John E. George

Chemical control of ticks on cattle and the resistance of these parasites to acaricides.

Toward the end of the nineteenth century a complex of problems related to ticks and tick-borne diseases of cattle created a demand for methods to control ticks and reduce losses of cattle. The discovery and use of arsenical solutions in dipping vats for treating cattle to protect them against ticks revolutionized tick and tick-borne disease control programmes. Arsenic dips for cattle were used for about 40 years before the evolution of resistance of ticks to the chemical, and the development and marketing of synthetic organic acaricides after World War II provided superior alternative products. Most of the major groups of organic pesticides are represented on the list of chemicals used to control ticks on cattle. Unfortunately, the successive evolution of resistance of ticks to acaricides in each chemical group with the concomitant reduction in the usefulness of a group of acaricides is a major reason for the diversity of acaricides. Whether a producer chooses a traditional method for treating cattle with an acaricide or uses a new method, he must recognize the benefits, limitations and potential problems with each application method and product. Simulation models and research were the basis of recommendations for tick control strategies advocating approaches that reduced reliance on acaricides. These recommendations for controlling ticks on cattle are in harmony with recommendations for reducing the rate of selection for acaricide resistance. There is a need to transfer knowledge about tick control and resistance mitigation strategies to cattle producers.

Detection and Characterization of Amitraz Resistance in the Southern Cattle Tick, Boophilus microplus (Acari: Ixodidae)

Abstract Amitraz, a formamidine acaricide, plays an important role in the control of the southern cattle tick, Boophilus microplus (Canestrini), and other tick species that infest cattle, dogs, and wild animals. Although resistance to amitraz in B. microplus was previously reported in several countries, the actual measurement of the level of amitraz resistance in ticks has been difficult to determine due to the lack of a proper bioassay technique. We conducted a survey, by using a newly reported technique that was a modification of the standard Food and Agriculture Organization larval packet test, to measure the levels of resistance to amitraz in 15 strains of B. microplus from four major cattle-producing states in Mexico. Low-order resistance (1.68- to 4.58-fold) was detected in 11 of those strains. Our laboratory selection using amitraz on larvae of the Santa Luiza strain, which originated from Brazil, achieved a resistance ratio of 153.93 at F6, indicating the potential for high resistance to this acaricide in B. microplus. Both triphenylphosphate and piperonyl butoxide significantly synergized amitraz toxicity in both resistant and susceptible tick strains. Diethyl maleate synergized amitraz toxicity in one resistant strain but had no effect on the susceptible strain and had minor antagonistic effects on two other resistant strains. Target site insensitivity, instead of metabolic detoxification mechanisms, might be responsible for amitraz resistance observed in the Santa Luiza strain and possibly in other amitraz resistant B. microplus ticks from Mexico. The Santa Luiza strain also demonstrated high resistance to pyrethroids and moderate resistance to organophosphates. Multiple resistance shown in this strain and other B. microplus strains from Mexico poses a significant challenge to the management of B. microplus resistance to acaricides in Mexico.

Resistance to coumaphos and diazinon in Boophilus microplus (Acari: Ixodidae) and evidence for the involvement of an oxidative detoxification mechanism.

Abstract The levels of resistance to two organophosphate acaricides, coumaphos and diazinon, in several Mexican strains of Boophilus microplus (Canestrini) were evaluated using the FAO larval packet test. Regression analysis of LC50 data revealed a significant cross-resistance pattern between those two acaricides. Metabolic mechanisms of resistance were investigated with synergist bioassays. Piperonyl butoxide (PBO) reduced coumaphos toxicity in susceptible strains, but synergized coumaphos toxicity in resistant strains. There was a significant correlation between PBO synergism ratios and the coumaphos resistance ratios. The results suggest that an enhanced cytochrome P450 monooxygenase (cytP450)-mediated detoxification mechanism may exist in the resistant strains, in addition to the cytP450-mediated metabolic pathway that activates coumaphos. PBO failed to synergize diazinon toxicity in resistant strains, suggesting the cytP450 involved in detoxification were specific. Triphenylphosphate (TPP) synergized toxicity of both acaricides in both susceptible and resistant strains, and there was no correlation between TPP synergism ratios and the LC50 estimates for either acaricide. Esterases may not play a major role in resistance to coumaphos and diazinon in those strains. Bioassays with diethyl maleate (DEM) revealed a significant correlation between DEM synergism ratios and LC50 estimates for diazinon, suggesting a possible role for glutathione S-transferases in diazinon detoxification. Resistance to coumaphos in the Mexican strains of B. microplus was likely to be conferred by both a cytP450-mediated detoxification mechanism described here and the mechanism of insensitive acetylcholinesterases reported elsewhere. The results of this study also underscore the potential risk of coumaphos resistance in B. microplus from Mexico to the U.S. cattle fever tick eradication program.

