Rhipicephalus ( Boophilus ) microplus resistant to acaricides and ivermectin in cattle farms of Mexico Rhipicephalus ( Boophilus ) microplus resistentes aos acaricidas e ivermectina nas fazendas de gado do México

Ticks and the diseases they transmit cause great economic losses to livestock in tropical countries. Non-chemical control alternatives include the use of resistant cattle breeds, biological control and vaccines. However, the most widely used method is the application of different chemical classes of acaricides and macrocyclic lactones. Populations of the cattle tick, Rhipicephalus (Boophilus) microplus, resistant to organophosphates (OP), synthetic pyrethroids (SP), amitraz and fipronil have been reported in Mexico. Macrocyclic lactones are the most sold antiparasitic drug in the Mexican veterinary market. Ivermectin-resistant populations of R. (B.) microplus have been reported in Brazil, Uruguay and especially in Mexico (Veracruz and Yucatan). Although ivermectin resistance levels in R. (B.) microplus from Mexico were generally low in most cases, some field populations of R. (B.) microplus exhibited high levels of ivermectin resistance. The CHPAT population showed a resistance ratio of 10.23 and 79.6 at lethal concentration of 50% and 99%, respectively. Many field populations of R. (B.) microplus are resistant to multiple classes of antiparasitic drugs, including organophosphates (chlorpyrifos, coumaphos and diazinon), pyrethroids (flumethrin, deltamethrin and cypermethrin), amitraz and ivermectin. This paper reports the current status of the resistance of R. (B.) microplus to acaricides, especially ivermectin, in Mexican cattle.


Introduction
Rhipicephalus (Boophilus) microplus (R. (B.) microplus) (Canestrini) is an endemic pest of cattle in tropical and subtropical regions of the world, causing major economic losses to cattle producers through direct physical effects on the parasitized animal and indirectly through transmission of infectious disease agents such as Babesia bovis, B. bigemina and Anaplasma marginale (SOLORIO-RIVERA et al., 1999;RODRÍGUEZ-VIVAS et al., 2004, 2005).
Chemical acaricides have played an essential role in control of this tick, but intensive acaricide use has favored the development of resistant populations (RODRÍGUEZ-VIVAS et al., 2006a, b).Macrocyclic lactones (MLs) have emerged as an alternative to mitigate the negative effects of ticks, including tick populations resistant to most acaricides (LANUSSE et al., 1997).Macrocyclic lactones are endectocides derived from Streptomyces avermitilis (avermectins) S. cyaneogriseus (milbemicins) and the genus Saccharopolyspora (Spinosyns) (LIFSCHITZ et al., 2002;LUMARET et al., 2012).In Mexico, the pharmaceutical industry reported that ivermectin is the preferred anthelmintic to control gastrointestinal nematodes in ruminants and it is also used to control cattle ticks (RODRÍGUEZ-VIVAS et al., 2014).However, as an effect from its intensive use, reports of R. (B.) microplus resistant to ivermectin have been documented in Brazil, Uruguay and Mexico (KLAFKE et al., 2006;CASTRO-JANER et al., 2011;PEREZ-COGOLLO et al., 2010a, b;FERNÁNDEZ-SALAS et al., 2012a, b).This paper reports the current status of the resistance of R. (B.) microplus to acaricides, especially ivermectin, in Mexican cattle.

Effect of Rhipicephalus (B.) microplus on Cattle Production
Approximately one billion cattle, most of which are in the tropics, are at risk from various tick species or tick-borne diseases (PEGRAM et al., 1993), causing significant production losses.Rhipicephalus (B.) microplus is an endemic pest of cattle in tropical and subtropical regions of the world, causing major economic losses to cattle producers through direct physical effects on the parasitized animal and indirectly through disease transmission of infectious agents such as Babesia bovis, B. bigemina and Anaplasma marginale (SOLORIO-RIVERA et al., 1999;RODRÍGUEZ-VIVAS et al., 2004, 2005).In addition to the costs of chemicals, labor, equipment and production losses associated with treatment, the cost of maintaining tick-free zones and boundaries is highly expensive (WHITE et al., 2003).
