Porcine Reproductive and Respiratory Syndrome

Porcine Reproductive and Respiratory Syndrome is caused by a virus (PRRS) and is of economic importance to the pig industry across the globe. This is because it can cause reproductive failure in sows and respiratory problems in weaners up to finisher pigs. 

Use this information to understand more about PRRS, how the virus spreads, and what impact it can have on the industry.

Porcine reproductive and respiratory syndrome (PRRS) is caused by a virus (PRRSV) and was first detected in South Humberside, UK, during 1991.

As the name implies, reproductive failure occurs in sows and respiratory problems occur in weaners up to finisher pigs.

PRRS is also referred to as blue ear disease, as cyanosis causes a blue colouring of the ears, vulva and abdomen.

The display of clinical signs can vary depending on the age, husbandry, herd’s immune status and the virus strain.

In acute cases, pigs can suffer from systemic effects, which include weight loss, fever between 39 to 41°C, lethargy, reduction in appetite and diarrhoea, along with respiratory effects, such as respiratory distress, pneumonia and laboured breathing. It can also affect reproduction, e.g. mummified foetuses, premature farrowing, stillbirth, abortion and pre-weaning death.

In some cases, pigs can show no symptoms (asymptomatic).

The virus that causes PRRS can be introduced into a herd via:

• Replacement pigs
• Vectors, such as flies, slurry, etc.
• Aerosol (windborne) over short distances

The virus does not survive well outside the host; although, once within a herd, can be transmitted as follows:

• Nose-to-nose contact (most likely method)
• From sow to piglet during pregnancy, with a higher probability of transmission the later the sow is infected during pregnancy
• Through fighting (via saliva and blood)
• By close contact with slurry and/or infected carcases
• In semen
• By birds and insects
• On contaminated clothes 

Persistently infected animals can shed virus and infect contacts.

Piglets born to sows that have been recently exposed to the virus may be born viraemic. Sows that are immune can transfer immunity to piglets through their colostrum. However, once this immunity wanes, usually at around 6–8 weeks of age, piglets can become infected if exposed to the virus.

Transmission from, and amongst, an infected herd means that the virus can persist for a long period of time. This can vary from months to years if new (susceptible) pigs are frequently brought in, due to shedding in urine and faeces, mammary secretions, semen and respiratory-tract secretions.

There are a greater number of diagnoses during the winter months as the virus likes wet, cool and dark weather conditions. Transmission levels also tend to increase at this time as the weather conditions can cause challenges in the cleaning and disinfection of housing, equipment and transport.

The impact of PRRS is extensive as it can negatively affect production on both the breeding and grower/finisher side, with health and welfare implications for both. Effects on breeding stock include:

Sows and gilts having more returns to service

• Smaller litter size
• Less piglets are born alive
• The output of piglets is further affected by abortions, increased stillborns, premature farrowing and mummification

Effects on grower and finisher stock can include:

• Greater pre-weaning mortality due to smaller, weaker piglets
• Greater mortality due to respiratory disease
• Higher incidence of other endemic diseases due to suppressed immunity

Financial implications

The financial impact can vary significantly. An EU survey suggested a cost of £3.50 per marketed pig and approximately £80 per sow.

In terms of herd costs, this would equate to £40,000 per annum, based on a 500-sow herd. The disease is estimated to cost approximately £26 million, based on 2018 population data.

There are four pillars important to controlling the virus that causes PRRS:

1. Herd management
2. Diagnosis and monitoring
3. Immunisation
4. Biosecurity

The primary objective at herd level is to raise negative piglets.

Semen

Semen is a source of the PSSR virus and so the selection of PRRS-negative semen is important to prevent transmission. Selecting semen from boars who undergo routine testing is a good way of achieving this.

McRebel

Management Changes to Reduce Exposure to Bacteria to Eliminate Losses (McRebel) is a set of management procedures which can be implemented when there is an outbreak to reduce the number of mortalities. These procedures include:

• Limiting cross-fostering or, if unavoidable, ensuring that it is done within 24 hours of parturition
• No movement of sows and their piglets between rooms
• Cull any piglets that are very ill
• Pigs should not be held back
• Implement an all-in all-out policy
• Ensure thorough cleaning and disinfection of pig accommodation and associated equipment

Gilts

Replacement gilts pose a big risk to the herd as they could potentially bring in the virus or introduce a new strain if they come from a positive farm. They are also susceptible to any PRRSV strain present on farm, and if introduced before immunity has been established, they facilitate recirculation of the virus. The aim of an acclimation programme is to expose the gilts at a young age to the resident PRRSV so that by the time the gilts are ready to move into the breeding herd, an immunity to the virus will have been established.
There are three methods for acclimation:

• Introduce gilts to PRRSV-positive nursery pigs, which allows direct transmission
• Vaccinate during acclimation
• Expose the resident PRRS in the form of a live virus injection

The benefit of the last option, from a population perspective, is that elimination of the resident virus is possible as immunity will begin at the same time.

Vaccines

Vaccination is an important aspect of limiting disease impact and maximising immunity.

Vaccination of more than 95% of the herd is required in order to be effective and mass vaccination is now the preferred approach.

There are two options for vaccination:

1. Inactivated vaccines were produced to try and reflect the diversity of the virus. However, there are mixed reviews surrounding efficacy.
2. Modified live virus (MLV) has a number of benefits. Firstly, when pigs suffer from a heterologous strain, the incidence of disease, along with the severity, is reduced. Secondly, when the vaccine contains the parent strain, there are fewer mortalities and the growth of the pig is not so adversely affected. Thirdly, there are fewer pigs that are persistently infected and therefore shedding when vaccinated repeatedly in an infected herd. This vaccine can be used in growing pigs, gilts and sows.