Control of sheep flystrike: what's been tried in the past and where to from here

Flystrike remains a serious financial and animal welfare issue for the sheep industry in Australia despite many years of research into control methods. The present paper provides an extensive review of past research on flystrike, and highlights areas that hold promise for providing long-term control options. We describe areas where the application of modern scientific advances may provide increased impetus to some novel, as well as some previously explored, control methods. We provide recommendations for research activities: insecticide resistance management, novel delivery methods for therapeutics, improved breeding indices for flystrike-related traits, mechanism of nematode-induced scouring in mature animals. We also identify areas where advances can be made in flystrike control through the greater adoption of well-recognised existing management approaches: optimal insecticide-use patterns, increased use of flystrike-related Australian Sheep Breeding Values, and management practices to prevent scouring in young sheep. We indicate that breeding efforts should be primarily focussed on the adoption and improvement of currently available breeding tools and towards the future integration of genomic selection methods. We describe factors that will impact on the ongoing availability of insecticides for flystrike control and on the feasibility of vaccination. We also describe areas where the blowfly genome may be useful in providing impetus to some flystrike control strategies, such as area-wide approaches that seek to directly suppress or eradicate sheep blowfly populations. However, we also highlight the fact that commercial and feasibility considerations will act to temper the potential for the genome to act as the basis for providing some control options.

Sheep death and loss of production associated with flystrike in mature Merino and crossbred ewes

In eight well-managed Australian Merino and crossbred sheep flocks the adult ewes were monitored frequently for flystrike. Occurrences of flystrike and severity of strike scores were recorded, whereas bodyweights and condition scores were recorded regularly across two wool production and reproduction cycles. In these unmulesed flocks, 17% of Merino and 5% of crossbred ewes were struck, with 2% of those struck dead within 7 days of detection of the strike. For those adult ewes that survived the immediate period of the strike, the rate of loss over the following year was slightly greater than for ewes that were never struck, particularly for those with a high severity strike. Flystrike also contributed to losses of up to 2.7 (Merino ewes) and 5.2 (crossbred ewes) kg of bodyweight, and 0.3 condition score units at the time of the strike. Bodyweights were still 2 kg lower by 3 months after the strike occurred for Merinos, although condition scores returned to average levels. For ewes struck between mating and lambing, flystrike was associated with significant losses of lambs. Flystrike at severity level 1 (light) had only minor effects on wool production, but severity score 3 (heavy) reduced clean fleece weight.

Use of a threshold of flystrike risk as a method for treatment intervention in the management of flystrike in sheep

Cost is important when determining preventative treatment for flystrike. This paper presents modelled output comparing two common treatment alternatives; either a fixed annual application date or waiting until a set proportion of the flock is struck. The Inverell, Gunning and Flinders Island regions of Australia, which provide differing strike conditions, were used for analysis. Where the period of annual strike risk was longer than the period of protection provided by chemical treatment, it was economically favourable (by 1–3%) to wait until a pre-determined proportion of the flock were struck before applying preventive treatment, rather than always treating on a fixed date. However, with spring or autumn shearing, where the season of strike risk was a similar length to the protective period, it was preferable to treat on a fixed date. For Flinders Island, with a shorter flystrike season than Inverell or Gunning, there were small benefits in treating at a pre-determined threshold, but selection of the correct intervention point was critical. In areas with lower levels of risk, where treatment may not be required every year, it appeared beneficial to set a threshold at a sufficiently high level to eliminate most treatment intervention except for years with high rainfall and humidity.

A method for estimating the economic value of changes in the risk of breech strike

An economic model of the costs of sheep flystrike has been modified to examine the effect of a given change in the risk of breech strike as if it applied to the entire wool-producing industry, rather than to individual wool producers. Within each region (high rainfall, sheep/cereal and pastoral zones), the model sets the proportion of the sheep population in low, medium and high risk categories and calculates the estimated level of strike for untreated sheep in those groups. The costs related to breech strike can be calculated according to the original model. The risk of breech strike can then be adjusted to any required level, the proportion of sheep in each category adjusted and the costs recalculated according to the adjusted risk. The model estimated that a 50% reduction in the risk of breech strike would decrease the number of sheep requiring preventive treatment to about one-third of previous levels, but only provide a small reduction in costs related to struck sheep, due to the large increase in the number of sheep no longer receiving preventive treatment. There would be only small reductions in costs related to crutching and mulesing sheep unless much larger reductions in strike risk occurred. The overall benefit of a 50% reduction in the risk of strike would be $0.23–0.27 per sheep. If this reduction in strike risk occurred across all regions this could provide a benefit to the Australian sheep industry of $15 million. The model found that if mulesing was not used in any of the sheep then the use of preventive treatment would increase so that 93% rather than only 60% received chemical treatment, and reduction of the risk of breech strike by 50% provided benefits of $0.40 per sheep. Therefore, the estimated benefits would be greater than $0.27 per sheep in situations where the current level of risk is very high.