In-home assessment of either topical fluralaner or topical selamectin for flea control in naturally infested cats in West Central Florida, USA

Flea allergy is considered the most common cause of dermatitis in dogs, and flea infestations in cats are also extremely common causes of irritation and pruritus leading to erythema, excoriations, papules and alopecia [1–4]. The flea species most commonly associated with these infestations in dogs and cats is the cat flea, Ctenocephalides felis felis [1, 2]. The cat flea is also a recognized intermediate host and vector of the common cestode Dipylidium caninum, as well as pathogens including Rickettsia felis, Bartonella henselae and Mycoplasma haemofelis [1–3]. Rapid and effective flea control is therefore, necessary to alleviate pruritus in cats and dogs and to help reduce the risk of disease agent transmission.

A new class of ectoparasiticides, the isoxazolines shows rapid residual speed of kill of fleas infesting dogs in both laboratory and field trials [5–9]. Recently a topical formulation of the isoxazoline fluralaner (Bravecto® Topical Solution, Merck Animal Health, Madison, NJ, USA) was introduced as an ectoparasiticide for dogs and cats [10–12]. This is the only topical isoxazoline formulation available to veterinarians in the USA at present. The extended retreatment interval of fluralaner, providing 12 weeks of protection against fleas, provides a new option to help veterinarians and cat owners deliver effective flea control. An extended duration of protection helps to improve owner compliance with veterinary parasite control recommendations [13].

The present study was conducted to evaluate the efficacy of topical fluralaner in naturally flea infested cats in subtropical Florida, USA. This area of Florida is known to carry a year round risk of intensive flea challenge. Selamectin (Revolution®, Zoetis, Whippany, NJ, USA), a flea control product with a monthly retreatment interval, was chosen as the positive reference control. The treatments were compared in their ability to eliminate existing flea infestations on cats, minimize pruritus and reduce dermatologic lesions and assess the impact on flea control within the home environment. Most homes in the area with cats also own dogs; therefore, co-habiting dogs were also treated for fleas. Dogs were treated with fluralaner in households with fluralaner treated cats. However, to match the monthly treatment recommendation of selamectin, dogs in these households were treated with the isoxazoline sarolaner (Simparica®, Zoetis, Whippany, NJ, USA). Sarolaner was selected for dogs based on the results of a prior flea efficacy trial conducted in the same area [7].

Initially 40 private residences were enrolled in the study, although four residences in the selamectin-sarolaner treatment group did not complete the study. Two homes were dropped within the first 2 weeks because of severe cockroach infestations and data from these households were excluded. The owners in one residence moved because of the infestation severity and owners in the other residence had a professional pest management company spray their entire home with an insecticide/insect growth regulator combination. The third residence, with two enrolled cats, was lost after the 28–30-day assessment because the home was vacated after a small electrical fire. The fourth home, with one enrolled cat and one dog, was lost following the day 56–60-day appointment because the owners moved to a new home. Data from these two residences and their enrolled pets were included in analysis up to the point they were lost from the study.

Additionally, there were three times during the study when data was not collected at a single counting period on an individual pet or an entire home because the animal was unavailable, or the owners were not at home that week. On day 7 data were collected from 19 of 20 homes in the fluralaner group and on day 14 data were collected from 17 of 18 homes in the selamectin/sarolaner group.

In the 20 homes in the fluralaner treatment group, 31 cats (mean 4.7 kg; range 2.9–6.7 kg) were officially enrolled on day 0. These cats were administered a mean topical dose of 58.9 mg/kg (range 40.7–86.3 mg/kg) fluralaner. There were also 18 dogs (mean 24.7 kg; range 4.8–37.7 kg) officially enrolled on day 0 and they were administered a mean oral dose of 36.6 mg/kg (range 26.5–52.4 mg/kg) fluralaner. Additionally, there were 28 cats and 14 dogs that were treated but did not meet qualifying criteria. These animals had < 5 fleas; spent the majority of their time outside the residence; or could not be safely handled. These households had a total of 91 pets (59 cats and 32 dogs) treated with fluralaner.

