Degree Type

Thesis

Date of Award

2020

Degree Name

Doctor of Philosophy

Department

Biomedical Sciences

Major

Population Sciences in Animal Health

First Advisor

Daniel Linhares

Abstract

Porcine reproductive and respiratory syndrome (PRRS) virus is a swine-specific pathogen that causes ongoing significant impact to the pig production. PRRS modified-live virus (MLV) vaccines are widely used to expose swine populations and mitigate the consequences of wild-type PRRS virus (PRRSV) infections. There are several factors affecting vaccine efficacy under field conditions, including timing of exposure, genetic diversity of wild-type PRRSV, and immune status of growing pigs at the time of infection. Thus, there is the need to further understand the dynamics of viral infections in growing pigs, as well as the effect of different immunization strategies upon specific scenarios encountered in field conditions. This dissertation was conducted with the general objective to evaluate the performance of growing pig populations vaccinated with different protocols of PRRS MLV vaccine and naturally challenged with wild-type PRRSV, in field conditions. The first Chapter of this dissertation presents background information on PRRSV impact on growing pigs, and presents important unmet needs regarding use of immunization strategies to mitigate the consequences of infection of growing pigs under field conditions. Chapter two introduces the impact of mass vaccinating sows from PRRSV positive-stable farms, using PRRS MLV vaccine, on the subsequent nursery pig production performance. Nursery pig batches that were started up to twelve weeks after the PRRS MLV mass sow vaccination events were classified into four post-mass sow vaccination groups, while batches started up to six weeks before the mass interventions were classified into the baseline production group. The post-mass sow vaccination groups had significantly higher mortality rate than the baseline group. The post-mass sow vaccination groups had lower number of pigs produced, but with a higher weight gain per pig, compared to the baseline group. Overall, the ratio of the total weight produced by the four post-mass sow vaccination groups over the weight produced by four baseline production groups of the same size was 1.02. According to the results of this study, mass vaccination of sows with PRRS MLV vaccine with the intent to immunize swine breeding herds to PRRSV was not associated with a negative impact on the productivity of the batches of nursery pigs flowing downstream. Chapter three identifies common patterns of wild-type PRRSV RNA detection on oral fluids samples collect from MLV-vaccinated growing pig populations, and assesses how the different wild-type PRRSV detection patterns in the field associates with the mortality rate of growing pigs. It was demonstrated that more than 90% of the batches of growing pigs were detected with wild-type PRRSV-2 (aka North-American), while 38% of the batches were detected with wild-type PRRSV-1 (aka European PRRSV). Growing pig mortality was higher in batches with wild-type PRRSV-2 detection in earlier stages of life. However, detection of wild-type PRRSV-1 was not associated with significant increase of growing pig mortality. Mortality rate of batches with wild-type PRRSV-2 detection in earlier stages of life was lower when pigs were vaccinated with two doses of PRRS MLV vaccine, compared to batches that received only one dose. Results presented in this study suggested that early wild-type PRRSV-2 exposure on pig populations was associated with higher wean-to-finish mortality. Additionally, results suggested that vaccination with two PRRS MLV doses was associated with lower mortality rate, when growing pig populations had early wt-PRRSV-2 exposure. Chapter four describes a retrospective cohort study comparing productivity and economic performance between batches of nursery pigs vaccinated with a full dose of PRRS MLV vaccine and batches vaccinated with a half dose of the same vaccine, following natural exposure to wild-type PRRSV. The full MLV dose group had significantly higher average daily gain than the half MLV dose group. However, no significant differences were detected on the mortality rate and on the feed conversion between study groups. Results suggested that the vaccination of pigs with one full dose of PRRS MLV vaccine was associated with improved nursery pig productivity and nursery pig profitability, compared to the vaccination with only half dose of the same vaccine. Chapter five consists of a randomized field trial to compare virus detection, productivity and economic performance between growing pig batches vaccinated with two doses or one dose of Ingelvac PRRS® MLV vaccine, in commercial swine operations. Under the conditions of this study, the proportion of wild-type PRRSV detection on oral fluids samples and the log counts of viral RNA per ml of oral fluids from the two doses group was lower than the one dose group, when batches originated from PRRSV positive-stable farms. The mortality rate of the two doses group was significantly lower than that of the one dose group, with a higher effect size on batches originated from PRRSV positive-unstable sow farms and on batches with higher frequency and with higher diversity of wild-type PRRSV detection. Overall, it was economically beneficial to use the second dose of MLV vaccination to immunize growing pigs compared to using only one MLV vaccination dose, especially when pig batches were originated from PRRSV positive-unstable sow farms. Under the study conditions, vaccinating growing pig batches with two doses of PRRS MLV vaccine was a useful strategy to immunize growing pigs against PRRSV, lowering the wild-type PRRSV detection, lowering mortality rate, and increasing profitability, compared to batches of growing pigs that received a single dose of the same vaccine. Results presented in this dissertation provide field-relevant information to guide veterinarians and producers to make evidence-based decisions to proactively influence herd immunity of growing pig populations at risk of wild-type PRRSV infection, and consequently maximize pig performance and increase the success of projects to control and eliminate PRRSV at region level.

DOI

https://doi.org/10.31274/etd-20210114-6

Copyright Owner

Cesar Augusto Amorim Moura

Language

en

File Format

application/pdf

File Size

109 pages

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