Porcine reproductive and respiratory syndrome (PRRS) in growing pigs may play an important role in PRRS outbreaks among sow herds, Jose Angulo, DVM, a managing veterinarian and PRRS specialist with Zoetis, told Pig Health Today.

PRRS control has traditionally focused on the breeding herd, but outbreaks of the disease still occur despite best efforts. After discovering that several swine veterinarians suspect PRRS in growing pigs is linked to outbreaks on previously negative sow farms, Angulo embarked on an epidemiology project aimed at investigating the association.

Using surveillance data from a pork producer, Angulo conducted a collaborative four-year retrospective analysis of multiple pig sites. The sites were located in an area encompassing about 12 miles, and all were part of the same production system, which had not vaccinated growing pigs or sows against PRRS. The analysis revealed a link between PRRS infection in growing pigs and sow farm outbreaks of PRRS. In other words, as the prevalence of PRRS in growing pigs increased, the risk of PRRS on sow farms increased.

“We can say… that growing pigs do represent a risk factor for sow farms,” and as a result, the prevalence of PRRS in growing pigs needs to be kept as low as possible, he said.

Angulo presented details about the analysis at the 2018 Allen D. Leman Swine Conference. There were 18 growing-pig sites surrounding five filtered sow farms. There were six outbreaks on the sow farms during the study period - two of the sow farms had one outbreak and two others had two outbreaks. The probability that a sow farm in the region would experience at least one outbreak in four years was 80 percent.

Of the growing-pig sites, 15 were positive for the PRRS virus (83 percent), and 14 of these had multiple PRRS outbreaks (93 percent). At least 11 different PRRS virus strains were identified in growing pigs during the four-year period.

Based on data collected, a filtered sow farm would have an outbreak about every 3.33 years, Angulo reported.

Vaccination timing, monitoring imperative

In his interview with Pig Health Today, Angulo said one of the obvious strategies for PRRS control in growing pigs is vaccination, but a better understanding is needed to reduce shedding of the PRRS virus and minimise clinical signs and the economic impact of PRRS.

Based on his experience, pigs need to be vaccinated at least three to four weeks before challenge to generate the required immune response. However, “…in cases where we have an early exposure in the nursery, we vaccinate at processing with good results,” Angulo said, noting that he believes the need to generate immunity against PRRS outweighs concerns about maternal-antibody interference with early PRRS vaccination.

He emphasised that besides vaccination, attention needs to be paid to other factors that may affect the risk for PRRS in growing pigs, such as farm management and biosecurity, and surveillance is vitally important to ensure the vaccination plan is working.

For instance, Angulo continued, it’s possible that viremic pigs vaccinated at processing may not get the full benefit of vaccination or that pigs vaccinated at weaning and then have early exposure to the PRRS virus won’t have enough time to generate an immune response. The only way to know is through monitoring, he said.

Unknown is exactly how growing pigs become infected, and that’s a lingering question Angulo is trying to answer through a collaborative project at the University of Minnesota, where he’s working toward an advanced degree. The project is aimed at identifying the incidence and infection patterns of PRRS virus in growing pigs in medium-density pig areas.

“Hopefully this project will shed some light on what risk factors are most important to minimise the spread of PRRS virus in growing pigs and minimise the risk of regional spread,” Angulo explained.