Dual infections of feeder pigs with porcine reproductive and respiratory syndrome virus followed by porcine respiratory coronavirus or swine influenza virus: a clinical and virological study. VanReeth K, Nauwynck H and Pensaert M (1996). Veterinary Microbiology 48: 325–335
What are they studying?
The present study was realized to determine the clinical effects of dual infections with PRRSV followed by PRCV or SIV. Another objective was to determine if PRRSV interferes with the replication of the PRCV or SIV.
How is it done?
36 10-week old pigs (seronegative for PRRS, SIV and PRCV) were assigned to 8 groups:
control group, PRRSV-only, PRCV-only, SIV-only, PRRSV+PRCV and PRRSV+SIV (3 groups).
The secondary infections (PRCV and SIV) were induced 3 days after PRRSV infection.
Pigs were daily monitored for fever, tachypnoea, dyspnoea, and coughing. They were also weighed daily and nasal swabs and blood samples were taken.
What are the results?
Fever was detected in PRRS-only infected pigs (one day).
In SIV-infected pigs fever lasted only one day; sporadic respiratory signs (when forced to move) and a transient increase in breathing rate (65/min) were also seen. However, when coinfected with PRRSV, the first group of pigs —PRRSV-SIV— showed high fever for 10 days, peaking at 41.4ºC.
They were severely affected and depressed and had also marked anorexia. These pigs showed remarkable respiratory disease between 4 and 10 days after coinfection.
The other two groups of PRRSV-SIV were moderately affected compared with the first group.
However, ADG of all 3 groups was affected when compared to control, PRRSV-only and SIV-only groups
PRCV did not induce any response. However, when pigs were already infected by PRRSV, an inoculation with PRCV produced fever for 9 days and pigs developed fever and respiratory disease.
Virus excretion for PRCV was similar regardless of PRRS status and the excretion of SIV in the PRRSV-SIV group was delayed by 2 days.
What implications does this paper have?
This study confirms the hypothesis that the clinical effects of a PRRSV infection may be exacerbated when co-infection with common respiratory viruses occurs. Also, it shows that PRRSv can aggravate the mild effects of other respiratory pathogens.
The view from the field by Enric Marco
Of late, it is relatively common to see fattening pigs in their final phase exhibiting respiratory symptoms that respond poorly to antibiotic treatments. In these situations we almost invariably think about the presence of a viral process complicated by secondary bacterial infections. In fact, if a lab test is required to find out the virus involved, the request is clear: Is it the PRRS virus? However, raising the possibility of a mixed viral infection such as the ones presented in this study is less common.
This work shows that the combination of PRRS virus with other viruses with respiratory tropism results in greater clinical severity. The possibility of the influenza virus being present in certain PRRS complicated cases is relatively frequently contemplated, but who thinks about the porcine respiratory coronavirus? We have known for years that it is a common pathogen found in herds and, in fact, it is the one that explains why there's no outbreaks of transmissible gastroenteritis in Europe yet no one remembers it when faced with respiratory symptoms. Could it explain some of the respiratory processes we see at the end of fattening? Removing the PRRS virus from a farm is not easy, but is it easy to eliminate the influenza virus or the respiratory coronavirus?
We tend to forget how important the correct implementation of batch management is, with a good practice such as the All-in All-out principle. It's nothing new, we have already old data that prove that the use of these type of techniques works (see table). However, in farms where there are several production batches at the same time (i.e., provided that we don't have a strict system of multiple phases), we might think we are applying the All-in All-out principle when, really, we are not. We enter all the units every day, jump in the pens, use the same brushes in different rooms and, if that were not enough, we treat the sick animals every day without changing the syringe and, worse, without changing the needle. These are the type of practices that make diseases become endemic, especially those of viral origin.
Holland is starting to apply —fairly strictly— what they call internal biosecurity —in other words, avoid spreading infections from one batch to the next—, and the way to achieve this is easy: we just need to handle each batch as if it were a different farm: boots and materials specific for each batch, and washing hands or gloves between batches. As for the daily routines, they always need to be carried out starting with the youngest pigs in increasing order of age. If there is a common hallway to all rooms, when the biggest pigs are being loaded, the windows to the other departments should remain closed in order to prevent aerosols generated by the departing pigs from entering the rooms. This is the only way to ensure that infections such as influenza, respiratory coronavirus or even PRRS disappear, or at least change their pattern, thus preventing mixed infections.
Commercial production does not seek perfect health, but a good management of the situation without costs compromising profitability.