Herd Health: Mapping Ontario's PRRS virus strains

Friday, August 8, 2008

The 'PRRS Project' is well on its way to tracing the new and virulent strains of PRRS virus that caused havoc in Ontario herds in 2004 and 2005

by S. ERNEST SANFORD 

It's almost three years since we embarked on "the PRRS Project" after the new and highly virulent strains of Porcine Reproductive and Respiratory Syndrome virus swept through Ontario in the fall of 2004 and the winter and spring of 2005, causing epidemic PRRS outbreaks in many herds.

PRRS virus mapping – funded by Ontario Pork and colloquially termed "PRRS Project #1" – was first off the mark and got started in January 2006.  I'll be providing updates on the other PRRS Project activities being undertaken by the Ontario Pork Industry Council Swine Health Advisory Board (OSHAB) at a later date.

The mapping project was designed as a case-control study. Cases were tracked via samples taken by veterinarians from herds experiencing PRRS outbreaks and submitted to the Animal Health Laboratory, University of Guelph. Control herds were identified by practicing veterinarians and included in the study.

The study was broken into two phases: a prospective portion which started in January 2006 and a retrospective portion which retraced data back to September 2004, the start of circulation of the current highly virulent PRRS strains which triggered the epidemic. Polymerase Chain Reaction (PCR) test on PRRS viruses and sequencing of open reading frame 5 (ORF 5) of the PRRS virus were conducted on all PRRS viruses from prospective and retrospective cases.

Participating producers were interviewed to collect management data and the geo-location of their herd was recorded.

PRRS viruses were sequenced from 455 case herds. Farm management data were collected from the 455 herds and 58 PRRS control herds via a telephone survey. The database encompassed 382 separate sites as multiple submissions occurred from some sites during the study period.

Three main risk factor groups were used as a preliminary risk factor analysis – herd demographics; frequency of purchasing breeding stock and number of sources; and biosecurity protocols related to trucking breeding stock in recipient herds.

Herds were classified according to whether they were farrow-to-finish, farrow-to-feeder, nursery only or finisher only.

The analyses conducted so far have been restricted to herds that have a farrowing component (i.e. farrow-to-finish, farrow-to-wean and farrow-to-feeder herds) as part of the operation. All sites were then classified in one of the following three categories:

• PRRS-negative (no PRRS virus circulating);
• PRRS-positive for field (wild-type) virus;
• PRRS-positive for any PRRS virus (wild-type or vaccine-like virus)

We found that PRRS-positive herds and PRRS-negative herds were similar in their risk profiles for frequency of delivery of replacements (gilts and/or boars) and number of external sources of replacements.

However, the case herds (herds with recent PRRS breaks) differed from the control (PRRS-negative and no recent history of PRRS breaks) in certain biosecurity practices related to trucking of breeding stock.  Among these were:

Truck washing and disinfecting
Compared with PRRS-negative herds, PRRS-positive herds were 10 times more likely to receive replacement gilts in trucks that were not washed. They were also four times more likely to receive stock in a truck that was not disinfected between deliveries.

Furthermore, wild-type viruses (as opposed to vaccine-like viruses) were more often found in herds which had their gilts delivered in trucks that were not always washed between deliveries.

Drying of trucks after washing
Herds which received gilts on trucks that were not dried for at least 24 hours after washing were twice as likely to be PRRS-positive as herds receiving deliveries in trucks that were dried.

Herd type as a risk factor
Farrow-to-finish and farrow-to-feeder herds were more likely to be PRRS-positive than farrow-to-wean herds. A farrow-to-finish herd was nine times more likely to be PRRS-positive than a farrow-to-wean herd. Wild-type PRRS virus infections were more likely to be in farrow-to-feeder than any other type of herd housing sows. The researchers cautioned, however, that since they had only one farrow-to-feeder control herd, the results for this particular parameter might be unnaturally skewed.

Of the 493 separate PRRS viruses sequenced for this study, 77 per cent were wild type and 23 per cent were vaccine-like viruses. The connectedness of the PRRS virus in one herd outbreak and its relationship to PRRS viruses in other herd outbreaks were a major reason for embarking on the PRRS Project and for initiating this particular study.  How are these various strains interrelated?  How were they connected to each other?  What did the researchers find after analyzing the sequences of the 493 PRRS viruses?  Here are a few clues starting to emerge.

When vaccine-like viruses are included, each isolate in the study population was identical (homologous) to 21 other viruses. Removing the vaccine-like viruses to focus on the wild-type viruses reduces each strain down to being homologous to nine other strains in the study.

It is of note that there was a very high variability in the number of identical (homologous) strains for each of the wild-type PRRS viruses. Some viruses were found only once and thus were not related (homologous) to any other virus identified in the herd outbreaks and included in the study. At the other extreme, however, one virus was homologous to 55 other viruses in the study. 

Tracking a virus like this should be highly informative when the second phase of analysis is conducted by the researchers. This second phase will target wild-type PRRS viruses linked to at least two homologous strains in the population tested. Tracking the spread of wild-type PRRS viruses in Ontario will be the next mission for the researchers.

From this, we can say that, until the researchers have done further analyses, it is too early to draw any firm conclusions from the results obtained from PRRS Project #1 so far. The PRRS Project, however, has come a long way since its start nearly three years ago.   

Starting with PRRS Project #1, we now have the foundation for tracing the different PRRS virus strains which have been circulating in the pig population since 2004.  Furthermore, we are on track to linking the relatedness of these different strains, their locations in different herds and possibly finding the common threads to track these viruses on how they have moved from one herd to another. 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