Porcine Reproductive and Respiratory Syndrome (PRRS) and Biosecurity

By DAHS - This article by DAHS looks at the causes of Porcine Reproductive & Respiratory Syndrome and methods of biosecurity to help prevention and control.
calendar icon 7 June 2004
clock icon 13 minute read


  • PRRS is a very common disease. It is commonly regarded as the most important pig disease affecting the global pig industry.

  • Costs are high – typically in an acute outbreak they can be US$255 per sow and US$15 per pig

  • Costs are ongoing after the initial acute outbreak – typically US$76 per sow

  • Transmission can occur by a number of means

  • Good Biosecurity is essential for effective prevention, control and eradication schemes

  • External Biosecurity prevents new infections, infection with new strains and can help maintain stable herds.

  • Internal Biosecurity reduces spread around a unit. It helps maintain herd stabilization, Test and Remove programmes, Herd Closure & Rollover programmes and partial depopulations

  • With the variety of spread individual Biosecurity plans need to be formulated for each farm

  • These should be based on broad-spectrum proven disinfectants ( Virkon®S and Hyperox®) plus the use of heavy duty detergents (Biosolve®)


Porcine Reproductive and Respiratory Syndrome (PRRS) is endemic in most areas of the world. Within infected countries 60-80% of herds are infected 1. The impact on the global pig industry has been huge, and PRRS is viewed by many pig veterinarians as the most economically important disease affecting the pig industry worldwide.

Costs of PRRS

In an acute outbreak in a breeding herd it has been calculated that the average cost per sow for the first year is US$255 (¡ê168). Importantly reduced breeding herd performance can continue over years in the sub acute and chronic phases. It is common to see losses of US$76 (¡ê50) per sow per year at this stage compared with pre-PRRS performance.

In the grower/finisher herd PRRS is important for its direct effects and secondary effects in combination with other pathogens (e.g. Mycoplasma hyopneumoniae, PCV2, Swine Influenza virus, Salmonella species and Streptococcus suis). It leads to reduced growth rates, increased mortality, medication costs and marketing problems. Depending on the level of impact of PRRS costs are estimated at US$6.25-15.25 (¡ê4.10-10.03) per pig.

PRRS will also increase the animal health costs needed to control secondary disease. In a 2700 sow operation in Poland costs associated with the prevention and treatment of secondary disease rose 60% over the first 12 months post infection. In the most acute phase they were four times higher than previously 9.

Transmission of PRRS

To establish effective biosecurity programmes for PRRS it is essential to understand how the virus is transmitted. It is a peculiar virus as it appears highly infectious, but poorly contagious. Given the right conditions, a dose as low as 10 virus particles by intranasal or intramuscular routes can infect young pigs 10. Conversely it is often difficult to intentionally transmit PRRS virus by housing infected and negative animals together.

Transmission of PRRSV can occur vertically, that is from one generation to the next, by infection of the embryo or foetus in utero. This can produce symptomless long-term carriers. These can have long term implications in the control of disease in endemically infected herds and can be a biosecurity breach.

Horizontal infection occurs most frequently from pig to pig. Infected pigs can secrete high levels of virus in nasal secretions, saliva, urine, semen, milk, blood and probably faeces. Pigs seem most readily infected intra-nasally, via cuts & skin penetrations, and intra-vaginally. Oral infection occurs but less readily.

The most common cause of infection is via pig movement. After this the next most important spread of infection appears to be via semen. Additionally indirect transmission can occur. The effect of temperature on the virus is important, with virus survival higher in cold conditions. Transport and vehicles appear to be potentially important 11,12. Transfer on the hands and clothing of operatives can occur but less readily and is easily controlled. Aerosol transmission was believed to be very important, but recent work suggests it can occur but not easily 12.

Flies and mosquitoes have been demonstrated to carry the virus and may have a role in transmission13,14. Other animals appear to be capable of carrying the virus mechanically but are unlikely to be actively infected. PRRSV can also be spread by equipment, especially that which enters the body (e.g. needles, tatooers, castration knives, and tail dockers) 15. These may be important in the transfer of infection within a farm.

Persistence of PRRSV in the Environment

The virus is quickly inactivated by drying, but it can remain infectious for varying periods under the right conditions of temperature, humidity and pH. It has been shown to have a half life of 139 hours at pH 7.5 and a temperature of 4¡ãC, but 1.4-3 hours at the same pH and 37¡ãC and only 6 minutes at 56¡ãC 16. At a practical level it is important to remember that, even in very hot weather, there will be many areas on farms and in vehicles where local conditions are suitable for the virus to survive for considerable periods of time outside the pig (e.g. high humidity). It survives less than 1 day on clean plastic, stainless steel, wood shavings, pig starter feed and denim cloth. However infectious virus has been detected in water up to 11 days after pigs were removed 17.

What is the Role of Biosecurity in PRRS Control?

There are various areas where biosecurity is a vital part of PRRS control.

  1. By preventing a farm or area becoming infected.

  2. Or by preventing infection of an existing infected farm within a new isolate.

  3. Preventing access of infection into an AI stud and hence on to its customer farms.

  4. Preventing the spread of infection into or out of an isolation facility.

  5. Reducing spread of virus around a farm from actively infected pigs to susceptible pigs - thus aiding All-in, All-out as a control measure.

  6. In Test and Removal 18 and Herd Closure & Rollover 19 methods of eliminating PRRS biosecurity is needed to prevent the elimination protocol from failing.

  7. It is an integral part of the McRebel protocols to reduce the impact of PRRS on infected farms.

  8. It helps maintain stable/inactive breeding herds. This will reduce the impact of PRRS and also allow elimination techniques to proceed more easily.

