Host resistance in cattle tick control

Cattle ticks are an important constraint on the livestock industry, particularly in tropical and subtropical areas, mainly because of the diseases they transmit and the costs of control. Conventional control is by means of acaricides; although there are still serious drawbacks, these can be minimized by a strategic approach. In this review Julio de Castro and Robin Newson look at alternatives. One is to make use of the host's innate abilities. In the simplest situation, where no control measures are applied, the animals develop their own protective immunity against ticks and tick-borne diseases (TBDs). Alternatively, breeds of cattle with naturally high resistance to ticks can be used, enhanced by selection. Methods of immunizing cattle against ticks are also under development. Ideally, ticks should be managed at an economically acceptable level by a combination of techniques, supported where necessary by vaccination against TBDs.     

Ecological aspects of cattle tick control in central Zambia

In ecological studies in central Zambia, both climate and ecotype affected population dynamics of tick species. Below average rainfall for several years caused a suppression in numbers of Rhipicephalus appendiculatus Neumann adults. Reduction in rainfall leading to changes in grazing patterns is thought to have been responsible for an increase in numbers of Amblyomma variegatum Fabricius adults in a grassland habitat. There were reasonable correlations between numbers of each tick species on individual hosts over 1 year old. However, there were no relationships between numbers of ticks and bovine lymphocyte antigens (BoLA).     

Effect of tick control on liveweight gain of cattle in central Zambia

Field trials were carried out during 1982-86 in two different ecotypes in central Zambia to determine the impact of tick control on the liveweight gain (LWG) of cattle. During the first 2 years of the trial a diamidide acaricide (Amitraz) sprayed at weekly intervals caused periodic depression in LWG in young animals. Thereafter a pyrethroid acaricide (cypermethrin) was used. During the 1984-85 tick season infestations of 50-120 adult Amblyomma variegatum Fabricius in untreated animals caused significant reductions in LWG. From the differences in LWG between treated and untreated cattle, the induced loss in LWG was estimated at 46-61 g per engorged female A.variegatum.     

Carbaryl resistance in Mexican strains of the southern cattle tick (Acari: Ixodidae)

Susceptibility to carbaryl in six Mexican strains of the southern cattle tick, Boophilus microplus (Canestrini), was evaluated with the Food and Agricultural Organization larval packet test. Tick strains from the cattle fever tick quarantine zone in Texas were more susceptible to carbaryl than to coumaphos or diazinon. Compared with the susceptible reference (Gonzalez) strain, Mexican tick strains demonstrated 10.9-59.5-fold resistance to carbaryl. Significant cross-resistance was found between carbaryl and the organophosphate acaricides coumaphos and diazinon. Bioassay results with synergists suggested that metabolic detoxification mechanisms did not play a major role in carbaryl resistance. Resistance to carbaryl was likely conferred by insensitive acetylcholinesterase. The implications of carbaryl resistance in tick eradication and control also are discussed.     

Partial strategic tick control within a herd of European breed cattle in the state of Rio Grande do Sul,southern Brazil

A trial is described, in the state of Rio Grande do Sul, southern Brazil, as one of a series suggested to investigate the effects of strategic but selective acaricide treatments of cattle within herds against Boophilus microplus. They are aimed at considering the repercussions of farmer attempts at immediate reductions in acaricide costs and the potential for creation of ‘refugia’ of untreated ticks. Half (Group 1) of a small experimental herd of European breed heifers were treated strategically against ticks, three times during the late spring–early summer and twice during autumn (southern hemisphere), with an injectable avermectin endectocide, designed to act directly against the first and third generations of parasitic B. microplus per ‘cattle tick year’ at this site, respectively. The consequent levels of infestations on all of the member cattle in their common pasture were monitored. Group 1 showed low to zero tick counts during the 28-day treatment interval periods and up to ca. 14 days after the last of such a series. Treated cattle, however, became re-infested outside of these periods and to levels that would be considered as unacceptable by farmers in the state. The untreated cattle (Group 2) showed infestations at generally higher levels, than their contemporaries, within and outside of the treatment periods. There were thus ample sources of larvae in the pasture, derived principally from falling, untreated engorged female ticks, re-infesting both the treated and untreated cattle. Advantages of maintaining chemically untreated cattle ticks within a herd, compared to their disadvantages as contaminants to classical strategic control procedures, merit re-evaluation, especially in relation to the recent, world-wide resurgence of acaricide resistance in B. microplus. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Footbath acaricide treatment to control cattle infestation by the tick Amblyomma variegatum

