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The effect of simulated rainfall on the efficacy of doramectin pour-on against nematode parasites of cattle
Veterinary Parasitology 86 (1999) 229–234
The effect of simulated rainfall on the efficacy of doramectin pour-on against nematode parasites of cattle T.L. Skogerboe a,∗ , V.C. Cracknell b , D.J. Walstrom a , L. Ritzhaupt c , V.K. Karle a a
Pfizer Animal Health Group, One Pfizer Way, Lee’s Summit MO 64081-2998, USA b Pfizer Ltd, Ramsgate Road, Sandwich, Kent CT13 9NJ, UK c Pfizer Animal Health Group, 235 East 42nd Street, New York, NY 10017, USA Received 24 March 1999; accepted 27 May 1999
Abstract Two studies were conducted with doramectin topically administered at 500 g/kg body weight to assess retention of therapeutic efficacy against nematode infections of cattle before, and after, simulated rainfall. In the first study, 50 heifers, with patent nematode infections, were allocated to one of five treatment groups. An untreated control group and one doramectin-treated group were not exposed to simulated rainfall. Simulated rainfall was applied at a rate of 25.4 mm of water in 35 min to three of the five groups: one group immediately before treatment, the second group 90 min after treatment, and the third group 24 h after treatment. Fecal samples were collected for determining egg counts 14 days after treatment. Percentage efficacy ranged from 97.3% to 100% in all treated calves, regardless of exposure to simulated rainfall. The second study involved 40 mixed-sex cattle that were allocated to one of four treatment groups (one saline control and three doramectin-treated groups). All cattle were confirmed to be free of nematode infections prior to oral gavage with infective larvae of Dictyocaulus viviparus, Cooperia oncophora, and Ostertagia ostertagi. Twenty-six days after infection, three groups were treated with doramectin pour-on and exposed to 20 mm of simulated rainfall over 40 min: one group 60 min before treatment, the second 20 min after treatment, and the third 40 min after treatment. Approximately two weeks after treatment, all cattle were necropsied for worm counts. In all treated groups, the percentage efficacy against O. ostertagi and D. viviparus was >99% to 100%. Percentage efficacy against Cooperia ranged from 97% to 98%. Results indicated that doramectin pour-on remains efficacious against nematodes of cattle when administered immediately before or after rainfall. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Doramectin; Cattle: Nematoda; Simulated rainfall; Therapeutic efficacy
∗ Corresponding author. Tel.: +1-816-246-2281; fax: +1-816-246-2402 E-mail address: [email protected] (T.L. Skogerboe)
0304-4017/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 0 1 7 ( 9 9 ) 0 0 1 4 6 - 6
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T.L. Skogerboe et al. / Veterinary Parasitology 86 (1999) 229–234
1. Introduction Doramectin is a macrocyclic lactone endectocide. Topical application of the drug in a pour-on formulation (DectomaxTM Pour-On, Pfizer) is highly effective against nematode parasites of cattle (Marley et al., 1999). This formulation can be used in treatment programs that are designed for the control of nematode infections in grazing cattle. For example, Vercruysse et al. (1998) have demonstrated that a two-dose program of doramectin pour-on (one treatment at spring turnout and another eight weeks later) is effective in the seasonal control of nematodiasis. Although it is not ideal to treat unsheltered cattle with topical parasiticides in rainy conditions, sometimes this practice cannot be avoided without disruption of farm management schedules. Also, treated cattle may subsequently be exposed to rainfall within a few hours of their return to pasture. In order to investigate the potential effect of such exposure, the nematocidal efficacy of doramectin pour-on has been evaluated under laboratory conditions in cattle exposed to artificial rain. Two different methods have been used to evaluate pour-on formulations of other macrocyclic lactone endectocides in this context. In one, described by Guerino et al. (1994), efficacy was assessed by egg count reduction in groups of naturally-infected cattle that were either exposed or unexposed to periods of simulated rainfall. In the other, described by Rolfe et al. (1997), efficacy was assessed by the comparison of worm counts in experimentally infected cattle that were either untreated or treated and exposed to periods of simulated rainfall. Doramectin was evaluated in two studies; each using one or other of these methods and the results reported in this paper. 2. Materials and methods 2.1. Animals — egg count reduction study The cattle used in the egg count reduction study were selected from a pool of 70 mixed-beef breed heifer calves between 5 and 12 months of age. 2.2. Experimental design, treatments, and simulated rainfall — egg count reduction study Fifty test cattle selected on the basis of fecal nematode egg counts and uniformity of body weight were randomly, and equally, allocated to five treatment groups. Groups of cattle were assigned to pens such that no contact was possible between cattle of different treatment groups. On Day 0, cattle in four of the groups received doramectin pour-on topically from the withers to the tailhead at a dose rate of 500 g/kg body weight. Cattle in the fifth group remained untreated though they were subjected to the same handling procedures as the doramectin-treated cattle. The untreated control group and one doramectin-treated group were not exposed to simulated rainfall. Cattle in three of the doramectin-treated groups were exposed to simulated rainfall, one group immediately before treatment, a second group beginning at 90 min after treatment, and a third group 24 h after treatment. Cattle were exposed to rainfall in their respective groups in a specially designed pen in which water was delivered from an overhead spray system. The system was designed and calibrated to
