Efficacy of Moxidectin 6-Month Injectable and Milbemycin Oxime/Lufenuron Tablets Against Naturally Acquired Toxocara canis Infections in Dogs*

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1 Efficacy of Moxidectin 6-Month Injectable and Milbemycin Oxime/Lufenuron Tablets Against Naturally Acquired Toxocara canis Infections in Dogs* Dwight D. Bowman, MS, PhD a Walter Legg, DVM b David G. Stansfield, BVSc, MRCVS b a CHK-R &D Polk Road Stanwood, MI b Novartis Animal Health P.O. Box Greensboro, NC ABSTRACT The efficacy of moxidectin injection (Pro- Heart 6 Sustained Release Injectable for Dogs, Fort Dodge Animal Health) against naturally acquired infections of Toxocara canis was compared with that of milbemycin oxime/ lufenuron tablets (Sentinel Flavor Tabs, Novartis Animal Health). Eighteen dogs with naturally acquired infections of T. canis were ranked by egg counts and randomly assigned to treatment with moxidectin (170 µg/kg), milbemycin (500 µg/kg)/lufenuron (10 mg/kg), or to an untreated control group (six dogs per treatment). Dogs were euthanized 7 days after treatment for recovery, identification, and enumeration of parasites by species. There was no apparent efficacy for moxidectin against T. canis. Conversely, milbemycin oxime/lufenuron was 91.5% effective against naturally occurring infections of this canine parasite. *Funding for this study was provided by Novartis Animal Health, Greensboro, North Carolina INTRODUCTION Milbemycin oxime/lufenuron in a tablet formulation (Sentinel Flavor Tabs, Novartis Animal Health) to be given monthly is approved for prevention of heartworm infections (Dirofilaria immitis), treatment and control of intestinal infections with whipworms (Trichuris vulpis) and ascarids (Toxocara canis and Toxascaris leonina), and control of hookworms (Ancylostoma caninum). Lufenuron, an insect development inhibitor, has activity against flea eggs produced by female fleas feeding on treated dogs. The product is given monthly at a dose calculated to provide 500 µg milbemycin and 10 mg lufenuron/kg body weight. A commercial formulation of moxidectin (Proheart 6 Sustained Release Injectable for Dogs, Fort Dodge Animal Health) is administered to dogs for the prevention of canine heartworm infections and for the treatment of existing larval and adult canine hookworm infections (A. caninum and Uncinaria steno- 281

2 Veterinary Therapeutics Vol. 3, No. 3, Fall 2002 cephla). The product is given by subcutaneous injection every 6 months at a dose calculated to deliver 170 µg moxidectin/kg. Although this injectable canine formulation does not have a claim of efficacy against canine ascarids (T. canis and T. leonina), an injectable formulation of moxidectin designed for use in cattle (Cydectin Injectable, Fort Dodge Animal Health) has been shown to be efficacious against T. canis when injected subcutaneously to dogs at 200 µg moxidectin/kg body weight. 1 Additionally, an oral paste formulation of moxidectin (Quest, Fort Dodge Animal Health) is effective against several species of equine parasites, including the large ascarid, Parascaris equorum. 2 The present study examined the efficacy of moxidectin given by subcutaneous injection at the dosage rate recommended for heartworm and hookworm control in dogs against naturally acquired adult T. canis infections. Efficacy of moxidectin was compared with that for a group that received no treatment and a group treated with milbemycin/lufenuron. MATERIALS AND METHODS Animals Eighteen random-source adult dogs (15 males and three females) of various breeds, harboring patent, natural parasite infections (as determined by fecal flotation), were acquired from sources that typically supply animals to the research facility. Except for natural parasite infections, all dogs were determined to be healthy by physical examination before dosing and were negative for heartworm microfilariae. Because a sufficient number of naturally infected dogs could not be identified at one time, the study was performed in two replicate groups of nine dogs, with three dogs per treatment in each replicate. In both replicates, dogs were individually housed in cages or runs from the time of treatment until necropsy. Housing, handling, and routine care conformed to standards established at the facility. Each animal was identified by a collar with an attached medallion bearing a unique identification number, and each cage was marked with the corresponding animal identification number. Except for the day of dosing, dogs were provided nutritious, palatable food ad libitum throughout the day, and food was removed in the evening for fasting overnight. Water was available ad libitum. Allocation and Treatments Fecal samples were collected for fecal egg counts on two occasions during the 7-day acclimation period (treatment day = Day 0). A mean fecal egg (T. canis) count was calculated for each dog from these two samplings and dogs within each replicate were ranked by these egg counts and randomly allocated to three groups of three dogs each. Treatments (moxidectin injectable, milbemycin oxime/ lufenuron, untreated control) were randomly allocated to each group thus formed. Moxidectin was provided as the commercially available injectable formulation designed for heartworm prevention in dogs. The product was given according to label recommendations at a dose that provided a minimum of 170 µg moxidectin/kg (Table 1). Milbemycin oxime/ lufenuron combination also was obtained from commercial sources and administered orally at a dose calculated to deliver 500 µg milbemycin oxime and 10 mg lufenuron/kg body weight. The control groups did not receive any treatment for parasites. Dogs of each replicate were individually weighed on their respective Day 1. On the morning of Day 0, a staff member experienced in injection techniques administered moxidectin according to label instructions to dogs assigned to that treatment. Dogs assigned to treatment with milbemycin oxime/lufenuron were fasted 282

