Better Pork - February 2007

Herd Health

PCVAD and PRRS dominate IPVS congress in Copenhagen

Papers presented at the 19th International Pig Veterinary Society Congress last July could have a significant impact on how these diseases are dealt with. Here are some take-home messages

by S. ERNEST SANFORD

Last July, I attended the 19th IPVS (International Pig Veterinary Society) Congress in Copenhagen, Denmark. Attendance figures exceeded all previous IPVS Congresses, with some 2,410 registered delegates. When accompanying persons, students, exhibitors and invited guests are added in, overall attendance approached 3,000, easily surpassing the previous record attendance of 2608 set in 2004.

The IPVS is held every second year in different countries around the world. Attending delegates vote for the location of the Congress four years hence. It has been held in North America on three occasions in the past - twice in the United States and once in Mexico, so it was particularly gratifying to all Canadians present in Copenhagen when Canada won the bid to host the 21st IPVS in Vancouver in 2010 on the very first ballot. The next IPVS will be held in Durban, South Africa, in 2008.

Papers on porcine circovirus associated diseases (PCVAD) and porcine reproductive and respiratory syndrome (PRRS) surpassed all other topics in the oral presentations section in Copenhagen. The following are some of the PCVAD papers that caught my attention and I think could have a significant impact in the field.

Correlation Between Clinical and Laboratory Diagnoses of PCVAD
A Danish group (Jorsal S.E., et al.) reported on a case-control study conducted in 2003-04. For the study, 74 herds exhibiting clinical signs of PCVAD were matched with 74 herds not exhibiting PCVAD. Three pigs were submitted to the Danish veterinary diagnostic laboratory from each of the 148 herds for necropsy and histopathological examination.

PCVAD was confirmed in 58 of the 74 PCVAD-positive case herds, but PCVAD was also confirmed in 19 of the 74 of the PCVAD-negative control herds. In 12 of the 19 control herds (i.e. “clinically-negative” for PCVAD) that had PCVAD, only one of the three submitted pigs had PCVAD. In six of those 19 control herds, two of the three submitted pigs had PCVAD and, in the 19th herd, all three submitted pigs were diagnosed with PCVAD.

As a result of the findings in this study, the declaration of PCVAD-free (PMWS) status for Danish SPF herds was cancelled in 2005.

There are at least two take-home messages of note here:

• There is a poor correlation between clinical observations and diagnosis at the farm level, versus the definitive laboratory diagnosis of PCVAD.
  
  
• As more pigs are submitted for a laboratory workup and diagnosis, the probability increases for a diagnosis of PCVAD.

PCVAD Pigs Fail to Develop Serum Neutralizing Antibodies against PCV2
Two different research groups presented work showing that pigs that develop PCVAD do not produce serum-neutralizing (SN) antibodies (Abs) to PCV2 after infection with the virus. Here’s a quick snapshot of what those two research groups presented.

The first, a Belgian team (Lefebvre D.J., Meerts P., et al.) in collaboration with Danish researchers (Nielsen J., Bøtner A., et al.) reported that pigs in which PCV2 replicated to very high levels did not have SN Abs. They also showed that the pigs which produced high levels of PCV2 SN Abs after PCV2 infection did not go on to develop PCVAD. These latter pigs remain sub-clinically infected with virus, produce high levels of SN Abs and do not develop PCVAD.

The second group, a Spanish research team (Fort M., Mateu E., Segales J.), took a somewhat different approach in their investigation. They took a one-time snapshot of the IgG (regular Abs that we usually measure) and SN Ab levels in 35 randomly selected, two-to-four-month-old pigs. They grouped these pigs based on whether they had already been PCV2-infected or not PCV2-infected. They then further divided the PCV2-infected pigs into PCVAD clinically affected versus non-clinically (i.e. sub-clinical) affected groups.

What they found was that both the PCV2-infected and non-infected pigs were able to produce SN Abs to PCV2. However, pigs that were already clinical PCVAD pigs developed significantly lower (or no) SN Abs to PCV2. The researchers postulated that the high sustaining SN Ab titres to PCV2 were probably involved in preventing PCV2 from multiplying to high levels in those pigs, thereby protecting them from becoming clinical PCVAD pigs.

They also pointed out that pigs, which did not develop high PCV2 SN Abs after infection, were still able to produce high levels of regular IgG Abs to PCV2. These findings further indicate that we would be fooling ourselves if we attempted to use the regular IgG Ab tests to measure protection against PCVAD.

So what does all this mean down on the farm? There are at least a couple of take homes that I deduced from the above studies.

We are possibly getting to the stage where we can measure something in the live pig that tells us which pigs will get PCVAD and which pigs will not. Remember that virtually all pigs in the world get infected with PCV2, so it has been very difficult to separate out those that will succumb versus those that will survive. Now we might be able to.

Secondly, just looking at the regular IgG PCV2 Abs is not good enough to predict which pigs will get PCVAD. But we’ve known that for some time now. BP

S. Ernest Sanford, DVM, Dip. Path., Diplomate ACVP, is a swine specialist with Boehringer Ingelheim Vetmedica (Canada) in Burlington. Email:esanford@bur.boehringer-ingelheim.com