PRRS control: Can we learn from Covid-19?

Porcine reproductive and respiratory syndrome (PRRS) is a single-stranded RNA virus, present across the globe and the cause of death and long-term disease among susceptible populations. Sound familiar?

PRRS, also known as blue ear, first occurred in Europe in the early 90s, and has an estimated herd prevalence of around 60% in Ireland. It results in significant losses due to growth retardation and immunosuppression in piglets and reproductive disorders in sows and gilts.

Since then, the pig industry has sought to control the disease through strict biosecurity and vaccination.

Transmission

PRRS is highly transmissable, with an R-value of between one and three. Like Covid-19, it is usually transmitted by close respiratory contact between individuals and within groups.

Herd to herd transmission of PRRS can occur following the introduction of carrier animals or the use of infected semen. This has been a significant cause of outbreaks in Ireland and regular testing of all donor boars is essential.

The virus can also be spread via contaminated equipment and there are reports that the virus can spread in the air for up to 3km.

This means that PRRS eradication programmes are only successful where farms are a good distance away from others, or have ventilation systems and stringent biosecurity practices in place to prevent reinfection.

Disease can continue to circulate within a farm due to clinically normal pigs being persistently infected with the virus. Sows infected at 85 to 90 days of pregnancy can give birth to carrier piglets which can shed the virus for a prolonged period.

Clinical signs

In younger pigs, PRRS is often subclinical and suppresses the animal’s immune system, predisposing them to secondary bacterial and viral infections.

Due to its major role in the multifactorial porcine respiratory disease complex (PRDC), it is indirectly responsible for large economic losses in finishing herds.

PRRS infection can cause severe reproductive damage, including premature farrowing, stillborn or mummified pigs, weak pigs that die soon after birth and delayed return to service. In addition to reproductive failure, sows and gilts may be sick, off feed, have raised temperature and no milk and, will occasionally die.

Diagnosis

Laboratory testing is the most effective way of detecting the level of PRRS circulating on farm. Blood or antibody testing is a simple way to determine the status of a herd: infected or free.

PCR testing detects part of the virus particle and therefore is very specific allowing farmers and vets to obtain a more accurate picture of virus circulation within the herd.

PCR tests are very sensitive and can detect very low quantities of the virus. Samples can be taken from wounds, blood samples or rope testing where the saliva from ropes sucked by pigs provides easy, non-invasive test samples.

Control

The key pillars for PRRS control on farm are strict biosecurity and hygiene measures, absolute adherence to ‘all-in, all-out’ pig flow, elimination of weak pigs, as they could be persistently infected, diagnostic testing and vaccination.

Immunisation by vaccination of breeding stock is the most cost-effective first step for the control of PRRS. To maintain a high level of immunity, it is recommended sows receive ‘booster jabs’ at regular intervals, either before every service, on a ‘6-and-60 programme’, or at regular 3 or 4-month intervals.

Gilt management is key to stability of the breeding herd. These gilts may have been exposed to disease and are potentially infected with field strain of PRRS. They need to be immunised prior to joining the rest of the breeding herd.

Early selection, administering two live vaccines four weeks apart and with the second vaccine at least five weeks before service has been seen to be the most effective means of control for PRRS.

To protect growing pigs, vaccination of piglets from 14 days of age is an effective way to control the effects of PRRS and improve performance. A single dose of attenuated or weakened live virus vaccine should provide protection against PRRS for the next 6 months.

PRRS vaccination can be undertaken at the same time as vaccination for the other three diseases of greatest economic importance in growing pigs, PCV2, M. hyo and Lawsonia intracellularis (ileitis).

Vaccination for all four diseases can be undertaken as a single administration or through the IDAL device from 3 weeks old. Obviously, all farms will have different disease timing and severity and vaccination programmes may differ.

As we have learned with Covid-19, while good biosecurity can control the spread of disease, just a single infected case can cause a devastating breakdown. Effective vaccination reduces the risk for the entire population and can check the circulation of the virus and promote productivity.

Viruses have a lot of family characteristics, but luckily these two have a number of differences as well. There is no evidence that Covid-19 has affected pigs anywhere in nature.