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Sheep Scab

Sheep scab, or psoroptic mange, has been identified as one of the five most important diseases for Scottish sheep farmers and is a notifiable disease in Scotland.

The disease is a form of allergic dermatitis caused by infestation of the skin surface with the scab mite Psoroptes ovis, causing intense irritation which severely compromises welfare and costs Scottish farmers an estimated £5.75M annually. 

Efforts to improve control of sheep scab are focussed on integrating policy driven legislation with emerging technologies to diagnose and protect against disease. Recent work has identified vaccination as being a realistic novel method of control for sheep scab but to better understand how to deploy the vaccine in the field we need to fully understand its effects on disease transmission and how to best integrate vaccination with diagnostic tools to achieve optimised control.

Vaccination against sheep scab

We will assess the epidemiological benefits of additional control options, specifically the vaccine under current development at Moredun. In order to achieve this we will take a two-pronged approach:

  1. First we need to better understand the transmission dynamics of sheep scab. Transmission occurs through direct contact with an infested animal or by contact with contaminated fomites in an infested environment. Our current understanding of transmission of sheep scab within a flock is limited and may be influenced by a number of factors, i.e. stocking density, stage of disease in infested animals and the length of time of exposure to infested animals. We will develop and extend models of existing data from previous penned trials, which assessed within flock transmission of sheep scab to naïve animals following the introduction of infested “index” cases. The models will then be validated using penned trials (planned within RD2.2.6 Animal Disease Epidemiology of the Strategic Research Programme (SRP)).
  2. These models will then be used to better understand the degree of efficacy required by the sheep scab vaccine in order for it to have a significant impact on disease control. Finally, the models will be used to quantify the impact of vaccination against sheep scab transmission.

Ultimately, this work will help inform the implementation of this suite of potential mitigation measures in control programs in Scotland, and also to help preparedness for future incursions of bovine psoroptic mange.

Sheep Scab diagnostics

Sheep Scab infestation may be asymptomatic and detection of mites in skin scrapings, the traditional diagnostic test is very unlikely to be successful. The inability to diagnose infection before disease has spread through the flock is an obstacle to effective infection control and disease prevention. An alternative method to diagnose infestation with sheep scab mites has been developed by researchers at the Moredun Research Institute, and involves the detection of antibodies in blood. This diagnostic blood test can reliably detect the presence of P. ovis on sheep even before clinical signs of disease are evident. Therefore, this blood test will be of particular use as an aid to control the highly infectious mite but a key issue will be the uptake of the test by Scottish farmers.

Predicting New Sheep Scab Test Uptake

Economic models have been used to predict whether farmers are likely to adopt the P. ovis blood diagnostic test for sheep scab. Using game theory, an economic modelling tool to analyse the outcome of strategic interactions between individuals, expected test prices should result in sufficient uptake by the farming industry to reduce the incidence of sheep scab by approximately 50%.

The demands placed on farmers to manage increasing information flows constitute a significant shift in farmer power from autonomy to compulsion, with farmers held accountable for information, often at the risk of financial penalties for failures to maintain accurate systems.

Key Points

  • Sheep Scab is a major issue for Scottish farmers and is a notifiable disease
  • We now have a sensitive and specific blood test to diagnose disease and subclinical infestation
  • EPIC are exploring how farmers may make decisions about using this new test to inform policy on improved control campaigns
  • Moredun have developed a prototype vaccine for Sheep Scab.
  • In order to better understand how to deploy the vaccine in the field we need to fully understand its effects on disease transmission and how to best integrate vaccination with diagnostic tools to achieve optimised control

Biosecurity

Highlighting the key biosecurity messages that are a critically important part of disease prevention and control, Battle of the Bugs focuses on sheep scab and BVD as examples of biosecurity in practice and its importance to farm profitability. The animation, >funded by Crown Estate Scotland, builds on the Biosecurity Big 5 project - an initiative lead by Moredun and Crown Estate Scotland. EPIC member Stewart Burgess contributed to the project sharing his technical expertise on Sheep Scab.

Moredun Research Institute Sheep Scab Video

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