First record of Rhipicephalus (Boophilus) microplus in Ghana, a potential risk to livestock production

Ticks are a public health threat due to their tendency to spread pathogens that affect humans and animals. With reports of Rhipicephalus (Boophilus) microplus invasion in neighbouring countries, there is the risk of this species invading Ghana through livestock trade. Previous identification of tick species in Ghana has been based on morphological identification, which can be ineffective, especially with damaged tick specimens or engorged nymphs. This study focused on the Kassena-Nankana District, which serves as a trade route for cattle into Ghana, to determine the presence of R. microplus. Three genera of ticks were identified as Amblyomma (70.9%), Hyalomma (21.3%) and Rhipicephalus (7.8%). The engorged nymphs that could not be identified morphologically were analyzed using primers that target the mitochondrial 16S rRNA gene. This study reports the first record of R. (B.) microplus in Ghana. Furthermore, R. microplus constituted 54.8% of the Boophilus species collected in this study. This finding is an addition to the diverse tick species previously collected in Ghana, most of which are of veterinary and public health importance. With reports of acaricide resistance in R. microplus and its role in spreading infectious pathogens, the detection of this species in Ghana cannot be overlooked. Nationwide surveillance will be essential to ascertain its distribution, its effects on cattle production, and the control measures adopted.

Phylogenetic insights on Mediterranean and Afrotropical Rhipicephalus species (Acari: Ixodida) based on mitochondrial DNA

A multigene phylogeny including 24 Rhipicephalus species from the Afrotropical and Mediterranean regions, based on mitochondrial DNA genes (COI, 12S and 16S), was constructed based on Bayesian inference and maximum likelihood estimations. The phylogenetic reconstruction revealed 31 Rhipicephalus clades, which include the first molecular records of Rhipicephalus duttoni (Neumann), and Rhipicephalus senegalensis (Koch). Our results support the R. pulchellus, R. evertsi and R. pravus complexes as more phylogenetically close to Rhipicephalus (Boophilus) than to the remaining Rhipicephalus clades, suggesting two main monophyletic groups within the genus. Additionally, the phenotypic resembling R. sanguineus s.l. and Rhipicephalus turanicus (Pomerantsev) are here represented by nine clades, of which none of the R. turanicus assemblages appeared as distributed in the Iberian Peninsula. These results not only indicate that both species include more cryptic diversity than the already reported, but also suggest that R. turanicus distribution is less extended than previously anticipated. This analysis allowed to improve species identification by exposing cryptic species and reinforced mtDNA markers suitability for intra/inter-species clarification analyses. Incorporating new species molecular records to improve phylogenetic clarification can significantly improve ticks' identification methods which will have epidemiologic implications on public health.

Morphological and molecular divergence of Rhipicephalus turanicus tick from Albania and China

Rhipicephalus turanicus is an important tick species potentially carrying tick-borne pathogens. Several tick species have obvious subspecies divergence. However few studies aimed to examine the existence of divergence within R. turanicus. Therefore, a detailed morphological and molecular analysis was conducted for comparing R. turanicus from the Mediterranean Basin (represented by Albania) and Central Asia (Northwestern China). Altogether 315 adult ticks of R. turanicus (103 from Albania and 212 from China) were morphologically and molecularly analysed. DNA samples were used for mitochondrial 16S rRNA and cox1 gene sequences analysis. In addition, as potentially genetic markers, three fragments including partial nad1-16S rRNA, nad2-cox1, cox1-tRNA-Lys, were designed and then phylogenetically analyzed. Based on detailed morphological observations, only basis capituli length:width ratio (females), the length, the width and the length:width ratio of the scutum (males) had differences between R. turanicus from China and Albania. Gene divergences of 16S rRNA, cox1, partial nad1-16S rRNA, nad2-cox1 and cox1-tRNA-Lys from China and Albania ticks were 3.53-4.84, 3.57-4.92, 3.57-4.07, 3.57-4.39 and 3.18-4.69%, respectively. The evaluated five genetic markers revealed two phylogenetic branches in R. turanicus. Obvious differences exist within R. turanicus based on morphological and genetic analysis. Three newly designed genetic markers (partial nad1-16S rRNA, nad2-cox1 and cox1-tRNA-Lys) in this study may be suitable genetic tools for identification and analysis in R. turanicus. Subspecies analysis of R. turanicus from other regions of the world should be initiated in the future.