Previous studies have shown that about 90% of adult Amblyomma variegatum Fabricius (Acari: Ixodidae) picked up daily by grazing cattle are still attached to the interdigital areas in the evening, when the animals return from pasture. It was therefore postulated that a targeted treatment, designed to kill the ticks attached to the feet, would limit infestation of the predilection sites. Footbaths filled with various pyrethroid formulations were used over 3 years, at the beginning of the rainy season (from mid-May to the end of July), to assess the efficacy of such a control method. It proved efficient in preventing the ticks from attaching to the predilection sites. Although five to 12 A. variegatum adults attached to each treated animal daily, and although the tick burden of the predilection sites of control cattle increased each day by four to 10 ticks, the average infestation of the predilection sites of treated cattle that were initially highly infested (over 100 ticks/animal) continuously decreased to reach a level of about 10-30 ticks/animal after 6-8 weeks of treatment. In herds with a lower initial tick burden (40-70 ticks/animal) this level was obtained within 2-3 weeks and the mean infestation subsequently remained consistently low. Footbath treatment carried out every other day during the adult peak infestation period should therefore greatly limit losses due to ticks. This method was appreciated by traditional livestock farmers, essentially because it is not time-consuming and because it requires only c. 200 mL aqueous formulation per animal at each passage. The cost of the acaricide needed to treat one animal during the peak infestation period was assessed at c. euro 0.20. This control method might also have an impact on some species of tsetse flies and mosquitoes, thereby contributing to trypanosomiasis and malaria control.     

BmiGI: a database of cDNAs expressed in Boophilus microplus, the tropical/southern cattle tick

We used an expressed sequence tag approach to initiate a study of the genome of the southern cattle tick, Boophilus microplus. A normalized cDNA library was synthesized from pooled RNA purified from tick larvae which had been subjected to different treatments, including acaricide exposure, heat shock, cold shock, host odor, and infection with Babesia bovis. For the acaricide exposure experiments, we used several strains of ticks, which varied in their levels of susceptibility to pyrethroid, organophosphate and amitraz. We also included RNA purified from samples of eggs, nymphs and adult ticks and dissected tick organs. Plasmid DNA was prepared from 11,520 cDNA clones and both 5' and 3' sequencing performed on each clone. The sequence data was used to search public protein databases and a B. microplus gene index was constructed, consisting of 8270 unique sequences whose associated putative functional assignments, when available, can be viewed at the TIGR website (http://www.tigr.org/tdb/tgi). A number of novel sequences were identified which possessed significant sequence similarity to genes, which might be involved in resistance to acaricides.     

Pharmacological characterization of a tyramine receptor from the southern cattle tick,Rhipicephalus (Boophilus) microplus

The southern cattle tick (Rhipicephalus (Boophilus) microplus) is a hematophagous external parasite that vectors the causative agents of bovine babesiosis or cattle tick fever, Babesia bovis and B. bigemina, and anaplasmosis, Anaplasma marginale. The southern cattle tick is a threat to the livestock industry in many locations throughout the world. Control methods include the use of chemical acaricides including amitraz, a formamidine insecticide, which is proposed to activate octopamine receptors. Previous studies have identified a putative octopamine receptor from the southern cattle tick in Australia and the Americas. Furthermore, this putative octopamine receptor could play a role in acaricide resistance to amitraz. Recently, sequence data indicated that this putative octopamine receptor is probably a type-1 tyramine receptor (TAR1). In this study, the putative TAR1 was heterologously expressed in Chinese hamster ovary (CHO-K1) cells, and the expressed receptor resulted in a 39-fold higher potency for tyramine compared to octopamine. Furthermore, the expressed receptor was strongly antagonized by yohimbine and cyproheptadine, and mildly antagonized by mianserin and phentolamine. Tolazoline and naphazoline had agonistic or modulatory activity against the expressed receptor, as did the amitraz metabolite, BTS-27271; however, this was only observed in the presence of tyramine. The southern cattle tick's tyramine receptor may serve as a target for the development of anti-parasitic compounds, in addition to being a likely target of formamidine insecticides.     

