THE EFFECT OF MEDIUM CHAIN FATTY ACIDS ON PORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME VIRUS
Porcine reproductive and respiratory syndrome virus (PRRSV) is estimated to cost the US swine industry $664 million in annual production losses. Therefore, the objective of this project was to evaluate the effect of MCFA on PRRSV replication using in vitro and in-vivo studies. The overarching hypothesis was that MCFA would inhibit or reduce viral replication of PRRSV infection in vitro and reduce viral load in-vivo. In the first experiment (Chapter 2), MARC-145 cells were used to determine the effects of individual MCFA (C6, C8, C10, and C12) exposure at concentrations ranging from 1-1000 µg/mL prior to and following inoculation of North American Type II (P-129) or European Type I (Lelystad) PRRSV. Viral replication was determined using FITC labeled IgG anti-PRRSV monoclonal antibody and TCID50 was calculated for each concentration. Data were analyzed using the Proc Mixed procedure of SAS. Incubation of MARC-145 cells with caproic acid (C6) at concentrations of 1-1000 µg/mL prior to and after inoculation with Type II North American (P129) or Type I European (Lelystad) PRRSV did not alter viral replication (P > 0.10). However, incubation of MARC-145 cells with caprylic (C8), capric (C10), and lauric (C12) acid prior to and after inoculation with Type I and Type II PRRSV did reduce viral replication at concentrations ranging from 100-1000 µg/mL. In general, the effective dose required to reduce (P < 0.05) viral replication (Log10TCID 50/mL) decreased as MCFA chain length increased. In experiment 2 (Chapter 3), the use of MCFA combinations (C8:C10; C8:C12; C10:C12; and C8:C10:C12) to reduce viral replication of PRRSV in MARC-145 cells was investigated. The MCFA combinations were analyzed at six different concentrations ranging from 50-500 µg/mL with North American Type II (P-129) and European Type I (Lelystad) PRRSV. Viral replication was determined as described in experiment 1 (Chapter 2) using FITC labeled IgG anti-PRRSV monoclonal antibody and Log10TCID50/mL was calculated for each concentration. Data were analyzed using the Proc Mixed procedure of SAS. Incubation of MARC-145 cells with MCFA combinations prior to and after inoculation with Type II North American (P129) and Type I European (Lelystad) PRRSV resulted in reduced viral replication at MCFA concentrations of 200-500 µg/mL and was concentration dependent. Reduction of viral replication with MCFA was further evaluated by independently incubating MARC-145 cells or PRRSV. Results indicated that viral replication was reduced when MARC-145 cells were incubated with MCFA and not when PRRSV was incubated with MCFA. In experiment 3 (Chapter 4), 112 mixed sex pigs (PIC 1050 females x PIC 359 sire), weaned at 21 d of age, weighing 7.5 ± 0.68 kg, were used in a 33d PRRSV challenge study. Pigs were blocked by body weight and sex and randomly assigned to one of four treatments in a 2x2 factorial design with pigs receiving 0 or 0.30% MCFA in the diet and placebo or PRRSV inoculation. Following a 5 d adjustment to diets and rooms, pigs were inoculated with either a placebo (sterile PBS) or Type II North American (P129) PRRSV (1 x 105, TCID50/mL) given in 1 mL each intranasal and IM injection. Each room contained 4 pens with 7 pigs per pen and an equal ratio of barrows to gilts within treatment. Diets were formulated to meet or exceed all nutritional requirements (NRC, 2012) and were fed in 4 nursery phases. Feed budgets by phase were 1.13 kg/pig in phase 1, 2.72 kg/pig in phase 2, 6.35 kg/pig in phase 3, and phase 4 fed until the end of the experiment. MCFA (C8:C12) were mixed in a 1:1 ratio (wt:wt), and then mixed with finely ground corn to prepare a premix added to diets at 0.60% to provide 0.30% total MCFA. Control diets used soybean oil mixed with finely ground corn at the same 0.60% inclusion to keep ME levels constant across treatments. Body weights, feed intakes, blood samples, and temperatures were determined or collected on d 0, 3, 7, 10, 14, 21, and 28 post inoculation. Sections of tonsil, lung, and intestines were collected at d 10 post-inoculation from 1 pig per pen and at d 28 from all remaining pigs. Data were analyzed using the PROC Mixed procedure of SAS with pen as the experimental unit for growth and performance measurements and pig as the experimental unit for viral load analysis. Serum viral load confirmed PRRSV was only detectable in challenged pigs. Body weights were not different (P > 0.05) between treatments prior to d 14 post inoculation. Body weights from d 14 to 28 post inoculation were reduced (P < 0.05) in PRRSV infected pigs compared to non-infected pigs. Overall ADG and ADFI were reduced (P < 0.05) for PRRSV infected pigs compared to non-infected pigs by an average of 18 and 28%, respectively. Body temperatures were not different between treatments. Viral load measured in the lung was not different (P > 0.05) between PRRSV infected treatments. Tonsil viral load was not different (P > 0.10) between PRRSV treatments. However, there was a trend (P ≤ 0.10) for an effect of day post inoculation with control-fed, PRRSV-infected pigs having higher viral loads at d 10 post inoculation compared to d 28 post inoculation. Overall, no effects of MCFA on PRRSV viral load or performance were observed during the in-vivo trial. MCFA was effective at reducing viral replication of PRRSV in MARC-145 cells in vitro. However, the results could not be confirmed in the in-vivo experiment. Porcine alveolar macrophages should be used to confirm the in vitro inhibition of PRRSV replication observed in MARC-145 cells. In order to fully understand the application of MCFA to inhibit PRRSV infection in pigs, more studies should be conducted to evaluate the form of MCFA as well as viral inoculation with field strains of PRRSV.