Development and validation of a simulation model for blowfly strike of sheep

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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.

Section: Discussionmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

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

Broughan

Wall

2007

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“…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.…”

Section: Discussionmentioning

confidence: 99%

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

Bisdorff

Wall

2008

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The methods used for the control of sheep blowfly strike (ovine cutaneous myiasis) and the farm management factors associated with strike prevalence were examined using data from questionnaire survey returns provided by 966 sheep farmers in Great Britain, based on the period between March 2003 and February 2004. Overall, 91% of participants treated prophylactically with insecticides against blowfly strike; 39% treated twice and 11% treated more than three times in the year. Insect growth regulators (IGRs) were the most commonly chosen product (40%), especially the IGR cyromazine. Only 12% of farmers opted to dip their sheep in organophosphate insecticide against fly strike and 2% of farmers reported applying inappropriate products against strike to their sheep, such as ivermectin or 'drenches'. Farmers worming their ewes more often were 0.8 times less likely to report blowfly strike, but those who wormed their lambs more often were 1.2 times more likely to report strike. Pure-breed flocks were 0.7 times less likely to record an outbreak of blowfly strike than cross-breed flocks. Strike was less likely in ewe flocks grazed at higher altitude; however, this relationship with altitude was not seen in lambs. The results show that insecticides remain the primary tool used by almost all farmers to prevent strike and that the type of insecticides used and means of application have altered dramatically over the past 15 years. However, the prevalence of strike has remained almost unchanged over this period. Clearly careful attention to the type and timing of insecticide application, in association with a detailed understanding of the husbandry factors that predispose sheep to higher strike risk, is essential to allow the optimal management of strike problems.

“…It is possible therefore that further improvements might be made if the basic model was re-fitted using revised and/or additional susceptibility functions, particularly in relation to drenching. In a simulation model of flystrike in the U.K., Wall et al (2002) used a combination of seasonal changes in fleece length plus data on nematode egg counts in the faeces of untreated sheep to model susceptibility to breech strike in ewes and lambs. In effect, this is similar to the procedures adopted in the current study, where susceptibility was judged on the basis of progressive temporal changes in faecal and/or urine stain following crutching or shearing.…”

Section: Termsmentioning

confidence: 99%

Estimating the incidence of fly myiases in Australian sheep flocks: development of a weather‐driven regression model

Wardhaugh¹,

Morton²,

Bedo³

et al. 2007

The blowfly, Lucilia cuprina Wiedemann (Diptera: Calliphoridae), is the primary myiasis (strike) fly of sheep in Australia. Most strike occurs in the anal-perineum area (crutch), but strike to the neck, shoulders, back and withers (body) is also important. Regression analysis was used to determine the extent to which the weekly incidence of flystrike can be explained by variations in fly abundance and/or recent changes in weather, pasture conditions or flock management. Strike and flock management data were collected by questionnaire surveys of 30-60 sheep properties in each of three major sheep-producing areas in southeastern Australia, namely, Gunning (southern New South Wales), Inverell (northern New South Wales) and Flinders Island (Bass Strait). After using simulation modelling to remove effects due to shearing, crutching and/or insecticide treatment, pasture growth index was found to be the most important explanatory variable affecting the incidence of all forms of myiasis. Others were average weekly air temperature, the amount and frequency of rainfall, relative humidity, dung quality index and a factor denoting seasonal effects. Together, these variables accounted for 48.4% of the variation in body strike, 56.8% of that in crutch strike and 51.9% of that in other forms of strike. Prediction was improved by the inclusion of additional lagged variables describing previous strike, fly abundance and fly activity. With these additions, the variation explained increased to 60.4% for body strike, 68.0% for crutch strike and 58.3% for other strikes.