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uniformly deliver the equivalent of one inch (25.4 mm) of rain over the area of the pen in 35 min. After rainfall exposure, cattle were returned to their respective group pens and held for the duration of the 14-day study. 2.3. Parasitological procedures — egg count reduction study Fecal samples for counts of trichostrongylid-type eggs per gram of feces (EPG) were collected from each calf on the day of treatment and 14 days later. Egg counts per gram of feces were performed using a sugar flotation/centrifugation technique (Cox and Todd, 1962). 2.4. Statistical analysis — egg count reduction study Estimated EPG were transformed to the natural log (count + 1) and analyzed by a mixed model procedure and contrasts made among the treatment group means (SAS, 1998). The least-squares means were back-transformed to geometric mean EPG. Efficacy was assessed by the egg count reduction from Day 0 to Day 14. Percentage reduction in egg count was calculated for each calf. This variable was analyzed using a mixed model procedure that estimated the least-squares means for each treatment group and contrasts were made among the treatment group means. The significance level for treatment comparisons was set at p ≤ 0.05. 2.5. Animals — necropsy study In the necropsy study, the cattle used were parasite-na¨ıve, mixed sex Friesian or Limousincross calves between 6 and 12 months of age. 2.6. Experimental design, treatments, and simulated rainfall — necropsy study Forty-five calves were experimentally infected on study Day 0 by oral gavage with infective third-stage larvae of Ostertagia ostertagi (20 000 larvae per calf), Cooperia oncophora (18 000 larvae per calf), and Dictyocaulus viviparus (1000 larvae per calf). The presence of patent gastrointestinal nematode infections was confirmed by using the modified McMaster technique on fecal samples collected 24 days later. From this group, the 40 cattle with the highest egg counts were selected, blocked by weight, and equally allocated to four treatment groups. Groups were assigned to pens such that no contact was possible between cattle of different treatment groups. Two days later (Day 26), calves in three of the groups received doramectin pour-on at a dose rate of 500 g/kg by topical application along the dorsal midline. Cattle in the fourth group were treated in a similar manner with saline solution. Cattle in the doramectin-treated groups were exposed to simulated rainfall, one group beginning at 60 min before treatment, a second group 20 min after treatment, and a third group 40 min after treatment. Simulated rainfall exposure was by treatment group in a special pen equipped with an overhead sprinkler system that was calibrated to deliver the equivalent of 20 mm of rain over the pen area in 40 min.
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Table 1 Percentage trichostrongylid-type egg count reduction in untreated cattle and in doramectin-treated cattle exposed and unexposed to simulated rainfall Treatment
Exposure to rainfall
Untreated None Doramectin Pour-On None Doramectin Pour-On Immediately before treatment Doramectin Pour-On 90 min after treatment Doramectin Pour-On 24 h after treatment
Number Pre-treatment (Day 0) Mean percentage reduction∗ of cattle mean egg counts in egg count (Day 0 to Day 14) arithmetic geometric 10 10 10
658 696 582
499.7a 548.2a 377.1a
10.2a 100.0b 99.4b
10 10
423 791
305.5a 597.0a
97.3b 99.3b
Within a column, means not sharing a common superscript letter are significantly different (p ≤ 0.05). Percentage reduction calculated for each calf using the formula 100 × (Ti Day 0 − Ti Day 14)/Ti Day 0, where Ti Day 0 is the EPG for calf i on Day 0 and Ti Day 14 the EPG for calf i on Day 14. The least-squares means of the treatment group percentage reductions are presented. a,b ∗
2.7. Parasitological procedures — necropsy study Calves were humanely slaughtered for worm counts in equal numbers from each group on Day 40 or Day 41 (14 or 15 days post-treatment). Parasites were recovered from the abomasum, abomasal digests, small intestine, and lungs by standard techniques (Wood et al., 1995). 2.8. Statistical analysis — necropsy study Efficacy was assessed by comparing worm burdens in doramectin-treated cattle to those in controls. Estimated worm burdens were transformed to the natural log (count + 1) and analyzed by a mixed model procedure and contrasts were made among the treatment group means (SAS, 1998). The least-squares means were back-transformed to geometric mean worm burdens and used to estimate efficacy. The significance level for treatment comparisons was set at p ≤ 0.05.
