Distributions of the vectors of heartwater, Amblyomma hebraeum and Amblyomma variegatum (Acari: Ixodidae), in Zimbabwe

The tick vectors of heartwater (Cowdria ruminantium infection) in Zimbabwe, Amblyomma hebraeum and Amblyomma variegatum, historically were believed to be confined to the low-lying regions of the south and north-west of the country. However, country-wide surveys performed in 19751980 and 19881991 demonstrated that both species were also established in western parts of the highveld plateau and had started to encroach on the predominantly heartwater-free central and eastern highveld regions. To determine the current distributions of both the vectors and evaluate the potential threat of heartwater to animals in the highveld, a survey of ticks infesting cattle was performed in 1996 at 2994 locations in small-holder and large-scale commercial farming areas throughout Zimbabwe. Amblyomma hebraeum was collected at 1329 locations, A. variegatum at 72 locations and both A. hebraeum and A. variegatum at 13 locations. The results demonstrated that A. hebraeum was present, as previously recorded, throughout the southern half of the country and appeared to have undergone further limited spread into the central and eastern highveld regions. Only the northern-most region of the country appeared to be free of this species. Amblyomma variegatum was collected mainly in the north-west, as previously recorded, but was also found at isolated locations across the central highveld region and along the eastern border with Mozambique. This species was, however, still absent from the southern half and the northern-most regions of the country. An overlap of the distributions of the two species existed within a zone along the southern and eastern regions of the distribution of A. variegatum. These results suggest that the vectors of heartwater are spreading and threaten to introduce heartwater into intensive livestock-producing regions of the country.Exp Appl Acarol 22: 725740 © 1998 Kluwer Academic Publishers     

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.     

Perception of breath components by the tropical bont tick,Amblyomma variegatum Fabricius (Ixodidae)

Wall-pore olfactory sensilla located in the capsule of Haller's organ on the tarsus of Amblyomma variegatum ticks bear cells responding to vertebrate breath: one of these sensilla contains a CO2-excited receptor and a second sensillum has a CO2-inhibited receptor. Each of these antagonistic CO2-receptors, which display typical phasic-tonic responses, monitors a different CO2-concentration range. The CO2-inhibited receptor is very sensitive to small concentration changes between 0 and ca. 0.2%, but variations of 0.01% around ambient (ca. 0.04%) induce the strongest frequency modulation of this receptor. An increase of just 0.001-0.002% (10-20 ppm) above a zero CO2-level already inhibits this receptor. By contrast, the CO2-excited receptor is not so sensitive to small CO2 shifts around ambient, but best monitors changes in CO2 concentrations above 0.1%. This receptor is characterized by a steep dose-response curve and a fast inactivation even at high CO2-concentrations (greater than 2%). In a wind-tunnel, Amblyomma variegatum is activated from the resting state and attracted by CO2 concentrations of 0.04 to ca. 1%, which corresponds to the sensitivity range of its CO2-receptors. The task of perceiving the whole concentration range to which this tick is attracted would thus appear to be divided between two receptors, one sensitive to small changes around ambient and the other sensitive to the higher concentrations normally encountered when approaching a vertebrate host.     

Perception of breath components by the tropical bont tick,Amblyomma variegatum Fabricius (Ixodidae)

Summary Wall-pore sensilla in the capsule of Haller's organ on foreleg tarsi of the tick, Amblyomma variegatum, show multicellular responses upon stimulation with human and bovine breath. Filtering breath through charcoal removes the stimulant for some of these receptors. Analysis by gas chromatography coupled with olfactory sensillum electrophysiological recordings indicates that an ethanol extract of the breath components trapped on charcoal contains a major stimulant eluting at the same retention time as H₂S. Two types of H₂S-sensitive receptors have been identified. They are housed in separate sensilla, and are called sulfide-receptor 1 and 2.Although, both receptor types are characterized by a high sensitivity to H₂S with an estimated threshold of ca. 0.1 ppb and a response range covering 5–6 log orders of magnitude, their overall response to sulfides and mercaptans is nevertheless dissimilar. The type 1 receptor fires slightly more upon stimulations with H₂S than type 2, whereas ethylmercaptan induces a stronger response from type 2, and dimethyl sulfide activates only receptor 2.In a bioassay, H₂S tested at concentrations of ca. 0.02 ppm and 1 ppm equally arouses 60% of resting ticks. Two-thirds of these ticks quest the air with their first pair of legs, and the remainder start active search. By contrast, H₂S at ca. l ppm in a mixture with CO₂ severely diminishes the locomotor stimulating effect of CO₂.     

