Blowfly strike prevention in sheep

Wesley Power, Elanco, discusses the causes of blowfly strike on farm and the preventative steps farmers can take to prevent a strike occurring.

A study by Lane et al, in 2015, listed seven conditions as being associated with the greatest loss in production and cost to the sheep industry in Australia. This included neonatal mortality, internal parasitism and dystocia but, flystrike was listed at number five. 

In the UK, despite prevention efforts, more than 75% of sheep farms are annually affected by flystrike, with a prevalence rate of 1.5% for ewes and 3% for lambs (French et al., 1992).

One of the principle aims of any living organism is to ensure the continuation of its own species. Blowflies are no different. After emerging, a female blowfly will mate almost immediately and look for a source of organic matter to lay her eggs.

These eggs usually hatch within 12 hours, in adequate environmental conditions, to form first stage maggots (larva, larval instar). First instar larvae scrape the outermost skin surface with spines attached to their oral cavity, creating skin irritation and exudation. These maggots then develop into second stage maggots, who acquire mouthparts with a nice set of teeth.

It is these second stage maggots, along with their third stage, bigger brothers and sisters, which create the typical lesions we see as blowfly strike. They create these lesions by literally eating the poor struck sheep alive. Sandeman et al (1987), have shown that the only predisposing factor for the primary species of fly to initiate fly strike, was wetting of the wool. In this part of the world, the primary species is usually the common greenbottle (Lucillia sericata).

But why do blowflies lay eggs on sheep? Very simply the fleece acts as a nice incubator for the eggs and larva (maggots) to develop. Gatenby et al (1983), showed that fleece temperature, and humidity vary linearly along with increasing fleece length from the skin, when the environmental temperature varies between 5 and 28 degrees celsius.

Besides disruption of the skin with its mouth parts, blowfly instar larvae can damage the skin by releasing proteolytic enzymes and toxification of the skin by releasing ammonia secretions. (Sandeman et al., 1987; Morris, 2005).

As we all know, blowflies are less likely to be attracted to a sheep with a nice clean fleece. They are attracted to a fleece that has been contaminated by organic matter, such as faecal staining in a lamb with scour or an oozing discharge from an existing wound. Faeces and urine produced by sheep lead to the emission of odorous compounds such as ammonia. When these excretions are combined with skin sebaceous secretions, they form a strong and lasting odour which attracts flies for oviposition (James, 2006).

All sheep farmers will be aware that strike can occur very quickly, often within 24 hours. How is this possible? A single adult female blowfly can lay up to 300 eggs at a time (approximately 3,000 eggs throughout the average 30 day lifespan). Therefore, one single fly could produce 3000 new flies.

If you then imagine that many strike cases involve multiple female flies laying eggs on one single sheep, it is easy to see how very quickly thousands of maggots could be present in a 24 hour period, in a single case of strike.

Blowfly strike causes

• Pain and suffering for the individual sheep;
• Secondary bacterial infections;
• Death;
• Production losses for the farmer;
• Increases in management time treating affected animals;
• Increases in veterinary costs.

As the old saying goes 'prevention is better than cure'. An effective control strategy takes an integrated approach equipped with mechanical control methods and the use of insecticides (Sandeman et al., 2014). Several mechanical methods have been adopted in the sheep industry to mitigate flystrike, that include shearing, dagging, and tail docking. Minimising faecal contamination of the fleece by controlling worms and rapid attention to skin wounds, including foot rot lesions, will reduce the attractiveness of the fleece for flies. Proper disposal of dead animals on farm is also important.

When considering which control strategy to take at farm level, we are looking to minimise the incidence of strike, but balance this with the associated cost. When it comes to cost, it is important to consider the production losses, replacement costs, labour and cost of treatments.

Lihou et al (2023) looked at the expected cost incurred from flystrike at different levels of risk. Not surprisingly, the model showed the lowest strike incidence was associated with prophylactic treatment of ewes pre-shearing and post-shearing, while also prophylactically treating lambs in early summer. They showed that reactive treatment, in the absence of prophylactic treatment, was never the lowest cost strategy, even in low-risk flocks. In low to moderate risk flocks, the most cost effective method was to treat ewes pre-shearing with a pyrethroid insecticide, followed by prophylactically treating lambs with a long acting IGR in the summer (June in this model). In high-risk flocks, the addition of an IGR, as a post-shearing ewe treatment, was considered the most cost effective.

Available products for the prevention and treatment of blowfly strike in sheep.

Products are only effective if applied correctly, which is detailed in the below video:

Dicyclanil works by preventing the first stage maggots developing into the second stage maggots. As first stage maggots have no mouthparts they are unable to create any damage to the flesh of the sheep, preventing the lesions we see in strike. Its mode of action is targeted at the lifecycle of flies, therefore has no activity against other ectoparasites.

Cypermethrin, Deltamethrin pour-ons and Diazinon dip, are broad spectrum insecticides, meaning they will kill multiple stages of the lifecycle of many insects. From a sustainability point of view, narrow spectrum products should be chosen over broad spectrum products.

It is recommended to use appropriate PPE when applying products. It is suggested not to shear sheep for two months post administration of a pour-on (3 months post dipping) and in general to handle sheep as little as possible post administration.