Control of viral infections relevant to piglet mortality

Funding: Svineafgiftsfonden; 1,825,000 DKK

Period: 2023

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.


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.


Activity 1: Control of virus in Danish pig herds
To reduce the spread of PRRS and influenza virus in the farrowing unit and hereby reduce piglet mortality and to wean robust piglets.

Activity 2: Optimal sample material for the surveillance of PRRSV
In connection with PRRSV eradication programs, it is a challenge to choose the right sample materials for detection of PRRSV, especially in the phases of the program where the prevalence of positive pigs and sows is low (1-5%). This is essential knowledge before negative breeding animals can be introduced into the sow herd. In the USA, they test large pools of tails and testicles (processing fluids), saliva samples collected from both the sow and piglet (family oral fluids) and wiping the udders of lactating sows (udder wiping).

A number of Danish veterinarians already uses processing fluids at sample material. In previous projects, it has been demonstrated that air sampling is also an option to test for viruses in the farrowing unit, but this has not been tested on a larger scale. Related with Covid-19, waste water is now used as monitoring material for the infection rate in humans. As PRRSV is also excreted in faeces and is detected in slurry, samples from the slurry tank could theoretically be used for an easy and cheap monitoring of PRRSV.

In this activity, it is investigated whether material from dead newborn pigs (e.g juice from tongues) can replace processing fluids on testicles from castration. In addition, it is being investigated whether monitoring of PRRS virus can be carried out by testing manure samples and air samples.

Activity 3: Surveillance of new viruses and virus variants in Danish pigs
The purpose in this activity is to ensure continuous monitoring of new viruses that could potentially pose a threat to Danish pig production, as well as to monitor the genetic mutations/recombinations of the viruses that are in the country, and hereby ensure optimal diagnostic options. Influenza virus is monitored though a grant from the Danish Veterinary and Food Administration and is therefore not covered by this activity. Of the new viruses in pigs, the greatest focus is on porcine parainfluenza virus 1 (PPIV-1). This virus was first detected in dead pigs from Hong Kong in 2013 and has in recent years been detected in the USA, Chile, Hungary, the Netherlands and Poland. A 2016-2017 survey in the US found that more than 40% of the examined pigs were positive for PPIV-1. A just recently published study from Germany examined 26 herds for this virus and it was detected in piglets in 6 herds, but the prevalence in other European countries are not known. Experimental studies have shown that this virus can cause severe respiratory symptoms in pigs and can therefore have great clinical significance. We therefore want to investigate if this virus is widespread in Danish pig herds.

Activity 4: Effective reduction strategies, full-line production herds, and the use of inactivated PRRS-vaccines
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
In the fight against PRRSV, there is still a lack of knowledge about transmission dynamics within and between herds. In order to assess the relative contribution of PRRSV transmission routes between herds in an epidemiological model, it is essential that the model is calibrated with data on PRRSV occurrence, identification of risk areas, estimation of the impact of quarantine, biosecurity (including SPF production) and immunization strategies. The epidemiological model will be able to estimate the risk of infection for individual herds and identify regions of high and low re-infection risk. Quantification of the different transmission pathways via the model can be used in the development of new disease control strategies aimed at the most important transmission routes in each specific herd and region. Thus, the results of the model will be able to assist advisers to plan and implement the most effective prevention and control strategies for PRRSV.


Three articles on PRRSV in the special issue of “Fagmagasinet Gris”, marts 2023:

  • ”Tjek for PRRS i farestalden”         
  • ”Sekventering for PRRS”
  • ”PRRS-model skal vise årsager til smittespredning”

 ”PRRS-virus i spytprøver” Fagmagasinet Gris” Juni 2023

Congress proceedings:

  • Immunological effects after boost vaccination with inactivated vaccines in Danish sov herds against PRRS. Poster ESPHM 2023
  • Gilt introduction on PRRSV-positive farms. ESPHM 2023. Oral presentation