Emerging Diseases:  Lessons from the UK PMWS Experience

 

Eric Neumann, DVM, MS, Director, Swine Health Information and Research, National Pork Board

 

 

Introduction

Surveillance for emerging animal diseases is an important tool for maintaining the health of livestock in the U.S.  Both industry and government share in the need for this tool as all farm sizes and types are likely to be affected, spread between farms and between regions of the country can be expected to be rapid, and access to existing and new export markets would be reduced.  In the swine industry alone, several diseases have emerged or “re-emerged in the last 15 years.  These include Porcine Reproductive and Respiratory Syndrome virus, numerous strains of swine influenza virus, E. coli F18, Salmonella DT104, Erysipelas rhusiopathiae, and porcine circovirus-associated diseases such as postweaning multisystemic wasting syndrome and porcine dermatitis and nephropathy syndrome.

 

Circovirus (PCV-2) associated diseases in swine have been recognized in many countries around the world, including the US and the UK.  The most well known PCV-2 associated clinical diseases have been termed Postweaning Multi-systemic Wasting Syndrome (PMWS) and Porcine Dermatitis and Nephropathy Syndrome (PDNS).

 

Based on the presence of PCV-2 and similar lesion descriptions, many believe that these clinical syndromes are the same in both the UK and the US.  However, distinct clinical differences do exist raising the issue as to whether the diseases exist in the US at all or if they simply differ in their significance in the respective countries.  PMWS primarily affects nursery pigs (6-12 weeks of age) in the UK while PMWS is more frequently associated with finishing or older nursery pigs (10-16 weeks of age) pigs in the US.  UK mortality rates associated with PMWS and PDNS has been described in the 20-40% range.  Mortality in the US and Canada typically ranges from 3-10%.  Britain’s Meat and Livestock Commission estimated the two syndromes cost its swine industry $31 million in 2001.

 

The National Pork Board in conjunction with USDA, sponsored a study trip to the UK in an effort to further understand the differences between the US and the UK in regard to the prevalence and severity of PMWS.  A cross-functional team of veterinarians, producers, epidemiologists, pathologists, and clinicians spent a week in the UK during November of 2002 to carry out this mission.  Representatives from industry, production, and government in the UK were instrumental in helping to coordinate our trip and we are very grateful for their help and participation.

 

Current Status of the Disease in the UK

The label “PMWS” contains such wide variation in actual clinical disease that it is important to define a “case.”  The scientific literature has suggested that the presence of wasting accompanied by histologic evidence of lymphoid depletion and presence of the virus defines PMWS.  In reality, a trip to an affected farm displays an array of clinical specimens that paint a much fuzzier picture.  For example, does a typical PMWS pig waste over a matter of a few days or a few weeks?  Is PMWS preceded by, followed by, or unrelated to the occurrence of PDNS in pigs/farms?  Does PMWS make other disease conditions worse or do other diseases make PMWS worse?  Is a PMWS pig predestined to become a case or is each new day in the life of a pig an independent event with respect to his risk of becoming affected? 

 

The relationship between PDNS and PMWS is far from understood.  Some producers and veterinarians adamantly believe PMWS infection on a farm begins with an outbreak of PDNS but that it would eventually disappear or “change into” very classical PMWS over a period of months.  Others are convinced of the opposite sequence of events.  Rarely do both syndromes show up on the same pig.  Occasionally, both syndromes can be represented in an affected group. 

 

Many other diseases seem to occur concurrently with PMWS.  Both PRRS and M. hyopneumoniae infections (PRDC in US terms) are common respiratory pathogens.  Systemic infections including PRRS, H. parasuis, and S. suis are also present.   The frequency of enteric infections occurring along with PMWS is unknown but is apparent in some groups.

 

Reproductive failure associated with PCV2 in the UK has been suggested but is not thought to be a frequent event.

 

What is the cause of PMWS?

Researchers in the US and elsewhere have produced convincing evidence that PCV2 alone can cause the lesions and clinical signs commonly seen in PMWS.  However, we also know that many farms in the US and the UK are positive for the PCV2 organism but don’t show the classical clinical signs of the disease.  Clearly, there must be more to the story than simply being PCV2 positive.

 

One theory is that additional risk factors must be present (in addition to the presence of PCV2) in order for clinical signs of PMWS to appear.  These possible triggers may include:

1. Other infectious agents including PRRSV, Haemophilus parasuis, Salmonella spp. and others.

2. Unique feeding practices or ingredients.

3. Various management or pigflow techniques.

4. The potential role of mycotoxins.

5. Genetic, parity, gender, and individual pig factors.

6. Gilt isolation and acclimatization procedures.

7. Weaning age.

The relative importance of any of these potential risk factors is unknown.  However, they should provide a good framework for further study of the disease in the US and the UK.

 

Challenges in Developing an Emerging Disease Surveillance System

Countries need to begin working together to protect their industries from emerging diseases that are yet-to-be identified, but can be diagnosed symptomatically in conjunction with defined case criteria.  While overly broad application of a system like this could be used inappropriately by countries wishing to limit their trade obligations, proper use of the same system could improve the future health of all country’s livestock industries.  Early detection of emerging diseases would be expected to minimize the impact on production and allow rapid implementation of prevention strategies. 

 

Development of emerging disease surveillance programs will require a coordinated effort by governments and an international exchange of information, scientists, diagnostic technology, and disease surveillance tools in order for creation of a successful model.  Participants should be encouraged to review information outlined in the 1999 Swine Futures Report for guidance.

 

Summary

The UK has clearly experienced a much severe form of PMWS, and at a higher prevalence than the US.  Research needs to be continued to sort out the importance of different risk factors as well as to elaborate the interaction between PCV2 and other swine pathogens.   The National Pork Board has identified the issues of disease surveillance, and detection of emerging diseases (specifically, PMWS) as important priorities for Pork Checkoff.  We will continue to fund work in these areas and help to support US swine producers in their need to maintain farms with the highest standards of health.