Each engorged female tick has been shown to reduce weight gain by 0.6 g in beef cattle (SUTHERST et al., 1983) of which 65 % was attributed to tick infestation (stress and anorexia from the irritation cause by the ticks) and 35 % to loss of blood taken by the ticks (SEEBECK et al., 1971).In Australia, Jonsson et al. (1998) estimated that each engorging female of R. (B.) microplus was responsible for the loss of 8.9 ml of daily milk production and 1.0 g of bodyweight..In the last study reported in Mexico, the estimated cost of production losses, mortality, hide damage and control of R. (B.) microplus and its transmitted diseases was estimated to be $48 million USD per annum (RODRÍGUEZ-VIVAS et al., 2005).
Current tick-control methods involve use of non-chemical and chemical methods, and the systematic application of two or more methods (integrated pest management).Chemical control is the most widely used and in most cases the only method available to the producer (RODRÍGUEZ-VIVAS et al., 2006a).The present work will focus on chemical control of ticks.

Chemical Control of Rhipicephalus (B.) microplus
Available chemicals used in the treatment of ectoparasites of veterinary importance act either systemically, following uptake of the compound from host tissues, or by direct contact with the target parasites following external application.Virtually all ectoparasiticides are neurotoxins, exerting their effect on the ectoparasite nervous system (TAYLOR, 2001) (Table 1).Traditional methods for the delivery of an acaricide treatment to cattle to control ticks required formulations such as an emulsifiable concentrate, wettable powder or flowable products that could be diluted in water and applied to cattle by a hand sprayer, spray race or through immersion of animals in a dipping vat.More recently, treatment possibilities include the use of pour-on products, injectables, an intraruminal bolus, acaricide-impregnated ear tag and pheromoneacaricide-impregnated devices attached in different ways to the host (GEORGE et al., 2004).
In Mexico, organophosphate acaricides were heavily used in the national tick eradication program between 1974and 1984(TRAPAPA, 1989)).The organophosphates used during that period include coumaphos, chlorpyriphos, chlorfenvinphos, diazinon and ethion.Pyrethroid and amitraz acaricides were introduced into Mexico in 1986; however, amitraz use was limited initially due to higher cost.The use of amitraz became more frequent after 1993 when SP resistance problems started to hinder the tick control efforts in Mexico.One chemical alternative for the control of ticks in Mexico is the use of MLs that exhibit an endectocidal spectrum of activity including acaricidal properties (RODRÍGUEZ-VIVAS et al., 2010).

Currently Major Used Acaricide Groups
Organophosphates and Carbamates.Organophosphates, i.e. coumaphos, and carbamates have similar effect acting on acetylcholinesterase (AChE) (CORBETT, 1974a, b).AChE is one of the most efficient enzymes, being capable of an extremely rapid rate of hydrolysis of acetylcholine and generation of the active enzyme (KWONG, 2002).Organophosphates are neutral esters of phosphoric acid or its thio analogue and act by inhibiting the action of AChE at cholinergic synapses and at muscle end plates.The OP mimics the structure of acetylcholine (Ach) and when it binds to AChE it causes transphosphoryla tion of the enzyme.The transphosphorylated AChE is unable to breakdown accumulating Ach at the post-synaptic membrane leading to neuromuscular paralysis (TAYLOR, 2001).Organophosphate compounds can be extremely toxic in ani mals and humans causing an inhibition of AChE.They are generally active against fly larvae, flies, lice, ticks and mites on domestic livestock and fleas and ticks on dogs and cats, although activity varies between compounds and differing formulations (MacDONALD, 1995).
