Cattle Tick Populations Research Articles

Ticks (Acari: Ixodidae) on marsh deer (Blastocerus dichotomus) at a conservation center: infestation and Rickettsia parkeri infection dynamics along nine years

The objective of this work was to evaluate the relationship between the populations of the ticks Amblyomma triste Koch, 1844, Rhipicephalus microplus (Canestrini, 1888), Amblyomma sculptum Berlese, 1888, the pathogenic bacteria Rickettsia parkeri and a marsh deer (Blastocerus dichotomus) population after its removal from a pristine environment. For this purpose, ticks were collected from the cervical region of deer at the Marsh Deer Conservation Center in Promissão, São Paulo State, Brazil for nine consecutive years (2000 - 2008). Deer in captivity at the Center were kept in 2,000 m² paddocks surrounded by two-meter-high fences in the Tiete-river marsh. In total, 1,012 ticks of 26 deer were collected. Prevalence of the species A. triste among tick-infested hosts was the highest in the first triennium but decreased to the second and further to the third triennium. In contrast, the R. microplus prevalence amidst infested host population, increased from the first to the third triennium and was the species that attained the highest infestation intensity. Amblyomma sculptum was the tick with the lowest infestation prevalence and intensity throughout the period. The change in the proportion between the two most prevalent species was attributed to the new environment, specifically its restricted size and within it a dry area more suitable for R. microplus. DNA of 424 ticks processed in 276 pools was tested for Rickettsia genetic material by polymerase chain reactions (PCR). Twenty samples of the study were positive for the rickettsial gltA gene. Of these, 18 were from A. triste ticks and revealed the presence of the ompA spotted fever group gene as well. Eleven samples were sequenced and showed 100% identity with R. parkeri sensu stricto. Two samples from R. microplus did not amplify ompA gene neither yielded product in a PCR specific for Rickettsia bellii. Sequencing of the gltA gene in the DNA of these two ticks was also 100% identical with R. parkeri s.s. In conclusion, the changes in the deer environment modified the tick populations but maintained, at least temporary, R. parkeri bacteria in A. triste ticks. Rhipicephalus microplus was refractory to the R. parkeri infection and bacterial DNA in this tick species indicated DNA spill over from other tick species. It was demonstrated that captive marsh deer may sustain cattle tick populations on its own. This would hardly occur under pristine conditions because of the inadequacy the marsh deer´s naturally humid habitat for the cattle tick. However, deer transportations of R. microplus ticks from one farm to another may occur whenever habitat loss pushes this wild animal towards farms.

Assessing HeberFast® Line Gavac, a lateral flow immunochromatographic system for the rapid detection of anti-Bm86 antibodies in Gavac vaccinated cattle

Rhipicephalus Boophilus microplus cattle tick is a scourge for livestock production. The infestations produced by this pathogen are incompletely contained by chemical treatments, with the associated environmental pollution risks. Vaccination against cattle ticks has emerged as a feasible and environmentally friendly strategy to control tick-borne diseases. In this setting, Gavac® vaccine has proven effective in decreasing cattle tick populations through antibody responses against the tick Bm86 antigen, as part of an Integrated Control Program. However, animal vaccination programs require easy and ready-to-use screening tests to follow up the immune response in vaccinated animals under field conditions. This study reports the evaluation HeberFast® Line Gavac, a lateral flow immunochromatographic system for the rapid detection of anti Bm86 antibodies in vaccinated cattle. The system was tested on 598 serum samples taken from immunized animals, arranged in three groups according to their anti-Bm86 antibody response in ELISA (209 high, 150 medium or 239 low and 100 samples from non-immunized animals. The HeberFast® Line Gavac system was assessed for sensitivity, specificity, and concordance against the ELISA reference test. Consistency was evaluated among production batches and inter-analyst reading-independent consistency at two moments: ten minutes after completing the test and after strip drying. The system showed high sensitivity (81.6%, 82.2%, and 81%), specificity (96.7, 94.6, and 93.3%), and agreement with the ELISA reference test (75%; 74%, and 71%) for high, medium and low anti-Bm86 sera, respectively. The effectiveness of the diagnosis was 87.6; 87.1; 85.9 for high, medium, and low antibody titers, respectively. Consistency among production batches and analysts was documented, and no significant differences between evaluation times were found. These results indicate that HeberFast® Line Gavac is a valuable tool for the serological surveillance of Gavac vaccinated cattle.

