By Felix Meister, D.Phil., Advisor Scotland
The beginning of the new calendar year offers an opportunity to reflect on the results of the health testing in red grouse carried out by GWCT Advisory Services Scotland in 2024.
Samples
Red grouse are routinely monitored for the presence of strongyle worms in the digestive tract and for antibodies to the Louping Ill Virus (LIV) in the blood. For strongyle worm testing, estates and farms traditionally provide two types of samples: During the shooting season, grouse guts are taken from shot birds and are analysed for worm numbers. At other times of the year, caecal pats can be collected either where grouse have been roosting out on the hill or from caught birds kept overnight. These are then analysed for strongyle egg numbers. LIV testing is done exclusively in the autumn via blood samples from freshly shot birds.
The testing carried out in 2024 deviated in many ways from this usual regime. The breeding success of grouse across Scotland and the North of England was exceptionally low, which led many estates and farms to cancel most or all of their shoot days for the 2024 season. As a consequence, the number of birds available for testing in the autumn was extremely limited. Only 371 gut samples were collected by or sent to GWCT for testing in 2024, which contrasts with 915 gut samples in 2023. Likewise, only 376 blood samples were taken by GWCT for LIV testing in 2024, whereas 914 were collected in 2023.
Some estates tried to compensate for the lack of gut samples by collecting caecal pats in the autumn. This led to a higher proportion of caecal-pat data. In 2024, 222 caecal pats were analysed, which represented 37% of the total strongyle worm testing of that year. In 2023, in contrast, only 107 caecal pats were analysed, which represented 10% of the total worm testing that year. To be sure, worm counts and caecal-egg counts are equally reliable methods, but the different proportions in the two years aptly reflect the low grouse breeding success in 2024.
Regions
Estates and farms commissioning GWCT for grouse health testing are assigned to one of seven regions to allow observing trends across these larger geographical units: Angus, Borders, Dee/Donside, Dumfries & Galloway, Highlands, Moray & Nairn, and Perthshire.
For strongyle worm testing, all seven regions supplied either caecal-pat or gut samples in 2024. The highest number of samples was supplied by Borders, which supplied more than half of all samples (52%, n = 308), followed by Perthshire with 20% (n = 116) of all samples.
For LIV testing, only Angus, Borders, Moray & Nairn, and Perthshire supplied blood samples. The highest number of samples again came from Borders with 49% (n = 185) of all samples, followed by Angus with 32% (n = 120).
Results
The 2024 results for strongyle worm testing (caecal egg counts and worm counts combined) and LIV testing are detailed, by region, in tables 1 and 2 respectively.
For strongyle worm testing, a total of 593 samples produced an overall mean worm burden of 68 worms per bird. This is considered a low worm burden, as no region had a mean worm burden above the critical threshold of 1,000 worms per bird, at which we would advise immediate action. Among the seven regions represented in 2024, Angus returned the highest overall worm burden with a mean of 327 worms per bird. Dumfries & Galloway and Moray & Nairn returned the lowest overall worm burden with a mean of 3 and 4 worms per bird respectively. It should be noted, however, that the sample sizes for these two regions were especially small.
For LIV testing, a total of 376 samples returned 39 positive cases (10%). This figure is cause for some concern, especially as no region tested was found to be entirely free of LIV. However, it should be noted that only four regions provided samples, and that the total percentage is largely driven by high prevalences recorded in Angus (21.7%) and Moray & Nairn (25%) respectively. Moreover, the sample size in Moray & Nairn was relatively small, meaning that the recorded high prevalence might be a local rather than regional phenomenon.
Table 1: Results of strongyle worm testing in 2023 and 2024
Table 2: Results of LIV testing in 2023 and 2024
Looking back: 2023
Tables 1 and 2 also detail the results of 2023. There would appear to be an increase in the overall worm burden, from 48 in 2023 to 68 in 2024, and in the overall LIV prevalence, from 7.2% in 2023 to 10.4% in 2024.
However, there are limitations when attempting to compare the results of the two years. Angus, which returned the highest worm burden among the seven regions in 2024, did not provide any caecal-pat or gut samples in 2023. Likewise, the LIV prevalence in 2024 was highest in Angus and Moray & Nairn, which were not tested in 2023. In 2023, on the other hand, the prevalence was highest in Dee/Donside and in Highlands, neither of which was tested in 2024.
The different representation of regions in 2023 and 2024 makes it difficult to discern any regional or national trends.
Looking ahead: 2025
Annual health testing in red grouse, combined with accurate pair counts in the spring and brood counts in the summer, is indispensable for making informed management decisions on the part of individual estates and farms. If carried out by GWCT, test results also feed into national datasets that have the potential to identify drivers in grouse population dynamics on a regional and national level.
However, as the differences in comparing the test results of 2023 and 2024 illustrate, testing needs to be comprehensive and consistent if it is to shed any light on the distribution of strongyle worms and LIV across Scotland (and the North of England). This may serve as a strong appeal to estates and farms with grouse-shooting interests to continue and expand their testing regime in 2025.
Land managers wishing to discuss their testing regime with one GWCT’s expert advisors are encouraged to contact scottishadvisory@gwct.org.uk.