Porcine reproductive and respiratory syndrome (PRRSV) and porcine epidemic diarrhea (PEDV) viruses are two of the most economically significant diseases to affect the U.S. swine industry today. Yet, little is known about how they impact pig nutrient digestibility, intestinal function and feed efficiency. Therefore, the overall objective of this thesis was to determine the impact of PRRSV and PEDV viruses, alone or in combination, on grower pig performance, tissue accretion and on intestinal function and integrity. To accomplish these objectives, a series of experiments were conducted and are outlined in three research chapters (Chapter 2, 3 and 4). In Chapter 2, an experiment was conducted using growing pigs to assess the longitudinal impact of PRRSV on growth performance, feed efficiency, tissue accretion and apparent total tract digestibility (ATTD). In Chapter 3 pigs were infected with PRRSV, PEDV or a combination of both, to determine their impact on growth performance, feed efficiency, ATTD and apparent ileal digestibility (AID). The final research chapter (Chapter 4) assessed the effects of PRRS and PED viruses on intestinal integrity and function.

The results from this research indicate that PRRSV significantly reduces overall growth performance and feed efficiency. It also reduces ATTD of nutrients and energy in an industry setting (Chapter 2), but did not reduce AID of nutrients (Chapter 3). Expectedly, PRRSV infection did not affect markers of intestinal health and integrity, but did improve active glucose and lysine transport in the jejunum (Chapter 4). Jejunum sucrase activity was also improved during the 21 day PRRSV infection. However, no differences in the gene abundance of nutrient transporters between PRRSV and healthy controls were reported (Chapter 4). As expected, PEDV infection reduced growth performance and feed efficiency in the first week post infection (Chapter 3). Although it reduces ATTD of dry matter and energy from control health pigs, PEDV did not alter nitrogen or organic matter ATTD. There were also no differences reported in AID of nutrients between control and PEDV pigs. As reported in Chapter 4, PEDV challenge negatively affects intestinal integrity and morphology, reducing villi:crypt ratios throughout the small intestine and decreasing absorptive surface area compared to the control and PRRSV pigs. Interestingly, PEDV did not alter brush border enzyme activity or mRNA abundance of nutrient transporters in the jejunum compared to control pigs (Chapter 4). However, active glutamine transport was favorably increased in PEDV infected jejunum samples. Co-infection with PRRSV followed by PEDV reduced growth performance and feed efficiency more severely than PRRSV or PEDV alone (Chapter 3). Dry matter, nitrogen and energy ATTD were reduced, while organic matter ATTD and AID parameters did not differ from healthy, control pigs (Chapter 3). Intestinal morphology and integrity was negatively impacted throughout the small intestine similar to pigs challenged with PEDV (Chapter 4).

In summary, both systemic-respiratory and enteric-based pathogen challenges (PRRSV and PEDV respectively) reduced growth performance and feed efficiency in growing pigs. However, co-infection with both PRRSV and PEDV showed an additive effect on pig performance. An interesting highlight of this work was that PEDV challenged nursery pigs still had brush border digestive enzyme and nutrient absorptive machinery present during major atrophy and thinning of the intestinal epithelium. This translated into minimal changes in AID and ATTD of nutrients and energy in PEDV-challenged pigs compared to healthy control pigs. Longitudinally, PRRSV infection impacted growth performance, protein accretion and feed efficiency in pigs. Our data indicated that PRRSV infected pigs needed two more weeks on feed to reach the same market weight as healthy control pigs and we estimated this to costs producers $6-12/head.