Attraction of Amblyomma variegatum (ticks) to the attraction-aggregation-attachment-pheromone with or without carbon dioxide

The responses of adult Amblyomma variegatum ticks released from various distances to different doses of the synthetic attraction-aggregation-attachment pheromone (AAAP) (made up of ortho-nitrophenol, methyl salicylate and nonanoic acid in paraffin oil), dispensed from the center of circular field plots, were studied in the presence or absence of elevated levels of CO₂. Up to 90% of the ticks released were attracted to the pheromone source in the presence of CO₂ within 3h. CO₂ alone was unattractive, similar to previous findings in Zimbabwe, but unlike results from a Caribbean A. variegatum population, which was significantly attracted to this signal. In the absence of CO₂, smaller but significant proportions of the released ticks were attracted to the pheromone, albeit more slowly, suggesting another variation in the responses of this bont tick to inter- and intra-specific signals. Our results are interpreted in the light of a study undertaken elsewhere demonstrating relatively high heterozygosity among tick populations. Possible directions of further research to explore the use of the pheromone in off-host control of the tick are also highlighted.     

Microorganisms in the ticks Amblyomma dissimile Koch 1844 and Amblyomma rotundatum Koch 1844 collected from snakes in Brazil

Knowledge about ticks (Acari) and screening of ticks parasitizing various hosts are necessary to understand the epidemiology of tick‐borne pathogens. The objective of this study was to investigate tick infestations on snakes (Reptilia: Squamata: Serpentes) arriving at the serpentarium at the Institute Vital Brazil, Rio de Janeiro. Some of the identified ticks were individually tested for the presence of bacteria of the genera Rickettsia (Rickettsiales: Rickettsiaceae), Borrelia (Spirochaetales: Spirochaetaceae), Coxiella (Legionellales: Coxiellaceae), Bartonella (Rhizobiales: Bartonellaceae), Ehrlichia (Rickettsiales: Anaplasmataceae), Anaplasma (Rickettsiales: Anaplasmataceae), and Apicomplexa protozoa of the genera Babesia (Piroplasmida: Babesiidae) and Hepatozoon (Eucoccidiorida: Hepatozoidae). A total of 115 hard ticks (Ixodida: Ixodidae) were collected from 17 host individuals obtained from four Brazilian states. Two species of tick were identified: Amblyomma dissimile Koch 1844 (four larvae, 16 nymphs, 40 adults), and Amblyomma rotundatum Koch 1844 (12 nymphs, 43 adults). Rickettsia bellii was found in A. rotundatum and A. dissimile ticks and Rickettsia sp. strain Colombianensi, Anaplasma‐like and Hepatozoon sp. in A. dissimile ticks. Among the tested ticks, no DNA of Borrelia, Bartonella, Coxiella or Babesia was found. The present findings extend the geographic range of Rickettsia sp. strain Colombianensi in Brazil and provide novel tick–host associations.     

Pathogenicity of 17 isolates of entomophagous nematodes (Steinernematidae and Heterorhabditidae) for the ticks Amblyomma variegatum (Fabricius), Boophilus microplus (Canestrini) and Boophilus annulatus (Say)

Entomopathogenous nematodes are well known biocontrol agents of insects. They live in the superficial layer of the soil, a location where ticks accomplish their molt and where they oviposit their eggs, making them, theoretically, the preys of infective larvae of nematodes. Seventeen strains of entomopathogenous nematodes: eight strains of Steinernema and nine strains of Heterorhabditis were placed in contact with each of the free living stages of three tick species: Amblyomma variegatum, Boophilus microplus and B. annulatus. The first two species were resistant to all the nematode strains that were tested, whereas B. annulatus was susceptible to all of them. Ovipositing females were more susceptible than females during the preoviposition period. There were no anatomical differences between the two species of Boophilus which can account for such differences in their susceptibility. The use of nematodes to control some species of ticks should be considered.     

Identification of two genes essential for sperm development in the male tick Amblyomma hebraeum Koch (Acari: Ixodidae)

In most ticks of the family Ixodidae, gonad maturation and spermatogenesis are stimulated by the taking of a blood meal. Previous work from this laboratory identified 35 genes that are up-regulated by feeding [Weiss, B.L., Stepczynski, J.M., Wong, P., Kaufman, W.R., 2002. Identification and characterization of genes differentially expressed in the testis/vas deferens of the fed male tick, Amblyomma hebraeum. Insect Biochemistry and Molecular Biology 32, 785-793]. The functions of most of these genes remain unknown. We used RNA interference technology to investigate the consequences of blocking the function of 13 of these genes. Attenuation of the expression of two of these in particular, AhT/VD 8 and AhT/VD 10, correlated with deformities in the testis and abnormalities in spermiogenesis. Furthermore, most females fed in the company of these males did not engorge properly and laid many fewer eggs, most of which were infertile.     

