Home > Dr Trish Holyoake Publishes Article on PMWS in Pork CRC’s Newsletter

Dr Trish Holyoake Publishes Article on PMWS in Pork CRC’s Newsletter

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Dr Trish Holyoake, University of Sydney has written an article in Pork CRC’s newsletter on how post weaning multi systemic wasting syndrome (PMWS) affects weaner pigs. They are as follows: 

It is now known that Porcine Circovirus Type 2 (PCV2) is necessary for the syndrome to be expressed. Australia remains one of the few countries where PMWS has not been diagnosed, despite PCV2 being widespread in the pigs. 

PMWS was first reported in Canada in 1991, with cases subsequently reported in Europe. 

The first cases of PMWS occurred in Denmark in 2001 and in Sweden in 2003. Norway had two cases in 2003 and managed to stamp these out. More cases appeared in the country following the eradication. 

New Zealand had its first case in the North Island in 2003 and subsequent cases in the South Island in 2006. Finland has only recently become affected (December 2007). 

Disease Syndromes 

A number of disease syndromes have recently been linked to PCV2 including reproductive failure, respiratory disease, enteric disease, congenital tremors and porcine dermatitis nephropathy syndrome. The role of PCV2 in these diseases is still under investigation.

In December 2004, a consortium of researchers from 16 organisations across Europe and North America were funded by the European Union to study diseases associated with PCV2. The consortium annually holds a Specific Support Action (SSA) workshop to disseminate key research findings and Dr. Trish Holyoake attended a recent SSA meeting in Budapest. In this article, she has summarised some key findings from the workshop. 

Immune Activation 

PCV2 is a very small virus and, like all viruses, it relies on the cells of its host (the pig) to multiply sufficiently to cause disease.

The results of genomic studies of PCV2 suggest that the virus only has DNA to (1) start replicating and (2) to put a capsid (‘shell’) around the end product. All other functions must be provided by the host cell.

From several experimental challenge models developed to reproduce PMWS in the laboratory situation, it is generally accepted that the presence of PCV2 alone is not enough to reproduce PMWS as seen on farms.

Other infectious or non-infectious factors are required for the development of the full clinical disease.

These ‘factors’ can be pig pathogens e.g. parvovirus and porcine reproductive and respiratory syndrome virus (PRRSV). They can also be non-infectious immuno-suppressors or immuno-stimulators. 

Given the wide range of potential ‘activators’, the risk of an individual pig becoming infected with PCV2 is high.

Full-blown PMWS may occur in a herd when many pigs are exposed to the immuno-modulator. This assumes the ‘right’ strain of PCV2 is present. 

Right Strain

There is currently no standard system for classifying different PCV2 strains, with different countries having their own way of naming them. One of the objectives of the European consortium is to standardise them.

It appears that differences exist in the virulence of PCV2 strains. For example, there has been an increase in the incidence of PCV2 strain ‘321’ (‘new’ strain) in Canada since 2004.

This has been associated with an increase in PCV2-related disease. However, these virulence differences have not fully explained the differences in clinical disease observed in the field. 

Vaccines available There are now four commercially available vaccines against PCV2 disease. The first registered was manufactured by Merial (Circovac) and is registered for use in sows pre-farrowing to boost maternally-derived protection in piglets against PMWS.

Approximately 25% of sows in France, where Circovac was first registered, are vaccinated and 17% in Denmark and Germany. The growth and reproductive performance in herds is reported to improve after vaccination commences (i.e. when ‘before’ and ‘after’ performance is compared).

Independent studies in Sweden, where the progeny of vaccinated and non-vaccinated sows have grown out at the same time, also suggest this vaccine improves the growth performance of pigs in herds with PMWS.

In 2008, Boehringer Ingelheim registered a 1-shot PCV2 vaccine for piglets (Ingelvac Circoflex) in the EU and this opened the door for other piglet vaccines to be used in Europe.

Feedback from veterinarians working on-farm is that the vaccines appear to work well. 

Herd Mentality

Despite extreme research effort, it’s still unclear why some herds get PMWS and others do not. With the heavy reliance of PCV2 on its host for replication, the pig itself must be very important in determining the ability of the virus to cause disease.

University of Copenhagen research into the role pig genetics play in disease susceptibility may help explain Australia’s freedom from PMWS.

The genotypes of early PCV2 isolates in Australia have been sequenced (Muhling et al., 2006) and appear to fit into the ‘422’ (‘old’) family of PCV2.

It would be useful to know the genotype(s) of more recent PCV2 isolates to understand why we have not had the ‘classical’ PMWS outbreaks other countries have had. 

Vaccine Option

Vaccines offer an excellent control option for PCV2 disease. It would benefit Australia to consider using these vaccines should PCV2 disease occur here.

Preliminary discussions now, with relevant government agencies and pharmaceutical companies, would speed up registration and importation for the future.

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