Multiple acaricide-resistant Rhipicephalus microplus in the semi-arid region of Paraíba State, Brazil

Rhipicephalus microplus is responsible for major economic losses in cattle ranching in Brazil, and the development of multiple acaricide resistance is a major concern to control this ectoparasite. To determine the resistance of R. microplus to acaricides in 26 ranches in the semi-arid region of Paraíba State, Brazil, larval assays were performed with cypermethrin, chlorpyriphos, amitraz, ivermectin and fipronil and resistance was observed in all locations. The frequency of populations resistant to cypermethrin, chlorpyriphos, amitraz and ivermectin was, respectively, 96 %, 72 %, 83 % and 92 % while none was resistant to fipronil. Resistance to one, two, three and four acaricides was recorded, respectively, in 4 %, 15 %, 31 % and 50 % of the farms. Errors in the control management of R. microplus determined the development of resistance to acaricides in all evaluated properties pointing towards development of new strategies for its control. In order to control multiple acaricide-resistant cattle tick populations in this area it is recommended to apply suppressive treatments with fipronil and/ or fluazuron.

Mutations in Rhipicephalus microplus GABA gated chloride channel gene associated with fipronil resistance

The tropical cattle tick, Rhipicephalus microplus, is one of the most damaging parasites that affects cattle in tropical and subtropical regions in the world. Tick resistance to acaricides is dispersed worldwide and a number of associated mutations in target site genes have been described. Phenylpyrazole (e.g. fipronil) and cyclodiene (e.g. lindane, dieldrin) insecticides both have the same mode of action blocking the GABA-gated chloride channel encoded by the GABA-Cl gene. A conserved mutation, rdl (resistance to dieldrin) is found across a number of arthropods resistant to cyclodienes and phenylpyrazoles. In ticks, the mutation T290 L, was identified in the second transmembrane (TM2) domain of the GABA-gated chloride channel of Australian cattle tick populations that are resistant to dieldrin. Recently, cross-resistance between fipronil and lindane was reported in R. microplus populations obtained from Uruguay and Brazil. The objective of the present study was to identify mutations in the GABA-Cl gene associated with fipronil resistance. Genomic DNA was obtained from engorged females from fipronil-susceptible and resistant populations sampled from Uruguay and Brazil (n = 166). Initially, it was searched the T290 L mutation described in Australia; however, this mutation was not detected in individuals from resistant populations from either country. The sequencing of a fragment of the GABA-Cl gene revealed nucleotide polymorphisms in fipronil- and lindane-resistant ticks in two populations from Uruguay and five from Brazil. Five amino acid substitutions were present in the resistant strains. Two different substitutions were found in an alanine residue (A286S and A286 L) that is homolog to rdl mutations in fipronil-resistant individuals of other arthropod species. Four other amino acid substitutions (S281 T, V317I, T328 A and A329S) were present in some resistant strains, always with the mutation A286S. This is the first documentation of mutations in the GABA-Cl gene associated with fipronil-resistant in R. microplus.

Immunological control of ticks and tick-borne diseases that impact cattle health and production

