PRRS

A virus affecting pigs of all ages

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Pig Diseases

Porcine Reproductive and Respiratory Syndrome

Also known as: PRRS, Blue Ear Disease, Porcine Epidemic Abortion and Respiratory Syndrome (PEARS), Swine Infertility and Respiratory Syndrome (SIRS) 

Porcine reproductive and respiratory syndrome (PRRS) causes late-term abortions in sows and respiratory problems in piglets and growing pigs resulting in enormous economic losses (Lyoo, 2015). It is one of the primary etiologic agents responsible for swine reproductive failure today (Althouse and Rossow, 2011) and is also referred to as swine infertility and respiratory syndrome (SIRS), porcine epidemic abortion and respiratory syndrome (PEARS), mystery pig disease or blue ear disease. It was independently discovered in Europe and the USA in 1991 and is now widespread. There are two recognized genotypes: type 1 or European-like (prototype Lelystad) and type 2 or North American-like (prototype VR-2332) (Butler et al., 2014). PRRS is caused by an enveloped, single-stranded positive-sense RNA virus. The viral disease primarily causes respiratory problems in piglets and growers, but also causing respiratory failure in susceptible sows.

The virus is primarily transmitted through animal contact, although airborne infection and transmission by infected semen via artificial insemination are well recognized routes of transmission (Benfield et al., 1999; Kristensen et al., 2004; Swenson et al., 1994; Ulyshen, 1994). In the UK, it is believed that the transmission is mainly by pig-to-pig contact or by air-borne transmission (up to 3 km distance).

Causes of PRRS

Pigs are susceptible to the virus through multiple routes, including oral, intra-nasal, intra-muscular, intra-peritoneal and vaginal infection (Benfield et al., 1999). However, pigs are not equally susceptible to PRRS virus by all routes of exposure; that is, routes differ in the probability that a given dose will result in infection (Hermann et al., 2005). The length of time that infected stock can carry and shed the virus through saliva, urine, semen and mammary secretions appears to vary from a few days to several months (Benfield et al., 1999). Once a herd becomes infected, PRRS virus establishes an endemic cycle of transmission that results in chronic or recurring episodes of reproductive and/or respiratory disease (Hermann et al., 2005).

Heat stress in conjunction with PRRS significantly influences the amount of time pigs spend lying down. PRRS virus affects physiological, behavioral, and performance in growing pigs. Social rank influences pigs’ immune responsiveness to PRRS, as it does with other diseases, such as Aujeszky disease, with sub-ordinate pigs suffering more than dominant individuals (Sutherland et al., 2007).

Clinical Signs of PRRS

PRRS causes an increased incidence of late-term abortions and stillbirths, reduced farrowing rates, and weak piglets and increased mortality in pre- and post-weaned pigs (Benfield et al., 1999; Brouwer et al., 1994; Mengeling, 1999). Anorexia, lethargy and fever may be observed in breeding and finisher animals, along with cyanosis (blueing) of the ears, vulva, tail, abdomen or snout (Cowart, 1995; Done and Paton, 1995). Suckling and recently weaned piglets exhibit respiratory problems or ‘thumping’, and show increasing susceptibility to other endemic diseases (Benfield et al., 1999; Done and Paton, 1995).

The immune response to either infection or vaccination with PRRS virus is unusually complicated. We know that PPRS supresses innate immunity, and causes abnormal B-cell proliferation, often lymphopenia and thymic atrophy (Butler et al., 2014). PRRS virus is frequently isolated during outbreaks from weak-born piglets affected by secondary bacterial diseases. In utero infection by PRRS virus makes piglets more susceptible to infection and disease following challenge by Streptococcus suis type II (Feng et al., 2001).

PRRS is the number one disease problem in major swine producing areas around the world. It is estimated to cost the industry 660 million dollars a year just in the USA with proportional losses recognized in other countries. This is attributed to the remarkable ability of PRRS virus to:

  1. Infect susceptible pigs at all stages of production
  2. Be shed in the semen of boars for extended periods of time
  3. Be easily transmitted between farms,
  4. Tolerate a high mutation rate
  5. Negatively modulate the host’s immune response (Butler et al., 2014)

Other respiratory diseases include:

  • Atrophic rhinitis
  • Pneumonia
  • Streptococcal infections
  • Swine influenza

Control and Prevention of PRRS

PRRS is extremely difficult to control under the usual conditions of commercial pig production, therefore the design of effective strategies to control and/or eliminate PRRS virus depend on an accurate and comprehensive understanding of virus transmission. As a general rule, transmission is highly dependent on the route of exposure and thus, the dose of virus (Hermann et al., 2005).