Identification of vertebrate volatiles stimulating olfactory receptors on tarsus I of the tick Amblyomma variegatum Fabricius (Ixodidae)

Bovine odour excites olfactory receptor(s) in a wall-pore olfactory sensillum on the anterior pit of Haller's organ in Amblyomma variegatum. Gas chromatography-coupled electrophysiology recordings from this sensillum reveal the presence of 4 active compounds in bovine odour. The two strongest stimulants were identified as 2-nitrophenol and 4-methyl-2-nitrophenol by gas chromatography-coupled mass spectrometry, and by matching electrophysiological activity of synthetic analogues. Synthetic analogues of known vertebrate-associated volatiles also stimulate other olfactory receptors in sensilla on the surface of tarsus I: a lactone receptor responding to -valerolactone and 6-caprolactone; different fatty acid receptor types responding best to either pentanoic acid, 2-methylpropanoic acid or to butanoic acid; three receptors responding to NH₃; and one receptor responding to 3-pentanone. Gas chromatography-coupled mass spectrometry analysis of vertebrate volatiles revealed presence of a number of these olfactory stimulants in concentrates of rabbit and steer odour, i.e. 2-methylpropanoic acid, butanoic acid, 3-methylbutanoic acid, pentanoic acid, and -valerolactone.Abbreviations GC-EL gas chromatography-coupled electrophysiological recording - GC-MS gas chromatography-coupled mass spectrometry     

Antimicrobial activity in the egg wax of the African cattle tick Amblyomma hebraeum (Acari: Ixodidae)

Eggs of the tick Amblyomma hebraeum Koch (Acari: Ixodidae) inhibited the growth of Escherichia coli and Serratia marcescens (Gram-negative bacteria) in solid culture, but not the growth of Staphylococcus epidermidis, and only marginally the growth of Bacillus subtilis (Gram-positive bacteria). When egg wax was extracted with chloroform/methanol (2:1), the extract contained antibacterial activity, but the denuded eggs did not. When assayed against bacteria in liquid culture, the aqueous phase inhibited the growth of S. epidermidis. However, the activity against E. coli was lost during extraction. The antimicrobial component of the aqueous phase was heat stable (100°C for 10 min), resistant to proteinase K (15 min at 55°C) and to pronase (30 min at 37°C). The antibacterial activity in the aqueous phase increased the permeability of the cell membrane of susceptible bacterial cells within 30 min. However, lysis of the cells was detected by optical density measurements (OD_{600 nm}) only after 1.5 h. The most evident cytological changes observed by transmission electron microscopy were a thickening of the cell wall and the appearance of numerous electron lucent areas within the cytoplasm of treated bacteria. Gené’s organ, the egg-waxing organ in ticks, grew enormously during the first 16 days post-engorgement, and gained antimicrobial activity by day 10 (when oviposition began). This suggests that Gené’s organ is the major source of the antibacterial substance in the egg wax. The vitellogenic hormone in A. hebraeum, 20-hydroxyecdysone, when injected into recently engorged females, did not stimulate growth of Gené’s organ or precocious secretion of antimicrobial activity.     

Rickettsia africae infection rates and transovarial transmission in Amblyomma hebraeum ticks in Mnisi,Bushbuckridge, South Africa

Rickettsia africae is a gram-negative bacterium, which causes African tick bite fever (ATBF) in humans. ATBF is a febrile disease mainly affecting travellers to southern Africa. This bacterium is known to be transmitted by Amblyomma hebraeum and Amblyomma variegatum ticks. In southern Africa, the principal vector is A. hebraeum. Febrile disease is a serious issue in the study area. There is a high prevalence of non-malaria illness caused by Rickettsia, so there is a need to have more knowledge on these species. Infection rates and transovarial transmission efficiency of R. africae in A. hebraeum ticks were investigated in a rural area of Mpumalanga province, South Africa. Adult and engorged A. hebraeum female ticks were collected from cattle. Larvae were collected by dragging a cloth at ground level using 100 steps, equivalent to an area of 100 m². Tick identification was performed according to standard taxonomic keys using a microscope. Engorged ticks were incubated to oviposit and egg masses were collected. DNA was extracted from the ticks, larvae and egg masses, and screened for gltA and ompA genes, using quantitative real-time PCR and conventional PCR, respectively. Positive ompA amplicons were sequenced and phylogenetic analysis showed 99.8-100% identity with R. africae. Infection rates were 13.7 and 12.7% for adults and larvae, respectively. Transovarial transmission of R. africae in A. hebraeum from this study was 85.7%. The results provide a clear indication that people living in the study area and travellers that visit the area are at risk of contracting ATBF.

