Roger Iván Rodríguez-Vivas

Acaricidal activity of extracts from Petiveria alliacea (Phytolaccaceae) against the cattle tick, Rhipicephalus (Boophilus) microplus (Acari: ixodidae)

The acaricidal activity of crude extracts and fractions from stems and leaves of Petiveria alliacea (Phytolaccaceae) was carried out on larvae and adults of the cattle tick Rhipicephalus (Boophilus) microplus using the larval immersion test (LIT) and adult immersion test (AIT), respectively. Methanolic extracts of stems and leaves of P. alliacea showed 100% mortality on the LIT bioassay. On the other hand, methanolic extracts of leaves and stem on the AIT test showed 26% and 86% of mortality, respectively, egg laying inhibition of 40% and 91%, respectively and hatchability inhibition of 26% and 17%, respectively. Purification of the active stem methanolic extract showed that the activity was present in the n-hexane non-polar fraction. Bioassay-guided purification of the n-hexane fraction produced 10 semi-purified fractions; fraction B had the highest activity against tick larvae (100% mortality). Gas chromatography-mass spectrometry demonstrated that the chemical composition of the active fraction B samples were mainly composed of benzyltrisulfide (BTS) and benzyldisulfide (BDS). These metabolites might be responsible for the acaricidal activity of stem extract of P. alliacea. However, further experiments to evaluate the acaricidal activity of BTS and BDS on larvae and adults of R. (B.) microplus are needed. Our results showed that P. alliacea is a promising biocontrol candidate as acaricide against R. (B.) microplus resistant strains.

First report of the cattle tick Rhipicephalus microplus resistant to ivermectin in Mexico.

Three cattle farms with ticks, Rhipicephalus microplus, thought to be resistant to ivermectin in Yucatan, Mexico were studied (SFDO, SPN, LUADY). Each field-population was collected and tested twice several months apart. The larval immersion test was used on the progeny of collected adult females to test the susceptibility to ivermectin. Dose-mortality regressions, lethal concentrations (LC), their confidence intervals and slope were estimated by probit analysis. Resistance ratios (RR) were determined in the three investigated populations at the LC(50) and LC(99) estimates. 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): 8.19-11.06) 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. There was no difference observed at the LC(50) nor LC(99) estimates at two different times of collection from any of the three populations studied. In conclusion, we report for the first time field populations of R. microplus resistant to ivermectin in Mexico.

First report of a Rhipicephalus microplus tick population multi-resistant to acaricides and ivermectin in the Mexican tropics

We document the presence of a Rhipicephalus microplus tick population resistant to acaricides (organophosphates (OP), synthetic pyrethroids (SP), amitraz) and macrocyclic lactones (ML) (ivermectin). Engorged females of R. microplus were collected from a cattle farm in Veracruz, Mexico, to evaluate acaricide and ivermectin resistance. The modified larval packet test (LPT) was used to detect OP (chlorpiriphos and diazinon) and SP (flumethrin, deltamethrin and cypermethrin) resistance and the larval immersion test (LIT) to detect resistance to amitraz and ivermectin. Both, LPT and LIT were performed twice at different times with different collected samples. Mortality data with ivermectin were subjected to probit analysis to obtain lethal concentrations and resistance ratios (RR) using an ivermectin-susceptible strain (Deutch) as a reference. The R. microplus population showed resistance to all acaricides tested, with different mortalities at the discriminate dose: chlorpiriphos (1%), diazinon (24.2%), flumethrin (92.8%), deltamethrin (94.2%), cypermethrin (98.0%) and amitraz (1.5%). The studied tick population also showed resistance to ivermectin with a resistance ratio at 99% of 9.58 and 6.52 in the first and second evaluation, respectively. We report for the first time a R. microplus population in Mexico with different levels of resistance to OP, SP, amidines (Am) and ivermectin. The uncontrolled use of these products in the study area may promote the complete failure of tick control within a short period of time.