Synthetic pyrethroids.The term "pyrethroid" is commonly used to designate a synthetic insecticide that is derived structurally from the natural pyrethrins.Pyrethroids are grouped into two categories (type I and type II) based on their distinct poisoning symptoms, effects on nerve preparations, and their chemical structures (NARAHASHI, 1986).Type I pyrethroids lack an α-cyano group which is present at the phenylbenzyl alcohol position of type II pyrethroids.Type I pyrethroids cause repetitive discharges in response to a single stimulus, while type II pyrethroids cause a membrane depolarization accompanied by a suppression of the action potential (SODERLUND et al., 2002).The mode of action of pyrethroids has been studied using vertebrate and non-insect invertebrate nerve preparations.Collectively, these studies show that pyrethroids cause prolonged opening of sodium channels in nerve, muscle and other excitable cells (CATTERALL, 2001), primarily by inhibiting channel deactivation and stabilizing the open configuration of the sodium channel (CATTERALL, 2001;SODERLUND et al., 2002;RAYMOND-DELPECH et al., 2005).Studies on the mechanism of action of pyrethroids on insect sodium channels expressed in oocytes and the molecular mechanism of knockdown resistance (kdr) confirmed that sodium channels are the target of SP insecticides (DONG, 2007).Type I pyrethroids (i.e.cismethrin and permethrin) appear to bind to resting or inactivated channels, shifting the voltage dependence of activation to more negative potentials and causing a slowlyactivating sodium current.These compounds also produce characteristic sodium tail currents following a depolarizing pulse that decay with first-order time constants (ZHAO et al., 2000).In contrast to these results, Type II pyrethroids exhibit profound use-dependent modification of sodium currents, which implies that these compounds bind preferentially to activate sodium channel states (TAN et al., 2002).The lethal activity of SP seems to involve action on both peripheral and central neurones, while the knock-down effect, is probably produced by peripheral neuronal effects only (CASIDA et al., 1983).
Cypermethrin, deltamethrin and cyhalothrin are examples of SPs that are effective on susceptible ticks (> 98% of efficacy).Flumethrin was designed for application to cattle as pour-on, but there is also an emulsifiable concentrate formulation that can be applied as a dip or spray.The active ingredient in the pour-on has a remarkable capacity for spreading rapidly on the skin and hair from points of application along the dorsal line of an animal to all areas of the body.The residual effect of treatment with flumethrin is extended if the pour-on formulation is applied (GEORGE et al., 2004).Flumethrin for the control of both one-host and multi-host ticks species on cattle is effective at relatively low concentrations compared to other SPs (STENDEL, 1985).
Phenylpyrazoles.Fipronil is a phenylpyrazole compound, which blocks transmission of signals by the inhibitory neurotransmitter, GABA, presents in insects (RAUH et al., 1990).Fipronil binds within the chloride channel and consequently inhibits the flux of Cl-ions into the nerve cell resulting in hyperexcitation of the insect nervous system (COLE et al., 1993).Additionally, fipronil and its metabolite fipronil sulfone block two types of glutamate activated  Orr et al. (2009) chloride channels in insects (ZHAO et al., 2005).Also, it acts in the reproductive process by altering both the structure and function of germinative cells of R. sanguineus females (OLIVEIRA et al., 2009).Fipronil is used worldwide for the treatment and control of flea and tick infestations on cattle, cats and dogs (TAYLOR, 2001;GEORGE et al., 2004).Fipronil applied as a pour-on to cattle infested with R. (B.) microplus had a therapeutic efficacy greater than 99 % in susceptible ticks (DAVEY et al., 1998).Insect growth regulators.These constitute a group of chemical compounds that do not kill the target parasite directly, but interfere with their growth and development.Insect growth regulators act mainly on immature stages of the parasites and then, are not usually suitable for the rapid control of established adult populations of parasites.Based on their mode of action they can be divided into: a) chitin synthesis inhibitors (benzoylphenyl ureas), b) chitin inhibitors (triazine/pyrimidine derivatives) and c) juvenile hormone analogues (TAYLOR, 2001).Fluazuron, a benzoylphenyl urea, is efficacious against ticks and some mite species.The adverse consequences for ticks on cattle treated with a pour-on of this acaricide are the reduction of the fecundity and fertility of engorged females to near zero, and mortality of immature ticks because they unable to moult to the next instar (GEORGE et al., 2004).