Towards the selection of cattle for tick resistance in Africa

Half-body tick collections and visual assessment of tick burdens were performed monthly over six months on 100 bulls at the Kenya National Boran Stud, Mutara Ranch, Kenya.Boophilus decoloratus predominated among several tick species infesting the animals. Burdens ofB. decoloratus and total tick burdens were highly correlated. Rankings of relative tick resistance among bulls were consistent from month to month. Rankings based on visual assessment were very close to those based on actual tick burdens. Animals with thin skins appeared to carry fewer ticks, but tick burden bore no relationship to coat colour. The results suggest that simple visual inspection of total tick burdens may be a suitable basis for the selection of tick resistance in cattle.     

Efficiency and cost of strategic use of acaricide for tick control in N'Dama cattle in The Gambia

The efficiency of strategic and strategic/selective applications of flumethrin spray formulation for controlling ticks were assessed, respectively, in two groups of fourteen N'Dama cattle (Group S and Group S/S) by comparison of the number of feeding ticks with thirteen untreated N'Dama cattle (Group U) over a period of 11 months (June 1996 to April 1997). During the expected peak of tick abundance, acaricide was applied fortnightly on the whole body in animals in Group S and only on the most infested body areas in cattle in Group S/S. Weight changes and skin lesions, directly associated with tick attachment, were recorded in cattle in the three groups. The costs of the two tick control schemes were estimated. Maximum level of ticks, all species together, feeding on cattle was observed in the rainy season. Both in Group S/S and Group S, cattle carried a lower (P < 0.001) number of feeding ticks than animals in Group U over the whole study period. Percentage of tick control, over the entire period of tick investigation, was satisfactory in both acaricide-treated groups, reaching 61.2 and 75.2% in Groups S/S and S, respectively. However, the proportion of control varied according to tick species or genus. Significantly lower prevalence of skin lesions was observed on the ano-genital and udder region in cattle in Group S/S (P < 0.05) and Group S (P < 0.01) in comparison with cattle in Group U. Mean amount of acaricide solution used and relative estimated cost of treatment in cattle in Group S/S were, respectively, 25- and 14-fold lower than those in cattle in Group S. At the end of the study, animals in Groups S/S and S were, respectively, 7.2 and 15.9 kg heavier than animals in Group U. The difference was statistically significant (P < 0.02) only between Groups S and U. However, the efficiency, low cost and derived benefits of the strategic/selective acaricide application scheme indicated that it might be the most cost effective.     

Ovicidal activity of topically applied acaricides against eggs of the southern cattle tick (Acari: Ixodidae)

Ovicidal activity of coumaphos, diazinon, dioxathion, chlorpyrifos, and crotoxyphos applied topically at 0.125, 0.25, and 0.5% (AI) to 1-d-old, 10-d-old, and 20-d-old eggs of southern cattle tick, Boophilus microplus (Canestrini), was determined. All acaricides produced some disruption of hatching of eggs in all age classes. Coumaphos, diazinon, dioxathion, and chlorpyrifos were most effective in reducing egg hatch of 1-d-old eggs but had little effect against 10- or 20-d-old eggs. Although crotoxyphos was comparable with other acaricides in reducing hatch in 1-d-old eggs, it caused significantly greater mortality (greater than or equal to 94.6%) in 20-d-old eggs than other compounds tested. Crotoxyphos applied at 0.5% (AI) is the only acaricide tested that provided sufficient ovicidal activity in all three egg age classes (greater than or equal to 85% reduction of hatch).