Phenotypic evaluation of genetic resistance to the tick Rhipicephalus (Boophilus) microplus in Argentine Creole cattle.

The objective of this work was to characterize the Argentine Creole cattle breed through the identification of individual phenotypic variations in the levels of infestation with Rhipicephalus (Boophilus) microplus. We evaluated 179 heifers exposed to successive artificial infestations from 2015 to 2018, achieving a total of 663 observations. Tick counts were assessed with the linear mixed model, considering year of evaluation, time of infestation, dam's age and nutritional status during the evaluated period as fixed effects. The average tick count value obtained allowed to classify the breed as highly resistant to the tick charge (99.3%). Although the previous nutritional condition of the animals did not affect the individual charge response, weight gain during the trial showed a significantly negative correlation. We conclude that the Argentine Creole breed is an attractive genetic alternative for cattle breeding in endemic regions, either as a pure breed or a cross-breed.

Assays to evaluate the transovarial transmission of Anaplasma marginale by Rhipicephalus microplus.

The aim of this study was to evaluate the vectorial competence of Rhipicephalus microplus to transmit Anaplasma marginale transovarially, by analyzing the results of three different but complementary assays. First, larvae of R. microplus were fed on a calf infected with the isolate S1P of A. marginale. The engorged females obtained were analyzed by PCR and incubated for oviposition. After hatching, larvae were analyzed by PCR and fed on susceptible splenectomized cattle. Although A. marginale was detected in the females, no A. marginale DNA was amplified from the larvae and transmission of A. marginale to cattle was not recorded. In the second experiment, R. microplus larvae were fed on cattle naturally infected with field isolates of A. marginale and experimentally infected with the isolate S1P of A. marginale. After detachment, engorged females were incubated for oviposition. The offspring were analyzed by PCR, with negative results. Finally, free-living larvae of R. microplus collected from pasture on farms with cattle infected with A. marginale were analyzed by PCR for Anaplasma infection. All samples analyzed were negative for A. marginale. The results of this work indicate that transovarial transmission of A. marginale by R. microplus is unlikely to occur.

Mutation in the RmßAOR gene is associated with amitraz resistance in the cattle tick Rhipicephalus microplus.

We aimed to describe the evolution of resistance to amitraz in Rhipicephalus microplus in the field and to test the association between amitraz resistance and the frequency of a mutation in the ß-adrenergic octopamine receptor gene (RmßAOR). We established six populations of Rhipicephalus microplus ticks in similar paddocks by the admixture of ticks from strains known to be susceptible and resistant to amitraz and synthetic pyrethroids. Each population was managed using one of three acaricide treatment regimes: always amitraz, always spinosad, or rotation between amitraz and spinosad. We used microsatellites to elucidate population structure over time, an SNP in the para-sodium channel gene previously demonstrated to confer resistance to synthetic pyrethroids to quantify changes in resistance to synthetic pyrethroids over time, and a nonsynonymous SNP in the RmßAOR, a gene that we proposed to confer resistance to amitraz, to determine whether selection with amitraz increased the frequency of this mutation. The study showed panmixia of the two strains and that selection of ticks with amitraz increased the frequency of the RmßAOR mutation while increasing the prevalence of amitraz-resistance. We conclude that polymorphisms in the RmßAOR gene are likely to confer resistance to amitraz.

In vitro diagnosis of the first case of amitraz resistance in Rhipicephalus microplus in Santo Tomé (Corrientes), Argentina.

In Argentina, the cattle tick Rhipicephalus microplus has already developed resistance to organophosphates and synthetic pyrethroids. However, no cases of amitraz resistance have ever been recorded in this country despite its heavy use. A recent failure of amitraz to control ticks in a farm located in Santo Tomé, province of Corrientes, resulted in the collection of samples for acaricide resistance diagnosis. The modified Drummond adult immersion test (AIT) and the larval tarsal test (LTT) were performed separately in Argentina and Switzerland to evaluate efficacy of amitraz and other acaricides. The AIT showed that oviposition in the Santo Tomé field isolate was not inhibited when it was challenged to 250 and 500 ppm amitraz, and 50 ppm deltamethrin. However, oviposition was reduced by 90.6% when this field isolate was challenged to a combination of 400 ppm ethion and 100 ppm cypermethrin. To confirm the results obtained with the AIT, 2 additional tick samples were collected and shipped to Switzerland for resistance diagnosis of amitraz, cypermethrin and flumethrin, using the LTT. With this bioassay, the resistance ratios of the 2 field isolates were 32.5 and 57.0 for amitraz and between 5.9 and 27.2 for the synthetic pyrethroids. Both in vitro bioassays confirmed amitraz and synthetic pyrethroid resistance in the Santo Tomé samples. These results account for the first evidence of amitraz resistance in R. microplus in Argentina.