Better Pork - April 2007

Herd Health

Different PCV2 isolates show differences in virulence

That’s just one of the findings from papers presented on Porcine Circovirus Associated Disease (PCVAD) at the 19th IPVS Congress in Copenhagen last July

by S. ERNEST SANFORD

This is a continuation of reports on some of the papers presented on Porcine Circovirus Associated Disease (PCVAD) at the 19th IPVS Congress in Copenhagen in July, 2006.

PCV2 Isolates Differ in Virulence
In this paper, Dr. Tanya Opriessnig, a member of the Iowa State University Circovirus research group led by Dr. Pat Halbur, presented clear experimental evidence that different Porcine Circovirus type 2 (PCV2) isolates can have different degrees of virulence.

The researchers took two PCV2 isolates, PCV2 #1 and PCV2 #2, and infected two groups of 14 pigs each with one or other of the two isolates They then used several measurements to evaluate the impact of the two isolates on the pigs. A third group of 14 pigs was left as uninfected controls.

Their experiment indicated that pigs infected with PCV2 #1 isolate developed significantly more severe PCVAD, identified clinically and by gross and histopathological examination, than the pigs infected with the PCV2 #2 isolate. The pigs infected with the PCV2 #1 isolate also seroconverted more quickly and had greater amounts of virus circulating in their blood and tissues than those infected with the PCV2 #2 isolate. 

The PCV2 # 1 isolate came originally from a pig with severe clinical PCVAD in 1998, whereas the PCV2 #2 isolate came from a pig with no clinical PCVAD in 2003.

Take-home message. The results of this study provide strong and very convincing evidence that there are differences in virulence and disease-causing abilities among the different isolates of PCV2 viruses.

PCV2 Virus Can be Transmitted via Raw Pork
Dr. Tanya Opriessnig of the Iowa State University PCVAD research team was again the presenter of this paper.

The study was conducted, in part, as a result of New Zealand (and Australia) claiming that outbreaks of PCVAD which occurred in herds on the south island of New Zealand in September, 2003, were the result of transmission of PCVAD to their domestic pigs from raw pork that came from PCV2-infected pigs imported from other countries. 

The researchers experimentally infected pigs with PCV2 virus. Then, 14 days later, they fed portions of loins, lymph nodes and bone marrow from the pigs experimentally infected with PCV2 to PCV2-naïve pigs. They fed these tissues individually and in various pooled combinations. 
The results were that PCV2 was transmitted to the PCV2-naïve pigs in all of the various feeding combinations used.

Take-home message. PCV2 can be transmitted to naïve pigs by feeding fresh pork or pork products to PCV2-naïve pigs. It should be noted that, because of the ubiquity of PCV2 in the pig population and the long viremia known to occur in pigs infected with PCV2, this ability has most likely been in existence for at least several decades, and almost certainly ever since the PCV2 virus has existed in pigs.

PCVAD Outbreak Pattern in Swedish “Satellite” Production Units
Some background is needed here. In Sweden, a common production system consists of sows housed in a central facility and rotated through piglet production units (called “satellites”).

This is how they work. Dry sows are kept in a central sow-rearing unit during breeding and gestation. Just before farrowing, sows are leased to the production satellites, where the piglets are birthed and reared to market weight. You can think of it as a sort of expanded version of our Ontario production “loops.” After weaning, sows are returned to the central unit for breeding and gestation.

This system works on a 16-week cycle at the satellites. Pregnant sows arrive three weeks before farrowing, their litters are weaned at five weeks of age, the satellite producers return the sows to the central unit, and the cycle is completed by the arrival at the satellite of a subsequent batch of sows eight weeks later.

The 16-week cycle allows for adequate cleanup and disinfection time between batches.  Recently, some satellites have gone to shorter cycles to get increased production levels, which inevitably results in shortened times for cleanup and routine hygiene measures.

The system in this study contained 22 satellites, of which four were diagnosed with PCVAD in the fall of 2004. Performance of sows in three of the PCVAD-affected satellites was compared with that of four non-PCVAD-affected satellites and the performance of 415 dams with litters affected by PCVAD was compared with performance of the same sows and their litters in the next mating.

Results: All four satellites diagnosed with PCVAD in 2004 were on shortened cycles (three on seven-week cycles; one on a three-week cycle) instead of the conventional Swedish 16-week satellite cycle.

Sow productivity was similar in the PCVAD-affected and non-affected satellites.  Postweaning mortality was higher in the PCVAD-affected herds, but those herds also had higher daily weight gains. Culling rates were similar in both groups. PCVAD was again diagnosed in the piglets of the next farrowing if the next farrowing for the sow was in a PCVAD-affected herd. Piglets from sows leased to a non-PCVAD herd for the next farrowing were not diagnosed with PCVAD.

The four PCVAD-affected satellites reverted to 16-week cycles and also removed soybeans and decreased protein content in the diet. One of the four affected satellites was no longer PCVAD-affected at the time of writing of the IPVS abstract. However, a fifth satellite, this one on a conventional 16-week cycle but with severe management problems, was diagnosed with PCVAD in December 2005. The other 17 satellites remain unaffected.

Take-home message. Increased production pressure and inadequate inter-batch cleanup and routine hygiene measures were singled out as major contributing factors to the precipitation of PCVAD in this system. It was also noted that reducing the production pressure (i.e. reverting back to a 16-week cycle) did not automatically guarantee resolution of the PCVAD situation. 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

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