Experimental & Applied Acarology, 23 (1999) 351–364 Evaluation of a pour-on formulation of fipronil against Boophilus annulatus (Acari: Ixodidae) under natural South Texas field conditions Ronald B. Daveya**, John E. Georgeb, James S. Hunter IIIc and Philippe Jeannind a USDA, ARS, Cattle Fever Tick Research Laboratory, PO Box 970, Mission, TX 78573, USA USDA, ARS, Knipling-Bushland US Livestock Insects Laboratory, 2700 Fredericksburg Rd, Kerrville, TX 78029, USA c Merial, Inc., 115 Transtech Drive, Athens, GA 30601, USA d Merial, Inc., Laboratoire de Toulouse, 4 chemin du Calquet, 31057 Toulouse Cedex, France (Received 12 August 1998; accepted 14 September 1998) b ABSTRACT A long-term field study (60 consecutive weeks) was conducted to determine whether a Boophilus annulatus (Say) population could be eradicated through repeated applications of a 1% pour-on formulation of fipronil on cattle held in an infested pasture. Animals treated repeatedly over time (seven applications) were infested with significantly fewer (p , 0.05) female ticks (5 mm in size) than untreated animals in all except one of the 57 weekly tick counts conducted after the first treatment was applied. As the number of treatments increased over time, there was a progressive decrease in the number of instances in which engorging females were observed on the treated cattle. Tick numbers obtained from sentinel cattle placed in pasture with untreated and treated cattle at various intervals throughout the study indicated that a single fipronil treatment had no observable adverse effect on the field tick population. However, after two treatments had been applied, there were always dramatically fewer ticks obtained from sentinel animals placed with treated cattle than were observed on sentinel animals placed with untreated cattle. Furthermore, tick numbers obtained from sentinel cattle placed with treated cattle over time indicated that the tick population in the pasture declined by stages. While a single treatment had no effect on the tick population, the application of two to four fipronil treatments at various intervals dramatically reduced the tick population, even though substantial numbers of females were still present on the sentinel animals. Ultimately, the application of five to seven fipronil treatments resulted in an extremely low tick population in the pasture, although total eradication of the population was not achieved during the study. The mean weight gain of treated animals was significantly higher (p , 0.05) than untreated animals during the study, indicating that the fipronil treatments had a highly beneficial impact on weight gain production. Exp Appl Acarol 23: 351–364 © 1999 Kluwer Academic Publishers Key words: Boophilus annulatus, Acari, fipronil, acaricide, pour-on, field study. * The U.S. Government’s right to retain a non-exclusive, royalty-free licence in and to any copyright is acknowledged. ** To whom correspondence should be addressed at: Tel: (956) 580–7262; Fax: (956) 580–7261; e-mail: rbdavey@main.rgv.net 0168–8162 © 1999 Kluwer Academic Publishers 352 R.B. DAVEY ET AL . INTRODUCTION The Boophilus spp. eradication programme, which has been carried out in the US during most of the twentieth century, has been phenomenally successful in eliminating these ectoparasites from all except eight counties that lie along the Texas–Mexico border (Graham and Hourrigan, 1977). During the past 20 years, the introduction and dispersal of fever ticks, which includes both Boophilus annulatus (Say) and Boophilus microplus (Canestrini), into and within the continental US has been prevented through strict adherence to procedures which require the systematic treatment of all livestock in a dipping vat charged with the organophosphorus acaricide coumaphos (USDA, APHIS, VS., 1978). Among the most important procedures for eliminating fever ticks are the quarantine management requirements associated with premises found to be infested. When a fever tick outbreak is detected, proper quarantine management procedures require that the owner/ producer eradicate the infestation on the premise by either dipping cattle or pasture vacation (George et al., 1995). The pasture vacation method requires the removal of all livestock from the infested premise for a period of 6–9 months, depending on the time of year the infestation is discovered. Conversely, the cattle dipping method allows the owner/producer to maintain livestock on the infested premise during the 6–9 month quarantine period, but it requires all livestock to be treated in a dipping vat charged with coumaphos every 14 days during the quarantine interval. Thus, all animals held within the infested premise receive a minimum of 14 acaricide treatments (6 month period) and up to a maximum of 21 acaricide treatments (9 month period) during the quarantine period. Both of these eradication procedures produce substantial hardships and/or significant expense to the owner/producer of the infested premise. If pasture vacation is chosen, the productivity of the land is lost during the 6–9 month quarantine. On the other hand, if systematic dipping of all animals is chosen, the expense of gathering and handling