VETERINARY ENTOMOLOGY Efficacy of the Ivomec SR Bolus for Control of Horn Flies (Diptera: Muscidae) on Cattle in South Texas

J. ALLEN MILLER,1, 2 RONALD B. DAVEY,3 DELBERT D. OEHLER,1 J. MATHEWS POUND,1 1 AND JOHN E. GEORGE

J. Econ. Entomol. 96(5): 1608Ð1611 (2003) ABSTRACT The concentration of ivermectin in the serum of Hereford heifers treated with a single Ivomec SR bolus reached a maximum of 8.8 Ϯ 0.9 ppb at 2 wk post-treatment. The single bolus treatment resulted in Ͻ10% mortality of adult horn ßies feeding on the blood of the treated animals over the 21-wk trial. Bioassays of the manure from treated cattle showed complete inhibition of development of immature horn ßies through week 19 post-treatment. When the trial was repeated using two Ivomec SR boluses/heifer, the concentration of ivermectin in the serum of the treated cattle reached a maximum level of 31.2 Ϯ 3.9 ppb at week 13 post-treatment. During the Þrst 17 wk of treatment, the use of two boluses/heifer resulted in 96.2 and 81.2% mortality of adult male and female horn ßies feeding on the blood of treated animals, respectively. From these studies, we conclude that a single Ivomec SR bolus used as an anthelmintic treatment can be expected to provide signiÞcant control of immature horn ßies developing in the manure, but not of adults feeding on the treated cattle.

KEY WORDS ivermectin, sustained release bolus, Hematobia irritans

THE AVERMECTINS ARE A CLASS of endectocides that in- in the manure for 10Ð14 wk (Miller et al. 1999). Miller cludes ivermectin (Campbell 1989), doramectin (Ver- et al. (1986) demonstrated that adult horn ßies were cruysse 1993), and eprinomectin (Shoop et al. 1996) killed when they were fed either bovine blood treated and are closely related to the milbemycins such as with ivermectin or blood taken from heifers subcuta- moxidectin (Webb et al. 1991, Scholl et al. 1992, Miller neously injected with the therapeutic dosage of iver- et al. 1994). Because of their chemistry and mode of mectin. It was also shown that males were more sus- action, broad spectrum of activity, and efÞcacy at ceptible than females. In addition to mortality, this extremely low dosages, the drugs represent an impor- study demonstrated that ivermectin in the blood diet tant class of compounds for control and management reduced the number and hatch of horn ßy eggs (Miller of arthropod pests of livestock. Ivermectin, the Þrst et al. 1986). of this class to be introduced, has been shown to The Ivomec SR bolus (Merck & Co., Rahway, NJ) control a broad variety of both endo- and ectoparasites (Zingerman et al. 1997) was introduced as a means of (Drummond 1985, Lasota and Dybas 1991). Schmidt sustained delivery of ivermectin to the rumen of graz- and Kunz (1980) determined that ivermectin inter- ing cattle. This ruminal bolus is a push-melt osmotic fered with development of larval horn ßies, Hematobia pump designed to deliver ivermectin at a rate of irritans (L.), in manure. Miller et al. (1981) showed 11Ð13mg/d over Ϸ135 d. The bolus has been shown that a single subcutaneous injection of ivermectin to to be efÞcacious in the control of several tick species cattle at a dose of 200 ␮g/kg controlled larval horn ßies on cattle (Taylor and Kenny 1990, Soll et al. 1989, in the manure for up to 4 wk after treatment. They also 1990). The objectives of the current study were to showed that when ivermectin was given daily at a dose determine the efÞcacy and persistence of the com- of only 1 ␮g/kg orally, it killed all horn ßy larvae in the mercial Ivomec SR bolus on adult horn ßies feeding on manure. Ivermectin delivered by injectable micro- the blood from treated cattle and on larval horn ßies spheres also inhibited development of larval horn ßies breeding in the manure produced by these cattle.