Diversity of tick species on domestic animals in Shandong Province, China, using DNA barcoding

Ticks are considered to be second only to mosquitoes as vectors of diseases. In recent years, severe fever with thrombocytopenia syndrome, a new emerging tick-borne disease has been detected in many areas of China, including Shandong Province, Eastern China. Here, we report the tick species diversity based on surveys between 2014 and 2016 covering 16 locations in seven cities of Shandong. Based on DNA barcoding, 1859 ticks belonging to three species were identified: Haemaphysalis longicornis, Rhipicephalus turanicus and Haemaphysalis verticalis. Samples of the same species clustered together in a neighbor-joining phylogenetic tree, with intraspecific distances between 0 and 3.0% and interspecific distances ranged between 15.5 and 24.3%. Goats and dogs were the major hosts of ticks and H. longicornis was regarded as predominant tick species of Shandong. In order to reduce tick populations and prevent tick-borne diseases, effective control measures should be implemented on human and domestic animals, respectively.

Genome-wide association study of tick resistance in South African Nguni cattle

Ticks and tick-borne diseases are among the main causes of economic loss in the South African cattle industry through high morbidity and mortality rates. Concerns of the general public regarding chemical residues may tarnish their perceptions of food safety and environmental health when the husbandry of cattle includes frequent use of acaricides to manage ticks. The primary objective of this study was to identify single nucleotide polymorphism (SNP) markers associated with host resistance to ticks in South African Nguni cattle. Tick count data were collected monthly from 586 Nguni cattle reared in four herds under natural grazing conditions over a period of two years. The counts were recorded for six species of ticks attached in eight anatomical locations on the animals and were summed by species and anatomical location. This gave rise to 63 measured phenotypes or traits, with results for 12 of these traits being reported here. Tick count (x) data were transformed using log10(x+1) and the resulting values were examined for normality. DNA was extracted from hair and blood samples and was genotyped using the Illumina BovineSNP50 assay. After quality control (call rate >90%, minor allele frequency >0.02), 40,436 SNPs were retained for analysis. Genetic parameters were estimated and association analysis for tick resistance was carried out using two approaches: a genome-wide association (GWA) analysis using the GenABEL package and a regional heritability mapping (RHM) analysis. The Bonferroni genome-wide (P<0.05) corrected significance threshold was 1.24×10(-6), with 2.47×10(-5) as the suggestive significance threshold (P<0.10) (i.e., one false positive per genome scan) in the GWA analysis. Likelihood ratio test (LRT) thresholds for genome-wide and suggestive significance were 13.5 and 9.15 for the RHM analysis. Six ixodid tick species were identified, with Amblyomma hebraeum (the vector for Heartwater disease) being the dominant species. Heritability estimates (h(2)) from the fitted animal and sire models ranged from 0.02±0.00 to 0.17±0.04 for the transformed tick count data. Several genomic regions harbouring quantitative trait loci (QTL) were identified for different tick count traits by both the GWA and RHM approaches. Three genome-wide significant regions on chromosomes 7, 10 and 19 were identified for total tick count on the head, total body A. hebraeum tick count and total A. hebraeum on the perineum region, respectively. Additional regions significant at the suggestive level were identified on chromosomes 1, 3, 6, 7, 8, 10, 11, 12, 14, 15, 17, 19 and 26 for several of the traits. The GWA approach identified more genomic regions than did the RHM approach. The chromosomal regions identified here as harbouring QTL underlying variation in tick burden form the basis for further analyses to identify specific candidate genes and polymorphisms related to cattle tick resistance and provide the potential for marker-assisted selection in Nguni cattle.

Rhipicephalus rossicus, a neglected tick at the margin of Europe: a review of its distribution, ecology and medical importance

Rhipicephalus rossicus (Ixodida: Ixodidae) is a three-host tick with a broad host spectrum that includes wild animals, pets, livestock and humans. Despite its local abundance in certain areas, most of the available information on R. rossicus was published decades ago, mainly by former soviet authors. Its distribution largely overlaps the Eurasian steppe. However, its range may be more extensive than is currently known because this species may have been misidentified as Rhipicephalus sanguineus, principally in areas where the latter species is present. Although R. rossicus has been occasionally reported to feed on people, little attention has been given to its medical importance. It has been shown to have a vectorial role in the transmission of Francisella tularensis, Crimean-Congo haemorrhagic fever virus and West Nile virus. However, the vectorial importance of R. rossicus may be significantly greater, mainly as the closely related species R. sanguineus s.l. is known to transmit a very wide spectrum of pathogens. The probably underestimated vectorial role of R. rossicus may represent a hidden public health threat.