3. Results 3.1. Fecal egg counts — egg count reduction study Results of the first study are presented in Table 1. Pre-treatment group geometric mean trichostrongylid-type egg counts ranged from 305.5 EPG to 597 EPG, with no significant (p > 0.05) differences among groups. Egg counts were reduced in doramectin-treated groups by 97.3% to 100%, relative to pre-treatment egg counts, with no significant (p > 0.05) differences in reduction among them. In contrast, the mean egg count in the untreated control group exhibited only a slight reduction (10.2%) that was significantly (p ≤ 0.05) less than the reduction in any of the treated groups.
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Table 2 Therapeutic efficacy of doramectin pour-on against nematodes in cattle exposed to simulated rainfall — geometric mean worm counts and percentage efficacies Parasite species
Saline controls geometric mean (range) n = 10
Exposure to simulated rainfall 60 min before treatment geometric mean (range) n = 10
20 min after treatment geometric mean (range) n = 10
40 min after treatment geometric mean (range) n = 10
Ostertagia ostertagi Efficacy∗
2352a (130–8040)
1b (0–160) 99.9%
1b (0–270) 99.9%
0b 100%
Cooperia oncophora Efficacy∗
6976a (4740–10360)
197b (0–4040) 97.2%
133b (0–2060) 98.1%
147b (0–960) 97.9%
Dictyocaulus viviparus Efficacy∗
15a (0–145)
0b 100%
0b 100%
0b 100%
Within a row, means not sharing a common superscript letter are significantly different (p ≤ 0.001). Efficacy calculated using the formula: 100 × (C − T)/C, where C is the geometric mean of the nematode burden from the saline-treated cattle and T the geometric mean of the nematode burden from the doramectin pour-on treated cattle. a,b ∗
3.2. Worm counts — necropsy study Worm counts and percentage efficacies from the second study are shown in Table 2. Geometric mean worm burdens in controls were 2352 for O. ostertagi, 6976 for C. oncophora, and 15 for D. viviparus. In treated cattle, geometric mean O. ostertagi burdens were ≤1 and D. viviparus burdens were zero for all three groups. Cooperia oncophora burdens ranged from 133 to 197. Statistical analysis showed that burdens of the respective species in treated groups were in all cases significantly (p ≤ 0.001) different from those in controls with no significant (p > 0.05) differences among treated groups. Efficacies were ≥99.9% for O. ostertagi, 97.2% to 98.1% for C. oncophora, and 100% for D. viviparus.
4. Discussion The egg count reduction study demonstrated that the efficacy of doramectin pour-on against natural infections of gastrointestinal nematodes was not markedly affected by exposure of the treated cattle to the equivalent of a very heavy shower of rain immediately before treatment, at 90 min after treatment, or at 24 h after treatment. Efficacy ranged from 97.3% to 99.4% for rain-exposed cattle compared to an efficacy of 100% for treated cattle not exposed to rain. Guerino et al. (1994) obtained similar results with moxidectin pour-on in the same test model. Efficacy at 21 days after treatment ranged from 94.5% to 99.4% in treated cattle exposed to rain either before treatment or up to 24 h after treatment compared to an efficacy of 99.8% in moxidectin-treated cattle that were not exposed to rain. Results of the worm count study showed that when treated cattle were exposed to rainfall, doramectin pour-on was ≥99.9% effective against O. ostertagi and D. viviparus infections.