The cattle industry is one of the most important agroeconomic activities in Mexico. The national herd is estimated to include approximately 33.5. million head of cattle. Ticks and tick-borne diseases are principal factors with a negative impact on cattle health and production. The most economically important tick species parasitizing cattle in Mexico are Rhipicephalus microplus, R. annulatus, and Amblyomma mixtum. Parasitism by ticks affects cattle health and production directly. Morbidity and mortality caused by tick-borne diseases augment the detrimental effect of tick infestation in cattle. Bovine babesiosis and anaplasmosis are the most important tick-borne diseases of cattle, which are caused by infectious agents transmitted by R. microplus and R. annulatus. However, there are no prophylactic therapies to control bovine babesiosis and anaplasmosis. Chemical control is the most common way to treat animals against ticks, and the use of acaricides can also help manage tick-borne diseases. However, the evolution of resistance to acaricides among cattle tick populations renders chemical control ineffective; which represents a challenge for sustainable ticks and tick-borne diseases control. The only anti-tick vaccine commercially available globally is based on the recombinant antigen Bm86. Because of its mode of immunity against R. microplus and R. annulatus, the Bm86-based vaccine also decreases the exposition of bovines to babesiosis and anaplasmosis. Research with Bm86-based vaccines documented high efficacy against R. annulatus, the efficacy levels against R. microplus varies according to the geographic origin of tick populations, and there is not effect against other ticks species such as Amblyomma spp. The impact of ticks and tick-borne diseases, the problem of chemical control due to acaricide resistance, and progress with anti-tick vaccine research efforts in Mexico are reviewed herein.

Molecular quantitative assay for esterase-mediated organophosphate resistance in Rhipicephalus microplus

The use of pesticides is the main tool to control infestations of the cattle tick Rhipicephalus microplus, and organophosphate (OP) is one of the most used compounds for this purpose. Carboxylesterases (ChEs) are targets for OP pesticides in arthropods, and acetylcholinesterase 2 (AChE2) and esterase 1 (EST1) are metabolic enzymes involved in the xenobiotic detoxification process. The increase in the synthesis of these enzymes can be detected by the quantitative polymerase chain reaction (qPCR) assay, which was used to identify cattle tick populations resistant to OP pesticides. For that, two field populations of R. microplus were used, one previously identified by the larval packet test (LPT) as OP −sensitive (LC50=0.13μg/cm2) and the other OP-resistant (LC50=8.14μg/cm2). To promote the OP enzyme detoxification, groups of 10 females of the resistant strain were immersed in solutions of diazinon in technical grade at concentrations of 1.0mg/ml, 2.5mg/ml, and 5.0mg/ml. The ticks that survived diazinon exposure were submitted to qPCR assay, which enabled observing an increase in AChE2 and EST1 synthesis in the OP-resistant strain when compared to the susceptible strain. The initial results of expression analysis suggest that the qPCR assay can discriminate OP-resistant and susceptible populations. The development and improvement of molecular diagnostic tests to identify pesticide resistant R. microplus populations are priorities and in the near future it will be important to expand the molecular targets involved in OP resistance, which could be used for better selection of effective strategies to control cattle tick populations.

Molecular Comparison of Cattle Fever Ticks from Native and Introduced Ranges, with Insights into Optimal Search Areas for Classical Biological Control Agents

Cattle fever ticks, the southern cattle tick, Rhipicephalus microplus (Canestrini), and cattle tick, Rhipicephalus annulatus (Say), are livestock pests endemic to Mexico and invasive along the Texas-Mexico border. Resistance to acaricide, alternate wildlife hosts, and pathogenic landscape-forming weeds present challenges for sustainable eradication of this pest in the U.S. Classical biological control is being explored as a strategy to control cattle fever ticks, especially on alternate hosts such as nilgai antelope, Boselaphus tragocamelus (Pallas), and white-tailed deer, Odocoileus virginianus (Zimmermann). Molecular genetic tools were used to compare populations of cattle fever ticks from native and introduced ranges to provide insights into optimal search areas for potential biological control agents. Accessions representative of invasive populations of southern cattle tick from subtropical Zapata, TX, and other parts of the invaded range including Brazil and Kenya matched most closely populations in Cambodia and the Philippines. Similarly, accessions of cattle tick from invaded range in Del Rio, TX matched closely with accessions from native range in Bulgaria and Romania. These regions should be prioritized for field exploration for biological control agents.

Modeling the impact of climate and landscape on the efficacy of white tailed deer vaccination for cattle tick control in northeastern Mexico.

