Excerpted from the thesis of PhD condidate Do Huu Dung

1. Brief introduction

 Name of the PhD Candidate: Do Huu Dung.

 Thesis title: Studies on epidemiology of Porcine Reproductive and Respiratory Syndrome (PRRS) in Viet Nam, 2007-2013.

 Branch index: Veterinary Epidemiology – Code: 62. 64. 01. 08.

 Institution: National Institute of Veterinary Research.

2. Contents of the abstract

2.1 Research question

Porcine Reproductive and Respiratory Syndrome (PRRS) is one of the most important diseases in the world’s pig industry. PRRS virus (PRRSV) is an RNA virus enormously diversified. In Viet Nam, since the first emergence in 2007, PRRS epidemic waves have led to serious losses to the national pig industry.

So far, studies on PRRS in Viet Nam have been mostly focused on the determination of characteristics of molecular biology, virulence and pathogenicity of causative agent, diagnosis and vaccine; on epidemiological aspects, there is a limited number of initial analyses describing the epidemiological features of PRRS for a particular area. In this study, we broadly analyzed the causative agents and epidemiological features of PRRS at national level by utilizing specimens/viruses and outbreak data from 2007 to 2013, to unravel the disease patterns, properties of circulating viruses and variants as well as the compatibility of these viruses to the vaccine strains.

2.2 Purpose and objectives

Research purpose: To gain better understandings on the disease patterns, emerging roots, risk of outbreak and dissemination, to provide scientific evidences for improving the effectiveness of PRRS control measures, and thus contributing to sustainability of livestock production in Viet Nam.

Research objectives: (i) PRRSV isolated from outbreaks in Viet Nam; (ii) Major epidemiological characteristics of PRRS epidemics in pigs.

2.3 Research methods

A range of methodologies and tools from both conventional and state-of-the-art epidemiological studies, molecular analyses, evolution, phylogenetic, phylogeographic; analyzing parameters of descriptive, spatial-time and molecular epidemiology; risk factor analyses and challenging experiments.

2.4 Major results and conclusions

The study has determined the origination and genetic characteristics of PRRSV and their variants circulating in Viet Nam: Group 1ab, Group 2ab originated from oversea, sharing a close relationship with PRRSVs circulating in China; Group 2c was locally indigenous variants; their emergences coincided with the 3 major epidemic waves of PRRS in 2007-2008, 2010 and 2012-2013. All PRRSVs isolated from outbreaks through- out Viet Nam belongs to the Type II (North America), sublinage 8.7; during this period, none of PRRSV Type I was detected from clinical outbreaks. These PRRSVs and those circulating in China originated from the same ancestors that emerged around 2005. These PRRSVs have a high rate of nucleotide substitutions (7,09 x 10^-3 sub/site/year).

PRRS disease patterns in Viet Nam have been described through estimating descriptive epidemiological parameters, analyzing of data by space and time dimensions: PRRS in Viet Nam has persisted in endemic pattern, mosaic distribution, intermingling between infected and free farms, periodically raising broad waves every 2-3 years, no clear seasonality. Initiative infection root was not fixed but could emerge at any location, high risk farms were those in close proximity with main roads, with farm size of above 200 pig heads and hiring workers from outside the village; outbreaks potentially occurring when the estimated dissemination ratio is greater than 1 and disease clusters appeared.

Intervention-epidemiological analysis has reconfirmed that vaccine is one of the important tools in control measures with the implementation of both for prophylaxis (by inactivated vaccines) and for outbreak response (by live attenuated vaccines), meanwhile, this study preliminarily evaluated the efficacy of a home-made inactivated auto-vaccine.

Scientific outcomes:

This is the first study that has revealed the overall characteristics of PRRS epidemiology in Viet Nam, covering descriptive epidemiological features of PRRS over the last 7 years, the infection roots, disease patterns, risk factors, and evaluating and developing intervention tools, to supplement scientific background for informing disease control policies.

PRRS outbreaks distributed widely, mosaic-intermingled between the exposed and the virus-free farms; causative agents of major outbreak waves were PRRSVs that share the same ancestors with the highly pathogenic PRRSV strains in China and the indigenous source including their offspring variants beyond the emergence and locally persisted; this was enhancing the complexity of epidemiological situation.

The risk of outbreak depended on the emergence of infection cores, which up to date could be appeared at any place. Close proximity to main roads, farms with more than 200 pigs and farms that hire workers from other villages are among the risk factors identified in this study; the emergence of outbreaks could be predicted to happen when the estimated dissemmination ratio is greater than 1 and the appearance of disease clusters. Vaccination deems to be an appropriate solution but the homology between circulating strains and the vaccine strain need to be ensured at high level.

Practical implications:

(i) Information generated by this study provides scientific basis to define, fine-tune or supplement solutions for PRRS control measures;

(ii) The successful application of methodologies and tools for PRRS epidemiological analyses could be beneficial to the studies of other emerging infectious diseases;

(iii) Dissertation manuscript can be used as training materials at undergraduate universities and for training programs.

New contributions:

This work bears three major new contributions:

(i) First overall and systematic description of epidemiology of PRRS in Viet Nam;

(ii) Sequenced a large amount of genes, determining the genetic diversity of PRRSV, identifying 3 major PRRSV strains in Vietnam, the motif of pesistancy beyond outbreak and co-circulation;

(iii)  Utilized successfully epidemiology methods and tools to reveal the global situation of an infectious disease important to Vietnam, outlining the relevant key-points for epidemiological surveillance. It was for the first time this study made the use of intervention epidemiology to evaluate efficiency of measures by vaccination, directly serving the control campaigns for PRRS and in general for emerging infectious animal diseases in Vietnam.