animals 14–21 times during the 6–9 month quarantine will be substantial. Consequently, the development of any acaricide and/or procedure that reduces the hardship and/or expense of the presently required procedures, while maintaining the integrity of the eradication programme, would be a great advantage. Within the past decade, there has been a slow decline in the development of new classes of chemical agents for possible use against fever ticks. The reasons for this decline are varied, but among the most important reasons are the extremely high cost of developing new chemical agents, the complex regulations associated with ensuring environmental safety and the increased prevalence of pesticide resistance. The phenylpyrazole agent, fipronil, developed by Merial, Inc., has been registered for use on cats and dogs for its excellent activity against fleas (Blagburn et al., 1994; Jeannin et al., 1994; Postal et al., 1995). Preliminary evaluations suggested that the material might also have activity against B. microplus (Hunter et al., 1994) and, recently, detailed animal trials confirmed that fipronil provided excellent therapeutic and residual activity against B. microplus infesting cattle (Davey et al., 1998). The purpose of the long-term field study reported here was to determine FIELD EVALUATION OF FIPRONIL AGAINST B. ANNULATUS 353 whether it would be possible to eradicate a field population of B. annulatus through the repeated application of fipronil at various intervals. If eradication could be achieved with substantially fewer treatments, while cattle remained within the infested premise, the expense of gathering and handling of animals would be reduced and the productivity of the land would not be lost. Thus, the use of fipronil in such a treatment regime would provide a distinct advantage over the procedures presently used. MATERIALS AND METHODS Studies were conducted under natural field conditions at the United States Department of Agriculture (USDA), Agricultural Research Service (ARS), Cattle Fever Tick Research Laboratory (CFTRL), Mission, Texas. The study site is located near the southernmost area of Texas in the phytogeographical zone referred to as the Rio Grande Plains (Hatch et al., 1990). Climatically, the area is characterized by short, mild winters, a mean of 130 days with temperatures of 32.8°C and a mean annual rainfall of 550 mm, occurring primarily in May–June and September–October (Everitt and Alaniz, 1982; National Oceanic and Atmospheric Administration, 1983). The study was initiated on 10 April 1996 and ended on 29 May 1997 (60 consecutive weeks), thus encompassing an entire annual climatic cycle. The study was conducted on two separate and individually maintained pastures, each containing approximately 6.9 ha. The vegetation within both pastures was typical of the rangeland found in the region, consisting of approximately 65% open buffel grass, Cenchurus ciliarus L. and approximately 35% various woody species. The woody vegetation consisted primarily of honey mesquite, Prosopis glandulosa Torrey, although whitebrush, Aloysia gratissima (Gill and Hook) Troncoso, blackbrush acacia, Acacia rigidula Bentham, cenizo, Leucophyllum fautescens (Berlandia) and huisache, Acacia fernesiana (L.), were present to a lesser extent. Prior to the initiation of the study both pastures were infested with B. annulatus ticks. Field infestations were developed by placing tick-infested cattle in each of the pastures on several occasions and allowing the animals to range freely over the pastures as the ticks detached from the cattle under natural conditions. Animals were maintained on the pastures for a period of 2 years, during which the tick populations were allowed to develop and stabilize naturally. After this time period animals were removed from the pastures. At the initiation of the study 16 Hereford heifer calves that were approximately 6–9 months of age and weighed approximately 200 kg each were purchased from local livestock sale facilities in the Texas Hill Country (outside of the Boophilus quarantine zone) to ensure that they had no prior exposure to Boophilus spp. ticks. Each animal had a uniquely numbered ear tag placed in the left ear to facilitate easy identification. The animals were transported to the CFTRL where, after they were allowed to acclimatize to the area, they were randomly assigned to one of two individual groups, each containing eight animals. On 14 March 1996 animals were 354 R.B. DAVEY ET AL . placed in each of the pastures. One group of animals was released into one pasture which was designated as the untreated control pasture because no acaricidal treatments were applied to the animals during the study. The remaining group of animals was released into the second pasture which was designated as the treated pasture because animals were treated with acaricide at various intervals throughout the study. After release, animals (both pastures) were allowed to range freely for a period of 4 weeks ( 6 1 day), so that they could acclimatize to the pasture setting and pick up larval ticks that were present in the pastures. After 4 weeks ( 6 1 day) had elapsed (10 April 1996) and 2 weeks ( 6 1 