In the 18 homes that remained in the selamectin/sarolaner treatment group for at least 4 weeks, 29 cats (mean 4.8 kg; range 2.4–9.2 kg) and 13 dogs (mean 22.2 kg; range 2.5–47.2kg) were enrolled. On day 0, cats were topically treated with a mean dose of 10.4 mg/kg (6.6–18.4 mg/kg) selamectin and dogs were orally administered a mean dose of 2.8 mg/kg (2.0–3.8 mg/kg) sarolaner. There were an additional 12 cats and 12 dogs in these residences that did not qualify for inclusion in the study for reasons previously described. These households had a total of 66 pets (41 cats and 25 dogs) administered group appropriate treatments.

Following a single topical fluralaner dose, 80.6% (25/31) of treated cats had no fleas recovered in comb counts on days 28–30 and 100% (31/31) of cats were flea free at 12 weeks (Table 1) and the number of flea free cats was significantly different from day 0 pre-treatment counts at every post-treatment assessment (Table 1). The number of flea-free fluralaner treated cats was significantly greater than the number of flea-free selamectin treated cats at every post-treatment assessment (Table 1). The percentage of selamectin treated flea free cats was 17.2% (5/29) at 4 weeks and 38.5% (10/26) at 12 weeks (Table 1). The number of cats with no fleas collected in comb counts (flea-free cats) following selamectin administration was significantly different from day 0 pre-treatment counts on days 7, 14, 40–45, 56–60 and 82–86, but was not significantly different from pre-treatment counts on days 21 and 28–30 (Table 1).

Dogs treated orally with either one dose of fluralaner or 3 monthly doses of sarolaner had remarkably similar flea count reductions (Table 2). Within 7 days of treatment, area flea counts on dogs were reduced by 99.7% in both treatment groups and flea populations on dogs were reduced by 99.6–100% at every counting period from 4 weeks to the end of the 12-week study (Table 2). Flea counts on treated dogs were significantly reduced from day 0 counts at every post-treatment assessment in both treatment groups, and there were no post-treatment differences in flea counts or number of flea-free dogs between the two treatment groups (Table 2).

Overall flea trap counts were similarly reduced in both treatment groups and there were similar numbers of homes where traps were flea free throughout the study (Table 3). The number of homes with flea free traps on days 7 and 14 was not different from day 0 in the fluralaner group and was not different from day 0 on days 7, 14 and 21 in the selamectin/sarolaner treatment group, likely due to continued emergence of flea stages developing in the household before the start of the study. At the 82–86 day count, 95.0% (19/20) of homes in the fluralaner treatment group and 68.8% (11/16) of homes in the selamectin/sarolaner treatment group had flea free traps (Table 3).

The ratio of female to male fleas collected in flea traps in homes with fluralaner-treated cats and dogs shifted over time. On day 0, prior to treatment, 58.0% of fleas collected in intermittent-light flea traps in fluralaner treatment group homes were female. At weeks 1, 2, 3, 4 and 6, females represented 62.3, 47.9, 38.7, 35.0 and 37.5%, respectively, of collected fleas. Both fleas collected on traps in fluralaner homes at week 8 and the single flea collected in a trap at week 12 were male. A different trend was observed in the selamectin/sarolaner treatment group homes. On day 0, prior to treatment, 51.2% of fleas collected in traps in these homes were female. At weeks 1, 2, 3, 4 and 6, females represented 54.3, 53.6, 48.7, 5.3 and 0.0%, respectively, of the fleas collected; however, the percentage of female fleas increased to 36.6% at week 8 and to 41.2% at week 12.

Total flea eradication was defined as a home with no fleas on cats or dogs, or in traps, and there were significantly more homes with total flea eradication in the fluralaner treatment group than in the selamectin/sarolaner group at all post-treatment assessments after the second week (Table 3). The proportion of homes with total flea eradication at 4 weeks post-treatment was 60.0% (12/20) in the fluralaner treatment group homes, significantly greater than the 5.6% (1/18) of selamectin/sarolaner treatment group homes. At the end of the 12-week study total flea eradication occurred in 95.0% (19/20) of fluralaner homes, significantly greater than the 31.3% (5/16) of selamectin/sarolaner homes (Table 3).

Client interviews showed that reservoir hosts for C. f. felis were commonly observed by pet owners on their properties. Many pet owners reported opossums (42.1%; 16/38), raccoons (57.9%; 22/38) and/or feral cats (81.6%; 31/38) in their yards.