Metagenomic profile of the viral communities in Rhipicephalus spp. ticks from Yunnan, China

Besides mosquitoes, ticks are regarded as the primary source of vector-borne infectious diseases. Indeed, a wide variety of severe infectious human diseases, including those involving viruses, are transmitted by ticks in many parts of the world. To date, there are no published reports on the use of next-generation sequencing for studying viral diversity in ticks or discovering new viruses in these arthropods from China. Here, Ion-torrent sequencing was used to investigate the presence of viruses in three Rhipicephalus spp. tick pools (NY-11, NY-13, and MM-13) collected from the Menglian district of Yunnan, China. The sequencing run resulted in 3,641,088, 3,106,733, and 3,871,851 reads in each tick pool after trimming. Reads and assembled contiguous sequences (contigs) were subject to basic local alignment search tool analysis against the GenBank database. Large numbers of reads and contigs related to known viral sequences corresponding to a broad range of viral families were identified. Some of the sequences originated from viruses that have not been described previously in ticks. Our findings will facilitate better understanding of the tick virome, and add to our current knowledge of disease-causing viruses in ticks living under natural conditions.

Identification and characterization of Rhipicephalus (Boophilus) microplus and Haemaphysalis bispinosa ticks (Acari: Ixodidae) of northeast India by ITS2 and 16S rDNA sequences and morphological analysis

To investigate and identify the ticks prevalent in the North East part of India, scanning electron microscope (SEM) and DNA sequence of nuclear second internal transcribed spacer (ITS2) and mitochondrial 16S ribosomal DNA (rDNA) were used. Based on the morphological and molecular analysis, the ticks infesting cattle of North East India were found to be Rhipicephalus (Boophilus) microplus and Haemaphysalis bispinosa. ITS2 and 16S rDNA sequence from R. (B.) microplus and H. bispinosa were amplified using universal and gene specific primers, sequenced and analysed. The length of the amplified ITS2 sequence of R. (B.) microplus and H. bispinosa, were found to be approximately 1,500 and 1,700 bp, respectively. The length of the 16S rDNA sequences in both the ticks was found to be similar in size, but they differ in their base pair constitutions. This is the first report of the nucleotide sequences of ITS2 and 16S rDNA of H. bispinosa. Phylogenetic analysis revealed that H. bispinosa is a close relative of H. longicornis. A polymerase chain reaction-restriction fragment length polymorphism diagnostic tool was developed based on HindIII digestion of ITS2 in order to facilitate the identification of these two species which cannot be distinguished once it is fully-fed. Present study describes the use of SEM and 16S rDNA/ITS2 based molecular analysis in identification and differentiation of fully fed tick species.

First record of the pantropical blue tick Rhipicephalus microplus in Namibia

The invasive pantropical blue tick, Rhipicephalus microplus, has recently been collected from cattle in Namibia. A cross-sectional study aimed at recording the geographic distribution of Rhipicephalus decoloratus and establishing whether R. microplus is present in Namibia was conducted towards the end of summer (March-April) 2013. Ticks were collected from cattle on 18 privately owned farms across a large geographical scale. Ticks were collected from three to five cattle per farm and species belonging to the genera Hyalomma and Rhipicephalus were recovered. Rhipicephalus decoloratus was present on all farms and R. microplus was recorded on four of the farms. The small numbers of R. microplus compared to R. decoloratus collected in the mixed infestations, suggests that the introduction events were recent.