Cattle ticks are distributed worldwide and affect animal health and livestock production. White tailed deer (WTD) sustain and spread cattle tick populations. The aim of this study was to model the efficacy of anti-tick vaccination of WTD to control tick infestations in the absence of cattle vaccination in a territory where both host species coexist and sustain cattle tick populations. Agent-based models that included land cover/landscape properties (patch size, distances to patches) and climatic conditions were built in a GIS environment to simulate WTD vaccine effectiveness under conditions where unvaccinated cattle shared the landscape. Published and validated information on tick life cycle was used to build models describing tick mortality and developmental rates. Data from simulations were applied to a large territory in northeastern Mexico where cattle ticks are endemic and WTD and cattle share substantial portions of the habitat. WTD movements were simulated together with tick population dynamics considering the actual landscape and climatic features. The size of the vegetation patches and the distance between patches were critical for the successful control of tick infestations after WTD vaccination. The presence of well-connected, large vegetation patches proved essential for tick control, since the tick could persist in areas of highly fragmented habitat. The continued application of one yearly vaccination on days 1-70 for three years reduced tick abundance/animal/patch by a factor of 40 and 60 for R. annulatus and R. microplus, respectively when compared to non-vaccinated controls. The study showed that vaccination of WTD alone during three consecutive years could result in the reduction of cattle tick populations in northeastern Mexico. Furthermore, the results of the simulations suggested the possibility of using vaccines to prevent the spread and thus the re-introduction of cattle ticks into tick-free areas.

Protective action of Tagetes minuta (Asteraceae) essential oil in the control of Rhipicephalus microplus (Canestrini, 1887) (Acari: Ixodidae) in a cattle pen trial

The Rhipicephalus microplus tick is globally regarded as the most economically important ectoparasite of livestock, and the evolution of resistance to commercial acaricides among cattle tick populations is of great concern. The essential oil derived from Tagetes minuta may be efficacious against cattle tick infestation, and the results of a cattle pen trial using this essential oil for the control of ticks are reported here. The chemical composition of the essential oil was determined by GC–MS and NMR spectroscopy analyses, which revealed the presence of four major components in the essential oil. These components represent more than 70% of the essential oil: limonene (6.96%), β-ocimene (5.11%), dihydrotagetone (54.10%) and tagetone (6.73%). The results of the cattle pen trial indicated significant differences among the average values of the analyzed biological parameters, including the number of ticks, the average weight of the ticks, the average egg weight per engorged female and larval viability. Treatment with the T. minuta essential oil prepared in this study promoted significant effects on all biological indicators analyzed. Based on the biological indicators, the essential oil showed 99.98% efficacy compared to the control group when used at a 20% concentration. The results obtained in this study suggest that the T. minuta essential oil is a potential R. microplus tick control agent and may be used to mitigate the economic losses caused by tick infestation.

A new approach to characterization of the resistance of populations of Rhipicephalus microplus (Acari: Ixodidae) to organophosphate and pyrethroid in the state of Minas Gerais, Brazil

The monitoring of resistance of cattle tick populations in Brazil to the chemical bases in use is largely limited to investigation of the phenotypic profile. There are few studies investigating the role played by the genotypic profile in acaricide resistance in the country. Therefore, the aim of the present study was to carry out molecular characterization and trace out the genetic profile of populations of Rhipicephalus microplus with respect to resistance to the organophosphate and pyrethroid chemical groups. For that purpose, larvae were genotyped belonging to 587 populations for pyrethroids and 306 for organophosphates, using the polymerase chain reaction technique. It was found that 75.49% and 97.44% of the larvae studied showed resistance to the organophosphates and pyrethroids, respectively. Among the populations resistant to pyrethroids, 91.9% were heterozygotes, showing that most of the resistant populations have only one allele responsible for resistance. Therefore, it is possible to conclude that the genotyped populations have high resistance to organophosphates, and even more so to pyrethroids. This information is fundamental for understanding the mechanisms of resistance of R. microplus to acaricides, to enable improvement of control techniques.