No adverse events reported for any of the 100 cats in this study, regardless of treatment administered; however, two treated dogs had adverse events reported during the study. One fluralaner treated qualifying dog vomited one day before and one day after treatment administration. One sarolaner treated non-qualifying outdoor dog was found dead by its owner 7 weeks into the study (3 weeks after the second dose). The owner suspected that one of her other larger dogs killed this dog. The cause of death could not be determined as the dog’s remains were disposed of prior to reporting of this event to the study personnel. No other adverse events in treated dogs were reported.

A single topical dose of fluralaner provided excellent flea control in cats, achieving > 96% reduction in flea counts, a significant decrease compared to baseline, within 7 days and 100% control at 12 weeks (Table 1). The residual activity of this medication was remarkable given the constant flea re-infestation pressure from the heavily infested indoor premises and the lack of restriction on the cats from going outdoors. Faced with similar re-infestation pressure, three consecutive monthly topical applications of selamectin were significantly less effective than fluralaner for flea elimination (Table 1).

The significantly greater fluralaner efficacy over selamectin (Table 1) was also observed in comparisons between the post-treatment flea count range and the proportion of flea free cats. Cats treated once with fluralaner had no more than 4 fleas recovered in comb counts, while one cat treated with selamectin had 50 fleas on day 21 and another cat had 8 fleas at 12 weeks after 3 monthly treatments (Table 1). Within 7 days of treatment, fleas were recovered in comb counts on 31% of cats treated with fluralaner, while almost 76% of selamectin treated cats had at least one flea on day 7 (Table 1). Following 3 monthly selamectin treatments, 38.5% of cats were free of fleas at 12 weeks, significantly fewer than the 100% of cats flea free at 12 weeks following a single fluralaner treatment (Table 1).

Flea control efficacy observed following topical administration of fluralaner to cats in the present study was very similar to results of a previously published multicentric study in the United States [12] when topically administered fluralaner provided 99.1% reduction in flea counts on cats within 4 weeks and was 99.0% effective at 12 weeks. However, the selamectin flea control efficacy in this study is lower than results reported in earlier laboratory and field studies [20–23]. In a multicentric field study published in 2000, monthly application of selamectin to cats provided 92.5, 98.3 and 99.3% efficacy at days 30, 60 and 90, post-treatment, respectively [23]. In contrast, monthly selamectin efficacy in this study was only 77.8, 91.0 and 91.3% at similar post-treatment time intervals. Reasons for this efficacy reduction are likely multifactorial and possibly include: severe re-infestation pressure of cats living in subtropical Florida; slower rate of residual speed of flea kill; and/or variable susceptibility of C. f. felis flea strains used or encountered in previous investigations.

Significant improvement of cat dermal lesions (SCORFAD) from baseline was observed in both fluralaner and selamectin treatment groups, however, this was achieved sooner (30 days post-treatment) in fluralaner-treated cats compared to selamectin-treated cats (60 days) although treatment groups were not significantly different (Table 5). Owner assessed pruritus severity was significantly reduced from baseline on all post-treatment collection days for both treatments, and fluralaner-treated cats had significantly less severe pruritus than selamectin treated cats on days 21 and 28–30 (Table 4). Improvement in owner-assessed pruritus correlates with reductions in flea numbers and is consistent with a reduction in flea bites. Owner pruritus assessment improved faster rate in both treatment groups than dermatologic lesion improvement (Tables 4 and 5), indicating that after effective flea treatment, the reduction in pruritus severity occurs more rapidly than repair to injured and damaged skin.

The flea control efficacies of the two oral isoxazoline formulations administered to dogs in this study were very high; were not significantly different throughout the study (Table 3); and were remarkably similar to results observed in previous in-home investigations conducted in Florida [7, 24]. A single oral dose of fluralaner or three-monthly oral doses of sarolaner provided over 99% efficacy against adult fleas within 7 days of treatment and efficacy in both groups was 100% at both the 8 and 12-week assessments (Table 3). Previously, in an almost identical study design, a single oral dose of fluralaner reduced flea populations on dogs by 99.0% within 7 days and by 100% at both the 8 and 12-week post-treatment assessments [24]. Similarly, a previous Florida field study with oral sarolaner provided 99.0% reduction in flea populations on dogs at 7 days post-treatments and then further monthly doses of oral sarolaner resulted in 99.9% efficacy at 8 weeks (there was no 12 week efficacy assessment) [7]. Therefore, oral isoxazoline treatments can deliver rapid and sustained flea control for dogs in subtropical Florida.