Sequencing a new target genome: the Boophilus microplus (Acari: Ixodidae) genome project.

Abstract The southern cattle tick, Boophilus microplus (Canestrini), causes annual economic losses in the hundreds of millions of dollars to cattle producers throughout the world, and ranks as the most economically important tick from a global perspective. Control failures attributable to the development of pesticide resistance have become commonplace, and novel control technologies are needed. The availability of the genome sequence will facilitate the development of these new technologies, and we are proposing sequencing to a 4–6X draft coverage. Many existing biological resources are available to facilitate a genome sequencing project, including several inbred laboratory tick strains, a database of ≈45,000 expressed sequence tags compiled into a B. microplus Gene Index, a bacterial artificial chromosome (BAC) library, an established B. microplus cell line, and genomic DNA suitable for library synthesis. Collaborative projects are underway to map BACs and cDNAs to specific chromosomes and to sequence selected BAC clones. When completed, the genome sequences from the cow, B. microplus, and the B. microplus-borne pathogens Babesia bovis and Anaplasma marginale will enhance studies of host–vector–pathogen systems. Genes involved in the regeneration of amputated tick limbs and transitions through developmental stages are largely unknown. Studies of these and other interesting biological questions will be advanced by tick genome sequence data. Comparative genomics offers the prospect of new insight into many, perhaps all, aspects of the biology of ticks and the pathogens they transmit to farm animals and people. The B. microplus genome sequence will fill a major gap in comparative genomics: a sequence from the Metastriata lineage of ticks. The purpose of the article is to synergize interest in and provide rationales for sequencing the genome of B. microplus and for publicizing currently available genomic resources for this tick.

Characterization of Acaricide Resistance in Rhipicephalus sanguineus (Latreille) (Acari: Ixodidae) Collected from the Corozal Army Veterinary Quarantine Center, Panama

Abstract Rhipicephalus sanguineus (Latreille) were collected from the Corozal Army Veterinary Quarantine Center in Panama and characterized for resistance to five classes of acaricides. These ticks were highly resistant to permethrin, DDT, and coumaphos; moderately resistant to amitraz; and not resistant to fipronil when compared with susceptible strains. Resistance to both permethrin and DDT may result from a mutation of the sodium channel. However, synergist studies indicate that enzyme activity is involved. The LC50 estimate for permethrin was lowered further in the Panamanian strain then in susceptible strains with the addition of triphenylphosphate (TPP), but not with the addition of piperonyl butoxide (PBO). This suggests that esterases and not oxidases are responsible for at least some pyrethroid resistance. Elevated esterase activity and its inhibition by TPP were confirmed by native gel electrophoresis. The LC50 estimate obtained for coumaphos in the Panamanian strain was not lowered further than what was observed for susceptible strains by the addition of TPP or PBO. This indicates that enzyme activity might not be involved in coumaphos resistance. Resistance to amitraz was measured through a modification of the Food and Agriculture Organization Larval Packet Test. All tick strains were found to be susceptible to fipronil.

The '4-poster' passive topical treatment device to apply acaricide for controlling ticks (Acari: Ixodidae) feeding on white-tailed deer.

Abstract A ‘4-poster’ device that attracts white-tailed deer to a bait source, and as they feed, allows a self-application of a pesticide to the head, ears, and neck to control ticks was designed, constructed, and tested. The device consists of a central bin containing bait to attract deer and two feeding and application stations. These stations each have one bait port and two vertical pesticide-impregnated applicator rollers. This design allows unrestricted vertical retraction of the head to minimize injury to the deer or damage to the posts supporting the pesticide application rollers. Observations using deer demonstrated ready acceptance and repeated use by both antlered and antlerless deer. Results of an initial trial indicate that control values for lone star ticks,Amblyomma americanum(L.), exceeded 92–97% on deer that used the device regularly.