day) prior to the initiation of acaricidal treatment (designated as week 2 2), animals from each pasture were gathered into penning facilities that were located within each individual pasture. To provide a basis for comparing the relative tick burdens present on treated and untreated cattle, the number of ticks on each animal within each pasture was counted by placing each individual animal in a squeeze chute and then carefully counting the total number of female ticks that were 5 mm in size on the entire left side of the animal’s body. The criteria of counting only ticks that were 5 mm was based on the fact that virtually all ticks of this size would detach within the following 24–48 h (Wharton and Utech, 1970), thus providing an estimate of the number of viable ticks that would likely return to the field to sustain or increase the population. Additional pretreatment tick counts were made at 5 (18 April 1996) and 6 (25 April 1996) weeks after the cattle were released in the pastures (weeks 2 1 and 0 prior to the initial acaricidal treatment, respectively). These pre-treatment tick counts provided a comparative estimate of the relative abundance of ticks in each pasture, as well as ensuring that all cattle (both pastures) were infested with all parasitic stages of B. annulatus ticks at the time acaricidal treatments began. The acaricide used in all treatments throughout the study was a 1% pour-on formulation of the phenylpyrazole agent, fipronil (RM1601H), which was applied to the treated cattle at a delivery rate of 1 ml per 10 kg body weight. On the day of the initial acaricidal treatment (25 April 1996, week 0), all animals (both pastures) were weighed individually to provide a basis for determining the amount of acaricide to be applied to the treated animals and to establish the weights at the beginning of the study period. Based on the animal weight, the appropriate volume of fipronil was measured into a graduated cylinder and then poured evenly along the mid-line of the back of each treated animal from the neck to the tail setting. This procedure was followed in all subsequent fipronil treatments applied to the treated animals. Following the initiation of fipronil applications to the treated animals, an assessment of the tick burden present on each animal (both pastures) was conducted by gathering the animals weekly (57 consecutive weeks), beginning at 1 week after the initial fipronil treatment. At each weekly gathering period the entire left side of each animal was carefully inspected and the total number of females ticks that were 5 mm were counted and recorded to provide a basis for comparison, as described FIELD EVALUATION OF FIPRONIL AGAINST B. ANNULATUS 355 above. This weekly tick assessment procedure provided a means of evaluating the effect of the fipronil treatments on the treated animals. The weekly tick counts also provided the basis for determining when subsequent fipronil treatments were applied to the treated animals. When adult ticks of 5 mm in size were found on any of the treated animals during a weekly tick count procedure and the time interval since the previous treatment was . 2 weeks, the animals were treated with fipronil at the following weekly gathering period, as described above. Use of this procedure as a model for timing additional fipronil applications to treated animals resulted in the application of six additional treatments during the study (total of seven treatments). The interval between the first (initial) and second treatments was 6 weeks, the interval between the fifth and sixth treatments was 13.5 weeks and the interval between the sixth and seventh (final) treatments was 7.5 weeks. The remaining four treatments were all spaced at 7 weeks ( 6 1 day) apart. Thus, applications of fipronil were applied to the treated cattle at weeks 0, 6, 13, 20, 27, 40.5 and 48 of the study. While the weekly tick counts provided a method of evaluating the effect of the fipronil treatments on the cattle held in the pastures throughout the study, they did not allow a means of assessing the presence and/or relative density of ticks in the field. Therefore, to facilitate evaluation of the field tick population, untreated sentinel cattle were placed in each of the pastures (untreated and treated) at regular intervals throughout the study. Each sentinel group of cattle consisted of two animals per pasture per interval, with a total of nine sentinel groups being placed in the field during the study period. The first group of sentinel animals was placed in the two pastures 4 weeks after the initial fipronil treatment was applied to treated cattle (week 0). Subsequently, a new group of sentinel animals was placed in each pasture at 6 week intervals throughout the remainder of the study. Each set of sentinel animals was placed in the pasture and allowed to range freely with the other cattle for a period of 2 weeks (14 days). Thus, using this procedure as a model, sentinel groups were placed in each pasture at 4–6, 10–12, 16–18, 22–24, 28–30, 34–36, 40–42, 46–48 and 52–54 weeks after the initial fipronil treatment was applied (week 0). After the 2 weeks (14 days) interval in the field, the animals within each