Roles Played by Esterase Activity and by a Sodium Channel Mutation Involved in Pyrethroid Resistance in Populations of Boophilus microplus (Acari: Ixodidae) Collected from Yucatan, Mexico

Abstract Pyrethroid resistance in Boophilus microplus (Canestrini) (Acari: Ixodidae) was studied by correlating discriminating-dose (DD) bioassay results and esterase activity or the frequency of a sodium channel mutation known to be involved in pyrethroid resistance in nine field strains of B. microplus from Yucatan, Mexico. Two tick strains (P67 and B74) were identified as susceptible to cypermethrin, deltamethrin, and flumethrin by DD, one strain (P65) was susceptible to cypermethrin and resistant to deltamethrin and flumethrin, and six strains were resistant to cypermethrin, deltamethrin, and flumethrin (T11, M10, C54, R49, B71, and T66). By using polymerase chain reaction, only 6.0 and 6.2% of resistance allele (R) was found in the susceptible strains (P67 and B74, respectively). In the T66 strain, with 100% of larval survival to the DD of pyrethroids as measured by the larval packet test (LPT), 98.0% of the gene pool contained the R allele. Positive correlations between the larval survival and the percentage of the R allele were found (deltamethrin r2 = 0.8875, P < 0.01; cypermethrin r2 = 0.8563, P < 0.01; and flumethrin r2 = 0.8491, P < 0.01). There were no significant correlations between the level of larval survival and esterase-based hydrolytic activity. It was concluded that within the B. microplus populations studied, resistance to flumethrin, deltamethrin, or cypermethrin was because of the novel sodium channel mutation (Phe→Ile amino acid substitution in the S6 transmembrane segment of domain III), and there was a correlation between tick mortality by pyrethroid exposure (larval survival) and the presence of R allele. It was not determined whether enhanced esterase-based hydrolytic activity was involved in pyrethroid resistance in the populations tested.

Studies on the effect of infection by Babesia sp. on oviposition of Boophilus microplus engorged females naturally infected in the Mexican tropics.

Three hundred and fifteen engorged female Boophilus microplus ticks (weight 210-250 mg) naturally infected with Babesia sp., in the Mexican tropics were monitored for egg production. Haemolymph samples were taken from each tick on the 5th day to the 16th day after collection to detect and estimate the infection with Babesia sp. kinetes. All ticks were held in darkness at 27+/-1.5 degrees C and 85-86% relative humidity. The infection rate of Babesia sp. was 20.3% (64/315). Fifteen ticks were considered heavily infected and 49 lightly infected. The pre-oviposition periods were 3.17+/-0.37, 3.18+/-0.25 and 3.17+/-0.25 days for heavily infected, lightly infected and uninfected, respectively (P>0.05). The numbers of eggs laid on the first day of oviposition were 252+/-53, 235+/-37, 54+/-23 for heavily infected, lightly infected and uninfected, respectively. There was a statistically significant difference between infected (heavily and lightly) and uninfected (P<0.05) ticks. Oviposition periods were 9.60+/-0.81, 9.50+/-0.72 and 9.36+/-0.48 days for heavily infected, lightly infected and uninfected, respectively (P>0.05). The average egg production of heavily infected, lightly infected and uninfected female ticks was 2640+/-103, 2574+/-123 and 2841+/-170 (P>0.05), respectively. These data imply that there is an adaptative tolerance between Babesia sp., and B. microplus under field conditions in the Mexican tropics.

Molecular survey of pyrethroid resistance mechanisms in Mexican field populations of Rhipicephalus (Boophilus) microplus

Susceptibility to synthetic pyrethroids (SP´s) and the role of two major resistance mechanisms were evaluated in Mexican Rhipicephalus microplus tick populations. Larval packet test (LPT), knock-down (kdr) PCR allele-specific assay (PASA) and esterase activity assays were conducted in tick populations for cypermethrin, flumethrin and deltamethrin. Esterase activity did not have a significant correlation with SP´s resistance. However a significant correlation (p < 0.01) was found between the presence of the sodium channel mutation, and resistance to SP´s as measured by PASA and LPT respectively. Just over half the populations (16/28) were cross-resistant to flumethrin, deltamethrin and cypermethrine, 21.4% of the samples (6/28) were susceptible to all of the three pyrethroids 10.7 of the samples (3/28) were resistant to flumethrin, 3.4 of the samples (1/28) were resistant to deltamethrin only and 7.1% (2/28) were resistant to flumethrin and deltamethrin. The presence of the kdr mutation correlates with resistance to the SP´s as a class. Target site insensitivity is the major mechanism of resistance to SP´s in Mexican R. microplus field strains, involving the presence of a sodium channel mutation, however, esterase-based, other mutations or combination of mechanisms can also occur.

