Clinical Efficacy of Selamectin in the Treatment of Naturally Acquired Infection of Sucking Lice (Linognathus setosus) in Dogs

Study Sites

Five veterinary practices, located in the middle and southern parts of Sweden, participated in the study, which took place from February 2001 to January 2002. Clinical examinations and treatments were carried out at those veterinary practices. The dogs were maintained at the homes of their owners for the duration of the study, which was 42 days. The study was approved by the Swedish National Board for Laboratory Animals and was conducted in accordance with good clinical veterinary practice.

Study Subjects

Twenty-one privately owned dogs infected with the sucking lice, L. setosus, were included in the study. To be eligible for the study, the dogs were at least 6 weeks old. Dogs with any concurrent diseases were not included in the study. Dogs that were regularly placed in daycare centers and dogs that had been treated with products effective against ectoparasites within the previous 60 days were also excluded.

All 21 dogs were clinically healthy, except for their lice infections. The presence of L. setosus was confirmed by microscopic examination at the Department of Parasitology, Swedish University of Agricultural Sciences, Uppsala, Sweden.

Test and Control Materials

The test product was a commercial formulation, containing 12% selamectin in an isopropanol/glycol ether base.^e The positive control product was a commercial formulation containing permethrin (744 mg/mL).^a Both products are licensed for use in dogs.

Study Design

At enrollment in the study, a physical examination was performed, the extent of lice infection was confirmed, and an assessment of clinical abnormalities (e.g., pruritus, erythema, crusting, scaling, papules, and alopecia) was made. Lice were counted at hair partings at potential predilection sites [Table 1]. A total of 17 sites were examined on each dog, with two spots counted at six of the sites; thus, the total number of counts per dog was 23. Only live lice were counted, and no distinction was made between nymphs and adult lice. Following identification of the louse species, the day of treatment for each dog was considered day 0.

Each dog was randomly allocated to one of the two treatment groups, using a previously prepared schedule. The list was produced by a random generator within SAS version 6.^f The schedule was available only to the person administering the treatments. The veterinary examiners were blinded to the treatment used.

Body weights were recorded on day 0, and the dose or volume of the medication administered was determined by the manufacturer’s recommendations. The hair was parted to allow direct application of both products to the skin. The selamectin treatment was administered on the midline at the base of the neck, according to the label instructions. Permethrin doses of 1 mL were administered on the midline at the base of the neck, and 2-mL doses were equally divided between the neck and the base of the tail, according to label instructions. The dogs were treated on only one occasion, regardless of which product was used.

Ten minutes after each dog was treated on day 0, it was reexamined by the evaluating veterinarian for any adverse reactions. The dog was subsequently returned to its owner, who was instructed to observe the dog at home and to contact the veterinarian if any abnormal clinical signs were seen. Confirmation that no signs occurred during the first week after treatment was obtained when the dog was returned to the clinic on day 7.

The dogs were subsequently examined on days 7, 28, and 42. At each visit, a physical examination was performed, clinical signs were assessed, and lice counts were done. The principal parameter of efficacy for the two products was the determination and statistical analysis of the reduction in the lice counts during the 42 days following treatment.

Data Analysis

For each animal at each examination date, the total lice count was obtained by adding the lice counts from all sites. A natural logarithm transformation was applied to the total lice counts prior to analysis. The transformed variable was analyzed using a linear mixed model for repeated measurements.^f A priori contrasts were used to assess the difference between the day 0 mean count and each of the posttreatment mean counts within each treatment group. Geometric means were calculated for each treatment at each day of the study by back-transformation of the least-squares means.^f These geometric means were then used to estimate the percentage reduction in lice counts at days 7, 28, and 42, using the following formula:

Because a substantial withdrawal of animals occurred in the permethrin treatment group, the proportion of animals completing the study in the two treatment groups was analyzed using Fisher’s exact test to examine the effect of treatment on completion of the study. All analyses were performed using a statistical analysis software program.^f A P value <0.05, two-sided, was considered significant.

Study Subjects

Twenty-one dogs were included in the study. Twenty different breeds, as well as a mixed-breed dog, were represented. Ages were known for 13 of the study animals and ranged from 4 months to 3 years. The mean age of the dogs was 2 years. The dogs ranged in weight from 4.4 to 60.2 kg. Table 2 gives details of the body weights and dosages administered for the 11 dogs treated with selamectin, and Table 3 gives the same data for the 10 dogs treated with permethrin. The dogs in the selamectin group ranged in weight from 7.4 to 37.0 kg. Dogs in the permethrin group ranged in weight from 4.4 to 60.2 kg. The mean weights of the two groups were 21.4 and 22.8 kg, respectively. The mean dose administered was 7.9 mg/kg (range 6.1 to 11.5 mg/kg) for the selamectin group and 85.7 mg/kg (range 24.7 to 169.1 mg/kg) for the permethrin group.

Tables 4 and 5 provide summaries of the total lice counts for each dog from each treatment group on days 0, 7, 28, and 42, as well as the times dogs were withdrawn from the study for perceived lack of treatment efficacy. Four permethrin-treated dogs were withdrawn from the study (one on day 7, one on day 8, one on day 22, and one on day 28). Two dogs were withdrawn because of continued presence of lice, and the other two were withdrawn because of no improvement in skin condition. One of the 11 selamectin-treated dogs was withdrawn from the study at day 28. The owner wanted a different treatment for his dog when 31 lice were found at the examination. These five cases were regarded as treatment failures. One dog in the permethrin treatment group was not examined on day 42. The owner of this dog was contacted by phone on day 42, and because the dog exhibited no more clinical signs of lice infection, it was not regarded as a treatment failure. When analyzed by the Fisher’s two-tailed test, the difference between the two groups in the numbers of dogs withdrawn from the study by day 42 was not significant (P=0.063).

Lice Counts

At day 7, no lice were found on the dogs treated with selamectin, but lice were found on three permethrin-treated dogs. At day 28, one of the 11 selamectin-treated dogs and one of the seven permethrin-treated dogs had lice. At day 42, all of the animals remaining in the study (10 selamectin-treated dogs and six permethrin-treated dogs) were clear of lice [Table 6].

In both treatment groups, the reduction in lice counts from the initial pretreatment values on day 0 to day 42 was highly statistically significant (P≤0.0001). To put these values into clinical context, however, it should be noted that the values are based on only a small number of dogs (10 selamectin-treated dogs and six permethrin-treated dogs at day 42). The day 42 data resulted in overall clinical efficacy values of 91% (10/11 dogs) for selamectin and 60% (6/10 dogs) for permethrin. There were no significant differences (P=0.06357) in lice counts between the two treatment groups at any time point in the study.

Clinical Signs

The most common clinical sign at initial diagnosis was pruritus, followed by erythema, crusting, and scaling. At day 0 in the selamectin group, 10 of 11 dogs had pruritus; in the permethrin group, all dogs were pruritic. Pruritus was also the longest-lasting sign during the treatment period and remained at day 42 in three dogs in both treatment groups [Table 7]. There were no statistically significant differences between the two groups.

Adverse Reactions

Each animal was examined before and 10 minutes after each treatment. The animals were given a physical examination on days 0, 7, 28, and 42. Owners were also instructed to return the animals to their veterinarians if any abnormal signs occurred between visits. No adverse reactions were seen at any of the examinations. One owner reported his dog as being very tired at home for a few hours after treatment with selamectin, but the dog was normal the following day.