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It is commonly accepted in the global swine industry that PRRSv is one of the major important pathogens that impacts cost of pig production. Recent data (from Tousignant et al) showed that PRRSv is highly prevalent in the U.S. swine industry with a profound seasonal pattern of outbreaks (fall / winter).
The obvious question from producers is “What to do with infected breeding herds? Is the right strategy to get a farm stable, that is, control clinical signs reducing the economic impact of the disease; or should a farm adopt strategies to go “negative”?" As for other questions in the epidemiological field, our answer for this one is “it depends”.
There is no “silver bullet-approach” that will be the right choice for all farms. For farms that need to wean PRRSv-negative pigs for some reason (ex. genetic multipliers), and for farms with great location biosecurity (i.e. in low pig density region), “going negative” would be an obvious choice. On the other hand, there are many farms that are highly exposed to PRRSv (i.e. high pig density region) and therefore with low chance of remaining PRRSv-negative that choose to remain positive. To such farms, the adopted strategy is focused on “cooling down” PRRSv infection, building strong herd immunity and therefore reducing the cost of living with PRRSv.
Studies from our group showed that if a sow farm gets infected, herds with pre-existing anti-PRRSv herd immunity (e.g. using attenuated vaccines or exposing the breeding herd with a wild-type PRRSv) had a significant lower production impact than those farms without PRRSv immunity. This finding has important practical implications. For example, it suggests that for farms that are repeatedly infected with PRRSv year after year, building herd immunity translates into decreasing total loss along the years.
Table 1 – Deciding between going negative or keeping a breeding sow farm “stable”.
| Strategy | Advantages | Risks |
| Go negative |
Lower cost of pig production. Better growth performance. |
When infected, the limited herd immunity leads to substantial economic loss due to pigs not weaned and lower growth performance. |
| Get stable | Production more stable and predictable (if farm gets a new PRRSv strain, the odds of having a strong total loss is less than that of “negative” farms because of the constant herd immunity level. | Productivity level might never equal that of “negative” farms. |
There are different tools than can be used to control and eliminate PRRSv from breeding herds, including partial depopulation and herd closure.
Herd closure is a method commonly used in North America to produce PRRSv-negative weaned pigs from positive breeding herds. Along with herd closure, with intent to expedite the herd immunity process, it is common to expose all breeding sows to a live, replicating PRRSv. This process is also known as load-close-expose.
Our group conducted a 3 year prospective study with breeding herds that adopted load-close-expose program with intent of producing PRRSv-negative piglets from acutely infected breeding farms. The herds used either live-virus-inoculation or modified-live virus vaccine as exposure method. We showed in phase 1 of the study that the factors associated with shorter time to produce PRRSv-negative pigs at weaning were a) herds treated with live-virus inoculation, b) herds that had prior immunity to PRRSv and c) farms assisted by a specific veterinary clinic. On phase 2 of the study, we reported that the factors associated with lesser production impact (measured as reduced total pigs weaned) were herds that were exposed with modified-live virus vaccine, herds assisted by specific veterinary clinic and herds that had prior PRRSv-immunity.
As the results of phase 1 and 2 were contradictory (live-virus achieved time-to-negative sooner but had more total loss than herds treated with modified-live virus vaccine), we developed economic (partial budget) models with data from the phases 1 and 2, and showed that on average, herds treated with modified-live virus vaccine had economic advantage over those treated with live-virus inoculation.
Thus, there are different ways to control and eliminate PRRSv. The details of the load-close-expose operation depend on farm layout, pig flow, technology available, quality of labor, expectations and objectives. The success rate of load-close-expose depends on the interaction between virus properties and implementation/ compliance level to the adopted protocol. Some production systems are working with success rate of over 90%, which is proof of concept that the strategy works under field conditions.
