Introduction

Porcine reproductive and respiratory syndrome (PRRS) is viewed by many veterinarians as the most important disease currently affecting the pig industry worldwide. First described in 1987 in the United States as "mystery swine disease", it spread rapidly, being reported in Europe in 1990 and subsequently across the world.

Infected herds may experience severe reproductive losses and/or increased levels of post weaning pneumonia with poor growth. The reproductive phase typically lasts for 2-3 months, however post weaning problems often become endemic. The reproductive disease is characterized by an abortion storm that affects both sows and gilts in the last term of gestation. Premature farrowings around 109 and 112 days of gestation are also observed. There is also an increase in the number of stillbirths and weak born piglets which results in a considerable increase of the pre-weaning mortality.

The respiratory phase has traditionally been seen in the nursery, especially in continuous flow nurseries. However, respiratory problems caused by PRRS virus can also be seen in the finisher as part of the porcine respiratory disease complex (PRDC). A reduction in growth rate, an increase in the percentage of unmarketable pigs and elevated post weaning mortality can be seen. Diagnostic findings indicate high levels of pneumonia, with the isolation of PRRS virus together with a wide variety of other microbial agents commonly seen as secondary infections. Bacterial isolates may include Streptococcus suis, Haemophilus suis, Actinobacillus pleuropneumoniae, Actinobacillus suis, Mycoplasma hyopneumoniae and Pasteurella multocida. Viral agents commonly involved include swine influenza virus and porcine respiratory corona virus. Affected pigs rarely respond to high levels of medication, and all-in/all-out systems have also failed to control the disease.

Control of PRRS virus in swine farms

Control of PRRS virus in swine farms has proven to be a very tedious process. Several strategies and protocols have been described. However the success of these strategies has been highly variable among farms.

All programs of PRRS disease control are based on understanding how the virus is transmitted. By identifying the source of the virus and where the virus re-circulates in the farm, PRRS disease can be controlled and in some cases, PRRS virus may also be eradicated.

Several characteristics of PRRS virus have to be considered in order to understand PRRS control programs:

• Transmission occurs mostly by direct contact between infected and susceptible pigs.
• The virus can remain in the blood of infected pigs for periods of 35 days or more. This long period of viremia is the most important factor responsible for PRRS spread. After the virus is cleared from the blood, it may still remain in the pig sequestered in immunological sites which may result in persistent infected pigs. However, the role of persistent infected pigs in PRRS transmission is uncertain.
• PRRS virus can cross the placenta during the last third of gestation and therefore, it can infect piglets in utero. Weak born piglets usually have considerable levels of virus in the serum.
• PRRS virus can be aerosol transmitted within very short distances. However, this route of transmission is not believed to play a significant role in PRRS virus transmission between farms in the US. Some epidemiological European studies have suggested that aerosol spread might have occurred in high dense swine areas with certain weather conditions. However this has not been proven to have occurred in the US.
• Transmission by semen can also occur. Boars can be intermittent shedders and can shed the virus in the semen for more than 100 days. However, transmission via semen appears more likely to occur at the early stages of infection due to the high dose of virus in the semen. As the infection progresses, the amount of virus in the semen decreases and if artificial insemination is used, the semen is extended and therefore the amount of virus is diluted. In such case, the risk of PRRS virus transmission via semen is decreased.
• Pigs recovered from a PRRS infection will develop an immune response which under normal circumstances will protect them from being infected again by the same virus strain.
• However, PRRS virus has the ability to change (by mutation or recombination) and therefore, new viral strains may arise in a farm. In such cases, cross protection between strains may not exist and new outbreaks may be observed in farms that had been previously infected.

Before starting any PRRS virus control program, the producer has to decide whether the objective is to "control" the clinical disease or to eradicate the virus.

Controlling the clinical disease in the breeding herd

The secret to control the clinical disease in the breeding herd is by managing the gilt pool.

Gilts are a very important source for virus introduction and circulation within the farm. There are two reasons for this: a) The gilts may be positive and shedding virus when they are introduced within the breeding herd. In this case the gilts will be a constant flow of external virus introduction into the farm. This situation may happen when the gilts have not been acclimatized in an isolation unit and have been introduced directly into the farm. b) When introducing negative or susceptible gilts in an unstable farm. The gilts will be infected during breeding and gestation and therefore will contribute to keep the farm unstable. Negative replacements have to be introduced only when there are no more signs of virus circulation in the breeding herd.

Therefore the practice of managing the gilt pool requires the existence of facilities to acclimatize the gilts before being entered in the sow herd. The objective in this stage is to make sure that gilts get infected by the virus at a young age so that, by the time they are enter into the sow farm, they have recovered from infection and do not longer shed more virus. This practice requires receiving the gilts at least 60 days previous to the date for these gilts to be introduced in the breeding farm. During the first 10-15 days the gilts are acclimatized by exposing them to cull sows, gilts or to nursery and finisher pigs and after that, during the last 30 days or more, gilts are allowed to recover from the infection. This program will result in the clinical stabilization of the sow farm. It is important to mention that these gilts must come from a consistent single source. Semen or boars are a risk as well and must come from the same farm as the gilts.

Eradicating PRRS virus from the sow farm

In order to be able to eradicate the virus, it is necessary to have a source of negative replacements. However, before introducing the negative replacements into the farm we need to make sure that there is no virus re-circulating in the sow farm. Usually, this will be achieved by closing the farm to the introduction of replacements during a period of 5-8 months and by allowing infected sows and gilts to recover from infection. The farm will be open to negative replacements when signs of virus re-circulation are no longer observed. However, virus re-circulation is not easy to measure, but it can be attempted by introducing negative animals as sentinels into the sow farm or by mixing cull sows (preferably low parities) with negative replacements in a building separated from both the gilt unit and sow unit. Negative replacements should be introduced when there are no signs of virus re-circulation in the sow herd. Negative check boars are excellent sentinels due to the extensive contact with most of the breeding herd.

A practice tip in order to maximize the time between the closure of the herd and the introduction of negative replacements and to minimize losses in production would be to breed and gestate off-site the new negative replacements. Negative replacements could be bred in an off-site facility and be introduced in the sow farm right at farrowing. This practice would allow an extension of 3.8 more months in the period during which the sow farm is being closed. Pregnant negative replacements should be introduced into the herd just prior to farrowing and be placed directly into the farrowing crate.

Eradicating PRRS virus from the nursery and finisher stages

The best way to eradicate PRRS virus from the growing stages is by doing a partial depopulation. Nursery depopulation is effective but only indicated in cases when the virus is only re-circulating in the nursery and no longer shed by the sows. Partial depopulation will not only result in the control of PRRS virus infection, but also in an improvement in production due to the elimination of other infections that may have also existed.

By performing a partial depopulation, a bubble is created in the system. This bubble may be walked through the finisher stage and therefore result in the elimination of the virus from the system.

Economic value of PRRS eradication

PRRS virus eradication has an important economic value not only because of the direct effect on disease but also due to the indirect losses.

Reports on farms that have eradicated PRRS virus have shown considerable improvements on the overall health of the farms. Mortality due to secondary infections has been shown to decrease and parameters such as feed conversion and average daily gain are considerable improved.

For farms selling breeding stock there is an increasing demand on PRRS negative replacements. Producers may look at this possibility as a way of overcoming the low prices that the commercial pigs are subjected to. The demand on PRRS negative replacements is expected to grow as the swine industry succeeds on eradicating PRRS virus from the swine farms.