Cuticular fatty acid profile analysis of three Rhipicephalus tick species (Acari: Ixodidae)

Cuticular fatty acids (CFA) are important constituents of the arthropod exoskeleton, serving as structural and defense components, and participating in intra-species communication. Here we describe for the first time a comparative analysis of the CFA profiles of three tick species of the genus Rhipicephalus: R. annulatus, R. bursa and R. sanguineus. CFA profiles were determined for R. bursa and R. sanguineus grown both on rabbit or calf, and for R. annulatus grown on calf. CFA composition was compared for each species before and after ethanol treatment, for different hosts of each species, and between the different species. Our data suggest that adsorption of the host's fatty acids changes the apparent CFA composition. Ethanol treatment efficiently removed the unbound fatty acids from the ticks and revealed the actual composition. Comparison between ticks grown on rabbit versus calf showed significant difference in the relative abundance of fatty acids C14 and 9,12-C18:2 for R. bursa, and a difference in the relative abundance of C14 for R. sanguineus. Comparison of the CFA between the three species revealed significant differences in the abundance of fatty acids C16, 9,12-C18:2, 9-C18:1, C18 and C20. Our results show that while the host had a minor effect on CFA composition within each species, significant differences were observed in the CFA profiles of different species. We suggest that CFA profiles may be used to distinguish between related species. CFA analysis can also be used in studies of communication and defense mechanisms in ticks and other arthropods.

Diagnosis of amitraz resistance in Brazilian populations of Rhipicephalus (Boophilus) microplus (Acari: Ixodidae) with larval immersion test

Among the ectoparasites of cattle, Rhipicephalus (Boophilus) microplus (Canestrini) (Acari: Ixodidae) remains a major cause of economic losses to livestock. The chemical control with acaricides is still the most efficient method available to control ticks. The aims of this study were to diagnose resistance to amitraz in 16 tick populations from the States of São Paulo (14) and Paraná (2), using the larval immersion technique (LIT), and evaluate the effect of synergists [piperonyl butoxide (PBO), diethyl maleate (DEM), triphenyl phosphate (TPP)] on amitraz resistant and susceptible strains of cattle tick. Most of the evaluated populations (68.7 %) showed to be resistant to amitraz, with resistance ratios ranging from 2.14 to 132. The results suggest that the test procedure by LIT is sensitive and adequate for detection and monitoring of amitraz resistance in cattle tick. No synergistic effect was observed for the synergists PBO, DEM and TPP, on the amitraz resistant (Poa) strain of cattle tick, indicating that increased detoxification metabolism was not involved in this resistance.

Displacement of Rhipicephalus decoloratus by Rhipicephalus microplus (Acari: Ixodidae) in the Eastern Cape Province, South Africa

The objective of the study was to establish to what extent the native tick species Rhipicephalus decoloratus has been displaced by the invasive introduced tick, Rhipicephalus microplus at two communally grazed areas in the Eastern Cape Province, South Africa. To this end ticks were collected monthly from five cattle over a period of 2 years and from 10 drag-samples of the vegetation over a period of 1 year at each locality. Whereas 10 years previously only R. decoloratus and no R. microplus had been recorded in the vicinity of the two sites, R. microplus now comprised the bulk of collections at both. Furthermore, significantly more R. microplus were collected from cattle at both localities during the 2nd year of the survey than during the 1st. In addition to 83 instances of intraspecific coupling, there were 17 instances of R. microplus males coupled with R. decoloratus females. Collections made from cattle and goats on two farms close to the study sites revealed that R. microplus was present on both host species and that it significantly outnumbered R. decoloratus on one of the farms. Rhipicephalus decoloratus and R. microplus larvae as well as larvae exhibiting characteristics of both species were collected from the vegetation.

A new species of Rhipicephalus (Acari: Ixodidae), a parasite of giraffes in Kenya

A new tick species belonging to the genus Rhipicephalus Koch, 1844 (Acari: Ixodidae), namely, Rhipicephalus walkerae n. sp., is described. The male and female of this species are similar to those of several species in the Rhipicephalus appendiculatus group but can be distinguished from them by the very dense pattern of medium-sized punctations covering the conscutum and scutum, long and narrow dorsal prolongation of the spiracular plate, and relatively short dorsal cornua; in addition, the male has long and narrow adanal plates without a posterolateral angle. R. walkerae is known only from Kenya, where the adults were collected from giraffes, Giraffa camelopardalis (L.).