Research suggests that pigs are extremely susceptible to infection via parenteral exposure (breaks in the skin barrier) and much less susceptible by all other routes investigated to date (Hermann et al., 2005). Normal pig behaviour such as bites and cuts are examples of potential parenteral exposures. Also standard practices including ear tagging, injections and teeth clipping will result in damage to skin, and therefore a potential route of infection for PRRS virus.

This in mind the primary source of infection with PRRS is via other infected pigs. Animals being introduced into the herd should be tested for the virus, and those found positive should not be brought into contact with non-infected stock. Strict all-in/all-out policies coupled with efficient cleansing and disinfection between batches of rearing, growing and finishing pigs can reduce losses associated with diseases such as PRRS and PMWS/PDNS (Bown, 2006).

Airborne transmission of PRRS virus from infected to non-infected is possible (Kristensen et al., 2004) and there is some evidence that airborne transmission may occur over distances to 3 km (Cowart, 1995). The potential for airborne transmission may increase the risk of outdoor units contracting and spreading PRRS.

Houseflies have also been identified as a route of transmission (Otake et al., 2004; Schurrer et al., 2004). Fly-born transmission may explain how PRRS virus is seasonally transported between farms. Even in cold weather mechanical transmission of PRRS virus can occur during coordinated sequences of events (Dee et al., 2002).

Transmission via infected semen is possible and should be considered carefully before using artificial insemination or natural service boars of unknown health status. Excretion of PRRS virus into semen appears to occur most frequently during the acute phases of the infection in the boar (Althouse and Rossow, 2011; Prieto et al., 1997).

For persistent cases, disease elimination strategies such as partial or whole herd depopulation may be necessary. Your vet can advise on this.

PRRS Vaccine

Over the past 20 years, a tremendous effort has been made to protect pigs from this economically devastating disease by developing such as killed, modified live, recombinant protein based, and DNA vaccines, but, only cell culture attenuated virus vaccine is practiced in the field with arguably limited efficacy (Lyoo, 2015). A vaccine which provides solid, protective immunity against PRRS virus has yet to be developed, nevertheless, to control PRRS vius spread in endemically infected herds, vaccination is regarded as an important intervention measure (Nodelijk et al., 2003).

Treating PRRS

Treatment options for PRRS remain limited and are supportive in nature. The main aim in the breeding unit is to limit mortalities among piglets and to maintain reproductive performance.

During an outbreak in a farrowing unit the following supportive measures could be taken but are often impractical in outdoor situations:

  • Electrolytes and antibiotics to scouring piglets
  • Avoid stress caused by handling of healthier piglets while providing nursing to weak individuals
  • Improve colostrum intake by manual administration twice within four hours of birth
  • Change service management:
    • no early service (before 21 days after farrowing)
    • high energy diet to weaned sows
    • re-serve early aborted sows (under 70 days) as soon as possible but wait 3 weeks with late aborted ones
    • keep a close eye on the pregnant sows to detect abortions
    • use AI for 6 weeks after an outbreak as boars are likely to be infertile for that period
  • Consider giving non-steroidal anti-inflammatory drugs (NSAIDs) to sows before farrowing to improve appetite and avoid early farrowing. Contact your vet for details
  • Consult your vet about prescribing in-feed antibiotics for sows throughout the acute outbreak to prevent and treat secondary bacterial infections

During an outbreak in a finishing unit the following supportive measures could be taken:

  • Strict all-in-all-out management
  • Segregation of different age-groups of pigs
  • Partial depopulation, in order to stop the continual re-cycling of PRRS virus into younger animals, as they lose colostral immunity
  • Talk to your vet about administering antibiotics to prevent and treat secondary bacterial infections

PRRS and Welfare

During an outbreak, nursing of affected piglets is advisable and improves survival.

Good Practice Based on Current Knowledge

To avoid an outbreak:

  • Prevent the introduction of PRRS virus entering the herd by maintaining good biosecurity and screen all stock entering the herd
  • Investigate any abortions and establish a good recording system for abortions on the farm

In the event of an outbreak:

  • Identify and isolate/remove infected stock promptly
  • Treat and nurse affected piglets (see Treatment)
  • Follow the treatment advice on changes in breeding management (see Treatment)

PRRS References
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