Modern control strategies for PRRS

Porcine respiratory and reproductive syndrome (PRRS) continues to have significant performance and economic effects on pig production. In fact, it is considered to be the most significant pig health problem across the globe.

Porcine reproductive and respiratory syndrome virus (Courtesy Boehringer Ingelheim Animal Health)
Porcine reproductive and respiratory syndrome virus (Courtesy Boehringer Ingelheim Animal Health)

This article appears in the November/December issue of Pig International. View all of the articles in the digital edition of this magazine.

Porcine respiratory and reproductive syndrome (PRRS) continues to have significant performance and economic effects on pig production. In fact, it is considered to be the most significant pig health problem across the globe. While focus has often been on the effect the virus has in the breeding herd, the economic impact on grow/finish units is thought to be even higher. For this reason control strategies are being designed to protect the whole herd, reducing the viral load in the pig population as a whole.


PRRS virus is an arterivirus characterized by the ability to kill macrophages. Macrophages would normally form part of a pig’s immune defense against viruses and bacteria, engulfing and digesting them. However, macrophages cannot destroy arteriviruses, which instead multiply within the cells eventually killing them.

The virus is spread either by contact with bodily secretions (nasal mucous, saliva, feces, urine, milk and colostrum), or can be airborne. The time that infected animal shed the virus depends on their age. It can be up to two months in growing pigs, while in adults it is normally only two weeks. 

Prevalence of PRRS

PRRS is endemic in most countries with only a few countries being free of the virus. In countries where the virus is present, all herds will be affected to some extent. There are two main types of the virus: Type 2, being present in the U.S., and a highly pathogenic version in Asia. Europe is mainly positive for Type 1, but there is some Type 2 in Germany, Denmark, Hungary, Belgium and the Netherlands. 

global PRRS distribution
There are two main types of PRRS affecting different countries around the world; other countries still remain virus free. | Boehringer Ingelheim Animal Health

Symptoms of PRRS

As the name suggests, PRRS is typified by respiratory and reproductive effects. The symptoms displayed will depend on the life stage of the pig. While reproductive symptoms are obvious during pregnancy, dry sows are more subtly affected, e.g., length of time to return to service. Similarly, respiratory disease can have a catastrophic effect in the young piglets, but sub-clinical effects can still retard growth in older animals.

symptoms of PRRS different life stages
Clinical signs of PRRS are dependent of the age of the pig.  

While losses in the breeding herd are obvious, the cost of PRRS in grow/finish units may be even higher. | Boehringer Ingelheim Animal Health

The severity of PRRS symptoms varies significantly in herds that are virus-positive. Some herds may display no outward signs of disease. Other herds may exhibit non-specific reduction in performance or show severe clinical signs and mortality. “The severity of PRRS is largely dependent on other diseases present in the herd,” confirmed PH Rathkjen, the senior global technical manager responsible for PRRS at Boehringer Ingelheim Animal Health. “For example, in Asia, where they have classical swine fever, mortality rates can be very high.” Other common complications include enzootic pneumonia and Streptococcus suis. The cost of treating these infections adds to the financial impact of PRRS.

Effects on profitability

In terms of economics, there have been a number of studies undertaken to show the effect on both breeding and grow/finish herds. In the U.S., PRRS is thought to be costing US$1.8 million per day, and 55 percent of that is from grow/finish units. These losses are due to the cost of treating respiratory disease, poorer growth, higher mortality and more culling, adding up to a cost of US$4.67 per pig. This is compared to a loss of US$114 per sow. In Europe, the cost to the breeding herd is similar at 100-200 euros per sow and 5-10 euros per marketed pig, dependent on the country. “The financial impact in Asia would be at least at the same level,” said PH Rathkjen. “Adding up to an estimated global cost of US$4 billion per year.” 

Persistence of the virus

Diagnosis of PRRS involves establishing the presence and activity of the virus in live animals. The virus is not persistent in a single animal and will be cleared over time. However, in order to eliminate it from a whole herd, control measures to homogenize immunity and reduce transmission need to be put in place. Diagnostic tests for the PRRS virus are improving all the time. As well as ELISA, more sensitive PCR and viral sequencing tests are now available. This allows producers to effectively monitor their herd status and implement control plans accordingly. 


There are several vaccines available on the market place sold to immunize virus-positive herds. In order to be effective, pigs should be vaccinated with the right product at the right time. “For the breeding herd, we recommend a mass vaccination of all sows every four months with a good modified live virus vaccine,” stated PH Rathkjen. “Then the piglets are vaccinated just once at around weaning.”

The direct benefits of vaccination have been demonstrated in the breeding herd and grow/finish. They include: reducing mortality, increasing the number of piglets weaned, improvement in feed conversion and growth rates. The indirect effect of vaccination is that the wild-type virus load is reduced. Viral shedding and transmission is reduced, helping not only the herd vaccinated but also others in the locality. However, vaccination is only part of the solution, other elements of a PRRS control program are just as important. 

Control strategies

Herd hygiene and biosecurity are key ways to prevent herds becoming infected. As the virus can be spread through contact with feces and urine, vectors such as equipment, machinery, vehicles, boots and clothing need to be controlled. Practices such as showering in and out, as well as wheel washing, need to be carefully considered.

Control strategies should focus on the grow/finish period as much as the breeding herd.

The fact that the virus can be transmitted in the air should not be ignored. “In the U.S. it is becoming commonplace for breeding units and boar studs to install air filters,” PH Rathkjen explained. “However, due to the volume of air required by growing pigs, it isn’t possible for them.” If new units are to be built, making sure they are distant from other pig producers would help to reduce the spread.

proportion of wild type PRRS detected
Vaccination with Ingelvac PRRS MLV reduced wild-type virus circulation. | Boehringer Ingelheim Animal Health

Movement of pigs is another obvious target for tailored management procedures. Knowing the PRRS status of animals coming onto the unit is essential, and semen should only be purchased from virus-negative studs. “The easiest way for the virus to enter a herd is via other pigs,” stressed PH Rathkjen. “For this reason a gilt management plan must always include a period of quarantine.” He recommended that gilts be vaccinated on arrival and again after four weeks, while being kept in quarantine for 12 weeks before introduction into the herd.

effect of piglet vaccination on growth

Pigs vaccinated with Ingelvac PRRS MLV grew faster, as shown in these three studies. | Boehringer Ingelheim Animal Health

A whole herd approach

The aim of a PPRS control program should be to reduce the risk of transmission, while supporting immunity. The goal is to maintain performance and optimize profit. Key factors include:

  • Tailoring biosecurity programs to reduce spread of the virus into and within the herd
  • Promoting immunity in the breeding herd
  • Improving the health status of growing pigs
  • Minimizing exposure to the virus by reducing circulation and shedding

For many, the use of vaccination is an increasingly viable option and one, that like other control strategies, should be applied to the whole herd. There are approximately ten times as many growing pigs as breeding animals, so by reducing infection in them, a significant effect on viral load can be made.

The take home message is to not underestimate the effect that PRRS can have on pig productivity as a whole. Therefore, in order to reduce its financial impact, whole herd control strategies should be put in place.

Learn more: Controlling pig disease with prevention chain approach

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