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Efficacy against C. oncophora was 97.2% in cattle exposed to the equivalent of a heavy shower of rain before treatment, 98.1% in cattle exposed 20 min after treatment, and 97.9% in cattle exposed 40 min after treatment. In a comparable study reported by Rolfe et al. (1997), ivermectin pour-on was >99% efficacious against induced infections of O. ostertagi and C. oncophora in cattle exposed to rain 10 or 60 min after treatment. In cattle C. oncophora is recognized as the dose-limiting species for the macrocyclic lactone endectocides (Egerton et al., 1981; Goudie et al., 1993). The World Association for the Advancement of Veterinary Parasitology (WAAVP) guidelines (Wood et al., 1995) for standards of anthelmintic efficacy, rates a product as highly effective with an efficacy of >98%, and effective when efficacy is 90–98%. The efficacy of doramectin pour-on formulation met these criteria for three of the most common nematode species of cattle, including the dose-limiting species, when cattle were exposed to rainfall conditions similar to those routinely encountered in commercial cattle operations.
Acknowledgements The authors thank Mervyn Jones, the personnel at Agriculture Training and Technology, Amarillo, Inc., TX, and Moredun Animal Health Ltd, Edinburgh, Scotland, for the significant roles they played in the conduct, reporting, and interpretation of these studies. References Cox, D.D., Todd, A.C., 1962. Survey of gastrointestinal parasitism in Wisconsin dairy cattle. J. Am. Vet. Med. Assoc. 141, 706–709. Egerton, J.R., Eary, C.H., Suhayda, D., 1981. The anthelmintic efficacy of ivermectin in experimentally infected cattle. Vet. Parasitol. 8, 59–70. Guerino, F., Clymer, B.C., Janes, T., 1994. Evaluation of the effect of simulated rainfall on the efficacy of moxidectin pour-on against fecal output of nematode eggs in cattle Proceedings of Australian Society for Parasitology Abstract No. 31. Goudie, A.C., Evans,. N.A., Gration, K.A.F., Bishop, B.F., Gibson, S.P., Holdom, K.S., Kaye, B., Wicks, S.R., Lewis, D., Weatherley, A.J., Bruce, C.I., Herbert, A., Seymour, D.J., 1993. Doramectin — a potent novel endectocide. In: Vercruyssse, J. (Ed.), Doramectin — A Novel Avermectin, Vet. Parasitol. 49, 5–15. Marley, S.E., Illyes, E.F., Keller, D.S., Meinert, T.R., Logan, N.B., Hendrickx, M.O., Conder, G.A., 1999. Efficacy of topically administered doramectin against eyeworms, lungworms, and gastrointestinal nematodes of cattle. Am. J. of Vet. Res 60, 665–668. Rolfe, P.F., Dawson, K.L., Nichols, G.N., Webster, M., Ryan, W.G., 1997. Efficacy of topical ivermectin following exposure of treated cattle to rain. Vet. Rec. 141, 269–270. SAS, 1998. SAS/STAT® Users Guide: Version 6.12. SAS Inst. Inc., Cary, NC. Vercruysse, J., Dorny, P., Claerebout, E., Weatherley, A.J., 1998. Field evaluation of a topical doramectin formulation for the chemoprophylaxis of parasitic bronchitis in calves. Vet. Parasitol. 75, 169–179. Wood, I.B., Amaral, N.K., Bairden, K., Duncan, J.L., Kassai, T., Malone, J.B., Pankavich, J.A., Reinecke, R.K., Slocombe, O., Taylor, S.M., Vercruysse, J., 1995. World Association for the Advancement of Veterinary Parasitology (WAAVP) second edition of guidelines for evaluating the efficacy of anthelmintics in ruminants (bovine, ovine, caprine). Vet. Parasitol. 58, 181–213.