     

Detection of Babesia caballi in Amblyomma variegatum ticks (Acari: Ixodidae) collected from cattle in the Republic of Guinea

A reverse line blot hybridisation (RLB) assay was applied to screen Amblyomma variegatum adult ticks (n = 504) collected from N'Dama cattle in the Republic of Guinea. In a PCR, the V1 hypervariable region of the 16S ribosomal RNA (rRNA) gene was amplified with a set of primers unique for species of the genera Anaplasma and Ehrlichia, and the V4 hypervariable region of the 18S rRNA gene was amplified with primers specific for members of the genera Theileria and Babesia. Amplified PCR products from A. variegatum ticks were hybridised onto a membrane, to which oligonucleotide probes species-specific for Ehrlichia/Anaplasma and Theileria/Babesia parasites were covalently linked. No pathogens belonging to Ehrlichia/Anaplasma species were found, while 10 DNA samples resulted positive for Babesia caballi and 5 samples for Theileria velifera. This is the first report of B. caballi in A. variegatum ticks. One of the B. caballi positive samples was sequenced. This new strain (BcabGuinea) showed a 97% similarity to the Z15104 B. caballi GenBank sequence.     

Attractivity of cattle infested by Amblyomma variegatum (Acari: Ixodidae) for conspecific adult ticks from the field in Guadeloupe

Four field experiments were conducted with creole cattle in Guadeloupe to study the attractivity for adult ticks of animals previously infested with males of Amblyomma variegatum producing aggregation attachment pheromones (AAPs). In each experiment, two groups of cattle were used; the first group was infested with adult A. variegatum and the second group was kept free of ticks and pheromones by acaricidal treatment and by washing the hair and skin of previously infested sites. Both groups were then exposed to natural infestation by ticks present in the environment or by ticks bred in the laboratory and released on the pasture. The number of additional ticks infesting animals were compared between the treated and control groups. There were no significant differences between both groups in the numbers of new male ticks infesting the animals during the first days following the beginning of the experiment. Thereafter, comparison of the slopes of infestation indicated that infested cattle were no more attractive for male ticks than uninfested hosts. Uninfested cattle were spontaneously and rapidly infested by male ticks. It is concluded that carbon dioxide or another host component was a major attractant for A. variegatum males. Female ticks attached on predilection sites exclusively in the presence of AAP-producing males, i.e. 5–10 days after the attachment of the first males. Based on these results we postulate that slow-release devices impregnated with pheromones have no additional attractive effect on A. variegatum males under Caribbean conditions. However, they may be useful in inducing the attachment of females, but only in the first days preceding the arrival of pioneer AAP-producing males. © Rapid Science Ltd. 1998     

Comparison of the attachment rates of males of the ticksAmblyomma hebraeum andA. variegatum to cattle,sheep and rabbits in the absence of aggregation-attachment pheromone

Losses in domestic ruminants caused by heartwater (Cowdria ruminantium infection) in Zimbabwe and Mozambique are greater when the vector isAmblyomma hebraeum than when the vector isA. variegatum. It has been suggested that the epidemiology of the disease may be influenced by the rates at which unfed adults of these two tick species attach to uninfested hosts (i.e. in the absence of the male-produced aggregation-attachment pheromone [AAP]). In this study we confined unfed males ofA. hebraeum andA. variegatum on uninfested cattle, sheep and rabbits and recorded their attachment rates. Males of both species attached more rapidly on cattle than on sheep or rabbits. Males ofA. hebraeum attached more rapidly than males ofA. variegatum on all three host species. The differences in the attachment rates between the two species were much greater on sheep and rabbits than on cattle. The findings suggest that in the absence of AAP, pioneer males of both tick species may attach to cattle, and pioneer males ofA. hebraeum may also attach to sheep. The differences in the attachment rates ofA. hebraeum andA. variegatum provide a possible explanation for observed differences in the epidemiology of heartwater associated with these two vector species.     

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.     

