PRRS virus
PRRS virus [31] has been isolated from boar semen [32–40]. Specific local antibodies against non-structural viral proteins (IgA) can also be detected in semen after infection, which is correlated with the duration of PRRS virus RNA excretion in semen [41]. Quantification of PRRS virus RNA in semen has been attempted using RT- nested PCR, but the biological significance of PRRS virus RNA in semen is still to be proved [42].
The disease affects boars at all ages and produces various clinical signs (Table 4) and testicular alterations after infection of immature sperm cells. PRRS virus is detected in the non-spermatozoa cell fraction of the ejaculate but not in the mature spermatozoa. The infected cells are responsible for the presence of infectious PRRS virus in semen [43]. It has also been reported that PRRS virus infection causes a significant reduction in semen quality (decreased sperm motility, a higher percentage of rejected ejaculates, and an increased number of spermatozoa with distal cytoplasmic droplets) (Table 5) [32,44–47]. The decrease in sperm motility is observed starting 2 weeks after the onset of clinical symptoms and returns to a normal level 10–12 weeks later (Figs. 2 and 3).
Viral excretion appears more frequently during the acute stage of the disease [48] but excretion for considerable periods of time has been described, especially between breeds (Table 6) [32,34,38,39].
The role of the boar in transmission of the disease was suggested for the first time in Great Britain [49] and France [50] after an ELISA test showed the boars in a major AI center to be seropositive. Extensive investigations were subsequently reported, indicating