Dogs administered oral sarolaner had significantly more fleas (geometric mean 37.9) than fluralaner treated dogs (geometric mean 26.9) prior to treatment (Table 3). However, this difference likely had no clinical effect on the study outcome because: dogs were the secondary host in this study, the efficacy of both isoxazolines in dogs was remarkably similar; efficacy of both products corresponded closely with previous in-home investigations; and pre-treatment flea counts for both groups fell within the range (20.4–51.0) of 4 recent Florida field investigations using an identical dog flea count methodology [7, 24–26].

Intermittent-light flea traps used in this investigation provide an estimate of flea biomass, flea reproduction and emergence in homes [27]. Following a single administration of fluralaner or three monthly administrations of selamectin/sarolaner to cats and dogs, 95.0% of homes in the fluralaner group and 68.8% of homes in the sarolaner/selamectin group had 0 fleas in traps at the end of the 12-week study, with both groups significantly different from baseline but not significantly different from each other (Table 3). Additionally, all 31 qualifying cats and all 18 qualifying dogs in fluralaner treated homes were flea free at the end of the study (Tables 1 and 2), showing that a single dose of fluralaner can completely eradicated fleas in these homes. All sarolaner-treated dogs were flea-free at the end of the study (Table 2); however, significantly fewer (10/26 or 38.5%) selamectin-treated cats than fluralaner treated cats were flea-free at the end of the study, despite receiving three consecutive monthly doses (Table 1). Significantly fewer (5/16 or 31.3%) selamectin/sarolaner treatment group homes achieved complete flea eradication compared to fluralaner treatment group homes (19/20 or 95.0%) (Table 3). In summary, these results show that highly effective control of fleas on both dogs and cats is essential for achieving flea control in the home.

Previous field studies have shown that halting flea reproduction leads to a shift in the sex structure of the household emergent flea population. An initially predominately female flea population will shift toward a male dominated population [25, 27]. This shift occurs because C. f. felis undergoes protogyny, where the first adults to emerge from a cohort of eggs are females, followed by both males and females and finally by an almost exclusively male emergent population [28]. In this study, flea sex ratio in fluralaner treatment group home traps shifted, as expected, from predominately female fleas observed during the first two weeks of the study, to a majority of male fleas between weeks 3–6, and finally to exclusively male fleas during weeks 6–12. A single male flea was the only adult collected in a trap between days 82–86 post-treatment; therefore, fluralaner treatment must have greatly reduced or possibly completely halted egg production [25, 27]. The selamectin/sarolaner treatment group home traps also had a similar shift in flea sex ratio toward a male dominated emergent adult population by weeks 3–6. However, the proportion of female fleas rebounded during weeks 8–12 with a marked increase in the proportion of female fleas collected in traps and an increase in the number of trapped fleas. This observation proves that flea reproduction was occurring in some selamectin/sarolaner homes after study week 8.

A challenge for veterinarians and pet owners is continued flea re-infestation pressure on cats and dogs from flea-infested free roaming dogs and cats and certain urban wildlife. Urban wildlife in North America, including opossums and raccoons, can be infested with C. f. felis. These animals move readily throughout urban environments and contaminate protected outdoor premises such as crawlspaces, decks, and under bushes with flea eggs [1, 2]. Although opossums and raccoons are nocturnal, 42.1% home owners reported observing opossums and 57.9% reported observing raccoons in their yards. In addition, almost 82% of home owners had observed cats, other than their own, in their yard. The frequent presence of potentially flea-infested hosts in areas inhabited by household dogs and cats shows that flea re-infestation pressure in this subtropical climate is substantial and continuous. Cats and dogs in this environment need to be maintained on life-long, year-round flea control to avoid the misery of flea infestation.

A single topical fluralaner application to cats living in an intense flea challenge environment was more effective than three consecutive monthly doses of selamectin for flea control on cats and for flea eradication in households. For dogs, both one dose of fluralaner and three doses of sarolaner delivered a very high level of flea efficacy with no significant flea control difference between the treatments. Successful household flea control requires effective treatment of both cats and dogs in the home.