3 TABLE 1. Dog Identification, Treatments, and Adult Toxocara canis Recovered at Necropsy Mean Pretreatment Treatment T. canis T. canis Group Egg Counts* Dog ID Sex Weight (kg) Dose Recovered Control Male 28.4 Untreated 10 Replicate Male 18.8 Untreated Male 31.0 Untreated Female 14.2 Untreated 6 Replicate Female 13.7 Untreated Male 26.7 Untreated 15 Geometric Grand Mean 7.1 Moxidectin Injection Male mg 49 Replicate Male mg Male mg Female mg 88 Replicate Male mg Male mg 7 Geometric Grand Mean (% Efficacy) 13.1 (0%) Milbemycin Oxime/Lufenuron Tablets Male lb tablet 2 Replicate Male lb tablet E2355 Male lb tablet E2490 Male lb tablet 0 Replicate Male lb tablet Male lb tablet 0 Geometric Grand Mean (% Efficacy) 0.6 (91.5%) *Eggs per gram of feces. overnight before dosing. On the morning of Day 0, whole milbemycin/lufenuron tablets hidden in food balls were administered orally by staff members experienced in oral dosing techniques. After dosing, dogs in this group were offered half of their usual morning ration. All dogs were observed approximately every hour for 3 hours after dosing with milbemycin/lufenuron for evidence of vomiting. Necropsy and Parasite Counts Seven days after treatment, all dogs in a replicate were euthanized and the intestinal tract was opened and thoroughly searched for 283

4 Veterinary Therapeutics Vol. 3, No. 3, Fall 2002 remaining T. canis using standard parasite recovery techniques. Contents from the lumen of the intestinal tract were washed through a 40-mesh sieve, and all worms encountered were recovered, enumerated, and identified according to stage and species. Other parasite species were present in some dogs; however, counts are presented only for T. canis. Analysis Efficacy of the two treatments was based on the percent reduction for T. canis as compared with the worm count for the control group. Parasite counts were transformed to the natural logarithm of (count + 1) for analysis and for calculation of geometric means. Efficacy of each treatment was calculated as follows: Percent efficacy = ([X c X t ]]/X c ) 100 where X c = geometric mean number of T. canis recovered in the control group and X t = geometric mean number of T. canis recovered in the treatment group. RESULTS All dogs remained healthy and completed the study. No vomiting occurred in any dog after oral administration of milbemycin oxime/ lufenuron tablets. A mean of 5.8 T. canis was recovered from the control group in Replicate 1 and a mean of 8.6 T. canis was recovered from controls in Replicate 2. The grand mean number of T. canis recovered from the control group was 7.1 (Table 1). All dogs treated with moxidectin injection had T. canis present at necropsy. A mean of 16.7 was recovered from dogs in Replicate 1 and a mean of 10.3 was recovered from dogs in Replicate 2. The geometric grand mean T. canis recovered from dogs treated with moxidectin was 13.1 (0% efficacy). Only three of the six dogs treated with milbemycin oxime/lufenuron had live T. canis present at necropsy. A mean of 0.8 worms was recovered from dogs in Replicate 1 and a mean of 0.4 worms was recovered from this group in Replicate 2. The geometric grand mean T. canis remaining at necropsy after treatment with milbemycin oxime/lufenuron was 0.6 (91.5% efficacy). DISCUSSION Moxidectin in the formulation and dosage used for prevention of heartworm and hookworm infection in dogs was ineffective against T. canis infections within 7 days after treatment in this study. In contrast, milbemycin oxime/lufenuron was highly effective, removing more than 90% of the worms present. The lack of efficacy for moxidectin against this parasite species is possibly related to the administration of the drug by the subcutaneous route. The subcutaneous route of administration might possibly minimize the direct effect of the compound on the unattached worms within the intestine. Early work with milbemycin oxime against T. canis has shown that the majority of worms are shed within the first 2 days after treatment and that all affected worms are passed within 5 days after treatment. 3 However, no information was found on the rate of worm kill with the administration of moxidectin. Thus, it is conceivable that the pharmacokinetics of the injectable moxidectin formulation could provide for achievement of peak levels in the intestines several days after treatment. If this does occur, a higher rate of efficacy might have been observed by performing necropsies on dogs treated with injectable moxidectin 3 to 7 days later than they were done in this study. Currently, milbemycin oxime is available in two formulations for prevention of canine heartworm disease (Interceptor and Sentinel Flavor Tabs ). In addition to heartworm prevention, both milbemycin formulations are effective 284

5 against canine ascarids and whipworms and provide control of canine hookworms. The latter of the two formulations also contains lufenuron, which provides control of flea populations by inhibition of egg development. CONCLUSION Moxidectin, given by injection at the dosage recommended for control of canine heartworms and hookworms, had virtually no effect on the number of adult T. canis in this study. In contrast, milbemycin oxime/lufenuron, given orally at the dosage rate recommended for control of heartworm and certain intestinal parasites of dogs, demonstrated excellent efficacy against the canine ascarid, T. canis. Additionally, this combination provides excellent efficacy against T. vulpis and A. caninum infections in dogs. REFERENCES 1. Martinez-Labat P, Garrido-Alcala Y, Vizzuett-Resendiz B: Prueba critica a la moxidectina como antinematodico alternativo en caninos. Vet Argentina 13: , Costa AJ, Barbosa OF, Moraes FR, et al: Comparative efficacy evaluation of moxidectin gel and ivermectin paste against internal parasites of equines in Brazil. Vet Parasitol 80:29 36, Bowman DD, Parsons JC, Grieve RB, et al: Effects of milbemycin on adult Toxocara canis in dogs with experimentally induced infection. Am J Vet Res 49: ,