Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/2008
Publication Date: 1/5/2009
Citation: Davey, R.B., Miller, J.A., Miller, R., George, J.E. 2009. Effect of rainfall exposure immediately after a single dip treatment with coumaphos on the control of Rhipicephalus (Boophilus) microplus (Acari:Ixodidae) on infested cattle. Journal of Medical Entomology. 46(1):93-99. Interpretive Summary: USDA, APHIS, VS Tick Eradication Program regulations mandate that tick infested cattle exposed to rainfall or water immediately following a dip treatment in coumaphos may not be moved until they are re-dipped with no rainfall or water exposure. However, there appears to be no published scientific evidence to support this regulation. Therefore, a study was conducted to determine the effects of various levels of rainfall on the efficacy of a single dip in the organophosphate acaricide, coumaphos immediately following treatment. Five treatment groups of cattle were evaluated in the study, four of which were treated with coumaphos at a concentration of 0.182% active ingredient. The treatment groups were as follows: (1) No treatment in coumaphos and no exposure to rainfall (negative control); (2) Treatment in coumaphos with no exposure to rainfall; (3) Treatment in coumaphos followed by exposure to 14.3 mm of rainfall; (4) Treatment in coumaphos followed by exposure to 28.6 mm of rainfall; and (5) Treatment in coumaphos followed by exposure to 42.9 mm of rainfall. Results of the study showed that the only treatment that provided the = 99% level of control that is acceptable for use in the eradication program was the treatment in coumaphos followed by no exposure to rainfall, which is the only procedure that is presently approved by the program. The level of control obtained against infested cattle exposed to 14.3, 28.6, and 42.9 mm of rainfall immediately following treatment was 83.7, 66.9, and 71.8%, respectively. These results suggest that exposure to the higher levels of rainfall (28.6 and 42.9 mm) will have a greater adverse effect on the efficacy of the coumaphos treatment than exposure to the lower level of rainfall exposure (14.3 mm). However, the most important fact is that exposure to any level of rainfall immediately following a dip treatment in coumaphos will reduce the level of control below the 99% level which is the minimum level of control for use in the program. Therefore, results clearly indicate that the regulation preventing movement of livestock following treatment if a rainfall event occurs should be maintained and strictly enforced to eliminate the risk of dispersing viable ticks outside the quarantine zone.
Technical Abstract: Efficacy of coumaphos applied as a single dip treatment at 0.182% active ingredient was determined against Rhipicephalus (Boophilus) microplus (Canestrini) in all stages of development on infested cattle that were exposed to various levels of rainfall immediately following treatment. One group of calves, to which all other groups were compared, remained untreated with no exposure to rainfall, thus serving as a negative control. A second group of cattle treated with coumaphos, but not exposed to a rainfall event, acted as a positive control. Each of three remaining groups of coumaphos treated cattle was exposed to 14.3, 28.6, and 42.9 mm of rainfall, respectively immediately following treatment. In the coumamphos treated group not exposed to a rainfall event, overall mean control (99.2%) was higher and mean female engorgement weight (200 mg), egg mass weight (43 mg), and index of fecundity (IF) (2.90) were all lower than any of the groups exposed to a rainfall event. Although there were few statistical differences in most measured parameters among the three groups exposed to rainfall, nevertheless, exposure to the lowest level of rainfall (14.3 mm) resulted in a substantially lower mean egg mass weight (65 mg) and IF (62.26) with higher control (83.7%) than was observed in ticks exposed to 28.6 or 42.9 mm of rainfall. This suggested that exposure to higher levels of rainfall was likely to reduce the efficacy of coumaphos substantially more than would occur as lower levels of rainfall exposure. Analysis of each parasitic development stage (larva, nymph, and adult) showed there was very little reduction in the efficacy of coumaphos against ticks in the larval stage (> 97% control), regardless of whether rainfall exposure occurred or not. However, rainfall exposure applied against ticks in either the nymphal or adult stage caused dramatic declines in control as a result of exposure to any rainfall. Thus, the movement of coumaphos treated that have been exposed to any level of rainfall would pose a very high risk of dispersing viable ticks into uninfested areas both inside and outside the quarantine zone.