Climate Change in Scotland - A Paradise for Parasites?

A special report researching issues surrounding fluke by Phillip Skuce, Moredun Research Institute

Fig1 (Image © Sinclair Stammers)

It may seem somewhat ironic to be discussing climate change in Scotland when we’ve just experienced one of the coldest winters on record but, apparently, “Climate is what you expect; weather is what you get” (Robert A. Heinlein). Either way, I think we would all agree that recent seasonal patterns appear to have changed in Scotland, as they have in other parts of the UK. Summers seem to be wetter than normal, winters milder, autumns stormier and springs earlier etc.

These subjective observations are, in fact, borne out by detailed studies of our climate over the past 30 years (SNIFFER Report 2006). Temperature has indeed increased (ave., max. and min); precipitation has also increased, with more extreme events; the number of ground frost days has fallen and the
grazing season has extended by as much as a month in all areas. The latest climate projections for the next 50-100 years predict warmer, drier summers and milder, wetter winters (UKCP09).

These changes play into the hands of the helminth (worm) Adult liver fluke parasites that infect our livestock as they have larval stages and, in some cases, intermediate hosts out in the environment that are exquisitely affected by their local micro-climate. Thus the abundance of the parasites themselves and the prevalence and transmission of the diseases they cause are directly affected by the prevailing climatic conditions.

In line with the changes in our weather patterns, we have seen parallel changes in prevalence, seasonality and geographic distribution of most/allthe major worm parasitic diseases of livestock. This is demonstrated by parasites such as the liver fluke, Fasciola hepatica, which affects both sheep and cattle (Fig. 1).

Fig. 2: Severe acute fluke damage in a sheep’s liver. The pale lobe of the liver on the right has been damaged by the migrating juvenile fluke, whereas the lobe on the left has been reduced to one large haemorrhagic blood clot. (Image reproduced by kind permission, Mr Rudolf Reichel, VLA)

Liver fluke infection can cause sudden death in previously healthy animals, especially sheep, as a direct result of liver damage (Fig. 2). More chronic infections cause significant production losses in both sheep and cattle, poor lambing/calving percentages, plus the fi nancialcost of liver condemnations at slaughter.

The fluke has a complicated two-host life-cycle involving a mammalian definitive host in which the adults live and lay eggs plus a mud snail intermediate host in which the larval stages develop and multiply (Fig. 3).

The life-cycle of the fluke is intimately linked to that of the snail– no snail, no fluke – and both appear to benefit from the current changes in climate. The incidence of fluke has increased dramatically across the UK in the past 15 years

Fig. 3: Liver fluke life-cycle (Image © Moredun Research Institute)

In Scotland, QMS has reported liver condemnations of ~45% in both sheep and cattle and as high as 95% in organic lamb. Disease has continued to increase in traditional fluke areas like the west and south-west of Scotland but has also spread into previously fluke-free areas like the east and north-east (e.g. Fig. 4). Similarly, we now see disease outwith traditionally high risk periods. For example, there have been reports of acute fluke, associated with the mass migration of juvenile fluke through the liver, in early spring. This would have been expected much later in the year (Oct-Nov) and may indicate increased overwintering survivalof infectious cysts and/or snails on pasture.

In line with the changes in our weather patterns, we have seen parallel changes in prevalence, seasonality and geographic distribution of most/allthe major worm parasitic diseases of livestock.

As well as increased prevalence and spread of familiar parasitic species like liver fluke, we are also seeing the incursion of more “exotic” species, such as the roundworm Haemonchus contortus (Figs. 5 & 6). This is a highly pathogenic, bloodsucking parasite of sheep more typically associated with tropical climates in the southern hemisphere. Roundworm infection or, in this case, ‘haemonchosis’ was previously restricted to the south of England and was considered extremely rare in Scotland. However, that appears to be changing because the SAC Veterinary Investigation Services diagnosed 16 of 74 Scottish outbreaks as haemonchosis in 2007. The parasite quite possibly reached Scotland through stock bought in from the south of England. However it got here, Haemonchus now appears to be completing its life-cycle under Scottish conditions.

