From exotic to endemic: a new disease of pigs
Severity indicator helps farmers and scientists gauge impact on farms.
Research is underway to tackle a new and mysterious disease that threatens young pigs in the UK and Europe. Scientists have taken the first steps toward understanding the disease by developing a model to assess the severity of post-weaning multi-systemic wasting syndrome (PMWS), a new but common disease in the UK which was first seen in England in 1999 and Scotland in 2002.
Unlucky siblings. The two smaller pigs top right are badly affected by PMWS compared to the larger, healthy pig in the foreground. All are from the same litter. Image: RVC
Scientists know very little about PMWS except that the symptoms are debilitating, unpleasant and painful. The disease strikes young pigs from about six weeks after they are removed from their mothers; they then lose weight, have difficulty breathing and suffer from symptoms such as fever and diarrhoea as they slowly become emaciated (ref 1, ref 2, ref 3).
Mortality is relatively high at up to 30%. The disease estimated to cost the UK £30M per year; across Europe the costs are between EUR562-900M.
Just a virus?
The precise cause of the disease is not known although a virus, porcine circovirus type 2 (PCV2), is associated with the condition. However, there is not a straightforward relationship between PCV2 infection and PMWS - many farms with the virus do not have sick pigs. Furthermore, the disease has yet to be artificially reproduced under controlled conditions, which is one of the tests used to identify an infectious agent.
"We know from other studies in Europe that they didn't have the disease before this virus occurred," says Professor Dirk Werling, who is based at the Royal Veterinary College (RVC), Hertfordshire, UK. "But the causative agent PCV2 on its own does not seem enough to cause disease, which is a typical feature of a multi-factorial disease," he says.
PMWS, seen in the pig top compared to a healthy one below, is a complex syndrome. Image: RVC
Thus, another infectious agent, such as bacteria, a mycoplasma, or another virus may be involved. "Genetic components of the pig might play a role as well," says Werling. "Indeed, anything that causes stress to the animal may cause the outbreak of PMWS, such as density of housing, not enough changes in diet, anything that we would also consider a stress."
Werling leads the £2.4M project aims to identify why PMWS occurs and to develop new methods to control the disease. It is funded by BBSRC through its Combating Endemic Diseases For Farm-Animal Sustainability (CEDFAS) programme (see 'Combating animal diseases' box) and includes government partner the British Pig Executive (BPEX), and two industrial partners, Pfizer Ltd (UK) and BioBest, a diagnostic company.
The project involves British pig farmers directly from the outset so that pigs and pig farmers in Britain will be the first to benefit from the research. Moreover, it is hoped that close collaboration with a major international pharmaceutical company will ensure that effective products will be made available to pig farmers around the world in the shortest possible time.
Professor Dirk Werling. Image: RVC
Werling says that when his team started their research in 2006 they knew nothing about the status of the disease England and that no one was researching it in Scotland or Wales either. "The first news was that PCV2 present in England on a shocking scale. Present in 99% of English farms before they started vaccinating," says Werling. ""We have a real endemic disease at the moment and we may really struggle to get rid of it or we may not get rid of it at all."
Because there was a large variability in how farms were affected by PMWS, it didn't make sense to distinguish between PMWS-infected farms and PMWS free ones; instead it seemed logical to collect data from farms and investigate risk factors for severity of the disease which lead to the PMWS severity calculator.
Two members of his team, Dr Barbara Wieland and second-year PhD student Pablo Alarcon-Lopez, visited 147 pig farms in England and quantified the severity of PMWS based on morbidity and mortality data and looked for the presence of PCV2.
Time on each farm was limited to a few hours, so Werling's team collected qualitative and quantitative data and included the views of the farmer, including questions to assess the farmer's knowledge of the disease. This, combined with the information from vets and misclassification trees to identify farms that had to be excluded from the analysis, formed the data. "Overall we were very pleased to see how much producers know about PMWS and most farms could be included in the analysis," says Werling. "This probably also reflects how much problems this disease syndrome has caused in the past."
Sense and surveillance
They then used the data to develop a PMWS severity calculator, which now can be used by farmers directly to assess the PMWS severity on their own farm. Although it's a relatively small step it's an important one: because the disease is so new, many farmers may over or underreport problems with PMWS. The severity calculator can now tell them when not to worry too much and when to call for help.
Furthermore, work with industrial partners is now underway to develop the severity calculator so it can be used to tackle the economic impact disease. For instance, the severity data might identify physical or biological risk factors common to badly affected farms, such as the presence or absence of other wild or domesticated animals. The model can also be used to assess the efficacy of PCV2 vaccination on a farm, and investigate the economic cost-benefits of new or existing control measures on a farm.
The PMWS severity calculator assesses the disease's impact on an individual farm. Click image for a larger version.
Currently, there are two vaccines for the disease. The British Pig Executive (BPEX), a part of the UK government's Agriculture and Horticulture Development Board that focuses on enhancing the competitiveness, efficiency and profitability for English pig producers, began a vaccination programme during the study which has yielded some early before-and-after indications of their effectiveness. For one of the vaccines, production data gets better. "Visiting farms after vaccination we see a general improvement. But the vaccine doesn't clear the virus so the animals are still infected," say Werling.
Werling's work has also revealed that there is no correlation between different viral strains and the severity of PMWS on a farm. In addition, the team is now also trying to re-create the disease under experimental conditions, using data obtained in the epidemiological and immunological analyses. The immunology strand of the project hopes to identify the changes the virus is making in three different cell types; the hope is that if a single response is found in different cells a biochemical signature pathway can be exploited to develop drugs. "If we are able to recreate the disease we can identify pathways for the virus and then we can interfere with them," says Werling. "And that's where the industrial partners come in."
Infected pigs seem to harbour PCV2 in immune cells, despite there being no obvious viral replication or functional differences in infected cells. But once PMWS occurs, significant changes in the subpopulations of immune cells occur, suggesting PMWS-infected pigs are unable to mount an effective immune response. However, at present the mechanisms by which the immune system is altered is not understood.
The £2.4M PMWS project is funded by BBSRC through its Combating Endemic Diseases For Farm-Animal Sustainability programme (CEDFAS). Launched in July 2007, the £11.4M project aims to tackle endemic animal diseases that undermine the UK farming and cost UK farmers (and consumers indirectly) hundreds of millions of pounds a year as well as causing significant animal welfare problems.
The 10 grants awarded to researchers will better scientific understanding of the behaviour and spread of diseases such as infectious bronchitis, bovine tuberculosis, and those caused by parasitic nematodes.
CEDFAS is led by BBSRC and is backed by the Scottish Executive; some individual projects have additional funding from Defra and industrial partners .
Werling is impressed by the CEDFAS call. "It was targeted directly to endemic diseases which I think this is something we should follow up and not lose again."
- Assessment and quantification of post-weaning multi-systemic wasting syndrome severity at farm level
- Postweaning multisystemic wasting syndrome (PMWS): the first year with the disease in Sweden
- Descriptive summary of an outbreak of porcine post-weaning multisystemic wasting syndrome (PMWS ) in New Zealand