Respiratory cryptosporidiosis in red grouse

Infection by cryptosporidiosis has impacted grouse, with infected birds having 51% lower survival and rearing 43% fewer chicks relative to healthy birds on the same moor. © Laurie Campbell

BACKGROUND

Infection by Cryptosporidium baileyi, a parasitic protozoan, causes respiratory cryptosporidiosis in red grouse. It was first diagnosed in 2010 and has spread with infection rates higher in young birds. Understanding underlying causes of disease emergence and routes of infection transmission are fundamental to its subsequent control. Cryptosporidia are protozoan parasites that are widespread among vertebrates, causing gastrointestinal diseases in mammals and reptiles and enteric, renal and respiratory diseases in birds. Two species, Cryptosporidium baileyi and C. galli, are restricted to birds. In most cases, pathogenic infection has been restricted to domestic poultry, captive-reared gamebirds or other birds kept in captive collections. In 2010, red grouse in northern England were clinically diagnosed with respiratory cryptosporidiosis following infection from C. baileyi.

Within three years of the first outbreak, our questionnaire survey to gamekeepers showed that signs of infection were detected from grouse on half of 150 grouse moors in northern England. Reported prevalence among shot birds, confirmed in the field by catching at night and visual screening, has been low, typically averaging 4% of birds. Infection by cryptosporidiosis has impacted grouse, with infected birds having 51% lower survival and rearing 43% fewer chicks relative to healthy birds on the same moor. Thus, should prevalence increase, respiratory cryptosporidiosis could markedly impact upon shooting bags and grouse moor economics.

To monitor disease prevalence, we examined 45,914 red grouse shot between 2013-18. Birds were sampled from 30 beats of 10 driven grouse moors in northern England. Grouse were aged, sexed and visually screened for signs of infection. Prevalence varied with age, being twice as high in juveniles (4.5%) as in adults (2.4%). It also varied nine-fold between moors from 1.0 to 8.6% and three-fold between years (see Table 1). Patterns of infection among grouse age groups and across years were consistent across study moors.

TABLE 1

The mean (± one standard error (se)) annual prevalence of signs of respiratory cryptosporidiosis among shot adult and juvenile red grouse in relation to the number of birds screened

Adults Juveniles Year N Screened Mean (se) Screened Mean (se)

2013 12 685 3.9 (1.2) 1,725 3.6 (0.7) 2014 59 4,029 3.3 (0.4) 11,205 7.4 (0.6) 2015 29 2,547 1.0 (0.3) 5,628 2.5 (0.4) 2016 24 1,809 1.1 (0.3) 4,575 2.2 (0.4) 2017 31 2,791 3.0 (0.6) 4,706 4.5 (0.7) 2018 9 919 1.2 (0.5) 1,377 2.3 (0.6) Total 164 12,780 2.4 (0.2) 29,216 4.5 (0.3)

*Annual means (± one se) are based on the number of shoot days when grouse were screened (N).

Our results are consistent with the concept that disease incidence is highest in naïve juveniles that have previously not been exposed to infection, with prevalence dropping as birds develop immunity. We found no evidence of increased disease prevalence over time. Indeed, samples taken in 2019 showed a further drop with less than 1% of shot birds examined showing symptoms. To date, fears of escalated disease prevalence, bringing with it increased mortality and lowered productivity that may have significant impacts on the economic viability of shoots, have not yet been realised. We will, however, continue annual screening for symptoms among shot birds, as our current time series of only seven years is too short to make future prevalence predictions.

On welfare grounds, we recommend the selective culling of diseased individuals, which can be distinguished by their unresponsive avoidance behaviour on approach, and improved hygiene at communal gritting stations used for strongyle worm control, which may form reservoirs of Cryptosporidium infection. Ultimately, respiratory cryptosporidiosis may be best managed by reducing currently high grouse densities, either through less intensive management, including only using anthelmintic treatments when necessary, increased shooting rates or both. Without it, given the increasing wider environmental concerns about the intensification of grouse moor management, such approaches may be enforced by tighter Government regulation. KEY FINDINGS

Respiratory cryptosporidiosis quickly spread from first diagnosis to infection of grouse on most moors in northern

England, where it impacts upon their survival and productivity.

Scares regarding the economic impact of infection on grouse shooting have to date been unfounded because prevalence has remained low, averaging 2.4% in adults and 4.5% in juveniles.

Infection may be minimised by selective culling of diseased animals, better hygiene at communal gritting stations and reductions in overall grouse densities.

David Baines David Newborn Michael Richardson

Disease incidence is highest in naïve juveniles that have previously not been exposed to infection, with prevalence dropping as birds develop immunity. © Laurie Campbell

ACKNOWLEDGEMENTS

We acknowledge the generous funding provided by Roger Henderson QC on behalf of the G & K Boyes Charitable Trust.