sentinel group were removed from the pastures (before any detachment of female ticks had occurred) and placed individually in 3.3 3 3.3 m stalls separated by 1.6 m high cinder block walls inside an open-sided barn. The sentinel cattle were then held in stalls for 28 days, during which all engorged females that detached were collected daily and counted and recorded. To provide a means of evaluating whether the fipronil applications had any effect on weight gain production in the treated animals as compared to untreated animals, all cattle (both pastures) were weighed at regular intervals throughout the study. As previously stated, the weight of each animal was obtained immediately prior to the initial fipronil treatment (week 0). Subsequently, animal weights were obtained at 6 week intervals throughout the study, except the last weighing interval which was conducted at week 57 when the study was terminated (total of 11 weight 356 R.B. DAVEY ET AL . recordings). Thus, the weight of each animal was obtained at weeks 0, 6, 12, 18, 24, 30, 36, 42, 48, 54 and 57 during the study. The data obtained from animals in the untreated and treated pastures during each weekly tick count and for each time the animals were weighed was subjected to statistical analysis. If statistical analysis indicated that the data was normally distributed, an unpaired t-test (p 5 0.05) was conducted. However, if the data was not normally distributed (as was frequently the case), a Mann–Whitney rank sum test (p 5 0.05) was conducted (Jandel Scientific Software, 1994). The value obtained for each animal within each treatment group was used as the experimental unit for each of the measured parameters. RESULTS Since all of the pre-treatment tick counts (weeks 2 2, 2 1 and 0) were conducted before any fipronil treatments were initiated on 25 April 1996 (week 0), they provided important baseline information on tick population densities prior to the initiation of fipronil treatments. In the first pre-treatment count (week 2 2, 10 April 1996) there were significantly more ticks (t 5 2.64, df 5 14 and p , 0.02) on animals in the treated pasture (382 ticks per animal) than were observed on cattle in the untreated pasture (198 ticks per animal). In the second pre-treatment count, conducted at week 2 1 (18 April 1996), although there were fewer ticks on cattle in both pastures, cattle in the treated pasture still produced more ticks (61 ticks per animal) than cattle in the untreated pasture (43 ticks per animal), but there was no difference (t 5 83, df 5 8.8 and p 5 0.13) between the two groups. At the final pre-treatment count (week 0, 25 April 1996), conducted immediately prior to the first fipronil treatment, the number of females on animals in both pastures was again lower than the previous week, but cattle in the untreated pasture produced more females (51 ticks per animal) than cattle in the treated pasture (25 ticks per animal), although, again, the difference between the means was not statistically significant (t 5 54, df 5 8.8 and p 5 0.16). The results of the pre-treatment tick counts showed that all animals in both pastures were harbouring active infestations of B. annulatus and that tick numbers in each pasture were similar, if not higher in the treated pasture, at the time the fipronil treatments were initiated. The fipronil treatments had a dramatic adverse effect on the ability of female ticks to survive to repletion on the treated animals (Fig. 1). The results of each weekly tick count, beginning 1 week after the initial fipronil treatment and continuing through to week 57, showed that there were significantly more (p , 0.05) female ticks (5 mm) observed on the untreated cattle than on the treated group during every week except one (week 20) throughout the study. Although the number of countable female ticks (5 mm) observed on untreated animals varied from week to week, some females were observed at each of the 57 weekly tick count intervals. However, of the 57 weekly counts made during the study, following the initiation of fipronil treatments, no countable female ticks (5 mm) were FIELD EVALUATION OF FIPRONIL AGAINST B. ANNULATUS 357 observed on any of the treated animals during 47 weekly counts, whereas countable ticks were observed on the treated animals on only ten occasions during the study (weeks 1, 5, 6, 7, 13, 19, 20, 27, 40 and 57). In relation to the fipronil treatments that were applied to the treated animals, these weekly tick counts showed that treated animals were found harbouring female ticks that would likely have reached repletion and detached on three occasions after one fipronil treatment was applied (weeks 1, 5 and 6), on two occasions after two fipronil treatments were conducted (weeks 7 and 13), in two instances after three treatments were made (weeks 19 and 20), on one occasion after four fipronil treatments were applied (week 27), in one instance after five applications of fipronil were conducted (week 40) and on one occasion after seven treatments were made (week 57), at which time the study was terminated. The tick numbers obtained from each of the nine sentinel groups placed with untreated animals over time were assumed to be representative of a natural tick population where no acaricidal treatments had been applied. The tick population in the untreated pasture remained at a relatively high level throughout the study, as reflected by the large number of female ticks recovered from eight of the nine Fig. 1. Mean number of engorging ( ≥ 5 mm) female Boophilus annulatus counted on the left side of each animal (±SEM) at weekly intervals on untreated cattle and cattle treated with a 1% pour-on formulation of fipronil at various intervals and held for 57 wk under natural field conditions in infested pastures. R.B. DAVEY ET AL . 