Producers today invest huge amounts of money to ensure that farms and pig flows are free from PRRS. Any failure to maintain this status can be catastrophic. In acute outbreaks it has been calculated that the average cost per sow in the first year is US$255, with an annual ongoing cost of US$76 per sow. Such alarming figures underline the importance of doing everything possible to prevent PRRS accessing pig herds.

Field experience and a series of studies have implicated live pig transport as an important method of spreading the PRRS virus onto farms and within pig flows. Dr Scott Dee and colleagues from the Swine Disease Eradication Center, University of Minnesota, USA, have published important results aimed at helping producers reduce this risk.

Their long series of studies has been designed to establish a practical approach to PRRS control on pig transport at a commercial level. While there is now evidence that various drying and heating techniques can help remove the virus from vehicles, the vast majority of businesses have little or no access to these processes. Even if they can access drying and heating, effective disinfection is still needed. The Minnesota work reflects the real-world biosecurity constraints which apply to the majority of modern commercial production systems.

Short hygiene period

Early work at the university had been done on 1:150 scale models of weaned pig transport trailers. While these studies confirmed that PRRS could be transmitted via a trailer and that this could be controlled by good disinfection, it was necessary to show that the results could be repeated in full-size, multi-deck pig transport vehicles.

Scott Dee's team realised that prolonged breaks for hygiene control are seldom achievable in transport scheduling, so they looked at a maximum turn-round time of 2 hours. Furthermore, the equipment and techniques had to be in common use within the industry, rather than specialist items, to help keep costs down and ensure better quality of work as operatives would be accustomed to working with them. To mirror field conditions, the water used was not hot. Cold water and disinfectant were applied by a low-pressure foaming system.

The choice of disinfectant also had to be a commonly used product that was suitable for this application, used at the dilution rates normal at field levels. The researchers chose the disinfectant Virkon S used at a dilution of 1%. It has been shown to have "no significant long-term corrosive effects" on common vehicle materials. The fact that this disinfectant has proven activity against a broad range of pig pathogens meant that their work was even more relevant to the situation in practice where producers are trying to control multiple pathogens and not just PRRS.

In trials lasting over 5 days, the researchers inoculated a total of 150 sites with PRRS virus (MLV strain) in an aluminium livestock trailer. Test sites in the trailer were chosen specifically to include areas such as corners, support braces and gate hinges that pose difficulties for access and proper application of disinfectant. An extremely high dose of virus was arranged. In fact, the level of infective virus used was 500 times greater than the levels previously determined necessary to infect pigs in model trailers.

Because normal working practices were being followed, the disinfection operatives were not told where the difficult sites were before they applied the 1% Virkon S solution via foaming using cold water. The investigators swabbed the contaminated sites to test for PRRS virus, both immediately and 120 minutes post-application of the foaming disinfectant. These swabs were checked for PRRS virus by PCR testing and positive results were confirmed using swine bioassay. On each test day, positive and negative controls were run to ensure test conditions were valid.

Truck biosecurity

The results obtained in this study show it is possible to control PRRS contamination of full-size pig trailers under practical constraints. Immediate tests of all sites inoculated with PRRS virus showed that they had been infected successfully. Two hours after application of the Virkon S, all sites were negative for infective PRRS virus. To quote the authors, the procedure "produced good inactivation of PRRS virus within the target time when cold water was used and disinfection applied by foaming". They also remarked that the system of applying the disinfectant was easy-to-use. The foaming allowed better and more accurate application of disinfectant under repeated commercial usage. Importantly, it also ensured that the operatives could see where they had applied the disinfectant.

The study answers one of the most important challenges of pig transport biosecurity by demonstrating that it is repeatedly and consistently possible, under field-type conditions and real-world constraints, to prevent transmission of PRRS virus in live-haul vehicles using an appropriate disinfection programme. This was in spite of the very high concentrations of PRRS virus used to infect the trailers. These results mean that biosecurity and transport managers can now develop and maintain practical truck biosecurity programmes that will control PRRS transmission via pig transporters. As live haulage poses one of the greatest risks in transport biosecurity, it can be concluded that the same protocols would work equally well in controlling the possible spread of the virus by other vehicles, such as those for feed, dead animals or the removal of organic waste.

The disinfecting was done at ambient temperatures. Many commercial truck washes will be working with cold water in freezing conditions, but the disinfectant used in this study remains active at lower temperatures when the activity of other classes of disinfectant is often reduced.

Some areas have not been covered by this study. For example, clean trailers were used, whereas live-haul trailers would normally have a high degree of organic material on them. For any disinfection programme to be successful it is essential that the organic material be removed which requires a comprehensive washing programme using a heavy-duty detergent.

Also, there was no use of mechanical drying in this trial. Previous work has shown that drying trailers, or at least heating them, can bring dramatic reductions in PRRS virus levels. However, this requires facilities which have a cost that many commercial organisations will not bear. Although heating and drying kills PRRS virus, it is likely that many other pathogens could survive. While heating and drying has a role to play in top-level truck biosecurity, it should be seen as additive to good disinfection and not a replacement for it. PIGI