Responses of the tropical bont tick, Amblyomma variegatum (Fabricius), to its aggregation-attachment pheromone presented in an air stream on a servosphere

Male Amblyomma variegatum ticks feeding on a host release a mixture of o -nitrophenol and methyl salicylate which serves to attract conspecifics. The behavioural responses of A. variegatum on a servosphere to these volatiles presented in an air stream are detailed here. In still air, ticks walked on all eight legs, but with long halts. In contrast, the air stream caused continuous walking and induced a reaching response where the forelegs actively sampled the air. Such reaching increased the angular velocity and reduced walking speed, effects that were amplified in the presence of vapours from o -nitrophenol and methyl salicylate in the air flowing over the ticks. Vapour from a 1:1 mixture of o -nitrophenol and methyl salicylate was attractive over a 10⁴-fold concentration range providing an increase in upwind displacement of 20–40%, significantly higher than the natural ratio where o -nitrophenol vapour predominates. Although the responses to o -nitrophenol vapour were variable when presented alone, this chemical was consistently attractive when delivered with steer hair odour – unattractive on its own. Moreover, the upwind walk to this combination did not cause a change in speed or angular velocity. This supports the hypothesis that the response to the pheromone is enhanced by host odour. Accepted: 16 October 1999     

Seasonal occurrence of the bont tick (Amblyomma hebraeum) in the southern lowveld of Zimbabwe

The seasonal occurrence of the bont tickAmblyomma hebraeum was recorded over 2 years on 20 Brahman cattle held in a 240-ha paddock at Mbizi in southeastern Zimbabwe. The cattle were infested with all life cycle stages throughout the study period, and no clearly defined seasonal patterns were recorded. Males remained attached for up to about 6 months, and consequently accumulated on the cattle and outnumbered females considerably. The cattle did not acquire resistance toA. hebraeum and the abundance of the adults increased steadily over the 2 years of sampling. The occurrence of adults and nymphs on hosts appeared to be largely independent of weather, and these stages were active over the entire range of temperature and relative humidity recorded during the study period. It was concluded that unfed adults and nymphs are able to engage in host-seeking irrespective of weather conditions as they do not await hosts on the vegetation but emerge from protected microhabitats in response to specific stimuli from hosts, notably, carbon dioxide and the aggregation-attachment pheromone emitted by attached males. This behavior may explain the reported absence of a consistent pattern of seasonal occurrence ofA. hebraeum at different locations in southeastern Africa.     

Molecular characterization of the vitellogenin receptor from the tick,Amblyomma hebraeum (Acari: Ixodidae)

We have identified the full-length cDNA encoding a vitellogenin receptor (VgR) from the African bont tick Amblyomma hebraeum Koch (1844). VgRs are members of the low-density lipoprotein receptor superfamily that promote the uptake of the yolk protein vitellogenin (Vg), from the haemolymph. The AhVgR (GenBank accession No. JX846592) is 5703 bp, and encodes an 1801 aa protein with a 196.5 kDa molecular mass following cleavage of a 22 aa signal peptide. Phylogenetic analysis indicates that AhVgR is highly similar to other tick VgRs. AhVgR is expressed in only the ovary of mated, engorged females, and is absent in all other female tissues and in both fed and unfed males. Unfed, adult females injected with a VgR-dsRNA probe to knock-down VgR expression experienced a significant delay in ovary development and started oviposition significantly later than controls. These results indicate that the expression of AhVgR is important for the uptake of Vg and subsequent maturation of the oocytes.     

Comparison of efficacy of American and African Amblyomma ticks as vectors of heartwater (Cowdria ruminantium) infection by molecular analyses and transmission trials

The ability of Amblyomma americanum, Amblyomma cajennense, Amblyomma maculatum, and Amblyomma variegatum to acquire and transmit Cowdria ruminantium infection was investigated. Uninfected nymphs were fed on clinically reacting C. ruminantium-infected sheep and then analyzed for infection by specific DNA detection assays and by tick transmission trials. By polymerase chain reaction (PCR), the mean infection prevalence of A. maculatum ticks (50.7%) was similar to that of A. variegatum, Elevage strain (43.5%; P = 0.83) and Petit Bourg strain (45.9%; P = 0.26) ticks. Though Amblyomma hebraeum were not tested by PCR, by DNA probe their infection prevalence was 94%. In contrast, A. americanum and A. cajennense ticks demonstrated very low susceptibility to C. ruminantium, and the prevalence of infection by PCR was approximately 1%. The higher susceptibility of A. maculatum and A. variegatum to C. ruminantium correlated with superior vector efficiency, depicted by similar prepatent periods and severity of disease transmissions to sheep. Amblyomma americanum and A. cajennense failed to transmit infection, confirming that low susceptibility to C. ruminantium correlates with the poor vector status of these species. These results highlight the importance of A. maculatum as a potential vector that is likely to play a major role in the establishment and maintenance of heartwater, if the disease were to be introduced to the U.S.A., Central, and South America.