The effect of simulated rainfall on the efficacy of doramectin pour-on against nematode parasites of cattle T.L. Skogerboe a,∗ , V.C. Cracknell b , D.J. Walstrom a , L. Ritzhaupt c , V.K. Karle a a
Pfizer Animal Health Group, One Pfizer Way, Lee’s Summit MO 64081-2998, USA b Pfizer Ltd, Ramsgate Road, Sandwich, Kent CT13 9NJ, UK c Pfizer Animal Health Group, 235 East 42nd Street, New York, NY 10017, USA Received 24 March 1999; accepted 27 May 1999
Abstract Two studies were conducted with doramectin topically administered at 500 g/kg body weight to assess retention of therapeutic efficacy against nematode infections of cattle before, and after, simulated rainfall. In the first study, 50 heifers, with patent nematode infections, were allocated to one of five treatment groups. An untreated control group and one doramectin-treated group were not exposed to simulated rainfall. Simulated rainfall was applied at a rate of 25.4 mm of water in 35 min to three of the five groups: one group immediately before treatment, the second group 90 min after treatment, and the third group 24 h after treatment. Fecal samples were collected for determining egg counts 14 days after treatment. Percentage efficacy ranged from 97.3% to 100% in all treated calves, regardless of exposure to simulated rainfall. The second study involved 40 mixed-sex cattle that were allocated to one of four treatment groups (one saline control and three doramectin-treated groups). All cattle were confirmed to be free of nematode infections prior to oral gavage with infective larvae of Dictyocaulus viviparus, Cooperia oncophora, and Ostertagia ostertagi. Twenty-six days after infection, three groups were treated with doramectin pour-on and exposed to 20 mm of simulated rainfall over 40 min: one group 60 min before treatment, the second 20 min after treatment, and the third 40 min after treatment. Approximately two weeks after treatment, all cattle were necropsied for worm counts. In all treated groups, the percentage efficacy against O. ostertagi and D. viviparus was >99% to 100%. Percentage efficacy against Cooperia ranged from 97% to 98%. Results indicated that doramectin pour-on remains efficacious against nematodes of cattle when administered immediately before or after rainfall. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Doramectin; Cattle: Nematoda; Simulated rainfall; Therapeutic efficacy
∗ Corresponding author. Tel.: +1-816-246-2281; fax: +1-816-246-2402 E-mail address: [email protected] (T.L. Skogerboe)
0304-4017/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 0 1 7 ( 9 9 ) 0 0 1 4 6 - 6
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T.L. Skogerboe et al. / Veterinary Parasitology 86 (1999) 229–234
1. Introduction Doramectin is a macrocyclic lactone endectocide. Topical application of the drug in a pour-on formulation (DectomaxTM Pour-On, Pfizer) is highly effective against nematode parasites of cattle (Marley et al., 1999). This formulation can be used in treatment programs that are designed for the control of nematode infections in grazing cattle. For example, Vercruysse et al. (1998) have demonstrated that a two-dose program of doramectin pour-on (one treatment at spring turnout and another eight weeks later) is effective in the seasonal control of nematodiasis. Although it is not ideal to treat unsheltered cattle with topical parasiticides in rainy conditions, sometimes this practice cannot be avoided without disruption of farm management schedules. Also, treated cattle may subsequently be exposed to rainfall within a few hours of their return to pasture. In order to investigate the potential effect of such exposure, the nematocidal efficacy of doramectin pour-on has been evaluated under laboratory conditions in cattle exposed to artificial rain. Two different methods have been used to evaluate pour-on formulations of other macrocyclic lactone endectocides in this context. In one, described by Guerino et al. (1994), efficacy was assessed by egg count reduction in groups of naturally-infected cattle that were either exposed or unexposed to periods of simulated rainfall. In the other, described by Rolfe et al. (1997), efficacy was assessed by the comparison of worm counts in experimentally infected cattle that were either untreated or treated and exposed to periods of simulated rainfall. Doramectin was evaluated in two studies; each using one or other of these methods and the results reported in this paper. 2. Materials and methods 2.1. Animals — egg count reduction study The cattle used in the egg count reduction study were selected from a pool of 70 mixed-beef breed heifer calves between 5 and 12 months of age. 2.2. Experimental design, treatments, and simulated rainfall — egg count reduction study Fifty test cattle selected on the basis of fecal nematode egg counts and uniformity of body weight were randomly, and equally, allocated to five treatment groups. Groups of cattle were assigned to pens such that no contact was possible between cattle of different treatment groups. On Day 0, cattle in four of the groups received doramectin pour-on topically from the withers to the tailhead at a dose rate of 500 g/kg body weight. Cattle in the fifth group remained untreated though they were subjected to the same handling procedures as the doramectin-treated cattle. The untreated control group and one doramectin-treated group were not exposed to simulated rainfall. Cattle in three of the doramectin-treated groups were exposed to simulated rainfall, one group immediately before treatment, a second group beginning at 90 min after treatment, and a third group 24 h after treatment. Cattle were exposed to rainfall in their respective groups in a specially designed pen in which water was delivered from an overhead spray system. The system was designed and calibrated to
