Why does boar semen become a vehicle for virus transmission? (Part Three)
Boar Semen - A Vehicle for Virus Transmission (Part Three)
Porcine semen as a vector for transmission of viral pathogens-Part 3
Dominiek Maesa,*, Ann Van Sooma, Ruth Appeltanta, Ioannis Arsenakisa, Hans Nauwynckb
aGhent University, Faculty of Veterinary Medicine, Department of Herd Health and Reproduction, Division of Reproduction, Merelbeke, Belgium
a Department of Reproduction, Obstetrics and Herd Health, Faculty of Veterinary Medicine, Ghent University, Merelbeke, Belgium
bGhent University, Faculty of Veterinary Medicine, Laboratory of Virology, Department of Immunology and Parasitology, Division of Virology, Merelbeke, Belgium
b Department of Virology, Immunology and Parasitology, Laboratory of Virology, Faculty of Veterinary Medicine, Ghent University, Merelbeke, Belgium
Keywords:
Boar semen, pig, artificial insemination, virus, review/examination
Semen, Pig, Artificial insemination, Virus, Review
Continued from the previous part...
2.2 Viruses present in boar semen that are not listed by the OIE
Viruses in porcine semen which are not in the OIE list
2.2.4. Atypical porcine pestivirus
Nonclassical swine fever pestiviruses
Pigs are susceptible to atypical porcine pestiviruses, which include bovine viral diarrhea virus (BVDV) and border disease virus; these RNA viruses cause related diseases in cattle and sheep. Pigs congenitally infected with these viruses may shed large amounts of virus. Terpstra and Wenswoort [35] isolated BVDV from the oropharyngeal fluid, urine and semen of congenitally infected and infertile boars.
Pigs are also susceptible to non-CSF pestiviruses including BVDV and border disease virus; these are RNA viruses which are associated with disease in cattle and sheep, respectively. Pigs congenitally infected with these viruses may shed large amounts of virus. Terpstra and Wenswoort [35] isolated BVDV from oropharyngeal fluid, urine, and semen of a congenitally infected and infertile boar.
Bungowannah virus is genetically distinct from classical swine fever (CSF) virus, but based on its molecular characteristics it can be included in the genus Pestivirus. In 2003, this virus caused severe disease outbreaks on two pig farms in Australia, mainly increased stillbirths and mummified fetuses and elevated pre-weaning mortality [72]. The risk of transmission of this virus through embryos or semen has not yet been clarified.
Bungowannah virus is genetically distinct from CSF virus, but molecular characterization supports its inclusion in the genus Pestivirus. The virus caused a severe disease outbreak characterized by an increase in stillbirths, preweaning mortality, and mummified fetuses in 2003 in two pig herds in Australia [72]. The risk of transmission by embryos or semen has not yet been evaluated.
2.2.5. Porcine enteric picornaviruses
Porcine enteric picornaviruses
Porcine enterovirus (PEV) and porcine teschovirus are both non-enveloped RNA viruses belonging to the family Picornaviridae. These two types of viruses are highly viable in the environment. Transmission of the virus between pigs occurs mainly through oral contact with contaminated feces. Semen may become contaminated during the collection process. Infection of boars with PEV may cause seminal vesiculitis, sperm abnormalities and reduced libido [32]. Except when infection occurs during pregnancy, clinical signs are usually not observed in sows. If infection occurs during pregnancy, it may lead to embryonic death, mummified fetuses, stillbirths and weak piglets [73].
Porcine enterovirus (PEV) and porcine teschovirus are nonenveloped RNA viruses belonging to the family of Picornaviridae. They are highly resistant in the environment. Infections are transmitted between pigs mainly by oral exposure to contaminated feces. Contamination of semen during collection is possible. Infections with PEV in boars may lead to seminal vesiculitis, sperm abnormalities, and a decreased libido [32]. In the sow, usually no clinical signs are observed, except if infections occur during pregnancy. In the latter case, infections may cause fetal death and mummification, stillbirth, and weak-born piglets [73].
Porcine teschovirus has been isolated from the reproductive tract of boars [32], and insemination of gilts with virus-carrying semen does not affect fertilization [74]. However, Dunne et al. [44] proposed that semen infected with PEV and porcine teschovirus can cause embryonic and neonatal piglet death. No subsequent studies have confirmed this finding.
Porcine teschovirus has also been isolated from the male genital tract [32], but insemination of gilts with contaminated semen did not affect fertility [74]. Dunne et al. [44] however suggested that semen contaminated with PEV and porcine teschovirus could cause embryonic and neonatal death. There are no recent reports that confirmed this finding.
