Control of viraemia and virus excretion in animals infected with an extremely aggressive strain of Porcine Reproductive and Respiratory Syndrome

The Lena strain is a highly pathogenic PRRSV subtype 3 strain that is found in some countries and that can have devastating effects when it infects a swine population negative for PRRS.

Even today, there are some unknown aspects of the Porcine Reproductive and Respiratory Syndrome, aspects such as the immune response or how and why the virus evolves; contentious issues such as mutation or possible combinations with wild-type strains or why there are differences between the aggressivity of some strains and others - these are some of the main priorities of the scientific community.

Controlling the Porcine Reproductive and Respiratory Syndrome is relatively easy if a series of measures are taken. These range from biosecurity to immunization and include the control of animal movements, hygiene and disinfection protocols or the management of gilts. All these measures are aimed at a single objective, which is to avoid infection, and this is achieved by reducing the presence of the virus in the environment, thereby minimizing its chances of reaching the animals.

Vaccination of the animals against the Porcine Reproductive and Respiratory Syndrome is key – it is not the only key, but it has been demonstrated that viraemia is reduced in vaccinated and infected animals and there is no doubt that this helps with control, indeed all over the world vaccination against this disease (especially with attenuated vaccines) helps to control the clinical signs of it. This is no different in the case of highly aggressive strains such as the Lena strain. Given the global distribution of the virus and the fact that trade in live animals takes place between four out of the five continents, it is possible to find highly virulent strains both in Asia and in Europe or the United States. One of these strains is the Lena strain, so named by H. Nauwynck's group at the University of Ghent.

As a leading company in the development of vaccines for pigs, one of our objectives is to create value for our clients in the control of the Porcine Reproductive and Respiratory Syndrome. One of the ways of creating this value is by verifying that our products, especially our new PRRS vaccine, are able to objectively achieve what we ask of them, which is to control homologous and heterologous infections under any circumstances. To this end, we are carrying out a joint project with researchers at the University of Ghent with the aim of demonstrating that UNISTRAIN® PRRS is able to reduce viraemiae in animals infected with a highly virulent strain such as the LENA strain.

In terms of materials and methods, 4-week-old piglets, clinically healthy and free from virus and antibodies against PRRS, were randomly assigned to two groups: in the vaccinated group the animals were intramuscularly vaccinated with UNISTRAIN® PRRS and the control group was left unvaccinated.

Four weeks after vaccination, all the pigs were intranasally inoculated with the highly pathogenic Lena strain (subtype 1.3; 82.3 % ORF5 homology to the vaccine strain; 10 5TCID50/ml). Blood samples and nasal swabs were collected from all the animals at 0, 3, 5, 7, 10, 14, 21 and 28 days post challenge (dpc) to assess viraemia (measuring virus titration of sera) and viral shedding (measuring virus titration of nasal swabs), respectively.

Viraemia and excretion were analysed using the non-parametric Mann-Whitney test (p<0.05).

The results of this trial confirmed that animals vaccinated with UNISTRAIN® PRRS obtained partial protection when challenged with a highly virulent strain of the Porcine Reproductive and Respiratory Syndrome.


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Which are the disadvantages of a prolonged weaning (40-45 days) in relation to PRRS virus transmission from sows to piglets?

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See also