This paper reports the results of research only. Mention of a com- mercial or a proprietary product in this paper does not constitute an Methods and Materials endorsement by the U.S. Department of Agriculture. 1 U.S. Department of Agriculture, Research, Education, and Eco- This study was conducted at the USDA, Agricultural nomics, Agricultural Research Service, Southern Plains Area, Research Service, Cattle Fever Tick Research Labo- Knipling-Bushland U.S. Livestock Insects Research Laboratory, 2700 ratory, Mission, TX, in conjunction with a study of the Fredericksburg Road, Kerrville, TX 78028Ð9184. efÞcacy of the Ivomec SR bolus against Boophilus 2 E-mail: [email protected]. 3 U.S. Department of Agriculture, Research, Education, and Eco- annulatus (Say). Two trials were conducted; one with nomics, Agricultural Research Service, Southern Plains Area, Cattle a single bolus/heifer and a second with two boluses/ Fever Tick Research Laboratory, Rt. 3, Box 1010, Edinburg, TX 78539. heifer. In conducting the research described in this October 2003M ILLER ET AL.: IVOMEC SR BOLUS FOR CONTROL OF HAEMATOBIA 1609 report, the investigators adhered to the Guide for the Care and Use of Laboratory Animals, as promulgated by the Institutional Animal Use and Care Committee of the Knipling-Bushland U.S. Livestock Insects Re- search Laboratory, Kerrville, TX. Two Ϸ7-ha (17 acre) pastures, consisting primarily of buffel grass (Cenchurus ciliaius L.), were used to conÞne treated and control cattle. Six Hereford heif- ers (207.6 Ϯ 16.5 kg) in the pasture designated as the treated pasture were each treated with a single Ivomec SR bolus using a standard balling gun on 11 September 1997. The six Hereford heifers (206.8 Ϯ 23.0 kg) in the pasture designated as the untreated pasture remained Fig. 1. Mean (ϮSE) concentration of ivermectin in se- untreated as controls for comparative purposes. rum of six heifers as a result of the treatment with one or two Before treatment (day 0) and at weekly intervals, Ivomec SR boluses. two serum samples were collected by venipuncture from all heifers using 13-ml Vacutainers (Becton Dick- inson, Franklin Lakes, NJ) until no ivermectin was the sample. For the bioassay, the manure samples were detected in any animal for two consecutive bleeding thawed to room temperature and 100 g were placed in times. Samples were analyzed using a high-perfor- waxed paper cups. One hundred horn ßy eggs, col- mance liquid chromatographic (HPLC) method de- lected from the laboratory colony over a 4- to 6-h veloped in our laboratory (Oehler and Miller 1989). period, were placed on each sample. The samples were The technique enables quantiÞcation of 2 ppb of iver- covered with tissue paper and incubated at 27 C and mectin in 5 ml of serum. 50% RH for 7 d. Pupae were then extracted by ßota- In addition to the samples taken for HPLC analysis tion. The number of pupae and adults subsequently of serum, every other week beginning with week 1 emerging were recorded for each sample. The percent post-treatment, two additional blood samples were control of adults produced from the manure bio- taken from each heifer using 13-ml Vacutainers con- assays because of the treatment was calculated using taining 0.5 ml sodium citrate solution (100 mg/ml). AbbottÕs formula. These were used for feeding caged horn ßies in a Because the single bolus trial provided a low level bioassay. For the bioassay, 50 pairs of 3- to 4-d-old horn of control of horn ßies feeding on the blood from ßies from the Kerrville laboratory colony were placed treated animals, this component of the study was re- in cylindrical plastic cages (1,400 ml) with screened peated with additional heifers using two Ivomec SR (20 by 20 mesh) tops and bottoms. The ßies in each boluses per animal. Six additional Hereford heifers cage were fed ad libitum on blood either from the (214.0 Ϯ 27.0 kg) were placed in the treated pasture treated or the control heifers. The blood was poured and given two Ivomec SR bolus on 20 May 1998. Six from the respective collection tubes into a shell vial additional Hereford heifers were placed in the un- (2 cc), covered with a small piece of saran screen (30 treated pasture (227.7 Ϯ 23.8 kg) and served as the by 30 mesh), and inverted onto the screened top of the untreated controls for this supplemental experiment. cage. A new vial of blood was placed on each cage each day. Dead ßies were removed daily from each cage Results and Discussion through a port in the cylindrical wall of the cage. A record of cumulative mortality by sex was maintained The concentration of ivermectin in the serum of for 4 d. Percent mortality because of the treatment was the cattle treated with the single Ivomec SR bolus determined by using AbbottÕs formula (Abbott 1925) reached a maximum of 8.8 Ϯ 0.9 ppb at 2 wk post- to correct for mortality in the control sample. In ad- treatment and steadily declined to 5.0 Ϯ 0.3ppb at 16 dition, an analysis of variance (ANOVA) was per- wk and 1.5 Ϯ 1.0 ppb at 19 wk post-treatment (Fig. 1). formed on the number of ßies surviving in the bioassay By week 21 post-treatment, no ivermectin could be using the mixed model procedure (SAS Institute detected in the serum. 1987). Figure 2 shows the corrected mortality of both male Manure was collected from the treated and the and female horn ßies as a result of feeding on the blood control heifers for bioassay against larval horn ßies. of the cattle treated with the single bolus. An ANOVA Samples were collected from the treated and the con- indicated that the blood from the treated heifers pro- trol heifers before treatment and at biweekly intervals duced signiÞcantly (F ϭ 24.82, df ϭ 1,140, P Ͻ 0.001) beginning with week 1 post-treatment. Although sam- greater mortality than blood from the control animals. ples were not identiÞed by the animal number, an Over the Þrst 17 wk post-treatment, the average mor- effort was made to collect the samples from six fresh tality was 12.5 Ϯ 5.5 and 8.0 Ϯ 2.1%, respectively, for manure droppings in the treatment and the control male and female horn ßies feeding on the blood from holding pens immediately after the heifers were gath- treated heifers. However, there was no signiÞcant ered for bleeding. Each sample was placed in a large, difference (F ϭ 1.75, df ϭ 1, 140, P Ͼ 0.23) in the sealable plastic bag, labeled, and frozen for preserva- response between male and female horn ßies because tion and to kill any horn ßy eggs that might be within of treatment. 1610 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 96, no. 5

