Sheep scab resistance worrying development for sheep sector

Sheep scab, caused by the mite psoroptes ovis, is a hugely important disease with the potential to significantly hit animal performance and impose serious financial losses.

Therefore, it is not surprising that a recent study which identified sheep scab resistance to macrocyclic lactones on farms in Wales and elsewhere in the UK has caused high levels of concern.

Widespread use of macrocyclic lactones, which offer broad spectrum parasite treatment, can also increase the rate of development of worm resistance to the three macrocyclic lactones – ivermectin, doramectin and moxidectin.

Bimeda has launched an initiative called Scab Control Awareness with Bimeda (SCAB). The initiative aims to generate a greater understanding of optimum control strategies for sheep scab and, in turn, reduce the rate of resistance developing to vital drugs.

The company held an information event earlier this year in the UK. The event also looked at a mobile plunge sheep dipping unit, with Bimeda backing greater use of plunge-dipping contractors to improve sheep scab control at farm level.

Optimum control

Dr Peter Bates, a specialist who has conducted a huge amount of research into external parasites, said it was important to understand the life cycle and disease characteristics in order to implement best practice control strategies.

Figure 1 gives an overview of the life cycle of the psoroptes ovis mite. The female mite lays two eggs per day for about 50 days and, with the life cycle taking just 14 days from egg to adult, the number of mites present can multiply rapidly.

Despite what is commonly perceived, the mites do not actually feed on blood. The mite secretes faeces which causes an immune response and the development of lesions.

Mites feed on the edges of these lesions and, over time, this is what causes lesions to expand. Sheep become very irritated and start to scratch vigorously and bite their wool, which also leads to wool loss and the characteristic symptoms that most farmers are accustomed to.

However, Peter said the disease can be present and spreading long before sheep exhibit these signs, with some sheep displaying no real signs of infection.

Explaining further, he said the subclinical phase occurs where mites are getting used to the sheep while an antigen response and lesions develop. The rate at which this response develops differs significantly between sheep, with some having a form of natural immunity.

The disease can also be slower to establish in some hill breeds, but this does not mean it is not present and spreading. Furthermore, he says most sheep exhibit an immune response at day 48 of infection and this can help to kill some mites and slow down the rate of spread of lesions over an animal’s body. It is for these reasons that the subclinical phase can last from a period of months to years.

It is a criminal offence in England and Wales to trade sheep that are visually infected, but, as sheep can pass under the radar, farmers need to be extra vigilant

There is also what is known as a regressive stage, where mites have spread over the surface area of an animal’s body and there is nowhere left to feed.

Lastly, there is an added complication of other external parasites, such as keds and lice for example, being present and changing the skin character, which can limit the presence of characteristic lesions caused by sheep scab.

These variable modes of action make the disease hard to identify and Peter said infection rates in a flock could range from 8% to 60%, with numbers showing characteristic symptoms as low as 1% to 5% respectively.

All of these factors combined make the disease hard to identify and can open the door for sheep that are infected but showing no signs to be introduced into a healthy flock.

Limiting entry

Given that sheep scab can pass so easily under the radar, Peter says farmers need to put more effort into ensuring all entry routes are minimised. He highlighted the main risk areas as infection being introduced through purchasing animals, with rams often overlooked, while store lambs can be a particular issue where purchased from a number of sources.

This can be compounded in the summer months when there is generally a lull in intensity of infection, with shearing capable of killing quite a proportion of mites and leading to a slow recovery time for mites to multiply. The other big area is infection spreading from neighbouring or stray animals, often a greater issue in commonage areas.

As such, Peter advised that it is important to be wary of purchasing animals irrespective of the time of year and to implement a robust quarantine period and effective health programme.

This includes dipping correctly, with the concentration of the dip tank and length of time animals spend immersed in water critical to its success.

Where animals only spend 20 seconds of the recommended minimum of 60 seconds in the dip tub, then there is likely to be 50% less chemical absorbed, while even with sheep spending 40 seconds in the tub, there will 38% less chemical absorbed.

Answering a question about the usefulness of sheep showers in controlling sheep scab, Peter advised that there is little official data on the suitability of showers to control scab.

He said there are three major constraints that prevent recommending showers for sheep scab. The first is no guidelines for the make-up or replenishment rate, it is unknown if the insecticide will penetrate through the fleece, while mites can also be present in an animal’s ears and only full immersion in a dipping tub can address this. He also raised the issue of incorrect use also increasing the risk of resistance developing.

Treatment avenues

This view was echoed by Lesley Stubbings, SCOPS an independent sheep consultant, who added: “Plunge dip products should not be used in showers or jetters. I know that is controversial, but it is illegal. There is no product licensed for use in alternatives [to plunge dipping] and if we lose dips to resistance we are in big trouble.”

Lesley said that the industry needs to get back to carrying out plunge dipping as a control measure. Putting the scale of the problem into perspective, she said there are more than 10,000 outbreaks of sheep scab in the UK each year, with a predicted cost of £20/ewe (€22.85) or £8m (€9.14m) annually.

It is a criminal offence in England and Wales to trade sheep that are visually infected, but, as sheep can pass under the radar, farmers need to be extra vigilant. Lesley said that farmers will need to work together more, particularly in hotspot areas where common grazing is contributing to the spread of scab. In such instances, sheep grazing in these areas will need to be convened together and dipped within a 24- to 48-hour period.

She highlighted the advent of mobile plunge-dipping contractors as a massive benefit in helping farmers to overcome problems. The primary treatment should be plunge dipping, with endectocides (injectable products) used very carefully due to the emergence of resistance. SCOPS is working with the industry to develop a new best-practice approach to sheep scab control and this will be available in due course.

New blood test option

Rebecca Mearns, senior veterinary adviser, Biopest. Sheep farmers now have another option in identifying scab, with a new Elisa blood test developed that can test for the presence of scab. Senior veterinary adviser with Biobest Rebecca Mearns said it is a serology test developed by scientists at Moredun Research Institute and works on the presence of an antibody response.

The test can identify the disease before clinical signs appear and will identify its presence within two weeks of mite infestation.

This then can be used to allow rapid treatment and control strategies to be put in place.

Rebecca said the test can be used strategically for numerous scenarios. It can be used to test individual animals when sheep scab is suspected and give an accurate diagnosis. It also has massive merits as a biosecurity tool, with purchased animals tested while in quarantine.

Farmers could also use it as a surveillance tool and Rebecca says this is particularly useful in common grazing and to avoid unnecessary treatments.

For flock surveillance, the recommendation is to sample 12 sheep from a group (of up to 2,000 sheep).

Individual scab blood samples cost €10.80 (£9.50), with a flock sample of 12 sheep costing €82 (£72) plus VAT (excluding vet and postage charges).

Contact your local vet for more details. Results take 10 days at the Biobest Lab in Scotland.