Submitted to: Experimental and Applied Acarology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 20, 2004
Publication Date: March 1, 2005
Citation: Davey, Ronald B., Miller, J. Allen, George, John E., Miller, Robert J. 2005. Therapeutic and persistent efficacy of a single injection treatment of ivermectin and moxidectin against Boophilus microplus (Acari:Ixodidae) on infested cattle. Experimental and Applied Acarology. 35:117-129. Interpretive Summary: The U.S. Boophilus eradication program relies exclusively on the use of the organophosphate (OP) acaricide, coumaphos to eliminate ticks at import facilities. However, the development of OP resistance in most B. microplus populations in Mexico has created a critical need in the program to identify, develop, and implement the use of alternative acaricides that will prevent the ingress of OP-resistant ticks into the country. A study was conducted to evaluate the effectiveness of a single subcutaneous injection treatment with either ivermectin or moxidectin applied at 200 µg/kg to cattle infested with all parasitic stages of B. microplus. While the overall therapeutic efficacy of both compounds was 99%, which meets the standards required for use in the eradication program, both materials provided significantly lower control (98%) against ticks that were in the larval stage of development at the time of treatment. In addition, neither material provided adequate residual control (<93%) for even 7 days after treatment. Therefore, results indicated that the use of these endectocides at import facilities would pose a risk of dispersing ticks to areas outside the permanent quarantine zone. However, repeated systematic treatments with either compound would be an excellent alternative to coumaphos in eliminating ticks from infested premises.
Technical Abstract: The effectiveness of a single treatment with either ivermectin or moxidectin was determined by administering a single subcutaneous injection of each endectocide at 200 µg per kg body weight to cattle infested with all parasitic developmental stages (adults, nymphs, and larvae) of Boophilus microplus (Canestrini). The percentage reduction in the number of females that reached repletion following treatment (outright kill) was 94.8 and 91.1% for ivermectin and moxidectin, respectively. In addition, the reproductive capacity of the females that did survive to repletion was reduced by >99%, regardless of the endectocide. Based on these two factors, the therapeutic level of control obtained against ticks on the cattle at the time of treatment was 99.0 and 99.1% for ivermectin and moxidectin, respectively. Engorged females recovered from either group of treated cattle weighed '5-times less than untreated females, and the egg masses produced by treated females weighed '5-8-times less than egg masses produced by untreated females. Partitioning of data into three separate 7-d post-treatment intervals allowed for an estimation of the efficacy of each endectocide against each individual parasitic development stage (adult, nymph, and larva). Results indicated that both endectocides were '99.7% effective against ticks that were in either the adult or nymphal stage at the time of treatment. However, the level of control against ticks in the larval stage of development at treatment was significantly lower at 97.9 and 98.4% for ivermectin and moxidectin, respectively. Analysis of the persistent (residual) activity of the two endectocides indicated that neither material provided total protection against larval re-infestation for even 1-wk following treatment. Against larvae infested 1-4 wk following treatment, the level of control with moxidectin ranged from 92.4% (1 wk) to 19.5% (4 wk). These control levels were higher at each weekly interval than ivermectin, which ranged from 82.4% (1 wk) to 0.0% (4 wk). The potential for the use of these injectable endectocide formulations in the U.S. Boophilus Eradication Program is discussed.