bolus treatment. Although treatment with two Ivomec SR boluses could be expected to provide signiÞcant control of blood-feeding adults for up to 17 wk, such a treatment would not be considered practical. In a recent study, Miller et al. (2001) drew similar conclu- sions with respect to the efÞcacy of the Ivomec SR bolus against Boophilus annulatus (Say).

Acknowledgments The authors thank D. Krska, H. Bazan, A. Pena, and H. Vasquez, Cattle Fever Tick Research Laboratory, Mission, Fig. 2. Mean (ϮSE) corrected mortality of male and TX, for invaluable assistance in handling of cattle and col- female horn ßies fed blood from cattle treated with one lection of samples. In addition, we thank A. Siebenaler, Ivomec SR bolus. Knipling-Bushland U.S. Livestock Insects Research Labora- tory, Kerrville, TX, for conducting the bioassays and his efforts in data analysis and preparation of Þgures. Ivermectin in the manure of heifers treated with the single bolus provided 100% control of developing horn ßy larvae for 19 wk post-treatment. Of the bioassays of References Cited the manure samples collected during week 21, two Abbott, W. J. 1925. A method for computing effectiveness showed 100% inhibition of larval development, two of an insecticide. J. Econ. Entomol. 18: 265Ð267. showed 41.2% and 47.2% corrected (Abbott 1925) in- Campbell, W. C. (ed.). 1989. Ivermectin and abamectin. hibition, and the other two showed no inhibition of Springer, New York. development. This variation between manure samples Drummond, R. O. 1985. Effectiveness of ivermectin for is not unusual toward the end of treatment efÞcacy. control of arthropod pests of livestock. Southwest. Figure 3shows the corrected mortality of both sexes Entomol. Suppl. 7: 34Ð42. of horn ßies as a result of feeding on the blood of the Lasota, J. A., and R. A. Dybas. 1991. Avermectins, a novel cattle treated with the two Ivomec SR boluses. An class of compounds: implications for use in arthropod pest control. Annu. Rev. Entomol. 36: 91Ð117. ANOVA showed that ßies fed on the blood from the ϭ ϭ Miller, J. A., S. E. Kunz, D. D. Oehler, and R. W. Miller. 1981. treated heifers had signiÞcantly (F 997.4, df 1,140, Larvicidal activity of Merck MK-933, an avermectin, P Ͻ 0.001) greater mortality than those fed on blood against the horn ßy, stable ßy, face ßy and house ßy. from the control animals. Average corrected mortality J. Econ. Entomol. 74: 608Ð611. for 17 wk post-treatment was 96.2 Ϯ 1.9% for males and Miller, J. A., D. D. Oehler, A. J. Siebenaler, and S. E. Kunz. 81.2 ϩ 2.1% for females fed on blood from treated 1986. Effect of Ivermectin on survival and fecundity of heifers. Male horn ßies demonstrated signiÞcantly horn ßies and stable ßies (Diptera: Muscidae). J. Econ. greater susceptibility to ivermectin than did females Entomol. 79: 1564Ð1569. (F ϭ 9.29, df ϭ 1, 140, P Ͼ 0.019). This result is Miller, J. A., D. D. Oehler, and P. J. Scholl. 1994. Moxidec- tin: pharmacokinetics and activity against horn ßies consistent with our previous observations (Miller et al. (Diptera: Muscidae) and trichostrongle nematode egg 1986). production. Vet. Parasitol. 