB/c mice to salivary gland extracts of three ixodid tick species (Acari: Ixodidae)

In vitro proliferation and cytokine production were investigated in BALB/c mice splenic cell cultures that were stimulated with concanavalin A (ConA) and lipopolysaccharide (LPS) and simultaneously exposed to salivary gland extracts (SGE) of unfed and partially fed adult ixodid ticks (Ixodes ricinus, Rhipicephalus appendiculatus, Amblyomma variegatum). Generally, tick SGE enhanced proliferation of unstimulated splenocytes and SGE of unfed ticks suppressed mitogen induced proliferation. Partially fed R. appendiculatus and A. variegatum suppressed ConA responses, while partially fed I. ricinus stimulated both ConA and LPS induced proliferation. A. variegatum and R. appendiculatus females slightly enhanced LPS responses 2 days after attachment but suppressed them at the end of the slow feeding phase. In 72 h ConA induced cell cultures, interferon gamma (IFN-γ) production was suppressed by SGE of all ticks, interleukin (IL)-10 production was enhanced by unfed I. ricinus and partially fed A. variegatum males and IL-5 production was enhanced by feeding R. appendiculatus females and A. variegatum males. The study revealed variability in the responsiveness of murine splenocytes to SGE of different ixodid tick species, whereby patterns of host immunomodulation within one tick species differed between sexes and changed during feeding.     

Tritrophic interactions between a fungal pathogen,a spider predator,and the blacklegged tick

The blacklegged tick Ixodes scapularis is the primary vector for the bacterium causing Lyme disease in eastern North America and for other medically important pathogens. This species is vulnerable to attack by fungal pathogens and arthropod predators, but the impacts of interactions between biocontrol agents have not been examined. The biocontrol agent Met52^®, containing the entomopathogenic fungus Metarhizium brunneum (=M. anisopliae), controls blacklegged ticks with efficacy comparable to chemical acaricides. The brush‐legged wolf spider Schizocosa ocreata is a predator of I. scapularis that reduces their survival under field conditions. We conducted a field microcosm experiment to assess the compatibility of Met52 and S. ocreata as tick biocontrol agents. We compared the fits of alternative models in predicting survival of unfed (flat) and blood‐fed (engorged) nymphs. We found the strongest support for a model that included negative effects of Met52 and S. ocreata on flat nymph survival. We found evidence for interference between biocontrol agents, with Met52 reducing spider survival, but we did not find a significant interaction effect between the two agents on nymph survival. For engorged nymphs, low recovery rates resulted in low statistical power to detect possible effects of biocontrol agents. We found that nymph questing activity was lower when the spider was active above the leaf litter than when the spider was unobserved. This provides the first evidence that predation cues might affect behavior important for tick fitness and pathogen transmission. This study presents field microcosm evidence that the biopesticide Met52 and spider Schizocosa ocreata each reduced survival of blacklegged ticks Ixodes scapularis. Met52 reduced spider survival. Potential interference between Met52 and the spider should be examined at larger scales, where overlap patterns may differ. Ticks were more likely to quest when the spider was inactive, suggesting the ticks changed their behavior to reduce danger.     

Selection of reference genes for quantitative RT-PCR studies in Rhipicephalus (Boophilus) microplus and Rhipicephalus appendiculatus ticks and determination of the expression profile of Bm86

Background  

For accurate and reliable gene expression analysis, normalization of gene expression data against reference genes is essential. In most studies on ticks where (semi-)quantitative RT-PCR is employed, normalization occurs with a single reference gene, usually β-actin, without validation of its presumed expression stability. The first goal of this study was to evaluate the expression stability of commonly used reference genes in Rhipicephalus appendiculatus and Rhipicephalus (Boophilus) microplus ticks. To demonstrate the usefulness of these results, an unresolved issue in tick vaccine development was examined. Commercial vaccines against R. microplus were developed based on the recombinant antigen Bm86, but despite a high degree of sequence homology, these vaccines are not effective against R. appendiculatus. In fact, Bm86-based vaccines give better protection against some tick species with lower Bm86 sequence homology. One possible explanation is the variation in Bm86 expression levels between R. microplus and R. appendiculatus. The most stable reference genes were therefore used for normalization of the Bm86 expression profile in all life stages of both species to examine whether antigen abundance plays a role in Bm86 vaccine susceptibility.