Approaches towards tick and tick-borne diseases control

Ticks are obligate haematophagous ectoparasites of wild and domestic animals as well as humans, considered to be second worldwide to mosquitoes as vectors of human diseases. Tick-borne diseases are responsible worldwide for great economic losses in terms of mortality and morbidity of livestock animals. This review concerns to the different tick and tick-parasites control methods having a major focus on vaccines. Control of tick infestations has been mainly based on the use of acaricides, a control measure with serious drawbacks, as responsible for the contamination of milk and meat products, as a selective factor for acaricide-resistant ticks and as an environmental contaminant. Research on alternatives to the use of acaricides is strongly represented by tick vaccines considered a more cost-effective and environmentally safe strategy. Vaccines based on the Bm86 tick antigen were used in the fi rst commercially available cattle tick vaccines and showed good results in reducing tick numbers, affecting weight and reproductive performance of female ticks which resulted in reduction of cattle tick populations over time and consequently lower reduction of the pathogen agents they carry.

Proteomics Approach to the Study of Cattle Tick Adaptation to White Tailed Deer

Cattle ticks, Rhipicephalus (Boophilus) microplus, are a serious threat to animal health and production. Some ticks feed on a single host species while others such as R. microplus infest multiple hosts. White tailed deer (WTD) play a role in the maintenance and expansion of cattle tick populations. However, cattle ticks fed on WTD show lower weight and reproductive performance when compared to ticks fed on cattle, suggesting the existence of host factors that affect tick feeding and reproduction. To elucidate these factors, a proteomics approach was used to characterize tick and host proteins in R. microplus ticks fed on cattle and WTD. The results showed that R. microplus ticks fed on cattle have overrepresented tick proteins involved in blood digestion and reproduction when compared to ticks fed on WTD, while host proteins were differentially represented in ticks fed on cattle or WTD. Although a direct connection cannot be made between differentially represented tick and host proteins, these results suggested that differentially represented host proteins together with other host factors could be associated with higher R. microplus tick feeding and reproduction observed in ticks fed on cattle.

Protective immunity against tick infestation in cattle vaccinated with recombinant trypsin inhibitor of Rhipicephalus microplus

The cattle tick, Rhipicephalus microplus, is regarded as the most economically important ectoparasite of livestock globally. Control is achieved primarily through the use of acaricides. This approach is hampered by the development of resistance to commercial acaricides among cattle tick populations. Vaccination against R. microplus infestation is another technology that can be integrated for effective cattle tick control. Proteins belonging to the Kunitz-BPTI family are abundant in cattle tick salivary glands, midgut, and ovaries. These organs are attractive targets for the development of a novel cattle tick vaccine. Efficacy assessment against cattle tick infestation in bovines using a vaccine containing the recombinant form of a member of the Kunitz family from R. microplus produced in a yeast expression system is reported for the first time here. The yeast Pichia pastoris was bioengineered to produce the recombinant version of a trypsin inhibitor that is expressed in cattle tick larvae (rRmLTI). Immunization with rRmLTI afforded 32% efficacy against R. microplus. The estimated molecular weight of rRmLTI was 46kDa. Structural homology to the native form of the larval trypsin inhibitor was documented by recognition of rRmLTI in Western-blots using polyclonal antibodies from mice immunized with cattle tick larval extract or rRmLTI. Bioinformatics analysis of the partial nucleotide and deduced amino acid sequences indicated that the rRmLTI closely resembles BmTI-6, which is a three-headed Kunitz protein present in cattle tick ovary and fat tissue.