A new species of Rhipicephalus (Acari: Ixodidae), a parasite of red river hogs and domestic pigs in the Democratic Republic of Congo

A new tick species belonging to the genus Rhipicephalus Koch, 1844 (Acari: Ixodidae), namely, Rhipicephalus congolensis n. sp., is described. Males and females of this species are similar to those of Rhipicephalus complanatus Neumann, 1911 and Rhipicephalus planus Neumann, 1907, but it can be distinguished from them by a pattern of dense medium-sized punctations on the conscutum and scutum. Males of R. congolensis may be distinguished by the following characters: posterior half of the marginal groove deep with a sharp outer edge; anterior portion of the groove shallow with rounded edges; posteromedian groove distinct, long, and deep; adanal plates broadly sickle-shaped; bluntly pointed posteromedian spur on coxa I; and posterolateral spur on coxa I slightly longer or subequal to posteromedian spur. Females of R. congolensis may be distinguished by the following characters: outer edge of cervical grooves smooth and not clearly defined either by slope or punctations; genital aperture broad, bowl-shaped, and tripartite in appearance, with central flap flanked on either side by an oval depression; and posteromedian spur on coxa I tapering to its apex. R. congolensis is known only from the Democratic Republic of Congo, where the adults were collected from red river hogs, Potamochoerus porcus (L.), and domestic pigs, Sus scrofa (L.), within the dense equatorial forest in the districts of Equateur and Tshuapa, in the province of Equateur.

Reinstatement of Rhipicephalus (Boophilus) australis (Acari: Ixodidae) with redescription of the adult and larval stages

Rhipicephalus australis Fuller, the Australian cattle tick, is reinstated and the adults and larvae redescribed from material collected in Australia. This long ignored boophilid was previously known as R. microplus Canestrini for specimens reported in Australia and New Caledonia. The adults of R. australis are easily recognized by a combination of characters, such as the ventro-medial spurs in the palpal segments of the male, and the abundant, plumose, pale white setae on the dorsum of the female. Other details, such as coxal and adanal shields are more variable among different populations and may lead to incorrect determinations. Larvae of R. australis are clearly smaller than those of R. microplus. The use of principal components analysis on body measurements leads to a clear separation of larvae of both taxa. A phylogenetic analysis based on 12S- and 16S-rDNA gene sequences supports the conspecificity of the neotype material on which the reinstatement of the species is proposed, and of the specimens used for previous interspecific crosses. R. australis is now known to be present in Australia, New Caledonia, the island of Borneo, Philippines, Sumatra, Java, New Guinea, Cambodia, and Tahiti. Both R. microplus and R. australis coexist in some countries in southeastern Asia. Given the extreme importance of these ticks for the cattle industry, field data on their distribution in the region are required to know the actual range of these species and to understand the evolution of the group.

Distribution of Rhipicephalus (Boophilus) microplus and Rhipicephalus (Boophilus) annulatus (Acari: Ixodidae) infestations detected in the United States along the Texas/Mexico border

Species identification and global positioning system coordinates of infestations of cattle fever ticks, Rhipicephalus (Boophilus) annulatus (Say) and Rhipicephalus (Boophilus) microplus (Canestrini), were determined for 790 specimens submitted to the National Veterinary Services Laboratory between 1 October 1999 and 30 September 2010. Cattle fever tick specimens obtained by personnel of the United States Department of Agriculture-Animal and Plant Health Inspection Service-Cattle Fever Tick Eradication Program from infested cattle and wildlife along the Texas/ Mexico border were submitted for identification, as required by the program. A geographic information system database was developed that incorporates location, collection, and infestation records. Submitted ticks came from 11 Texas counties and were comprised of 19.5% R. (B.) annulatus and 80.5% R. (B.) microplus. Maps produced from this study locate and define the parapatric boundary between R. (B.) annulatus and R. (B.) microplus.

Development and validation of a PCR-RFLP test to identify African Rhipicephalus (Boophilus) ticks

The cattle tick Rhipicephalus (Boophilus) microplus has recently invaded West Africa and caused anxiety amongst farmers in Ivory Coast, as livestock production was severely affected. The introduction of this tick species has remained unnoticed for several years, as all the members of this genus are very similar in appearance. To overcome the cumbersome morphological identification of the four closely related R. (Boophilus) spp. in the region, a PCR-RFLP test, based on a part of the second internal transcribed spacer ribosomal DNA (ITS2), was developed. The molecular tool was successfully validated with a large number of ticks recently collected from West Africa and that were identified both morphologically and genetically. The tool developed is simple, fast, reliable and reproducible; hence it can be routinely applied for species identification.

