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Culicoides spp. (Diptera: Ceratopogonidae) are the main vectors of Bluetongue and Schmallenberg virus disease, which are infectious diseases of ruminants. Recently, both viral diseases occurred for the first time in Germany and caused substantial suffering and large economic losses, primarily in sheep flocks. None of the currently available insecticides is registered to control infestations with Culicoides midges in ruminants. This study aimed to assess the safety and efficacy of a pour on formulation of a deltamethrin preparation (Butox® 7.5 mg/ml pour on, MSD Tiergesundheit Germany) against midges in sheep.
In a double blind study, a two hectare pasture in Gross Kreutz (Brandenburg) was divided by electric fences into two parts. A double fence formed a 2 m wide corridor, ensuring that contact between the two sheep flocks was prevented. Each pasture housed a group (A, B) of 20 female Merino sheep with approximately 40 kg body weight.
The treatment groups were treated with 10 ml Butox® 7.5 mg/ml pour on during five consecutive days. For this purpose, the respective subgroup (A1-A5, B1-B5) of three sheep plus one spare sheep was separated from the main group for 24 hours in an extra fenced section of the pasture to prevent the carry-over contamination of the veterinary product to untreated sheep. The treatment area was equipped with a calf igloo to protect the treated sheep against rainfall and a potential washing out of the insecticide.
Sheep of the control group received a similar treatment, but were not treated with Butox® 7.5 mg/ml pour on.
The midge catches were carried out on day 1, 7, 14, 21, 28 and 35 after the initial treatment, each time within a time window of 2 hours before to 2 hours after sunset.
For the midge collections, a subgroup of three sheep was led inside a crush pen which was located under a 3x3 m marquee (drop trap). All four side walls of the drop trap could be raised and fastened or quickly dropped. The sheep were kept in the drop trap for 15 minutes with the walls raised, followed by 20 minutes under the closed drop trap with the walls dropped. With the aid of an aspirator, all insects inside the drop trap were collected from the tent walls and the ceiling for 15 min. A single collection took about 1 hour in total, allowing four catches on one evening. Collections were not made on days with rainfall, temperatures below 10 °C or wind speed above 5m/s.
A calculation of the efficacy of the insecticide was made by comparing the total number of collected midges and the total number of collected engorged midges from the treatment and from the control group. During the field study of six weeks, a total of 19,057 midges were caught on 22 catching days in both groups. 7,026 midges were caught in the treatment group, with 103 engorged midges (1.47 %). In the control group, 12,034 midges were caught, including 757 engorged midges (6.29 %). The midges belonged to the subspecies Avaritia Fox (86,62 %) or Culicoides Latreille (13,38 %). The catches on day 1 and 35 after the treatment showed no substantial differences between both groups. Significant differences regarding the number of engorged midges were observed from day 7 to day 28 between the treatment group and the control group. The total number of midges collected in the treated group on day 7, 21 and 28 after treatment was lower in comparison to the control group.
The Butox® 7.5 mg/ml treatment effectively reduced the number of engorged midges by 86.39 %, and the total number of midges by 41.47 % over the entire study period.
These results show a high efficacy of Butox® 7.5 mg/ml pour on against midges infesting sheep. The pour on treatment of sheep with Butox® 7.5 mg/ml may be useful during outbreaks of animal diseases associated with midges. Additional safety measures should be implemented for an increased protection of the animals.