What you should know about the main viruses causing reproductive failure in pigs

Wednesday, February 10, 2016

The most common and costly viruses causing abortions and reproductive failure are Porcine Reproductive and Respiratory Syndrome Virus and Porcine Parvovirus. Here's a primer on how infection occurs, is diagnosed and treated

by ERNEST SANFORD

Porcine Reproductive and Respiratory Syndrome Virus (PRRS virus). PRRS is characterized by reproductive disorders consisting of late-term abortions, premature farrowings, stillborns, mummifications and weak liveborn pigs. High piglet mortality and respiratory disease in neonates to market pigs make up the other half of the syndrome.

PRRS is caused by an RNA virus. It first presented as abortion storms in herds in North Carolina in 1987 and also in Ontario and Quebec in the same year. Outbreaks then occurred across Western Europe in 1990 and 1991 and subsequently in most countries with a significant swine industry worldwide. It quickly became the most important production-limiting disease in North America, costing the U.S. swine industry more than $650 million annually.

The PRRS RNA virus mutates at a very high rate, causing it to evade 100 per cent protection provided by vaccines. As a result, control measures have evolved over the last 25 years that involve various combinations of management techniques with and without vaccination. An entire textbook could be devoted to PRRS, so I have purposely given just a "bare bones" introduction to it here.

Porcine Parvovirus (PPV). PPVs occur worldwide wherever pigs are reared. PPV infection causes stillbirths, mummified fetuses, small litters and infertility, the characteristics of the SMEDI (stillbirths, mummified fetuses, embryonic deaths and infertility) combination of reproductive failure events. PPV is a small (from Latin, parvus, meaning small) DNA virus that is very stable in the environment.

PPV infection occurs by inhalation or during mating. The virus crosses the placenta of sows and gilts that are infected at time of mating. Embryos and fetuses die in utero if infected prior to 67 days of gestation. Embryos and fetuses that die before 33-35 days of gestation are usually resorbed completely and the sow returns to heat at an irregular interval (longer than 21 days since the last heat).Those dying after 33-35 days of gestation may become mummified, stillborn or, occasionally, may be aborted. After initially infecting one fetus in the uterus, infection can thereafter spread from fetus to fetus in utero.

The virus is shed in urine, feces and nasal secretions from two weeks after infection.  Sows develop solid immunity after infection and pass passive immunity, via colostrum, to suckling piglets that lasts for four to six months.

Clinical signs associated with PPV infection in a herd are infertility, small litters, mummies, stillborns and sometimes abortion. Pseudopregnancies (false pregnancies) and irregular returns to estrus may also signal PPV infection.

Gilts, young sows and naïve pigs introduced into a PPV-infected herd are prime targets for PPV infection. Boars transmit infection via semen. Semen and embryos are sources of infection in uninfected herds.

As for diagnosis, an outbreak characterized by mummies and stillborn pigs, birth of small litters, especially in gilts in the herd, but not mature sows, with few or no outright abortions, signals a probable PPV infection. Confirmation is made by fluorescent antibody (FA) tests on lungs from mummified or stillborn pigs.

Susceptible breeding stock is routinely vaccinated with killed, adjuvanted PPV vaccine to prevent PPV disease and outbreaks of PPV-induced reproductive failures. Breeding stock is then revaccinated every breeding cycle.

Enterovirus infections. Enteroviruses were initially identified as the agents responsible for the SMEDI reproductive failure syndrome prior to the discovery of PPVs, which ultimately proved to be the true SMEDI agents. Enteroviruses cause outbreaks of polio encephalomyelitis (polio) in the piglets of naïve gilts. Polio encephalomyelitis in pigs is similar to polio in humans. Enteroviruses are common in swine herds worldwide and do cause some of the SMEDI problems, but much less frequently than PPVs.  Since enteroviruses are common in swine herds, clinical problems occur in litters of naïve gilts recently introduced into a herd. The SMEDI problem does not recur in the second and subsequent litters as the gilts acquire immunity to the resident enteroviruses in that particular herd.

Infection of fetuses occurs after viremia in the dam or via infected spermatic or ovarian fluids. Infection is unlikely to occur in immune sows, but maternal immunity does not protect against intra-uterine infection of fetuses. Infection of fetuses prior to 35 days of gestation results in delayed returns to heat. Infection after 35 days of gestation results in mummification, abortion or malformation of the fetuses, stillbirths or death.

Delayed heats, abortions, mummified, dead and stillborn piglets or litters with small numbers of piglets may follow introduction of new breeding stock into a herd. Affected sows will farrow normally in subsequent pregnancies. Apart from reproductive failure, other clinical signs are not seen in enterovirus infection in sows.

When it comes to diagnosis, viral (e.g. PPV) and bacterial (e.g. Erysipelothrix spp., Leptospira spp.) causes of abortion must be ruled out. Where possible, enterovirus should be isolated from frozen or chilled fetuses. If serological tests are available, seroconversion in serum (blood) samples of affected sows and/or presence of enterovirus antibodies in thoracic or abdominal fluids from aborted fetuses can be used for diagnosis.

Isolation and quarantine of incoming breeding stock is the best way to prevent introduction of infection into a naïve herd. Feedback of aborted fetuses and fetal membranes at least three weeks prior to service is practised to build immunity during an active outbreak. There are inherent dangers of unknowingly transmitting other disease agents during feedback procedures.

Aujeszky's disease (pseudorabies). Aujeszky's disease virus is endemic in many countries worldwide and is a major cause of respiratory disease and reproductive failure. It has never been present anywhere in Canada, although the threat of its entry into Canada was always a concern because it was endemic and clinically active in the domestic and wild pig populations in the United States.

However, since its eradication from the U.S. swine population in 2004, the likelihood of it entering Canada has been greatly reduced. Furthermore, there are active eradication efforts in most countries with endemic Aujeszky's disease. As a result I shall not be discussing Aujeszky's disease any further in this article.

There are several other viruses that are associated with reproductive failure in swine, including: encephalomyocarditis virus, cytomegalovirus and swine influenza virus, but I have restricted myself here to the main viruses involved in cases of abortion and general reproductive failure in swine.

Most common of these viruses is PRRS virus, the most production limiting and economically important disease in North America, responsible for widespread reproductive failures and respiratory disease, and PPV, which causes stillbirths, mummified fetuses, small litters and infertility; the SMEDI combination of reproductive failure events.

Enteroviruses were originally believed to have been the cause of the SMEDI syndrome of reproductive failure until porcine parvoviruses were discovered and replaced enteroviruses as the usual cause of SMEDI, Enteroviruses cause the same  clinical signs of stillbirths, mummified fetuses, small litters and infertility as porcine parvovirus. BP

S. Ernest Sanford, DVM, Dip Path, Diplomate ACVP is a swine veterinary consultant based in London