53: 133Ð143. This study demonstrated that treatment of cattle Miller, J. A., R. B. Davey, D. D. Oehler, J. M. Pound, J. E. weighing Ϸ200 kg with a single Ivomec SR bolus George, and E. H. Ahrens. 1999. Control of Boophilus yielded signiÞcant mortality of larval horn ßies in the annulatus (Acari: Ixodidae) on cattle using injectable manure and was only marginally effective against microspheres containing ivermectin. J. Econ. Entomol. blood-feeding adults. Effective control of blood- 92: 1142Ð1146. feeding adults would not be expected with a single Miller, J. A., R. B. Davey, D. D. Oehler, J. M. Pound, and J. E. George. 2001. The Ivomec SR Bolus for Control of Boophilus annulatus (Acari: Ixodidae) on Cattle in South Texas. J. Econ. Entomol. 94: 1622Ð1627. Oehler, D. D., and J. A. Miller. 1989. Liquid chromato- graphic determination of ivermectin in bovine serum. J. Assoc. Off. Anal. Chem. 72: 59. SAS Institute. 1987. SAS/STAT guide for personal comput- ers. Version 6 ed. SAS Institute, Cary, NC. Schmidt, C. D., and S. E. Kunz. 1980. Testing immature laboratory-reared stable ßies and horn ßies for suscepti- bility to insecticides. J. Econ. Entomol. 73: 702Ð703. Scholl, P. J., F. S. Guillot, and G. T. Wang. 1992. Moxidectin: systemic activity against common cattle grub (Hypoder- ma lineatum) (Diptera: Oestridae) and trichostrongyle nematodes in cattle. Vet. Parasitol. 41: 203Ð209. Shoop, W. L., J. R. Eggerton, C. H. Eary, H. W. Haines, B. F. Fig. 3. Mean (ϮSE) corrected mortality of male and Micheal, H. Mrozik, P. Eskola, M. H. Fisher, L. Slayton, female horn ßies fed blood from cattle treated with two D. A. Ostlind, B. J. Skelly, R. K. Fulton, D. Barth, S. Costa, Ivomec SR boluses. L. M. Gregory, W. C. Campbell, R. L. Seward, and M. J. October 2003M ILLER ET AL.: IVOMEC SR BOLUS FOR CONTROL OF HAEMATOBIA 1611

Turner. 1996. Eprinomectin: a novel avermectin, for use Vercruysse, J. (ed.). 1993. DoramectinÑa novel avermec- as a topical endectocide for cattle. Int. J. Parasitol. 26: tin. Vet. Parasitol. 49: 1Ð122. 1237Ð1242. Webb, J. D., J. G. Burg, and F. W. Knapp. 1991. Moxidectin Soll, M. D., I. H. Carmichael, G. E. Swan, and S. J. Gross. evaluation against Solenopotes capillatus (Anoplura: 1989. Control of induced infestations of three African Linognathidae), Bovicola bovis (Mallophaga: Trichodec- multihost tick species with sustained-release ivermectin. tidae) and Musca autumnalis (Diptera: Muscidae) on Exp. Appl. Acarol. 7: 121Ð130. cattle. J. Econ. Entomol. 84: 1266Ð1269. Soll, M. D., G. W. Benz, I. H. Carmichael, and S. J. Gross. Zingerman, J. L., J. R. Cardinal, R. T. Chern, J. Holste, J. B. 1990. EfÞcacy of ivermectin delivered from an intraru- Williams, B. Eckenfoff, and J. T. Wright. 1997. The minal sustained-released bolus against natural infesta- in vitro and in vivo performance of an osmotically con- tions of Þve African tick species on cattle. Vet. Parasitol. trolled delivery systemÑIvomec SR Bolus. J. Control. 37: 285Ð296. Rel. 35: 1Ð11. Taylor, S. M., and J. Kenny. 1990. An ivermectin sustained release bolus in cattle: its effects on the tick Ixodes ricinus. Med. Vet. Entomol. 4: 147Ð150. Received for publication 4 June 2003; accepted 10 June 2003.