Therapeutic and persistent efficacy of a long-acting (LA) formulation of ivermectin against Rhipicephalus (Boophilus) microplus (Acari: Ixodidae) and sera concentration through time in treated cattle

Authors: Davey, Ronald; Pound, Joe; MILLER, J - Retired ARS Employee; Klavons, Jerome

Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/30/2009
Publication Date: 4/19/2010
Citation: Davey, R.B., Pound, J.M., Miller, J.A., Klavons, J.A. 2010. Therapeutic and persistent efficacy of a long-acting (LA) formulation of ivermectin against Rhipicephalus (Boophilus) microplus (Acari: Ixodidae) and sera concentration through time in treated cattle. Veterinary Parasitology. 169(1-2):149-156.

Interpretive Summary: There is a critical need in the Cattle Fever Tick Eradication Program (CFTEP) for the development of a treatment method that would lengthen the interval between treatments of livestock on infested premises that is presently required (every 14 days), as a means of reducing producer costs necessitated by frequent gathering of animals, while still achieving eradication of the ticks. A long-acting formulation of ivermectin was tested to evaluate the concentration-time profile of ivermectin in the sera of treated cattle, the therapeutic efficacy, and the persistent efficacy of the product against Rhipicephalus (Boophilus) microplus ticks. Cattle were treated with a single subcutaneous injection of the LA ivermectin material at a dose of 630 µg per kg of body weight. Blood samples were taken from treated cattle at various intervals after treatment and cattle were infested with ticks at various intervals both before treatment and after treatment to determine efficacy. Sera samples showed the material peaked at a level of 26.2 ppb at 11 d after treatment and remained above the threshold level for control of feeding ticks (8 ppb) for 42.6 d after treatment. Therapeutic efficacy against ticks infested on the cattle at the time of treatment was > 99.9%, which is suitable for use in the CFTEP. Persistent efficacy results showed that the level of control was > 99.9% against ticks infested at 14 d after treatment, however, control was well below the 99% level required by the CFTEP against ticks infested at 28-70 d after treatment. It was estimated the use of this material would lengthen of time between treatments to 31 d, thus doubling the presently required 14 d treatment interval. However, this interval fell far short of the 75 d label claim of the product in preventing re-infestation.

Technical Abstract: Concentration-time profile, therapeutic, and persistent efficacy of a single subcutaneous injection of cattle with a long-acting (LA) formulation of ivermectin at a concentration of 630 µg per kg of body weight was determined against Rhipicephalus (Boophilus) microplus. Ivermectin sera concentration increased to 13.0 ppb within 1 d after treatment and peaked at 26.4 ppb at 11 d post-treatment. Ivermectin levels remained above the threshold level for control of feeding ticks (equal to or greater than 8 ppb) for 42.6 d after treatment. Therapeutic efficacy was > 99.9%, and tick number, index of fecundity and fertility, engorgement weight, and egg mass weight of treated ticks were dramatically less than those of the untreated group. The persistent efficacy indicated tick number and reproductive capacity of ticks infested on treated animals at 14 and 28 d post-treatment were less than untreated ticks, whereas engorgement weight and egg mass weight remained lower than that of untreated ticks 49 d post-treatment. However, the level of control against ticks infested at 14 d after treatment (99.9%) was the only post-treatment infestation interval that provided the required 99% control necessary for use in the U.S. tick eradication program. The 14 d post-treatment infestation was also the only interval at which infested ticks were exposed to ivermectin in the levels above the threshold level of 8 ppb for the entire parasitic development period. Cattle would have to be treated at intervals of no more than 31 d apart to ensure that no viable ticks could reach repletion and detach from the host. Although this treatment interval is > 2-fold longer than the present treatment requirement (14 d), it is dramatically less than the label claim for the LA ivermectin formulation of 75 d of prevention against re-infestation.