Present and Future Technologies for Tick Control

Abstract: Arsenic dips were the first effective method for controlling ticks and tick‐borne diseases, and were used in many parts of the world for over 50 years before resistance to the chemical became a problem. Until organochlorine products became available about 1946 as alternatives to arsenic, significant losses occurred in cattle herds exposed to arsenic‐resistant tick strains. Since the discovery of organochlorines, virtually every chemical group of pesticides developed for the control of arthropods is represented among the list of products employed for the control of ticks on cattle. The evolution of tick resistance to acaricides has been a major determinant of the need for new products. The variety of procedures for treating animals with acaricides ranges from dipping cattle to injecting systemic acaricides, but regardless of the treatment method used, producers need to know and follow proper application procedures to derive maximum benefits. The possibility of stocking with cattle breeds that acquire pronounced resistance to ticks or using recombinant antigen antitick vaccines are the most promising alternatives to acaricides. Most ranchers depend completely on acaricides to control ticks, but do not have access to guidelines on how to make a profit from their tick control program or how to detect and resolve problems with resistance to acaricides. Extension programs are needed to help ranchers manage animal health problems, including how to control ticks and tick‐borne diseases.

Threat of Foreign Arthropod-Borne Pathogens to Livestock in the United States

Abstract There are many exotic animal pathogens throughout the world that, if introduced into the United States, could have a significant detrimental impact on the health of livestock, agricultural economy, the environment, and public health. Many of these pathogens are arthropod-borne and potential vectors are readily available in the United States. A number of these arthropod-borne pathogens are discussed here as examples that illustrate the potential risk and the consequences of inadvertent introductions. Several International agencies have a role in global surveillance and in controlling animal diseases should they begin to expand their range. The risk to the United States is considerable. We propose that the United States invest in the improved infrastructure needed to reduce the risk of foreign arthropod-borne pathogens. Current U.S. programs focus on the exclusion of pathogens through regulation of animal movements and products, surveillance, especially trained animal disease diagnosticians, research support, international cooperation and, should pathogens enter our country, the resources for their prompt eradication. We suggest that the United States needs to develop a comprehensive, updated strategic plan to assess all aspects of current and future requirements, objectives, and resources needed to protect its national interests.

Modification of the Food and Agriculture Organization Larval Packet Test to Measure Amitraz-Susceptibility Against Ixodidae

Abstract Modifications of exposure time, substrate, and formulation were made to the Food and Agriculture Organization Larval Packet Test (LPT) to determine a combination suitable for measuring the susceptibility of Boophilus microplus (Canestrini) to amitraz. Exposure time influenced the slope of the dose–response when paper was used as a substrate for amitraz. However, time did not influence the dose–response slope when nylon fabric was used as an amitraz substrate. Formulated amitraz produced results with less deviation from the log-probit model than technical amitraz. The combination of formulated amitraz and nylon fabric as a substrate for amitraz produced results that best fit the log-probit model. The modified FAO procedure (formulated amitraz/nylon substrate combination) was used to assay a Brazilian strain of B. microplus and a Panamanian strain of Rhipicephalus sanguineus (Latreille). Resistance ratios (95% CI) of 26.3 (25.7–26.9) and 7.3 (5.5–9.9) were calculated for the B. microplus and R. sanguineus strains, respectively. A discriminating dose of 0.03% amitraz was determined for B. microplus. This technique will help to locate amitraz resistant tick populations, provide data for improved control practices, and aid in the discovery of resistance mechanisms through synergist studies and verification of molecular techniques.

First report of organophosphate-resistant Boophilus microplus (Acari: Ixodidae) within the United States.

Abstract Boophilus microplus (Canestrini), collected from Starr County, Texas, were determined to be resistant to the organophosphorus acaricides coumaphos and diazinon. Initial bioassay results from wild-collected ticks produced a probit regression slope (SE) of 3.96 (0.22), which was different from that obtained from a susceptible reference population 6.97 (0.38). Resistance ratios (RRs) (95% CI) indicated that the population was resistant to coumaphos 3.6 (3.4–3.8), 5.0 (4.5–5.5), and 6.5 (5.4–7.7) at the LC50,90,99, respectively. A second collection of wild ticks made 12 d after all cattle in the infested pasture were treated with coumaphos produced a slope (SE) that was not significantly different from a susceptible laboratory reference population. A second bioassay found these ticks to be resistant to diazinon, RR (95% CI) = 7.1 (6.5–7.7), 11.7 (10.3–13.3), 17.7 (14.5–21.5) at the LC50,90,99, respectively. The slope (SE) generated from the diazinon bioassay with the resistant ticks was different than that of a reference strain, 2.98 (0.12) and 6.09 (0.35), respectively. The high-dose strategy used by the Cattle Fever Tick Eradication Program was able to eradicate coumpahos-resistant B. microplus after just two treatments of coumaphos, 12 d apart.