It is not just the types of parasite infecting our livestock and the numbers of animals that are being infected that has changed. We have also observed changing seasonal patterns of parasitic disease, again possibly as a result of changing weather patterns.

Fig. 5 Adult female Haemonchus contortus (Image © Moredun Research Institute)	Fig. 6: Adult Haemonchus in the stomach of a sheep. The small red worms are clearly visible. (Image reproduced by kind permission, Dr NeilSargison, R(D)SVS)

For example, the main worm found in sheep in the UK (Teladorsagia circumcincta), traditionally caused ill-thrift in lambs in late summer/early autumn but more lambs are being diagnosed with this worm in spring in Scotland, even on the east coast. This may be because the larvae of the worms are surviving over winter on pasture – it will be interesting to see what happens after this very cold winter.

Similarly, under favourable climatic conditions, the eggs of another worm (Nematodirus battus) can all hatch together which can cause severe “storms” of scouring and death in young lambs. The incidence of scouring due to infection with this worm has increased steadily from the mid-1990s to reach a peak in 2006, when there was an exceptionally warm early summer. So the appearance of the infectious larvae was timed perfectly to cause maximum impact on that year’s crop of lambs.

Thus we have observed increased incidence of parasitic disease, the appearance of more “exotic” species, the increased geographical spread and changing seasonalpatterns of disease. Climate change may wellbe implicated, however, it is not the only explanation.

There has been a significant increase in the transportation of livestock, especially post-FMD, and these livestock will take their parasites with them. Unless, routine quarantine drenching operations are in place, these parasites will establish rapidly on their new premises.

Furthermore, the appearance and spread of parasites that are resistant to the few wormers we have available means that some parasites will undoubtedly slip through the net and contribute to the changing disease patterns we are seeing. This is particularly true of the roundworms, in which resistance is widespread but increasingly also applies to fluke.

Finally, changes in farm management practices, for example, environmental schemes aimed at retaining or introducing wetlands to preserve biodiversity or to act as carbon sinks to offset agricultural emissions, can provide ideal habitats for liver fluke and its snail intermediate host.

So, is climate change all bad news for farmers and good news for parasites?

Well, the very fact that these parasites are so exquisitely affected by climatic conditions means that the diseases they cause are essentially predictable. Over the years, a number of parasite forecasting systems, linking incidence to prevailing weather patterns, have been developed. In their present-day incarnation, these forecasts are available online through organisations like the National Animal Disease Information Service (NADIS) which is sponsored by QMS, the other Levy Boards and industry.

NADIS provides up-to-date, monthly regional animal health bulletins aimed at raising awareness and providing an indication of specific disease risk. These are available on the QMS animal health website.

In years gone by, farmers would have been able to apply a “blueprint” for effective parasite control, with particular treatments at certain prescribed times of the year.

However, parasite control nowadays represents more of a moving target. Farmers need to monitor the parasite situation on their farms and make best use of all available information, including regional disease forecasts, localclimatic conditions and, in consultation with their vet, devise customised parasite control strategies tailored to the parasite problems on their respective farms.

Sources of additional information

An online handbook of climate trends across Scotland. Scotland and Northern Ireland Forum for Environmental Research (SNIFFER); http://climatetrendshandbook.sccip.org.uk

Baird, G. Animal Health implications of climate change in Britain. Veterinary Times, Nov 2, 2009, pp 18-19

Kenyon, F., Sargison, N.D., Skuce, P.J., Jackson, F. (2009) Sheep helminth parasitic disease in south eastern Scotland arising as a possible consequence of climate change. Vet Parasitol. 163(4):293-297

Sargison, N.D., Mitchell, G.B., Jackson, F., Gilleard, J.S. (2003) Nematodiriosis and spring teladorsagiosis in lambs. Vet Record 152(25):788

UK Climate Projections – Key findings. http://ukcp09.defra.gov.uk

First Published in Research & Development 2010