VETERINARY ENTOMOLOGY A Sustained Release Gel Formulation of Doramectin for Control of Lone Star Ticks (Acari: Ixodidae) and Horn Flies (Diptera: Muscidae) on Cattle 1 K. H. LOHMEYER, J. A. MILLER, J. M. POUND, AND J. A. KLAVONS USDAÐARS, Knipling Bushland U.S. Livestock Insect Research Laboratory, 2700 Fredericksburg Road, Kerrville, TX 78028 J. Econ. Entomol. 102(2): 804Ð808 (2009) ABSTRACT A gel formulation formed by incorporating technical doramectin into a 10% hy- droxypropyl methylcellulose aqueous solution was used to subcutaneously inject steers at varying dosages. Doramectin serum concentration of steers receiving 600 ␮g (AI)/kg body weight declined from 21.9 ppb at 0.5 wk to below detectable at 8 wk postinjection. The 1,200 ␮g (AI)/kg injection resulted in serum concentrations of 29.1 ppb at 0.5 wk and declined to 0.5 ppb at 8 wk postinjection. Both the 600 and 1,200 ␮g (AI)/kg injections provided 100% inhibition of index of fecundity (IF) in adult lone star ticks, Amblyomma americanum L. (Acari: Ixodidae) through week 8, after which inhibition declined to 79.4 and 45.3%, respectively, during the 12th week posttreatment. For steers treated at 600 ␮g (AI)/kg, mortality of adult horn ßies, Hematobia irritans L. (Diptera: Muscidae), declined from 16.9% during week 2 to 3.1% during week 7 postinjection. The blood from steers treated at 1,200 ␮g (AI)/kg resulted in a similar decline in mortality of blood fed adult horn ßies from 29.4% during week 1 to 4.0% during week 7. The 600 ␮g (AI)/kg treatment provided complete control of larval horn ßies in the manure for 9 wk, whereas the 1,200 ␮g (AI)/kg injection gave complete control for 14 wk posttreatment. The doramectin gel formulation provided long-lasting delivery of doramectin to cattle and extended control of lone star ticks and larval horn ßies. Such a simple and inexpensive formulation could be useful in tick eradication programs by reducing the frequency of gathering cattle. KEY WORDS Amblyomma americanum, Hematobia irritans, endectocide, delivery system The availability of highly potent endectocides pro- Miller et al. (1983) described a sustained release vides unique opportunities for development of novel implant for delivery of ivermectin with extended ef- delivery systems for management of livestock pests. Þcacy against larval horn ßies, Hematobia irritans L. The avermectins such as ivermectin (Campbell 1989), (Diptera: Muscidae), and tick pests on livestock. doramectin (Goudie et al. 1993), eprinomectin Boyce et al. (1992) demonstrated that the implant was (Shoop et al. 1996), and the closely related milbemy- effective against Psoroptes mites in free-ranging big- cins such as moxidectin (Webb et al. 1991, Scholl et al. horn sheep. Although implants are commonly used in 1992, Miller et al. 1994) are the most typical of this the livestock industry, an injectable formulation is group that have found signiÞcant utility in the animal considered more desirable because of reduced trauma health industry. EfÞcacy at dosages in the microgram to the animal and greater ease of administration. Con- per kilogram range has been shown effective against sequently, Miller et al. (1998) developed bioabsorb- a variety of livestock pests (Drummond 1985, Lasota able, injectable microspheres for delivery of ivermec- and Dybas 1991). Despite the potential of using the tin over an extended period (Ͼ12 wk) to cattle and potency of these compounds to develop long-lasting demonstrated long-lasting effectiveness against adult formulations, few such products have become avail- lone star ticks, Ambylomma americanum L. (Acari: able, with the exception of the Ivomec SR Bolus Ixodidae), feeding on cattle. They later showed that (Merck & Co., Rahway, NJ) and a long-acting inject- this formulation could be used to eliminate Boophilus able formulation of moxidectin (Soll et al. 1989, annulatus (Say) (Acari: Ixodidae) from treated cattle Zingerman et al. 1997, Miller et al. 2001, Cleale et al. and the infested pasture where they were held (Miller 2004, Yazwinski et al. 2006). et al. 1999). Similar formulations of both ivermectin and moxidectin have been used to deliver low dosages to dogs for the prevention of heartworm for at least 6 This article represents the results of research only. Mention of a mo (Clark et al. 2004, Lok et al. 2001). proprietary product does not constitute an endorsement or a recom- mendation by USDA for its use. To reduce the cost of the polymers, solvents, and 1 Corresponding author, e-mail: [email protected]. labor for production of the injectable microsphere April 2009 LOHMEYER ET AL.: DORAMECTIN GEL FOR CONTROL OF TICKS AND FLIES 805 formulations, we conducted research to develop a Tick Bioassay. Standard lone star tick (LST) bioas- simpler formulation for extended delivery of an aver- says were conducted to determine efÞcacy of the mectin. The objective of the research reported here treatment against feeding adults. A stockinet sleeve was to 1) develop a bioabsorbable gel formulation for was glued onto the side of each animal by using con- sustained release of doramectin, 2) determine the tact cement at least 24 h before infestations. At each drug-serum proÞle over time for such a formulation, 3) infestation, 30 mating pair of laboratory-reared LST determine the persistence of the formulation and eval- adults were placed in the stockinet sleeve on each uate its ability to inhibit reproduction of adult lone star animal. Infestations were initiated on the day of treat- ticks feeding on cattle, and 4) determine the efÞcacy ment and at 2-wk intervals. The ticks were examined and persistence of the formulation against adult horn periodically to determine percentage of attachment ßies feeding on cattle and immature horn ßies devel- and to collect detached engorged females. The num- oping in the manure of treated cattle. ber of engorged females, the weight of each female, the weight of the egg mass produced, and the esti- mated hatch of eggs were used to determine percent- age of inhibition of the index of fecundity (IF) (Davey Methods and Materials et al. 2001). Calculation of the IF provided a means of Formulation. Hydroxypropyl methylcellulose estimating the reproductive capacity of ticks recov- (HPMC) (Methocel, The Dow Chemical Company; ered from each steer by incorporating the number of Midland, MI) was prepared as a 10% solution in deion- ticks collected, the egg mass weight of sampled ticks, ized water. Two 10-ml plastic syringes were attached and the estimated percentage of hatch of the eggs to each end of an 18-gauge ϫ 2Ð7/8-in. micro-emul- produced by females into the formula. Records were sifying needle (Popper & Sons, Inc., New Hyde Park, kept on each tick from each animal. The engorged NY). One syringe contained 5 ml of prepared HPMC females and subsequent egg masses were held in an gel along with the required amount of technical do- environment of 27ЊC, 85% RH, and a photoperiod of ramectin (PÞzer Animal Health, Exton, PA). The sus- 14:10 (L:D) h. The percentage of inhibition of IF due pension was emulsiÞed by conducting a back-and- to treatment was calculated using a modiÞed AbbottÕs forth motion of the two syringe pistons until no formula (Abbott 1925) reported by Davey et al., 2001. doramectin particles were visible. The required dos- Horn Fly Bioassay. Adult horn ßy bioassays were age for each animal was prepared just before injection conducted by feeding 100 laboratory-reared horn ßies along with syringes containing 5 ml of the HPMC gel in 1.4-liter acrylic cages with a screened top and bot- only to be used for the control steers. tom by using the inverted blood vial technique de- Treatment. Six Ϸ200 kg Hereford steers were di- scribed by Miller et al. (1986). Flies had been reared vided into three groups of two animals each and held at the Kerrville facility by using techniques described in stanchions in an enclosed research barn. The steers by Lohmeyer and Kammlah (2006). Flies were held in were fed twice daily with alfalfa cubes at the rate of a rearing room at 27ЊC, 85% RH, and a photoperiod of Ϸ3% of body weight and were provided water ad 14:10 (L:D) h. The mortality of the ßies fed on blood libitum. Two steers were treated with a 5-ml subcu- from each animal was recorded at 96 h, and percentage taneous injection at a dose of 600 ␮g doramectin/kg of control was calculated using AbbottÕs formula (Ab- body weight, two steers were given an injection at a bott 1925). In addition, manure samples collected dose of 1,200 ␮g doramectin/kg body weight, and the weekly from each animal were used for larval horn ßy remaining two were given a 5-ml injection of the gel bioassays (Miller et al. 1986). For this bioassay, 100 only. The injections were administered in the pres- horn ßy eggs were placed on 100-g samples of manure capular area on the left side of the neck of each animal in waxed paper cups for each dose. Cups of manure in a downward direction. In conducting the research were held in a rearing room at 27ЊC, 85% RH, and a described in this report, we adhered to the Guide for photoperiod of 14:10 (L:D) h. After 7 d, the pupae the Care and Use of Laboratory Animals, as promul- were extracted using a ßotation technique, placed in gated by the Institutional Animal Use and Care Com- small paper cups, and the adults allowed to emerge. mittee (IAUCC) of the Knipling-Bushland U.S. Live- Corrected percentage of inhibition of larval horn ßies stock Insects Research Laboratory, Kerrville, TX.