The impact of PRRSV on feed efficiency, digestibility and tissue accretion in grow-finisher pigs

The economic losses caused by Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) infection are estimated to cost the US swine industry more than $640 million annually (USDA, 2008). While significant advances have been made through research efforts to enhance our understanding of PRRSV at the animal health, immunological and genomic level, this disease still remains a significant issue in the US swine industry.

Although we clearly know that PRRSV attenuates ADG of production pigs, its direct impact on feed efficiency, nutrient and energy digestibility, and whole body lean and fat accretion in grow finisher pigs has been poorly characterized. Therefore, the overall objective of this project was to characterize the impact PRRSV challenge has on grow-finisher pig feed efficiency, energy and nutrient digestibility, and tissue accretion rates.

To achieve this objective, 60 Choice Genetic maternal line gilts (33 ± 3 kg BW) where selected. These gilts came from 30 sows and consisted of live weight matched littermate pairs. After a nursery period, the litter mates were then split into two separate barns and placed into five pens (n = 6 pigs/pen). Pigs in both barns were fed corn-soybean-DDGS diets containing the non-digestible marker titanium dioxide. All pigs had ad libitum feed and free access to water. Pigs were of high health status and PRRSV naïve. One barn was then inoculated intramuscularly with a live field strain of PRRSV (PRRVS+) and the other barn with saline (PRRSV-). Blood samples were collected for PRRSV titers and feed intake and body weight gains were recorded for each pen over a 70-80 day performance period. At 19-21 days post inoculation (dpi), total tract fecal samples were collected for nitrogen and energy digestibility analysis. To longitudinally assess the impact of PRRSV on tissue accretion, at -1 and 80 dpi, all 60 pigs were sedated and scanned using a dual X-ray absorptometry machine (DXA). The DXA scan enabled us to measure whole body fat, lean, protein, water, and bone mineral content in a non-destructive way. DXA scan data was then corrected to account for gut and bladder fill using regression equations.

All pigs were PRRSV naïve before the study started and the 30 PRRSV- pigs remained PRRSV negative throughout the study. Within the first seven dpi, the PRRSV+ gilts had a log adjusted PRRSV genomic titer of 4.6 and as expected they had all seroconverted by 35 dpi. No mortalities or secondary infections were reported in the study. The impact of PRRSV challenge on pig performance is shown in Table 1. As expected, body weights where negatively affected by the virus challenge and growth rates were reduced by 20% compared to the PRRSV- pigs (P < 0.01). PRRSV+ gilts also had significantly (P < 0.05) reduced ADFI and G:F compared to their healthy counterparts over the test period (-7% and -15%, respectively, Table 1). This was especially apparent over the first 28 dpi (data not shown). Proximate analysis of feed and fecal grab samples highlight that apparent total tract digestibility of dry matter, nitrogen and energy were all significantly reduced at 19- 22 dpi (P < 0.001). The digestibility coefficients for dry matter (81.5 vs. 79.8), nitrogen (80.1 vs. 78.0) and energy (78.1 vs. 76.3) were all reduced by 3-5% for PPRSV- vs. PRRSV+ pigs, respectively. Knowing that PRRSV infection alters ADG, we wanted to know if the composition of gain was affected. Longitudinal DXA scan analysis showed that PRRSV+ gilts had significantly lower lean, protein, fat and bone accretion rates compared to their PRRSV- counterparts (Table 1).

In summary, this ongoing project is clearly demonstrating that PRRSV infection reduces not only ADG, but also ADFI, coefficients of apparent total tract digestibility, and feed efficiency in grow-finisher gilts. Furthermore, lean/ protein and fat accretion rates all appear to be affected to a similar extent. Additional blood analysis and carcass data from this project will allow us to better understand the metabolic impact of PRRSV in pigs and to calculate the economic impact of this health challenge in a grow-finisher production setting. This work has been supported by the National Pork Board grant #12-163 and Choice Genetics.