358 TABLE 1 Number of engorged Boophilus annulatus females per animal recovered from sentinel cattle placed in pastures with untreated animals and animals treated with 1% fipronil pour-on at various intervals Sentinel group number Weeks in which cattle were placed in pasture after initiation of the study No. of fipronil treatments applied to treated cattle when sentinel cattle were in pasture 1 2 3 4 5 6 7 8 9 4–6 10–12 16–18 22–24 28–30 34–36 40–42 46–48 52–54 1 2 3 4 5 5 5–6 6 7 Mean no. of // ticks recovered from sentinel cattle held with the indicated treatment group Untreated Treated 4,880 4,701 191 2,199* 4,000 18,957 5,473 7,568 2,104 6,893a 56 171 100 1 10 1 2 2 a Indicates that the number of ticks collected are represented by a single animal, because the second animal died before any female ticks detached. sentinel animal groups in the untreated pasture (Table 1). The single instance in which tick numbers were low (191 ticks per animal) occurred at 16–18 weeks after the study began (sentinel group 3). However, the time interval during which this group of sentinels was placed in the untreated pasture corresponded to the hottest period of the summer (31 July–14 August), when natural tick populations are generally at their lowest levels of the year (R.B. Davey, unpublished data). One other factor that should be noted with regard to sentinel animals in the untreated pasture is associated with sentinel group 4 (22–24 weeks after the study began). Although the number of ticks recovered from this group was somewhat lower (2199 ticks) than most of the other sentinel groups in the untreated pasture, the number is representative of a single animal because the second animal in this group died before any female detachment occurred. When the tick numbers obtained from each of the nine sentinel groups placed with treated cattle were compared with the corresponding sentinel groups placed with the untreated cattle, it provided a means of evaluating the effect the fipronil treatments had on the field tick population in the treated pasture (Table 1). In eight of the nine sentinel groups placed in the untreated and treated pastures over time, there was a dramatic difference between the number of ticks obtained from the untreated and treated groups in each case. Tick numbers obtained from the first sentinel animal group, placed in the treated pasture at 4–6 weeks following the initial fipronil treatment, were considerably higher (6893 ticks) than the tick numbers obtained from the corresponding sentinel group in the untreated pasture (4880 ticks per animal). Consequently, the application of a single fipronil treatment appeared to have little or no observable adverse effect on the tick population in the FIELD EVALUATION OF FIPRONIL AGAINST B. ANNULATUS 359 treated pasture. However, it should be noted that the number of ticks obtained from sentinel group 1 in the treated pasture was representative of a single animal because the second animal in the group died prior to any female detachment. In contrast, the tick numbers obtained from sentinel groups 2 and 4–9 placed in the treated pasture were dramatically lower than the tick numbers obtained from the corresponding sentinel groups placed in the untreated pasture in each case. The only instance in which tick numbers obtained from animals in the two pastures were not drastically different was obtained from sentinel group 3, placed in the pastures at 16–18 weeks, after three fipronil treatments. However, the results obtained in this group of sentinel animals was not surprising because, as has been stated previously, these animals were in pasture at a time when tick populations in the field would be expected to be at a low level, even in the absence of acaricidal pressure. The number of female ticks recovered from each of the nine sentinel groups placed in the treated pasture over time also provided an indication of the progressive effect each subsequent fipronil treatment had in reducing the tick population in the pasture, such that the population decline appeared to progress in three distinct phases. The first phase, which was represented by the tick numbers Fig. 2. Mean weight gain (±SEM) of untreated cattle and cattle treated at various intervals with a 1% pour-on formulation of fipronl and held for 57 wk under natural field conditions in pastures infested with Boophilus annulatus. 