Porcine circovirus (PCV) was first discovered in the early 1970’s. The virus was later named PCV type 1 and was shown to infect swine but not cause pathogenesis. A variant strain of PCV was described and named PCV type 2 (PCV2) in the late 1990’s and is now known to be involved with a number of conditions collectively known as porcine circovirus associated diseases (PCVAD). Recently, research has focused on reproductive failure as a component of PCVAD particularly in gilts and low-parity sows. Clinical signs of PCV2 include, for example, irregular returns to estrus after mating, abortion, and reduced litter size with increased proportions of stillborn pigs and mummified fetuses. Confirmatory diagnosis as a cause of reproductive failure requires pathology and detection of PCV2 in fetal tissues, such as cardiac tissue.
A large proportion of gilts and sows in major pork-producing countries are bred using artificial insemination (AI). In an 11-month old boar from a stud with a history of illness and infertility, PCV2 antigen was demonstrated in the testes and accessory sex glands. In a large-scale study, semen from a total of 472 boars housed at five commercial studs in Austria and Germany or used for on-farm semen collection and AI on 30 Austrian farms was collected and 18.2% of the samples tested positive for PCV2 in semen; Of the boars testing positive for PCV2 in semen, 57.5% were serologically positive for PCV2 specific IgG when blood was sampled. This likely indicates that the boars were exposed to the virus early in life as none of the boars were positive for PCV2 specific IgM an indicator of an acute, more recent infection. Because of widespread distribution of PCV2 in boars used for AI, it is logical to speculate that a possible route for transmitting PCV2 is semen collected from infected boars that is subsequently used for breeding gilts and sows. Shown in figure 1 is a diagram depicting the movement of PCV2-contaminated semen from infected boars at a stud to sow farms. Although the boar stud is relatively small (100 head) it still supplies the weekly semen needs of 12, 1,500-sow farms. Thus, in production systems employing AI, PCV2 can be spread quite rapidly.
Figure 1: An example of the spread of PCV2 from a boar stud to commercial sow farms. 100-head Boar Stud ⇒ 20 AI doses/boar/week ⇒ 2,000 total AI doses/week ⇒ 1,500-sow farm ⇒ 3 week weaning ⇒ 75 females mated/week ⇒ 2.2 AI doses/mating ⇒ 165 AI doses needed/week
Several research trials support the concept that PCV2 is spread from boar to sow via infected semen. Gilts became infected after raw semen from PCV2-positive boars was administered via intraperitoneal injections; however the dose of PCV2 that was infectious in that bioassay did not cause infection in gilts via AI. In other trials, however, semen spiked with PCV2 was used for AI in gilts, resulting in antibodies to PCV2 and reproductive failure including decreased conception rates, an increase in the number of stillborn pigs and mummified fetuses.
In theory, some reproductive failure in gilts or sows that are mated using AI with PCV2 positive semen could be secondary to effects of PCV2 on sperm characteristics. Research does not support this hypothesis. In a study comparing semen characteristics in PCV2-positive boars actively shedding PCV2 in semen with those in PCV2-positive boars that did not shed PCV2, no difference was detected between groups in the proportion of live or morphologically normal sperm cells. Likewise, addition of PCV2 to semen had no effect on sperm motility and the proportion of viable cells.
In an experiment conducted in our laboratory at Virginia Tech, semen collected from PCV2-positive boars that received either no vaccine or were vaccinated with a commercial killed vaccine against PCV2 was evaluated using a computer-assisted sperm analysis (CASA) system. Measures of sperm motion are considered important correlates to male fertility in several species. For example, the average velocity measured over the actual point to point track followed by the sperm cell (VCL) has been shown to be a significant predictor of fertility in humans. In swine VCL and the path velocity of the smoothed cell path (VAP) have been shown to be positively correlated with litter size. Our research however indicates no effects of vaccination against PCV2 on any characteristic of sperm motion, including VCL and VAP, as determined by a CASA system for PCV2-positive boars (figure 2). Moreover, vaccination had no effects on volume of the ejaculate, sperm concentration, total sperm cells, or morphological characteristics of sperm cells (figure 3).
Figure 2: Effect of vaccination of PCV2-positive boars against PCV2 on characteristics of sperm motion
Figure 3: Effect of vaccination of PCV2-positive boars against PCV2 on sperm morphology
To summarize, PCV2 can be shed into semen by infected boars and research suggests that the virus can then be transmitted to gilts and sows by AI causing reproductive failure. There is no evidence that PCV2 in semen has effects on various characteristics of sperm cells. Moreover, vaccination of PCV2-positive boars against PCV2 does not alter semen quality.