Two distant Antioquian cattle farms where systemic and topical
acaricides had previously failed to control infestations by Rhipicephalus (Boophilus) microplus were studied. An initial in vivo study was conducted using single
subcutaneous injections with a long-acting formulation of
ivermectin (630 μg/kg).
Injections were made at 3-month intervals on animals at each farm to evaluate the therapeutic and persistent efficacy of
ivermectin against R. microplus. Body tick counts and reproductive parameters of semi- or fully engorged females (≥5 mm) were assessed at 10-day intervals, and since no negative control group could be included, values were compared against those for day 0. Although there was an overall reduction of 50%-75% in tick numbers that persisted for 30-40 days, it was not significantly different at one of the farms and not enough to afford protection from severe infestations. The engorgement weight and egg mass weight of ticks from treated animals were significantly lower throughout the 50-day posttreatment period. Egg hatch was not significantly reduced posttreatment and remained at levels of 80%-90%. A random selection of 9 out of 28 commercial formulations of
ivermectin sold in Colombia were analyzed by High Performance Liquid Chromatography (HPLC). All were within the expected labeled concentration (±15% deviation) of 1% and 3.15%
ivermectin except for one. A popular unregistered
injectable widely used in both farms and labeled as "natural
pyrethrin", was found to contain 10.5%
ivermectin. An adult immersion test was conducted to evaluate the efficacy of topical
acaricides to recommended concentrations of five commercial products and/or their combinations. Efficacy was determined by comparing the reproductive index of each treated group to that of the control group.
Cypermethrin (150 ppm) was completely ineffective at both farms.
Amitraz (208 ppm) exhibited low and intermediate efficacies of 14% and 56%. The combination of
amitraz (100 ppm) and
cypermethrin (150 ppm) was less efficacious than the
amitraz alone. A generic product based on
amitraz + citronella (208 ppm + 10 ppm, respectively) was shown to be less efficacious than the name-brand
amitraz product. Products containing the
organophosphate chlorpyrifos or
trichlorfon exhibited intermediate efficacies of approximately 60% at the Tarso farm. We conclude that at these two locations, there is a high degree of resistance to many of the
acaricides available in Colombia and confirm suspicions that
ivermectin is no longer able to eliminate
tick infestations.