  9. It allows the benefits of total de-populations and partial de-populations to be achieved and then maintained.

Prevention of Entry of PRRS – External Biosecurity

Good geographical location and separation from other pig enterprises will reduce the risks of infection. The farm should have a secure perimeter with access controlled, preferably through one point. Personnel entering should ideally shower, but definitely change clothing and footwear. At the entrance to the farm and each building boots should be cleaned and dipped in a footdip containing Virkon® S (DAHS) 20. Prior to entry and while moving about the farm operatives should cleanse their hands with a biocidal hand cleanser (DAHS Hand Hygiene System).

As the major method of transmission of PRRS is by movement of pigs, farms free from PRRS should only purchase pigs from known and tested free sources, which have high levels of biosecurity. Purchased breeding stock should be isolated, acclimatised and allowed to recover in a separate biosecure facility over a period of 60-150 days. Personnel dealing with these pigs should ideally not work on the main unit, and if they have to they should deal with isolation at the end of the day using separate clothes and showering prior to re-entering the main unit.

Semen should only be purchased from a stud with known negative PRRS status. The stud should have a comprehensive programme of testing of pigs and semen for PRRS. Stud biosecurity needs to be of the highest level along the lines described here.

Vehicles approaching the farm should have been cleaned and disinfected using a comprehensive vehicle biosecurity programme (DAHS Vehicle Biosecurity Programme). This is most important with vehicles that have been carrying pigs, and appropriate downtimes should be applied especially after carrying PRRS positive stock. All vehicles approaching the farm should be sprayed or pass through a wheel dip containing Virkon® S. Special attention should be paid to wheel arches and steps. Drivers should be supplied with boots and overalls.

Correctly constructed (preferably separate) loading and unloading bays are required. These should be cleaned using Biosolve® (DAHS) and disinfected using Virkon® S or Hyperox® (DAHS) prior to use and immediately after use. Correct protocols for loading bay use are essential with drivers not entering the unit and staff not entering the vehicle or its ramp. Footdips (Virkon® S) are required both sides of the loading bay. Feed vehicles should deliver to the perimeter of the unit and the unit should have its own blower hoses. Other vehicles should be kept away from the unit. Special care is needed over the collection facilities for dead pigs.

Any equipment entering the unit is a risk. Dee showed how easily PRRS could enter a unit on a box passed through a reception area 11. Ideally as much equipment as possible should have an outer wrapping which can be removed prior to it entering the unit. Equipment should not be shared between units. If it is it needs to be thoroughly cleaned and disinfected with Virkon® S. Electrical equipment is a special risk because it cannot be wetted – this should be wiped with cloths with Virkon® S. Contractors¡¯ tool boxes are another risk.

Access of other animals should be controlled. Exclusion of insects and flies is difficult on many units, but may well be essential. Rodent control needs maximising. Pork meat products could potentially carry PRRS, although they are of far greater risk for other diseases (e.g. Classical Swine Fever/Hog Cholera), and they should be excluded from the farm. Do not share medicines or medicine delivery equipment between farms. As water can carry PRRS virus it is important to sanitise water systems. This can be achieved using Virkon® S at 1:1000 and pigs can still drink this. At times of risk Aerial disinfection in the presence of pigs using Virkon® S will reduce the risk of aerial infection.

Control of Spread of PRRSV Around the Unit – Internal Biosecurity

Control of the spread of PRRSV around a unit will heavily depend on adopting the correct management procedures. Farrowing, weaners, growers and finishers should all be operated on an All-in, All-out system with thorough Terminal disinfection between fills. This will include good cleaning using a heavy duty detergent (Biosolve®) followed by disinfection using Virkon® S or Hyperox®.

Many of the points pertinent to external biosecurity remain important here. These include control of staff and vehicle access and movement. Staff movement between buildings and sections should be minimised. Appropriate changes of overalls and boots should be made. Footdips (Virkon® S) and hand hygiene facilities should be placed outside every building/room. Any equipment moved between buildings should be thoroughly cleaned and then disinfected with Virkon® S. Where pigs are moved between buildings the passages should be cleaned and disinfected before and after movement.

Aerial disinfection can prevent spread of PRRSV, and with the short distances involved on a given farm it can have a real role to play in reducing spread in acute situations. It can be done 2 or 3 times daily if necessary. Water systems need regular sanitising and can be disinfected with Virkon® S as a part of the terminal programme.

Control of other animals is important. Other pet and production animals including cats should be excluded from the unit. Rodent control should be via a comprehensive programme (The IPM® Rodent Control Programme – DAHS). Flies and mosquitoes should be excluded where possible.

Equipment used in invasive procedures, such as injections, castrations, tattooing and teeth clipping and tail docking need to be frequently changed or cleaned and disinfected. Virkon® S is a good disinfectant for this.


Biosecurity has an integral part to play in controlling PRRS, the most economically important pig disease in modern production. Transmission of PRRS occurs easily and by many methods, so complete biosecurity programmes need to be designed for individual farms and systems to minimise the risk of spread. If this is done biosecurity can prevent farms from becoming infected with PRRS and be an integral part in modern control and elimination programmes for PRRS.

Activity of Virkon® S against pathogens mentioned in this article

Agent Virkon® S Activity Test Number Test Country
PRRS 1:700 81 UK
Mycoplasma hyopneumoniae (EP) 1:800 29 UK
PCV2 1:200 200 USA
Influenza viruses 1:320 11 UK
Streptococcus suis 1:150 24 UK
Salmonella spp 1:100 152 USA
Classical Swine Fever virus 1:150 170 UK

Further Information

For further information on PRRS Click Here. More detailed information is also available by following the links in this article.


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Source: Dupont Animal Health Solutions - May 2004

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