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uniformly deliver the equivalent of one inch (25.4 mm) of rain over the area of the pen in 35 min. After rainfall exposure, cattle were returned to their respective group pens and held for the duration of the 14-day study. 2.3. Parasitological procedures — egg count reduction study Fecal samples for counts of trichostrongylid-type eggs per gram of feces (EPG) were collected from each calf on the day of treatment and 14 days later. Egg counts per gram of feces were performed using a sugar flotation/centrifugation technique (Cox and Todd, 1962). 2.4. Statistical analysis — egg count reduction study Estimated EPG were transformed to the natural log (count + 1) and analyzed by a mixed model procedure and contrasts made among the treatment group means (SAS, 1998). The least-squares means were back-transformed to geometric mean EPG. Efficacy was assessed by the egg count reduction from Day 0 to Day 14. Percentage reduction in egg count was calculated for each calf. This variable was analyzed using a mixed model procedure that estimated the least-squares means for each treatment group and contrasts were made among the treatment group means. The significance level for treatment comparisons was set at p ≤ 0.05. 2.5. Animals — necropsy study In the necropsy study, the cattle used were parasite-na¨ıve, mixed sex Friesian or Limousincross calves between 6 and 12 months of age. 2.6. Experimental design, treatments, and simulated rainfall — necropsy study Forty-five calves were experimentally infected on study Day 0 by oral gavage with infective third-stage larvae of Ostertagia ostertagi (20 000 larvae per calf), Cooperia oncophora (18 000 larvae per calf), and Dictyocaulus viviparus (1000 larvae per calf). The presence of patent gastrointestinal nematode infections was confirmed by using the modified McMaster technique on fecal samples collected 24 days later. From this group, the 40 cattle with the highest egg counts were selected, blocked by weight, and equally allocated to four treatment groups. Groups were assigned to pens such that no contact was possible between cattle of different treatment groups. Two days later (Day 26), calves in three of the groups received doramectin pour-on at a dose rate of 500 g/kg by topical application along the dorsal midline. Cattle in the fourth group were treated in a similar manner with saline solution. Cattle in the doramectin-treated groups were exposed to simulated rainfall, one group beginning at 60 min before treatment, a second group 20 min after treatment, and a third group 40 min after treatment. Simulated rainfall exposure was by treatment group in a special pen equipped with an overhead sprinkler system that was calibrated to deliver the equivalent of 20 mm of rain over the pen area in 40 min.
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Table 1 Percentage trichostrongylid-type egg count reduction in untreated cattle and in doramectin-treated cattle exposed and unexposed to simulated rainfall Treatment
Exposure to rainfall
Untreated None Doramectin Pour-On None Doramectin Pour-On Immediately before treatment Doramectin Pour-On 90 min after treatment Doramectin Pour-On 24 h after treatment
Number Pre-treatment (Day 0) Mean percentage reduction∗ of cattle mean egg counts in egg count (Day 0 to Day 14) arithmetic geometric 10 10 10
658 696 582
499.7a 548.2a 377.1a
10.2a 100.0b 99.4b
10 10
423 791
305.5a 597.0a
97.3b 99.3b
Within a column, means not sharing a common superscript letter are significantly different (p ≤ 0.05). Percentage reduction calculated for each calf using the formula 100 × (Ti Day 0 − Ti Day 14)/Ti Day 0, where Ti Day 0 is the EPG for calf i on Day 0 and Ti Day 14 the EPG for calf i on Day 14. The least-squares means of the treatment group percentage reductions are presented. a,b ∗
2.7. Parasitological procedures — necropsy study Calves were humanely slaughtered for worm counts in equal numbers from each group on Day 40 or Day 41 (14 or 15 days post-treatment). Parasites were recovered from the abomasum, abomasal digests, small intestine, and lungs by standard techniques (Wood et al., 1995). 2.8. Statistical analysis — necropsy study Efficacy was assessed by comparing worm burdens in doramectin-treated cattle to those in controls. Estimated worm burdens were transformed to the natural log (count + 1) and analyzed by a mixed model procedure and contrasts were made among the treatment group means (SAS, 1998). The least-squares means were back-transformed to geometric mean worm burdens and used to estimate efficacy. The significance level for treatment comparisons was set at p ≤ 0.05.
