This study demonstrated that type 1 and type 2 PRRSV were able to infect germ cells of male reproductive organs from infected boars. Infection of PRRSV may be depended on the presence of viral receptors on the surface of germ cells. PRRSV enters porcine alveolar macrophages via the receptor-mediated endocytosis in vivo
. Four receptors of PRRSV have been identified on porcine macrophages: heparan sulphate, sialoadhesin (CD169), CD 163, and CD151
[12–14]. Further studies are needed to determine the presence of these receptors or other possible receptors on germ cells of male reproductive organs.
The simultaneous detection of viral nucleic acid and protein of type 1 PRRSV by ISH and IHC, respectively, in spermatogonia and other cells indicates that PRRSV replicates in these cells. No significant differences in the type 1 and type 2 PRRSV-positive cells in the male reproductive organs were observed in the present study. These observations contrast with those regarding the respiratory disease in growing pigs
[4, 5, 15], in which type 2 PRRSV was found in higher titers in the respiratory organs, especially alveolar macrophages. These results suggest that type 2 PRRSV may have more affinity for the macrophage lineages. However, there is no significant difference in the affinity of type 1 and type 2 PRRSV for cells of non-macrophage lineages, such as germ cells of male reproductive organs.
This study demonstrated that PRRSV were localized prominently in spermatogonia and their progeny, and apoptosis were also seen in these germinal cells. These results agree with previous findings in which the major site of PRRSV replication and apoptosis were germinal cells; spematogonia, spematocytes, and spermatids
. Spermatogonia are one of the most important potential targets because these cells do not constitutively express biologically active interferon
. PRRSV was most consistently found in the cell fractions of semen from vasectomized and nonvasectomized boars
[17, 18]. These observations suggest that PRRSV excretion in semen results from viral replication in the reproductive tract of the boars or originates from other organs or tissues in a cell-free or cell-associated state. Our results were also shown that germinal cells are the major contributors of cell-associated PRRSV in ejaculates.
The origin of PRRSV in spermatogonia and their progeny was most likely to the hematogenous spread of the virus because PRRSV did not infect the epithelial cells lining the ducts or the glands of the male reproductive tracts. PRRSV antigens were predominantly detected in macrophages in testes and other male reproductive tracts
. PRRSV may use macrophages as a vector for spread of infection to germinal cells within testes. Of the blood vessels, monocytes and endotheial cells have been associated with viremia
[19, 20]. Because monocytes continue to circulate, virus dissemination in reproductive organs such as testes occurs via infected monocytes. PRRSV viremia contributes to viral distribution throughout the reproductive tissues
. Alternatively, direct infection from infectious virions in the peripheral blood is also a distinct possibility given the highly vascular nature of the testes.
There is no significant difference in the histopathological lesions of type 1 and type 2 PRRSV-infected boars. The main histopathological lesions are desquamation and multinucleated giant cells of germ cells in both type 1 and type 2 PRRSV-infected boars. These characteristics lesions were also described in previous study
. The observed desquamation of germ cells indicated the disruption of spermatogenesis
. The multinucleated giant cells, consisting of degenerated and necrotic speromatocytes and/or spermatids
 are the consequence of the particular mode of cytokinesis of dividing germinal cells. The failure of cytokinesis is one of the causes of origin of multinucleated giant cells
Analysis of ejaculates by IHC demonstrated that PRRSV protein was present in spermatogonia and their progeny, and non-sperm cells. The origin of non-sperm cells was identified as macrophages
. This result is in agreement with previous findings that PRRSV is present in semen mainly in a cell-associated fashion
[10, 23, 24]. Detection of PRRSV-infected spermatogonia and their progeny in ejaculated semen indicated that the virus is certainly released from the seminiferous epithelium
 and that the semen may contribute to spreading the virus to the sows
. Several studies have demonstrated seroconversion of sows and gilts bred with infected boars or inseminated with semen from such boars or with experimentally contaminated semen
Boar vaccination against the PRRSV infection is important preventive tool because of the prolonged seminal shedding of virus
[13, 26]. Vaccination with the modified live PRRS vaccine reduced or eliminated shedding of wild-type PRRSV in challenged boars by day 50 following vaccination
. Nevertheless, the cross-protection between two genotypes is another issue because type 1 PRRSV emerged in Asian countries
[31–33]. It has been reported that the type 2 PRRSV-based modified live vaccine is more effective against type 2 PRRSV than the type 1 PRRSV in vaccinated and challenged boars
This is the first study to compare the amount of PRRSV cDNA shed in semen of type 1 and type 2 PRRSV-infected boars. Quantitative differences in the shedding patterns of the two viral genotypes were not observed under the experimental conditions. There was no significantly difference in viral replication or the microscopic testicular lesions associated with the two genotypes. Therefore, two genotypes of PRRSV do not differ significantly in their virulence toward the male reproductive system of pigs. However, our results should be interpreted carefully because the pathogenesis of PRRSV can vary greatly among different strains of the same genotype
[4, 35]. In addition, it should be pointed out that this experiment used one strain of virus of each genotype, which has been passaged serially in MARC-145 cells. It is important to use inoculate which have undergone minimal passage in a porcine line ideally alveolar macrophages because the pathogenicity of the PRRSV field isolate became attenuated when propagated in MARC-145 cells
. Further studies are needed to use several strains of virus in each genotype to compare the pathogenesis in male reproductive organs.