Survey of pyrethroid and organophosphate resistance in Brazilian field populations of Rhipicephalus (Boophilus) microplus: Detection of C190A mutation in domain II of the para-type sodium channel gene

The cattle tick Rhipicephalus (Boophilus) microplus causes expressive damage to livestock in Brazil and other countries. Its control is becoming more difficult due to the development of resistance in populations. Early detection of resistance can help in developing effective control strategies. This study evaluated the susceptibility of R. microplus to cypermethrin and chlorpyriphos and was the first attempt to identify the mechanism of resistance (target site insensitivity) in cattle tick populations from Minas Gerais state (Southeastern Brazil). Engorged female ticks were collected from 10 ranches within the state of Minas Gerais, and susceptibility was evaluated with the larval packet test (LPT) using technical grade cypermethrin and chlorpyriphos. It was possible to analyze LPT results of seven populations. Target site insensitivity was investigated in all 10 isolates by using molecular approaches for detection of the T2134A substitution within the domain III S6 segment and the C190A in the domain II S4-5 linker from the para-type sodium channel gene. LPT showed that all seven populations were resistant to cypermethrin with resistance ratio (RR) ranging from 16.0 to 25.0 and 85.7% were resistant to chlorpyriphos (RR=2.2–15.6). Although the T2134A mutation was not detected, the C190A mutation was highly prevalent, being present in 82–100% of the alleles sampled in field populations. A significant correlation was found between the LC50 values for cypermethrin and the frequency of the C190A mutation suggesting that it might be responsible for the phenotypic resistance detected.

Facilitative Ecological Interactions Between Invasive Species: Arundo donax Stands as Favorable Habitat for Cattle Ticks (Acari: Ixodidae) Along the U.S.-Mexico Border

The cattle tick, Rhipicephalus (Boophilus) spp. is a key vector of protozoa that cause bovine babesiosis. Largely eradicated from most of the United States, the cattle tick continues to infest south Texas, and recent outbreaks in this area may signal a resurgence of cattle tick populations despite current management efforts. An improved understanding of the dynamic ecology of cattle fever ticks along the U.S.-Mexico border is required to devise strategies for sustainable eradication efforts. Management areas of the cattle tick overlap considerably with dense, wide infestations of the non-native, invasive grass known as giant reed (Arundo donax L.). Here we show that stands of giant reed are associated with abiotic and biotic conditions that are favorable to tick survival, especially when compared with other nearby habitats (open pastures of buffelgrass (Pennisetum ciliare) and closed canopy native forests). Overhead canopies in giant reed stands and native riparian forests reduce daily high temperature, which was the best abiotic predictor of oviposition by engorged females. In sites where temperatures were extreme, specifically open grasslands, fewer females laid eggs and the resulting egg masses were smaller. Pitfall trap collections of ground dwelling arthropods suggest a low potential for natural suppression of tick populations in giant reed stands. The finding that A. donax infestations present environmental conditions that facilitate the survival and persistence of cattle ticks, as well or better than native riparian habitats and open grasslands, represents an alarming complication for cattle fever tick management in the United States.

Applicability of in vitro bioassays for the diagnosis of ivermectin resistance in Rhipicephalus microplus (Acari: Ixodidae)

The applicability of laboratory bioassays to diagnose ivermectin (IVM) resistance in Rhipicephalus microplus was evaluated. Adult immersion tests (AITs), larval immersion tests (LITs) and larval packet tests (LPTs) were performed to characterise the effects of ivermectin toxicity on adults and larvae of a susceptible reference strain. The AIT was determined to be a reasonable assay but requires a large number of individuals to attain interpretable results. The LIT and LPT were validated with an IVM resistant strain, revealing resistance ratios (RRs) of 6.73 and 1.49, respectively. In a field survey, nine different populations of cattle tick from the states of São Paulo and Mato Grosso do Sul, Brazil, were analysed with the LIT. Populations without previous exposure to ivermectin exhibited RRs between 0.87 and 1.01. Populations previously exposed to IVM showed RRs between 1.83 and 4.62. The LIT was more effective at discriminating between resistant and susceptible populations than the LPT. The use of the LIT is recommended for the diagnosis of ivermectin resistance in R. microplus.

