The incidence of livestock disease is influenced by landscape and wildlife, but how well do we understand how they are linked together?
Chloe Palmer reports...
For many years, veterinary researchers have understood that the occurrence of diseases and parasites is inextricably linked to the ecosystem they inhabit.
Climate, landscape, predator and prey relationships and habitat type all influence whether disease hosts can thrive or decline.
The increasing prominence of conservation veterinary medicine as a recognised science has sought to quantify these relationships, and shed light on how and why some common animal health issues are becoming more widespread.
Independent consultant vet Iain Richards was a practising vet for more than 20 years before completing his masters degree in conservation medicine.
Gaining an in -depth understanding of ecology has allowed him to explore how changes in landscape, vegetation, climate and animal populations can influence livestock disease.
Mr Richards says: “It is important to look at what is changing within an ecosystem and what diseases might be released as a result.
“For example, if land becomes boggier as a consequence of a land management change to encourage a particular species or to provide natural flood management opportunities, fluke becomes more of an issue.
“If we plant more trees, we are likely to see an increased incidence of Lyme disease. Similarly, if we reintroduce species such as the lynx, what might be the impact on diseases?”
Mr Richards is concerned about the rise in the incidence of fluke as wetter summers and mild winters have become more commonplace.
“We must look at a sustainable model for the control of fluke, because we no longer have any drugs which work completely to control the parasite. The dry summer last year illustrated the importance of climate, as the number of snails which carry the fluke reduced dramatically.
“Trying to identify the exact ecosystem which supports the snail and the migrating fluke should help us to better understand how we might manage these areas to reduce the risk of infection,” Mr Richards says.
He suggests this information would allow farmers to plan their land management to ensure high risk animals are not grazing in areas which are most likely to be infested with infected snails.
Similarly, understanding which habitats do not support the mud snail is also helpful.
He says: “Studies suggest fluke infection is unlikely to occur on land with a pH of less than five. At Foulshaw Moss, Cumbria, despite being a very wet site, the acid conditions mean the fluke cycle cannot be completed.”
The role of ‘predators’ on the mud snail should not be discounted either, according to Mr Richards.
He says: “One farmer in Northumberland had never experienced any problems with fluke until one year when there had been an avian flu outbreak in the area. His ducks had been culled as a result and it appeared they had been eating the snails, so when the ducks disappeared, the snail population increased.”
Work on the occurrence of Lyme disease has shown a similar relationship with biodiversity. There are inevitably many reasons for the growing number of cases of Lyme disease, but land use may be just one contributing factor.
“Areas supporting low levels of biodiversity often support higher populations of rodents which carry Lyme disease and other viruses.
“Land use changes favouring species which are the competent hosts could lead to an increase in the incidence of the disease.”
The interactions between hosts, predators and disease are complex and the availability of research relevant to common livestock diseases and parasites in the UK is still patchy.
Studies carried out in South East Asia explored the links between transmission of the Nipah virus and its host, the fruit bat and livestock.
“Research showed that when native forests were cleared to make space for mango plantations, the fruit bats came closer to livestock when they came to feed on the mangoes.
“They were then able to pass the virus to the livestock, causing increased incidence of infection.”
“This finding is relevant as it demonstrates that a change in an environment can put pressure on a population which happens to be a competent host for a virus.
“This can cause this species to then pass the virus back to farm livestock, increasing the likelihood they will contract the infection,” says Mr Richards.
Other factors, such as climate, can also combine to favour particular species, which in turn, have an effect on disease levels in farm livestock.
“In some parts of the UK, we have seen a marked increase in the area of maize grown, coupled with a series of milder winters.
“Maize provides a plentiful high energy palatable food source for badgers and their numbers have been further encouraged by the benign winter weather in recent years.
“They have no natural predators and so, unsurprisingly, badger numbers have increased markedly, and we have also seen a rise in the incidence of bovine TB,” says Mr Richards.
Climate change may be a factor here and Mr Richards points to the outbreak of the Bluetongue virus in the UK in 2006 as another likely effect of the increased summer temperatures.
“The temperatures in northern Europe are normally too cold to allow transmission of the BTV8 strain. However, climate change led to the virus migrating to midges’ salivary glands and into livestock.
“If we continue to see consistently warmer weather, it is likely we will need to be alert to movements of diseases which until now have not been cause for concern.”