3. Results 3.1. Fecal egg counts — egg count reduction study Results of the first study are presented in Table 1. Pre-treatment group geometric mean trichostrongylid-type egg counts ranged from 305.5 EPG to 597 EPG, with no significant (p > 0.05) differences among groups. Egg counts were reduced in doramectin-treated groups by 97.3% to 100%, relative to pre-treatment egg counts, with no significant (p > 0.05) differences in reduction among them. In contrast, the mean egg count in the untreated control group exhibited only a slight reduction (10.2%) that was significantly (p ≤ 0.05) less than the reduction in any of the treated groups.
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Table 2 Therapeutic efficacy of doramectin pour-on against nematodes in cattle exposed to simulated rainfall — geometric mean worm counts and percentage efficacies Parasite species
Saline controls geometric mean (range) n = 10
Exposure to simulated rainfall 60 min before treatment geometric mean (range) n = 10
20 min after treatment geometric mean (range) n = 10
40 min after treatment geometric mean (range) n = 10
Ostertagia ostertagi Efficacy∗
2352a (130–8040)
1b (0–160) 99.9%
1b (0–270) 99.9%
0b 100%
Cooperia oncophora Efficacy∗
6976a (4740–10360)
197b (0–4040) 97.2%
133b (0–2060) 98.1%
147b (0–960) 97.9%
Dictyocaulus viviparus Efficacy∗
15a (0–145)
0b 100%
0b 100%
0b 100%
Within a row, means not sharing a common superscript letter are significantly different (p ≤ 0.001). Efficacy calculated using the formula: 100 × (C − T)/C, where C is the geometric mean of the nematode burden from the saline-treated cattle and T the geometric mean of the nematode burden from the doramectin pour-on treated cattle. a,b ∗
3.2. Worm counts — necropsy study Worm counts and percentage efficacies from the second study are shown in Table 2. Geometric mean worm burdens in controls were 2352 for O. ostertagi, 6976 for C. oncophora, and 15 for D. viviparus. In treated cattle, geometric mean O. ostertagi burdens were ≤1 and D. viviparus burdens were zero for all three groups. Cooperia oncophora burdens ranged from 133 to 197. Statistical analysis showed that burdens of the respective species in treated groups were in all cases significantly (p ≤ 0.001) different from those in controls with no significant (p > 0.05) differences among treated groups. Efficacies were ≥99.9% for O. ostertagi, 97.2% to 98.1% for C. oncophora, and 100% for D. viviparus.
4. Discussion The egg count reduction study demonstrated that the efficacy of doramectin pour-on against natural infections of gastrointestinal nematodes was not markedly affected by exposure of the treated cattle to the equivalent of a very heavy shower of rain immediately before treatment, at 90 min after treatment, or at 24 h after treatment. Efficacy ranged from 97.3% to 99.4% for rain-exposed cattle compared to an efficacy of 100% for treated cattle not exposed to rain. Guerino et al. (1994) obtained similar results with moxidectin pour-on in the same test model. Efficacy at 21 days after treatment ranged from 94.5% to 99.4% in treated cattle exposed to rain either before treatment or up to 24 h after treatment compared to an efficacy of 99.8% in moxidectin-treated cattle that were not exposed to rain. Results of the worm count study showed that when treated cattle were exposed to rainfall, doramectin pour-on was ≥99.9% effective against O. ostertagi and D. viviparus infections.
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Efficacy against C. oncophora was 97.2% in cattle exposed to the equivalent of a heavy shower of rain before treatment, 98.1% in cattle exposed 20 min after treatment, and 97.9% in cattle exposed 40 min after treatment. In a comparable study reported by Rolfe et al. (1997), ivermectin pour-on was >99% efficacious against induced infections of O. ostertagi and C. oncophora in cattle exposed to rain 10 or 60 min after treatment. In cattle C. oncophora is recognized as the dose-limiting species for the macrocyclic lactone endectocides (Egerton et al., 1981; Goudie et al., 1993). The World Association for the Advancement of Veterinary Parasitology (WAAVP) guidelines (Wood et al., 1995) for standards of anthelmintic efficacy, rates a product as highly effective with an efficacy of >98%, and effective when efficacy is 90–98%. The efficacy of doramectin pour-on formulation met these criteria for three of the most common nematode species of cattle, including the dose-limiting species, when cattle were exposed to rainfall conditions similar to those routinely encountered in commercial cattle operations.
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