Formamidines.The main member of this group is amitraz, which is a formamidine-like acaricide that acts at octopamine receptor sites in ectoparasites resulting in neuronal hyperexcitability and death (EVANS; GEE, 1980).The overstimulation of octopamine synapses in the CNS results in tremors and convulsions in juvenile and adult arthropods.At sublethal levels these compounds influence egg-laying, feeding activity and raise the excitatory state of the arthropod (ORR et al., 1990).It is toxic against mites, lice and ticks in domestic live stock.In cattle, for example, amitraz has been widely used in dips, sprays or pour-on for mulations for the control of single-host and multi host tick species (TAYLOR, 2001).Amitraz continues to be one of the most popular acaricides for the control of R. (B.) microplus in Australia, southern Africa and Latin America (JONSSON;HOPE, 2007).Amitraz is unstable in dipping vats, but adding sufficient calcium hydroxide or hydrated lime to raise and maintain the pH of the vats at 12, insures the stability of the active ingredient (GEORGE et al., 2004).
Mixture of acaricide groups.Several OPs synergize the toxicity to R. (B.) microplus of cypermethrin and deltamethrin.The reduction in concentration of a relatively expensive SP used with a relatively cheap OP acting as a synergist provided an efficacious, inexpensive product for the control of OP-resistant tick populations (SCHNITZERLING et al., 1983).In Australia, the product combination of cypermethrin + chlorfenvinphos and deltamethrin + ethion remains on the market (GEORGE et al., 2004).In Brazil, there are a number of different formulations based on mixtures of OPs and SP, for use in dipping vats, aspersion or in pour-on formulations.The most common are formulated with chlorpyriphos and cypermethrin, with or without a synergist (i.e.pyperonylbutoxide).Also, it is common to find in the market associations between two organophosphates (i.e.chlorpyriphos and dichlorvos).Recently it was launched in the Brazilian market a pour-on formulation of fluazuron in association with abamectin (SINDAN, 2013).The use of these mixtures is very popular in Brazil, in a survey conducted among cattle producers of the state of São Paulo, 30.2% of them reported the use of OPs and SPs mixtures to control cattle ticks (MENDES et al., 2011).In Mexico, mixtures of acaricides are available in the market and cymiazole + cypermethrin is one of the most used (RODRÍGUEZ-VIVAS et al., 2006a).One value of these mixtures may be their possible use for the control of both ticks and the horn fly.
One reason to research acaricide mixtures and synergized formulations is their potential to control populations of R. (B.) microplus resistant to one of the acaricides in the formulation under consideration.In this way, combination products could prolong the useful life of certain classes of acaricides (BARRÉ et al., 2008;RODRÍGUEZ-VIVAS et al., 2013).Knowles (1982) noted that the formamidine pesticides chlordime form and amitraz can act as a synergist of organophosphate, organochlorine, carbamate, and pyrethroid pesticides.Subsequent publications confirmed the synergism between amitraz, and pyrethroids against ticks (LI et al., 2007).Prullage et al. (2011) demonstrated acaricidal synergism between amitraz and fipronil.Recently, Rodríguez-Vivas et al. ( 2013) found strong synergism with the mixture of cypermethrin + amitraz + pyperonyl butoxide and this mixture was the most effective killing larvae resistant against pyrethroids in vitro (96.7-100% of larval mortality), and >95% control that persisted for 28 days post-treatment against R. (B.) microplus infesting cattle under field conditions in the Mexican tropics.Although the mode of action of pyrethroids and formamidines is different, Liu and Plapp (1990) demonstrated that formamidines act as target-site synergists of pyrethroids by modifying binding cooperativity in target tissues of insects.
Although it is a potentially useful tool to combat acaricide resistance, a product based on an acaricide mixture has to be used rationally (RODRÍGUEZ-VIVAS et al., 2013).Such a strategy needs to be part of an integrated tick control program that incorporates other technologies to minimize the economic impact of R. (B.) microplus and other ticks affecting cattle in a sustainable manner (RODRÍGUEZ-VIVAS et al., 2005).For example, synergized mixtures of acaricides may also potentiate the effect of anti-tick vaccines among populations of R. (B.) microplus that are already resistant to certain pesticides (NARI-HENRIOUD, 2011;KISS et al., 2012).Stewardship of acaricidal combination products is required because it has been demonstrated that some agriculturally damaging insect species can develop resistance to pesticide mixtures (IRAC, 2012).Further studies on the effect of selection pressure of acaricide mixture on the development of resistance in both laboratory and in field populations of R. (B.) microplus are needed.

