Investigation of methods for protection of horses in jet stalls against Culicoides biting midges

Abstract

Culicoides biting midges (Diptera: Ceratopogonidae), specifically Culicoides (Avaritia) imicola Kieffer and Culicoides (Avaritia) bolitinos Meiswinkel have been implicated as vectors of African horse sickness virus (AHSV) and equine encephalosis virus (EEV) in southern Africa. Intercontinental trade is a potential mechanism whereby midge-borne viruses, such as AHSV, may be introduced into immunologically naive horse populations. Horses in containerised air transport systems (jet stalls) may be at risk of exposure to Culicoides midges during international export from South Africa. The World Organization for Animal Health (OIE) has recently recommended that during export from, and transit through, African horse sickness (AHS) endemic countries or zones, measures of a physical and chemical nature are applied to protect horses from Culicoides midge attack. To contribute to information on effective measures of protection and to generate data on the effect of these on welfare of horses in jet stalls, the efficacy and safety of alphacypermethrin-treated high density polyethylene (HDPE) mesh applied to jet stalls as a method for protection of horses against Culicoides midges was investigated at the Faculty of Veterinary Science, Onderstepoort. Firstly, the repellent and insecticidal efficacy of alphacypermethrin-treated HDPE mesh against Culicoides midges was determined using Onderstepoort 220V downdraught black light traps and a contact bioassay. Three traps were operated overnight in four replicates of a 3 x 3 randomised Latin square design near horses. Both an untreated and alphacypermethrin-treated HDPE mesh significantly (P < 0.05) reduced the numbers of Culicoides midges, predominantly C. imicola, collected in the light traps by 4.2 and 7.2 times, respectively. A repellent effect of the alphacypermethrin-treated mesh was not confirmed because the number of midges collected in the light traps with untreated and alphacypermethrin-treated HDPE mesh were not significantly different (P = 0.656). Bioassay of the insecticidal contact efficacy indicated median C. imicola mortality of 100% from 30 and 10 min following exposure to the alphacypermethrin-treated HDPE mesh for 1 or 3 min, respectively. In the bioassay, mortality was significantly higher (P = 0.016) at 5 min post exposure in the midges exposed to the alphacypermethrin-treated mesh for 3 min (74.8%) compared to the 1 min exposure group (59.5%). Secondly, the efficacy of alphacypermethrin-treated HDPE mesh applied to jet stalls against Culicoides midges was determined by mechanical aspiration of midges from horses and using light traps in four blocks of a 3 x 2 randomised design. The alphacypermethrin-treated HDPE mesh applied to the stall significantly (P = 0.008) reduced the number of Culicoides midges, predominantly C. imicola, mechanically aspirated from horses housed in the stall. The mesh reduced the Culicoides midge attack rate in the treated stall compared to the untreated stall and a sentinel horse by 6 times and 14 times, respectively. The number of Culicoides midges and C. imicola collected in light traps from the untreated and alphacypermethrin HDPE mesh-treated stalls did not differ significantly (P = 0.82). Finally, the effect of alphacypermethrin insecticide-treated HDPE mesh on ventilation and welfare of horses housed in jet stalls was determined under temperate, climatic conditions. Jet stall microclimate, clinical variables and faecal glucocorticoid metabolites (FGM) of 12 horses were monitored during overnight housing in either a treated or untreated jet stall in two blocks of a 2 x 3 randomized crossover design. Temperature difference between the treated stall and outside differed significantly from the difference between the untreated