360 R.B. DAVEY ET AL . obtained from the first group of sentinel animals (4–6 weeks), showed that the field tick population remained at a high level following the application of the initial fipronil treatment (week 0), as previously noted. The second phase of the population decline in the treated pasture, which was represented by tick numbers obtained from sentinel groups 2–4 (10–12, 16–18 and 22–24 weeks, respectively), showed that the application of two, three or four fipronil treatments dramatically reduced the number of ticks in the field, but there was still an active albeit small tick population present in the treated pasture. The third phase of the population decline in the treated pasture, represented by the extremely low tick numbers obtained from sentinel groups 5–9, placed in the treated pasture at 28–30, 34–36, 40–42, 46–48 and 52–54 weeks respectively, after five to seven fipronil treatments had been applied, showed that the tick population in the field was extremely low. At the beginning of the study (week 0), prior to the application of any fipronil treatments, there was no significant difference (t 5 1.5, df 5 14 and p 5 0.15) between the mean weight of the untreated animals (198.2 kg) and that of the treated group (218.6 kg) (Fig. 2). Likewise, 6 weeks after the study was initiated and the first fipronil treatment had been applied, there was still no difference (t 5 1.07, df 5 14 and p 5 0.3) in the mean weights of the two animal groups (untreated group 5 226.8 kg and treated group 5 242.7 kg). However, beginning at 12 weeks after the study began and for every weighing interval thereafter (weeks 18, 24, 30, 36, 42, 48 and 54), the mean weight of the untreated group was significantly less (p , 0.05) than the weight of the treated animals which were treated two to seven times with fipronil. At the end of the study (week 57), after seven fipronil treatments had been applied to the treated cattle, the mean weight of the treated animals (468.6 kg) was significantly greater (t 5 3.4, df 5 14 and p 5 0.005) than that of the untreated animals (400.0 kg). Thus, from an overall perspective, the treated animals gained a mean of 48.2 kg per animal more than the calves in the untreated group during the study, which was a significant difference (t 5 3.0, df 5 14 and p 5 0.009) in the weight gain between the two animal groups. DISCUSSION This long-term field study demonstrated that repeated treatments of 1% fipronil pour-on, applied to cattle held in a pasture that was heavily infested with B. annulatus larvae, were highly effective. As the number of fipronil treatments increased over time, the number of instances (determined by weekly tick counts) in which engorging females (5 mm) were observed on treated animals decreased, indicating that the treatments had a progressively protective value against larval reinfestation while cattle were in the pasture. As a result of the protection afforded by the repeated fipronil treatments against larval reinfestation, the field tick population was severely depleted to a level that was near eradication by the end of the study. These results were consistent with another study conducted with 1% fipronil pour-on, in which it was reported that after a single treatment very few FIELD EVALUATION OF FIPRONIL AGAINST B. ANNULATUS 361 larvae of the closely related species B. microplus were able to reach repletion following treatment and none of the surviving females produced viable offspring (Davey et al., 1998). Another factor worth noting in this study is the difference in weight gains obtained between the treated and untreated cattle. It is true that differences in vegetative composition of the two pastures could have accounted in part for differences in weight gain of the two groups of animals. However, the fact that the vegetative composition of both pastures was very similar suggests that at least a portion of the increased weight gain observed in the treated cattle was a result of the reduced physiological stress that is often associated with a heavy tick infestation on the animal. Other studies have shown similar results with regard to increased weight gains obtained from treated animals (Remington et al., 1997), thus the significant weight gains obtained from treated cattle in this study were not surprising. In areas of the world such as Mexico, Central and South America, Africa and Australia, where the objectives of acaricidal treatment are aimed at reducing the Boophilus tick population to a low level (control programmes) while maximizing weight gain production and protecting animal health, the use of 1% fipronil pour-on in a treatment regime like the one used in this study would provide excellent results. Even in tropical areas where reproduction occurs throughout the year, it seems likely that tick numbers could be reduced to extremely low levels by applying as few as seven fipronil treatments during the course of a year. Indeed, depending on the tick density that was deemed acceptable, it might be possible to achieve suitable population levels with even fewer than seven treatments. Consequently, a treatment scenario like the one used in this study would reduce the tick burden on treated animals, as well as decreasing the tick population in the field to a low level. Thus, since the treatments would result in significantly higher weight gain in treated cattle, it seems