Identification of a mutation in the para-sodium channel gene of the cattle tick Rhipicephalus microplus associated with resistance to flumethrin but not to cypermethrin

A mutation in the domain II S4–5 linker region of the para-sodium channel gene has been associated previously with synthetic pyrethroid (SP) resistance in the cattle tick (Rhipicephalus microplus) in Australia. This is a C→A mutation at nucleotide position 190, which results in a leucine to isoleucine amino acid substitution (L64I). In a survey of 15 cattle tick populations with known SP resistance status, sourced from Queensland and New South Wales in Australia, there was a strong relationship (r=0.98) between the proportion of ticks carrying the L64I homozygous resistant genotype and the survival percentage after exposure to a discriminating concentration of cypermethrin in the bioassay, as expected. However, among populations resistant only to flumethrin, the L64I homozygous genotype was not found. The sequence obtained for a 167bp region including domain II S4–5 linker in flumethrin-resistant ticks identified a G→T non-synonymous mutation at nucleotide position 214 that results in a glycine to valine substitution (G72V). The frequency of the G72V homozygous genotype in each population was found to be moderately related to the survival percentage at the discriminating concentration of flumethrin in the larval packet test (r=0.74). However, a much stronger relationship between genotype and resistance to flumethrin was observed when the heterozygotes of L64I and G72V were added to the G72V homozygotes (r=0.93). These results suggest that there is an interaction between the two mutations in the same gene, such that flumethrin resistance might be conferred by either two copies of the G72V mutation or by being a L64I and G72V heterozygote.

Changes in climate and habitat suitability for the cattle tick Boophilus microplus in its southern Neotropical distribution range

We addressed the possible effects of several climate scenarios on habitat suitability (HS) for the cattle tick Boophilus microplus and the probability of producing permanent populations from introduced females of that tick in central parts of Argentina, using both a correlative model (derived from climate predictors) and a mechanistic (life cycle) model. There was high correlation (R2 = 0.866) between HS-derived and life cycle outputs for HS values higher than 0.52, suggesting that HS is a good estimator of the life cycle of the tick above a critical threshold of HS values. Scenarios with increased temperatures increased suitable habitats for the tick in southern parts of the study region, extending below parallel 34 degrees S, but suitable habitats remained limited in the west. A concurrent increase in rainfall produced a further increase of HS in these areas. Results from the life cycle model suggest that in areas of suitable habitat, permanent cattle tick populations are most probable if engorged females are introduced during mid-summer.

A simulation study of the effects of acaricides and vaccination on Boophilus cattle–tick populations

This paper describes a simulation model to evaluate different control strategies for Boophilus microplus. The model combines a dynamic life-history module for tick-population dynamics with other modules for vaccination, sterile-hybrid larval release and use of acaricide dipping vats. The tick life-history module considers the cattle's nutritional level and allows for distribution of ticks by age at all stages of growth. Appropriately, the model was sensitive to host resistance and to host-nutritional status. The validity of the life-history module – as well as that of the vaccination and acaricide dipping-vats modules – was demonstrated by comparing simulated and real data for several geographical locations in Cuba and Brazil. Optimum tick-control strategies for the first year of vaccination were designed and the effect of long-term vaccination on tick population was also studied.

Control of parasitic arthropods

The review examines the history of control measures for the most important arthropod parasites of vertebrates, and discusses possible future trends. Control measures are discussed under the headings of discretional and strategic suppression and eradication, particular attention being focussed on screw-worm and cattle tick ( Boophilus) eradication and the bearing of social and economic factors on the success of campaigns. The importance of economic loss assessments is stressed in relation to the practicability of bringing control measures to bear on parasitic arthropods, but caution is indicated, due to fluctuations in the market value of stock and rises in the costs of pesticides. Stress is laid on the importance of chemicals in control, and the world spectrum of arthropod resistance to pesticides is examined. Detailed consideration is given to the problem in relation to sheep blowflies and cattle ticks. Prospects for future control of biting and nuisance flies, which pose particular problems, are discussed. The potential value of dung beetles in this field is considered to be high. Management of sheep blowfly populations is currently being approached along a number of lines which may eventually interact, including genetic manipulation, novel forms of chemical control, depilation of strike-susceptible areas and the selection of easy-care sheep. In the management of cattle tick populations, past, present and potential problems of acaricide resistance and the existing and potential role of tick-resistant cattle are major considerations in the development of integrated control measures, particularly for Boophilus spp.