Macrocyclic Lactone Groups
MLs are broad-spectrum antiparasitic drugs, extensively used in veterinary medicine.They are known as "endectocide" compounds based on their unique activity against external and internal parasites (SHOOP et al., 1995).The avermectins, milbemycins and spinosyns are collectively referred to as MLs which comprise several classes of chemicals derived from cultures of soil micro-organisms.Such compounds are extensively and increasingly used in veterinary medicine and agriculture (LUMARET et al., 2012).
Avermectins inhibit the gamma-aminobutyric acid (GABA) neurotransmission at two or more sites in nematodes, blocking interneuronal stimulation of excitatory motoneurons and thus leading to a flaccid paralysis (TURNER; SCHAEFFER, 1989).The drug is believed to block nerve signals by interfering with the glutamate-gated chloride (GluCl) channel receptors (found only in invertebrates), which make them likely to affect membrane stability (MARTIN et al., 2002).In arthropods, the avermectins interfere with the transmission between nervous and muscular cells, because the GABA receptors are located at the neuromuscular junction.In vertebrates, it stimulates the release of GABA in neurons, but as these are usually in the brain and thus protected by the blood/brain barrier, the drug is exceptionally safe for mammals (ÕMURA, 2008).In the mite Tetranychus cinnabarinus, the major resistance mechanism to avermectin B1 was the increasing activities of carboxylesterases and glutathione-S-transferase and the increase of mixed function oxidase of odemethylase activity (LIN et al., 2009).
Ivermectin and abamectin are generally used to control the ecto-and endoparasites (mites and nematodes) of livestock and antifilarial chemotherapy in humans.Ivermectin is the most widely used avermectin.Ivermectin's exceptional effectiveness has led to some negative aspects being identified.The drug is only partially metabolized in the alimentary tract and residual active ingredient can appear intact in the faeces at levels depending on how the drug is formulated and applied.In animal health, injectable and pour-on formulations are particularly long-living and can prove toxic for 2-8 weeks, especially as avermectins are not water soluble but bind tightly to particulate matter.Due to ivermectin's potency, the residues can cause mortality in a diverse range of invertebrates, especially beetles and fly larvae.ML, especially ivermectin is eliminated in high concentrations in the excreta of treated animals, causing negative effects on dung beetle populations (DIERSMANN et al., 2006;SUÁREZ et al., 2009).Adult individuals of a number of coprophagous beetle species (i.e. the genera Aphodius, Copris, Euoniticellus, Onitis and Onthophagus) exhibit low mortality when exposed to ivermectin, but these species also experience substantial reductions in fecundity, larva survival and imago adult emergence rates (HEMPEL et al., 2006;OECD, 2010).
Milbemycins.Moxidectin, the most important milbemycin, is a semisynthetic methoxime derivative of nemadectin, a fermentation product of Streptomyces cyanogriseus subsp.noncyanogenus.Chemically, avermectins differ from each other by chain substitutions on the lactone ring, whilst milbemycins, which are structurally related, differ from the avermectins through the absence of a sugar moiety from the lactone skeleton (ROCK et al., 2002).Milbemycin oxime is used against intestinal nematodes in dogs and cats, against adult heartworm in dogs, and against ectoparasites in companion animals.Milbemectin (a mixture of ≥ 70% milbemycin A4 and ≤ 30% milbemycin A3) is an insecticide and acaricide effective against all development stages of mites.In common with other ML, moxidectin displays a high affinity for the glutamate-gated ion channels specific to invertebrates.These glutamate-gated binding sites apparently occur in close proximity to GABA-gated chloride channels, and the macrolide endectocides may increase GABA-gated sites as well (LUMARET et al., 2012).
Spinosyns.Spinosyns are tetracyclic-macrolide compounds produced from the fermentation of soil-dwelling bacterial species in the genus Saccharopolyspora (LEWER et al., 2009).The active ingredient spinosad consists of a mixture of spinosyns A and D (thus its name, spinosAD) (KIRST, 2010).The unique mechanism of action of spinosyns involves the disruption of nicotinic acetylcholine receptors.However, the spinosyn A, the major constituent of the insecticide spinosad, does not interact directly with known binding sites of insect nicotinic receptors.Orr et al. (2009) suggested that spinosyn A exerts its insecticidal actions via a novel mode of action.

