Control of viral infections relevant to piglet mortality

Funding: Svineafgiftsfonden; 1,825,000 DKK

Period: 2022

Aim: The project aims to develop and implement knowledge-based counseling tools to control the most important virus infections in Danish pig production, specifically aimed at reducing piglet mortality.

Background

PRRSV is still the most important virus infection under Danish conditions. In many herds, PRRS is dormant in the sows, whereas in other herds it is a constant challenge. One way to control PRRS is with the use of modified live virus (MLV) vaccines, of which sales have exploded in the past year. As a result of infection with a recombinant PRRSV in 2019, many herds experienced a doubling of piglet mortality for several months. Therefore, it is obvious to increase efforts to avoid introduction of PRRS to sow herds. The outbreak in 2019 showed that it is problematic to use MLV vaccines as they are used now, and it can be questioned whether the manufacturers get value for money. In this project we will generate knowledge from samples collected from infected herds, and translate this knowledge into optimized guidelines for practical use in the herds. In addition, there will be a focus on how to avoid, discover and handle PRRS.

Viruses other than PRRSV can also attack and increase mortality among piglets and make them more susceptible to other diseases, but the sows are also vulnerable to virus infections. In such cases, infected sows will no longer be capable of taking optimal care for their piglets, which will also increases piglet mortality. Especially gilt litters are vulnerable to this. As an example, a recent Ph.D. study showed 1) that influenza virus was circulating in all sections of sow herds despite mass vaccination against influenza; 2) that piglets played a major role in the introduction of virus into the herds; and 3) that there was a clear correlation between the viral status in piglets at the end of the quarantine and the presence of viruses in the farrowing pen. Thus, there is a great potential that an optimized piglet immunization will have a positive effect on piglet survival.

In recent years, a large number of new viruses and new variants of known viruses have been detected in pigs. Additionally, there has been a sharp increase in cases of porcine parvovirus (PPV) detected in fetuses. Through sequence analysis, we have demonstrated that this increase coincides in time with the spread of a new genotype that in some studies has shown to be more virulent and may even escape immunity induced by the classics vaccines. The investigation of the outbreak of PRRSV at both the Hatting and other boar stations was only possible because of the extensive characterization of PRRSV in Denmark in the recent years, partly funded by Svineafgiftsfonden. We therefore propose that this ongoing monitoring of variants of the major viruses should be continued in order to provide optimized - and independent - advice regarding choice of vaccination strategies and other control measures.

Activities

Activity 1: Optimal immunization of young female pigs

The purpose of this activity is to investigate the effect of the usual vaccination strategies of young female pigs for the protection of gilts and piglets by time of farrowing.

Activity 2: Optimal sample material for PRRSV surveillance - an extension of the activities from 2021

In connection with the PRRSV reduction programs (Activity 5), it is a challenge to choose the right sample materials for the detection of PRRSV, especially in the phases of the program where the prevalence of positive pigs and sows is low (1-5%). In the USA, testing of large pools of tails and testicles (processing fluids), saliva samples collected from both the sow and piglets (family oral fluids) and wiping of udders of lactating sows (udder wiping) are used and these methods are now also increasingly used in Denmark. The purpose of this activity is to validate the use of these methods with a view to optimal sampling frequency, number of samples, optimal sample storage during transport and validation of the laboratory tests used to test the samples in the laboratories (SSI, Kjellerup laboratory and KU).

Activity 3: Surveillance of new viruses and virus variant in Danish pigs –a continuance and extension of the activities from 2021

The purpose of this activity is to ensure ongoing monitoring of new viruses that could potentially pose a threat to Danish pig production, and to monitor the genetic mutation / recombination of the viruses, as well as to ensure optimal diagnostic options.

Activity 4: Effective PRRSV reduction strategies, full-line production herds, and use of inactivated PRRSV vaccines –new activity

Today, virtually all partial sanitization programs is carried out using modified live PRRS vaccines. However, there are also inactivated PRRS vaccines, which may be used as a supplement to the live vaccines in connection with partial sanitizations for PRRSV. The advantage of inactivated vaccines is that they do not spread between the pigs and there is no risk of mutations as is the case with live vaccines. The purpose of this activity is therefore to investigate whether supplementation with inactivated vaccines can increase effectiveness.

Activity 5: Development of a model for regional reduction of PRRSV

One of the biggest risk factors for becoming infected with PRRS virus is airborne infection with viruses from nearby infected herds. The reduction program will therefore focus on systematic and regional reduction of PRRSV. In order to be able to divide Danish pig herds into regions where, overall, there is the least risk of re-infection, there is a need to develop a mathematical model. As a basis for the model, a risk assessment will be made partly with a focus on the individual herd and partly with a focus on a region. The risk assessment will include information on different variants of PRRSV that can be identified via sequence analysis of parts or the entire virus, which is identified in work package 3. The database can be used to make infection tracking of new introductions of viruses in herds / regions that have completed the reduction, including clarifying whether it was the "old" virus that persisted in the herd. The database can also be used to identify the spread of new variants, including recombinations, and can be used as a partial basis for the selection of vaccines.