likely that the increased revenue obtained when cattle were marketed would largely if not completely offset the costs associated with the repeated fipronil treatments. Within the context of using fipronil pour-on in areas where control of the population is the main objective, it seems worthy of noting that similar results to those obtained in this study could be expected in areas where treatments are applied to tick populations that are organophosphorus (OP) or pyrethroid (P) resistant. Although the tick strain used in this study was not resistant to any acaricide, a prior study using the same formulation of fipronil reported that there was no altered susceptibility observed when fipronil was tested against OP- and P-resistant B. microplus (Davey et al., 1998). Thus, in areas such as Mexico, where both OP and P resistance has been reported (Aguirre et al., 1986; Bull and Ahrens 1988; Harris et al., 1988; Fragoso et al., 1995), there is no reason to assume that these resistant tick populations could not be reduced to extremely low levels under a treatment scenario like the one used in this study. Although the results obtained in this study strongly indicate that the repeated application of fipronil was highly effective in controlling B. annulatus in infested 362 R.B. DAVEY ET AL . pastures, the fipronil treatments did not completely eradicate the field tick population. However, the results indicated that the most probable explanation for the inability to achieve complete eradication in the field was not due to a lack of efficacy, but to incorrectly timed intervals between fipronil applications. Each retreatment of cattle with fipronil was conducted 1 week following the observance of 5 mm females on the animals. This treatment interval was based on previous findings (conducted in barn trials) that indicated that fipronil provided 6–8 weeks of complete residual control (100%) under controlled conditions (Davey et al., 1998). However, as this study progressed, it became increasingly evident that the residual activity of fipronil under natural field conditions was less than 6–8 weeks. Consequently, it is almost a certainty that the treatment intervals followed in this study allowed some females to reach repletion and detach back into the pasture, thus allowing the field population to be sustained at a low level throughout the study. The results did show that engorging females (5 mm) were present on the treated animals at an interval of 5 weeks on only one occasion (at 5 weeks after the initial treatment) during the entire study. Therefore, it seems reasonable to speculate that if all fipronil treatments had been fixed at 5 week treatment intervals throughout the study, it may have been possible to eradicate the field tick population by applying the same seven treatments that were used in the study. Based on the results of this study, the use of a fixed 5 week treatment interval would likely have prevented any reproductively capable females from being returned to the pasture to sustain the tick population. In a 5 week treatment interval scenario, the number of treatments during the standard 6–9 month quarantine period would be seven to ten treatments, instead of the presently required 14–21 treatments. Thus, the cost associated with gathering and handling livestock would be reduced by at least 50% and the productivity of the land would not be lost by having to vacate livestock from the premises. From the perspective of the eradication programme, if such a treatment regime (5 week treatment intervals) were successful, it would still provide a distinct advantage over the present requirements of treating animals at 2 week intervals. Furthermore, this 5 week treatment regime would also be better than that reported in another field study, in which it was stated that B. microplus could be eliminated using moxidectin pour-on by applying treatments at 3 week intervals (Remington et al., 1997). Additional research will be necessary to confirm the validity of this 5 week treatment regime. However, since the distinct possibility exists that eradication could be achieved, the additional research seems warranted. ACKNOWLEDGEMENTS The authors gratefully acknowledge the technical assistance of Horacio Bazan, Adolfo Pena, Homero Vasquez and Michael Moses, without whose diligence in making tick counts and the weighing and handling of animals the study would not have been possible. FIELD EVALUATION OF FIPRONIL AGAINST B. ANNULATUS 363 Note This paper reports on the results of research only. Mention of a commercial or a proprietary product in this paper does not constitute an endorsement by the US Department of Agriculture. REFERENCES Aguirre, J., Sobrino, A., Santamaria, M., Aburto, A., Roman, S., Hernandez, M. and Ortiz, Y.A. 1986. Resistencia de garrapatas en Mexico. In Seminario Internacional de Parasitologia Animal, H. Cavazzani and Z. Garcia (eds), pp. 282–306. Cuernavaca, Mor. Mexico. Blagburn, B.L., Hendrix, C.M., Vaughan, L.L., Lindsay, D.S. and Hunter, J.S. III. 1994. Efficacy of a topical formulation of fipronil against Ctenocephalides felis felis in experimentally infested dogs. 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