Resistance of Rhipicephalus (B.) microplus to Acaricides
In Mexico, R. (B.) microplus has developed resistance to all main classes of acaricides in past decades due to intensive use of chemical acaricides (RODRÍGUEZ-VIVAS et al., 2012).Resistance to OP acaricides first developed in the 1980s in Mexico, and resistance to SP emerged in the 1990s.Amitraz was introduced along with SP to control OP-resistant ticks in 1986.Initially, Amitraz was not widely used, owing to its higher cost, but its use became more prevalent and intensive after SP resistance was discovered in 1993 (FRAGOSO-SANCHEZ et al., 2011).The first case of Amitraz resistance in R. (B.) microplus from Mexico was confirmed in 2001 at a ranch in the state of Tabasco (SOBERANES et al., 2002).The first report of R. (B.) microplus resistant to fipronil in Northern States of Mexico was made by Miller et al. (2013).Rodríguez-Vivas et al. (2007) studied 217 field populations of R. (B.) microplus and determined the prevalence (measured by bioassays) of farms with resistance to SPs, OPs and amitraz in the southern Mexico, and they found that SP resistance such as deltamethrin, cypermethrin and flumethrin was one of the most serious problems in the Mexican tropics (from 66 to 96 % farms showed resistance to SPs).Furthermore, Rodriguez-Vivas et al. (2006a) studied 98 field populations of R. (B.) microplus in Yucatan, Mexico and found that 63, 61 and 59 % of those tick populations were resistant to flumethrin, deltamethrin and cypermethrin, respectively.The findings of R. (B.) microplus resistant to all three major classes of acaricides in Mexico underscore the seriousness of the resistance situation and the importance of having a resistance management strategy in Mexico.

Resistance of Rhipicephalus (B.) microplus to Macrocyclic Lactones
The first reports of tick resistance to MLs were made in Brazil.Martins and Furlong (2001) reported cross-resistant R. (B.) microplus to doramectin, ivermectin and moxidectin in a field trial.Klafke et al. (2006) reported a R. (B.) microplus population resistant to ivermectin in the State of Sao Paulo, Brazil.The evaluated population (Barra Alegre) showed a LC 50 value significantly higher than the susceptible strain and a resistance ratio (RR) of 3.78 (3.57-4.12).Recently, Klafke et al. (2011) evaluated the applicability of laboratory bioassays to diagnose ivermectin resistance in R. (B.) microplus.Studying nine different populations of cattle tick from the states of São Paulo, Rio Grande do Sul and Mato Grosso do Sul, Brazil, the authors found that populations without previous exposure to ivermectin exhibited RR between 0.87 and 1.01.However, populations previously exposed to ivermectin showed RR between 1.83 and 4.62.Klafke et al. (2010) used four selection methods to increase the RR to ivermectin of a Brazilian R. (B.) microplus strain.After ten generations, the RR increased from 1.37 to 8.06.This work showed for the first time that it was possible to increase ivermectin resistance in R. (B.)As a significant finding of this study, it was identified that farms that used MLs ≥4 times per year were more likely to develop R. (B.) microplus resistant to ivermectin (OR = 13.0;p = 0.0028).Recently, Fernández-Salas et al. (2012b) reported for the first time a R. (B.) microplus population in Mexico with different levels of resistance to OP, SP, Amitraz and ivermectin.The studied tick population showed a RR at 99% to ivermectin of 6.52-9.58.
