The effect of farm management strategies on the incidence of sheep strike in Britain: a simulation analysis

Sheep breed and shearing influences attraction and blood-feeding behaviour of Culicoides (Diptera: Ceratopogonidae) on a UK farm

Background

Culicoides biting midges (Diptera: Ceratopogonidae) are responsible for the biological transmission of arboviruses of international importance between ruminant livestock. These arboviruses include bluetongue virus (BTV) and Schmallenberg virus (SBV), which have emerged in unprecedented outbreaks in northern Europe. The impact of breed and shearing of sheep on Culicoides: host contact rates has not been investigated in detail and has the potential to influence arbovirus transmission and control measures employed to limit spread.

Methods

Attraction of Culicoides to Hartline and Hartline/Suffolk cross-breed sheep was compared using 224 drop trap collections over 22 nights and 181 catches from sheared or unsheared Hartline/Suffolk ewes were made over 17 nights to compare Culicoides activity and rates of blood engorgement.

Results

A total of 31,314 Culicoides was collected in the two trials and females of the subgenus Avaritia represented over 96.9% of individuals collected. Attraction to breed was dependent upon species of Culicoides and physiological status, with a significantly greater number of individuals collected on the cross-breed sheep. Shearing of sheep did not significantly increase or decrease the number of Culicoides attracted but increased the rate of successful engorgement.

Conclusions

Both breed and shearing were shown to influence Culicoides biting rate on sheep. These data are useful in a direct context in understanding the likely impact of control measures against arboviruses including BTV and SBV and additionally in providing data from field-based studies to enable modelling exercises of arbovirus transmission and spread.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3003-5) contains supplementary material, which is available to authorized users.

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.

Climate change and parasitic disease: farmer mitigation?

Global climate change predictions suggest that far-ranging effects might occur in the population dynamics and distributions of livestock parasites, provoking fears of widespread increases in disease incidence and production loss. However, several biological mechanisms (including increased parasite mortality and more rapid acquisition of immunity), in tandem with changes in husbandry practices (including reproduction, housing, nutrition, breed selection, grazing patterns and other management interventions), might act to mitigate increased parasite development rates, preventing dramatic rises in overall levels of disease. Such changes might, therefore, counteract predicted climate-driven increases in parasite challenge. Optimum mitigation strategies will be highly system specific and depend on detailed understanding of interactions between climate, parasite abundance, host availability and the cues for and economics of farmer intervention.

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

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.

Myiasis in sheep in The Netherlands

In 1999, among 164 randomly selected Dutch sheep farmers, a questionnaire was carried out to estimate the prevalence of myiasis in sheep and to investigate factors associated with the occurrence of myiasis. The total number of sheep and/or lambs on the reference date 1 August 1999 was 12,200: 5243 ewes, 225 rams, 3393 ewe- and 3339 ram lambs. On 86 (52.4%) of the farms 349 (2.9%) of all sheep and/or lambs contracted myiasis, of which two died. Myiasis was seen significantly more frequently on farms with over 25 ewes compared to smaller farms. Cases of myiasis were detected from April to September, with a peak (47.1%) in August. Occurrence of myiasis was most frequently associated with hot and humid weather and was mainly observed (69.1%) in the area around the tail.Ewes and ewe lambs had significantly more myiasis when compared with rams and ram lambs. There was no relationship with tail docking, with breed, with the time of shearing, with the kind of soil (clay, sand, etc.), with the environment (bush, trees, water, etc.), with the type of treatment (pour on, dipping, spraying), the used insecticides (synthetic pyrethroïds, diazinon, cyromazin, etc.), the number of preventive treatments, the time of treatment or the number of observations on the herd (once a day, once a week, etc.).

The effects of oviposition aggregation on the incidence of sheep blowfly strike

The distribution of a parasite population within its natural host population can have a significant influence on the dynamics of both the host and parasite populations. The majority of parasite species are typically distributed in an aggregated manner within the host population, leaving most hosts lightly infected and a few hosts supporting very large parasite burdens. This paper presents a consideration of the effects of aggregation on the incidence of ovine cutaneous myiasis caused by the sheep blowfly, Lucilia sericata (Meigen). Using simulation analysis, the mechanisms causing larval aggregation are included in the model, allowing the consequences for control to be investigated. By explicitly incorporating host susceptibility, it becomes apparent that early in the season, strategies targeting the blowfly population may prove more effective in suppressing strike levels, whereas later in the season, treatment of the sheep population may be more beneficial. The analysis also shows that the greater the degree of aggregation, the fewer sheep that become struck and, hence, suggests that increasing the heterogeneity in susceptibility amongst a flock of sheep restricts strikes to relatively few sheep. Providing the highly-susceptible sheep could be identified, concentrating strikes on a low number of sheep would allow fewer sheep to be treated, leading to a more efficient means of controlling the blowfly population and suppressing strike.