Type 1 PRRSv induces aggravation of infection by a mild virulent Actinobacillus pleuropneumoniae serotype 2 strain. Norbert Stockhofe-Zurwieden, Henk Wisselink, Jacob Post, Eefke Weesendorp, Johanna M.J. Rebel. 2013. International PRRS Symposium
What are they studying?
They study the predisposing and aggravating pathogenesis mechanisms of a PRRSv (Lelystad strain) followed by an App (serotype 2) infection.
How is it done?
An experimental co-infection study with a genotype 1 subtype 1 PRRSv (Lelystad strain) followed by an App serotype 2 infection was induced. 3 groups of 10 pigs were inoculated: (group 1: PRRSv/App 2; group 2: PBS/App 2; group3: PBS/PBS). Pigs were clinically monitored during 7 days following App infection.
What are the results?
The presence of PRRSv significantly increased the cases of pneumonia compared to App infection alone. 60% of PRRSv/App and 20% of PBS/App infected pigs showed signs of fibrinous-necrotizing pleuropneumonia..
All PRRSv infected pigs developed fever 4 hours after App infection, whereas pigs infected only with App did not show increased body temperatures. This could be due to a reported increase in CD14 expressing cells in the lungs following PRRSv infections, and might explain a higher susceptibility to lung infections by gram-negative bacteria in PRRSv infected pigs.
What implications does this paper have?
From these results, it can be suggested that PRRSv predisposes pigs to an aggravation of infection by App, and it could create a more permissive environment for bacterial colonization.
The field view by Enric Marco
Numerous scientific studies have confirmed the interaction between PRRS and pathogenic bacteria that primarily affect the respiratory tract including, but not limited to, H. parasuis, S. suis or M.hyopneumoniae. The result of this interaction is always a worsening of the symptoms, with the consequences this has on the field. However, the fact that these studies used American strains of PRRS —which have a higher tropism for the respiratory system, causing a clinical picture with a clear respiratory component—, often goes unnoticed. In contrast, the most common clinical picture of infections caused by European strains mainly involves the reproductive system, being the respiratory component, albeit present, less severe. This paper demonstrates that European strains may also interact with bacteria affecting the respiratory system, thus aggravating the clinical picture.
We all know that pleuroneumonia control in Actinobacillus pleuropneumoniae-positive herds is not easy. In fact, it is one of the clinical pictures that gives practitioners the most headaches due to its high relapse rate and cost of treatment, usually affecting animals in the second half of the fattening period. For this very reason, App control is an essential link to control pleuroneumonia in PRRS-positive herds. Our aim is to keep the farm status as "stable inactive" according to the PRRS herds classification, where, despite having evidence that the sows have come in contact with the virus (ELISA-positive), their piglets are not viraemic at weaning (PCR-negative). As long as this status is achieved and good biosecurity maintained between batches of different ages in the growth and fattening phases, it should be possible to obtain fattening pig herds without PRRS circulation, even though the animals come from a positive source. This would be the first step to take in order to keep pleuropneumoniae infection under control.
Of course we must not forget that it is essential, to achieve this goal, to have a homogeneous population of sows as regards immunity against PRRSv and App, which entails the implementation of a good gilt adaptation process. Other pathogens such as M. hyopneumoniae also need to be under control —easily achieved via piglet vaccination—, as we know they play a major role in all respiratory processes.