Fly abundance and climate as determinants of sheep blowfly strike incidence in southwest England

Abstract

The relationships between abundance of the blowfly Lucilia sericata (Meigen) (Diptera: Calliphoridae), climate, animal management procedures and the incidence of cutaneous myiasis (blowfly strike) in sheep were examined in three sheep pasture systems in southwest England during the summers of 2002 and 2003. In each year, flies were collected using liver-baited sticky targets, daily weather and routine husbandry practices were noted and the age-class of each animal infested and body position of each strike were recorded. On sites where no strike control was used, 5.8-12.1% of ewes and 5.7-15.8% of lambs were struck. Ewe strikes predominated at the beginning of the season. The incidence of ewe strikes was significantly associated with higher mean temperature, rainfall and shearing; shearing was associated with a 95% reduction in the risk of ewe strike. In lambs, the incidence of strike was significantly related to higher fly abundance, ewe shearing, treatment and mean ambient temperature. Lambs were 4.6 times more likely to be struck after the ewes had been shorn than before; however, the strongest relationship was with mean L. sericata abundance. Average minimum threshold temperatures of 9.5 degrees C for lamb strikes and 8.5 degrees C for all strikes were extrapolated, below which oviposition did not occur. Over 80% of ewe strikes occurred in the breech region in 2002, as did 100% in 2003. However, in lambs both body and breech strikes occurred in both years. The distribution of lamb strikes appeared to change over time, with breech strikes predominating in May, June and July and body strikes occurring increasingly later in the season. The incidence of lamb breech strikes was significantly associated with higher L. sericata abundance and ewe shearing but there was no relationship with weather conditions. By contrast, the risk of body strike in lambs was significantly associated with higher blowfly abundance, higher rainfall and higher maximum temperatures. The relationship between strike incidence and L. sericata abundance is important because it enhances our understanding of strike incidence patterns and management of this disease. Clearly, any factors that facilitate larger L. sericata populations, such as inappropriate carcass burial or increased average ambient temperatures, are likely to increase the incidence of strike.

Multi-Level Determinants of Parasitic Fly Infection in Forest Passerines

“…Our finding that climatic variables, namely precipitation and temperature, were positively associated with mean larval abundance coincide with previous myiasis studies [6], [7], [34]. The time lags involved might provide clues on the duration of parts of the life cycle of the fly, which is so far ignored.…”

Sheep blowfly strike risk and management in Great Britain: a survey of current practice

“…Clearly, the aetiology of sheep strike is complex and results from the interaction between fly abundance and sheep susceptibility, both of which are, in turn, affected by weather, geography and animal husbandry factors, such as flock size, breed, age class, the timing of shearing and the complex interactions between worm burden, faecal soiling and anthelmintic use (Wall et al , 2002; Broughan & Wall, 2007a, 2007b). Insecticides remain the primary approach used by sheep farmers to prevent unacceptable levels of strike and animal suffering.…”

Blowfly strike: biology, epidemiology and control

“…Shearing was shown to be associated with a 95 per cent reduction in the risk of ewe strike by L sericata , and again this was considered to be largely the result of reduced humidity in the fleece of the shorn animal (Broughan and Wall 2007a). The presence and length of a tail have also been recognised as important risk factors for strike; strike incidence was shown to be over five times higher in lambs where the tail had not been removed, compared to docked lambs (French and others 1994).…”