At present, the molecular basis of resistance to MLs is not well understood.Insensitivity of the GluCl receptor, which prevents drug binding to its target site, has been associated with ivermectin resistance in some nematodes and arthropods (McCAVERA et al., 2009;KWON et al., 2010).In arthropods, MLs resistance is also associated with an increase in oxidative metabolism.Recently, it has become evident from molecular, biochemical and pharmacokinetic studies that the most important molecules involved in all of these processes are ATP-binding cassette (ABC) transporter proteins (BOURGUINAT et al., 2011).ABC transporters comprise a superfamily of membrane-integrated proteins expressed in all organisms, from bacteria to humans.These proteins are responsible for pumping xenobiotic and endogenous metabolites through extra-and intracellular membranes, thereby reducing cellular concentrations of toxic compounds (HOLLAND; BLIGHT, 1999).Recently, Pohl et al. (2011) demonstrated that the ABC transporter efflux pump is a defense mechanism against ivermectin in R. (B.) microplus.
The use of MLs to control ticks and gastrointestinal in cattle is expected to continue to increase in Mexico, due to convenience (broad spectrum), increasingly accessible prices (especially generic versions of MLs) and lack of effective chemical alternatives.A better understanding on spatial distribution, risk factors and possible cross-resistance associated to ivermectin resistance in the Mexican tropics is needed (PEREZ-COGOLLO et al., 2010a).Strategies of resistance management need to be pointed to keep the selection pressure at a minimum level, while still achieving an appropriate control of the parasite.Strategies involving the early detection of resistance and use of tactics designated to increase the useful life of MLs are recommended.

Conclusions
In Mexico many field populations of R. (B.) microplus are resistant to multiple classes of antiparasitic drugs, including OPs, SPs, amitraz and ivermectin.Ivermectin-resistant populations of R. (B.) microplus have been reported in Latin America, especially in Mexico (in two states, Veracruz and Yucatan).Although ivermectin resistance levels in R. (B.) microplus from Mexico were generally low in most cases, some field populations of R. (B.) microplus exhibited high levels of ivermectin resistance.Strategies involving the early detection of resistance and the use of integrated tick control are recommended.

Table 1 .
Mode of action and target sites for currently used acaricides and macrocyclic lactones.
Castro-Janer et al. (2011)nditions.Castro-Janer et al. (2011)studied the susceptibility of 18 R. (B.) microplus field populations from different Uruguayan counties to ivermectin by using the larval immersion test.Five tick populations presented a higher RR 50 (1.35-1.98)compared with the susceptible tick reference strain Table 2 shows the resistance ratio estimates at 50% and 99% for ivermectin resistance in R. (B.) microplus subjected to the larval immersion in two states of Mexico.Perez-Cogollo et al. (2010a) reported for the first time field populations of R. (B.) microplus resistant to ivermectin in Yucatan, Mexico.The authors studied three cattle farms (SFDO, SPN, LUADY).The LUADY (RR 50 : 2.04 and 2.29, RR 99 : 2.67 and 3.55), SPN (RR 50 : 3.55 and 3.68, RR 99 : 8.19 and 11.06) and SFDO (RR 50 : 6.84 and 8.59, RR 99 : 54.17 and 87.86) ticks had significantly higher LC 50 /LC 99 than the reference susceptible Deutch strain, demonstrating resistance in the field-collected populations.Furthermore, there was significant difference between LC 50 /LC 99 of the SFDO, SPN and LUADY tick populations, which indicates not only the presence of resistant populations, but also different levels of resistance to ivermectin in the field populations studied.Furthermore, Perez-Cogollo et al. (2010b) conducted a survey to evaluate the resistance level of 30 field populations of R. (B.) microplus to ivermectin at cattle farms with history of MLs use in Yucatan, Mexico.The authors found that field populations of R. (B.) microplus demonstrated various levels of resistance to ivermectin.The top three resistant populations were CHPAT (RR 50 =10.23,RR 99 =179.6),SDGO (RR 50 =7.37,RR 99 =115.3)and FND (RR 50 =7.09,RR 50 =50.22).However, the level of ivermectin resistance in most tick samples from cattle farms in Yucatan, Mexico were relatively low.Fernández-Salas et al. (2012a) determined the status of resistance or susceptibility to ivermectin in 53 populations of R. (B.) microplus collected in another state of Mexico (Veracruz).

Table 2 .
Resistance ratio estimates at 50% and 99% for ivermectin resistance in Rhipicephalus (Boophilus) microplus subjected to larval immersion test in two states of Mexico.