Resistencia de la garrapata Boophilus microplus a los ixodicidas

Resumen es: La resistencia de las garrapatas a los ixodicidas es uno de los principales problemas que afectan a los productores bovinos en el subtropico y tropico, d...

Prevalence of benzimidazole resistant nematodes in sheep flocks in Yucatan, Mexico

Thirty-eight sheep flocks, located in three municipalities in the Eastern Yucatan, Mexico, were surveyed for gastrointestinal nematodes resistant to benzimidazole (BZD) anthelmintics (AH). On each flock, 30 sheep were randomly distributed into two groups of 15 animals: albendazole group (5mg/kg BW) and untreated control group. Animals were refrained from any food (either browsing/grazing or supplement) for a period of 16 h prior to treatment. Faecal egg counts (FEC) and larval cultures were performed 10 days after anthelmintic treatment. Percentage reduction and 95% confidence intervals were determined. Anthelmintic resistance (AR) was declared when the percentage reduction in FEC was <95% and the 95% confidence interval was <90%. AR was suspected when only one of the two criteria was met. The survey indicated that AR occurred in 15.8% (n=6) (95% confidence interval=+/-11.6%) and was suspected in 23.7% of the farms (n=9) (95% confidence interval=+/-13.3%). Post-treatment larval cultures indicated that Haemonchus was the only resistant genus. The questionnaire survey showed that most farmers (92%) considered their sheep a secondary activity to cattle production. The majority of farmers (97.4%) treat their animals according to visual appreciation of weight. However, most farmers (79%) treat their flocks at very low frequencies (<3 times per year). Drug rotation was performed every 12 months or more by 84.2% of farmers. Anthelmintics used were: macrociclic lactones (47.4%), BZD (39.5%), levamisol (10.5%) and 1 farmer used closantel.

Factors affecting the prevalence of mange-mite infestations in stray dogs of Yucatán, Mexico.

The aim of the present study was to determine the factors affecting the prevalence of mange-mite infestations in stray dogs of Yucatán, Mexico. The study was carried out in 200 stray dogs of Mérida capital city of Yucatán, Mexico. Four samples (head, thoracic-abdominal area, extremities and ear) were taken from each animal by skin scraping and examined microscopically in 10% KOH solution to detect the presence of mites. Mites were also collected from the external ear canal of dogs using cotton-tipped swabs. The prevalence of different mite species was calculated. A primary screening was performed using 2xK contingency tables of exposure variables. All variables with P< or =0.20 were analyzed by a logistic-binomial regression. The overall prevalence was 34%. Demodex canis (23.0%) was the most frequent mite, followed by Sarcoptes scabei var. canis (7.0%) and Otodectes cynotis (3.5%). The following factors were found: body condition (bad, OR: 5.35, CI 95%: 1.66-17.3; regular, OR: 3.72, CI 95%: 1.39-9.99) and the presence of macroscopic lesions of dermatosis (OR: 42.80, CI 95%: 13.65-134.24).

The effect of management factors on the seroprevalence of Anaplasma marginale in Bos indicus cattle in the Mexican tropics.

A cross-sectional study with a two-stage design and proportional distribution was carried out to determine the effect of management factors on the seroprevalence of Anaplasma marginale in Bos indicus cattle in the Mexican tropics. Serum was obtained from 384 cattle aged 1– 2 years on 92 farms. The number of samples was proportionally distributed according to the number of farms in eastern Yucatan. Antibody activity against A. marginale was assessed using the card agglutination test. A primary screening using a 2×K contingency table of the exposed variables with the outcomes was performed. All variables for which p<0.20 were included in a fixed-effects log regression. The seroprevalence in the cattle was 69.75% (SE±0.02). Sixty-four per cent of the farms had a seroprevalence ≥75%. The risks related to managemental factors were stocking density (≥1 animal/ha, OR = 10.94), type of acaricide (pyrethroids, OR = 3.8), dipping interval (>60 days, OR = 0.13) and type of veterinary instruments used (needles, scalpels, ear tattoos, and dehorners, OR = 0.17).