Taxonomical studies of ticks infesting wild rodents from Asir Province in Saudi Arabia

Ticks infesting rodents in Asir Province, which is about 3000 meter above sea level, were surveyed in Wadi Dalaghan and Wadi Bin Hachbal. They were examined from September to December 2006, where ten local life baited traps were distributed for 3 days each month. The rodents were Acomys c. dimitatus (20), Meriones rex (19) & one Gerbillus cheesmani. Fifty three nymphs were dropped off from the rodents in the laboratory 3 to 12 days post-trapping. Forty eight nymphs were reared to adults for identification and 5 ones died. The reared ticks were Rhipicephalus turanicus and R. sanguineus. The medical and veterinary importance was discussed.

Biological differences within Rhipicephalus appendiculatus Neumann (Acari: Ixodidae populations in Kenya

Differences in biological parameters of Rhipicephalus appendiculatus Neumann, the brown ear tick, were investigated. They included engorgement weight of larvae, nymphs and females, fecundity, size of unfed females and mean larval and nymphal duration post-repletion on rabbits to completion of the moult. Ticks were collected from four geographically isolated zones in Kenya and from nymphs and females, size of unfed females and weight of egg batches per tick were noted (p < 0.05). It is concluded that R. appendiculatus in Kenya differ in some important biological characters. These features that showed differences might have important implications in the control of the vector as well as East Coast Fever (ECF) epidemiology.

Effect of burning on the numbers of questing ticks collected by dragging

Sixteen experimental burn plot replicates, in groups of four, in four landscape zones of the Kruger National Park, South Africa, and from which wildlife are not excluded, have been subjected to fixed, regular burning regimens since 1954. In 1999, a study to determine the effect of burning on ixodid ticks questing for hosts from the vegetation of the plots was initiated, and six sub-plots, with identical histories, within each of two of the burn plot replicates in Combretum collinum/Combretum zeyheyri woodland on granite, were selected. With few exceptions these 12 sub-plots, as well as unburned vegetation adjacent to each of the replicates, were sampled for ticks at monthly intervals for a period of 39 months by dragging with flannel strips. The existing regimen of burning during August or during October on individual sub-plots was continued during this time. A total of 14 tick species was recovered from the plots of which nine could be considered major species. Sufficient numbers for statistical analysis of only eight species were, however, collected. Burning appeared to have little short-term effect on the number of ticks recovered. In the longer term, the response varied from no change, an increase, or a decrease in the numbers of ticks collected each year after burning. Tick species, life cycle, seasonality, questing strategy, host preference and host utilization of the habitat were important determinants of the effect of burning.

A survey of free-living ixodid ticks on a commercial game farm in the Thabazimbi District, Limpopo Province, South Africa

Free-living stages of ticks on a commercial game farm in the Thabazimbi District, Limpopo Province, South Africa, were collected by drag-sampling with flannel strips during the period September 2003 to August 2004. A total of 5 tick species was collected from 4 sites. Boophilus decoloratus was the most abundant species, followed by Amblyomma hebraeum. Seasonal abundance of the ticks was quantified and an optimum time to implement control measures against the ticks is proposed.

Molecular differentiation of metastriate tick immatures

Hard ticks, family Ixodidae, are divided into two groups, the Metastriata and the Prostriata, based on morphological differences. In the United States, there are four medically important genera of the Ixodidae: Ixodes, Amblyomma, Dermacentor, and Rhipicephalus. Ixodes is the only genus in and representative of the Prostriata, whereas the latter three genera are members of the Metastriata. All developmental stages of the Prostriata can be easily differentiated from the Metastriata using morphology. Similarly, the three Metastriate genera are highly identifiable as adults, yet as immatures, the discriminating characteristics can be difficult to use for differentiation, especially if the specimens are damaged or engorged with blood. All three Metastriate genera represent medically important vectors, thus accurate differentiation is necessary. To this end, we have developed a multiplexed-PCR diagnostic assay that, when combined with RFLP analysis will differentiate between the Metastriate genera--Amblyomma, Dermacentor, Rhipicephalus, and Haemaphysalis based on the length of the PCR amplicon and subsequent restriction digestion profile. The intended use for this diagnostic is to verify morphological identifications, especially of immatures, as well as to identify samples destroyed for molecular analysis, which will lead to more accurate field data as well as implication of vectors in disease transmission.