2.2.6. Torque teno virus
Torque teno virus
Two types of torque teno virus (TTV) have been found in pigs: torque teno sus virus 1 (TTsuV1) and 2 (TTsuV2). Both are DNA viruses belonging to the newly created family Anelloviridae, genus Torque teno virus. Porcine torque teno viruses (TTsuVs) are estimated to be ubiquitous in pig populations worldwide [75]. Currently, no related clinical signs appear with TTsuV infection. However, TTsuV is associated with porcine circovirus (PCV)-associated disease [76,77]. Torque teno virus is transmitted both vertically and horizontally. The virus can be detected in fetal tissues and blood, semen and colostrum [36], thereby demonstrating the possibility of vertical transmission. Independent semen samples were collected from 100 boars at artificial insemination centers and tested for TTsuV1 and TTsuV2 by conventional PCR [37]. Four of the samples were positive for TTsuV1 by PCR, five were positive for TTsuV2, and after co-infection, both TTsuVs were detected. The route of fetal infection remains unclear, but the relevant evidence suggests that it may be due to virus-carrying semen or placental transmission.
Two species of TTV have been described in pigs: torque teno sus virus types 1 (TTsuV1) and 2 (TTsuV2). They are DNA viruses that belong to the newly created family anelloviridae, genus Iotatorquevirus. It is considered that TTsuVs occur in pig populations worldwide [75]. At present, no clinical signs are associated with TTsuV infection. However, TTsuVs have been linked to PCV-associated disease [76,77]. Torque teno sus viruses are apparently transmitted both vertically and horizontally. They can be detected in fetal tissues and blood, semen, and colostrum [36], indicating the potential for vertical transmission. Single semen samples from 100 boars from an AI center were analyzed by conventional PCR for TTsuV1 and TTsuV2 [37]. Four samples were PCR positive for TTsuV1 and five for TTsuV2, whereas in a single coinfection, both TTsuVs were detected. The route of fetal infection is uncertain but may reflect virus-contaminated semen or transplacental transmission.
2.2 Viruses present in boar semen that are not listed by the OIE
Viruses in porcine semen which are not in the OIE list
2.2.7. Ebola virus
Ebolavirus
Reston ebolavirus is an enveloped RNA virus belonging to the order Mononegavirales, family Filoviridae. In 2008, a severe disease outbreak occurred in swine herds in the Philippines and was reported to be suspected of being caused by porcine reproductive and respiratory syndrome virus (PRRSV). However, after detailed diagnosis it was found that the disease was caused not by PRRSV but by Reston ebolavirus (REBOV), and REBOV-specific antibodies were detected in farm workers [30]. The broader implications of this REBOV outbreak remain to be determined. Subsequent experiments showed that although the virus replicates in pigs after REBOV infection, it may not produce symptoms. The mechanism of REBOV transmission between pigs is not yet clear. The shedding routes of REBOV are also unclear. In nonhuman primate models, REBOV is shed in all body fluids, including semen [29]. Whether REBOV can be transmitted to sows through semen is unknown. Compared with REBOV, Zaire ebolavirus has been shown to cause severe lung disease in 5- to 6-week-old pigs, causing respiratory distress [42].
Reston ebolavirus is an enveloped RNA virus, classified in the order mononegavirales, family filoviridae. In 2008, severe disease outbreaks suspected to be caused by PRRSV were reported in swine herds in the Philippines. However, detailed diagnostic work revealed that not PRRSV but REBOV had caused the disease and that REBOV-specific antibodies were found in farm workers [30]. The broader significance of this outbreak of REBOV remains to be determined. Subsequent experimental challenge suggested that REBOV infection in pigs may be asymptomatic despite replication of the virus. The mechanism of transmission of REBOV to pigs or between pigs is unknown. Routes of REBOV shedding by pigs are also not known. In nonhuman primate models, REBOV is shed in all body fluids, including semen [29]. Whether REBOV is shed by boars in semen or transmitted to recipient females is unknown. In contrast with REBOV, Zaire ebolavirus has been shown to cause severe lung pathology in 5- to 6-week old pigs leading to respiratory distress [42]. 2.2.8. Porcine endogenous retrovirus
Pig endogenous retroviruses
Porcine endogenous retrovirus is an enveloped RNA virus belonging to the family Retroviridae, genus Gammaretrovirus. Up to 8% of mammalian genomic DNA is derived from retroviruses [78]. In pigs, three subtypes of retrovirus (PERV) have been identified: PERV-A, PERV-B and PERV-C. PERV-A and PERV-B are ubiquitous in pig populations, while the inter-species and intra-species distribution of PERV-C varies [38]. ⟦23 No retrovirus with exogenous horizontal transmission has yet been found in pigs. However, recombination between PERV-A and PERV-C has been discovered. PERV A/C recombinants do not exist as proviruses in the host germline and are therefore considered potential exogenous viruses [79]. Complete A/C proviral genomes are present in the somatic cells of some pigs, indicating that they can replicate. The possible transmission of PERVs between pigs is not yet fully understood. However, because PERVs are embedded in the genome, this indicates that they are transmitted through semen.
Pig endogenous retroviruses are enveloped RNA viruses of the genus Gammaretrovirus in the family retroviridae. Up to 8% of mammalian genomic DNA is believed to be retroviral in origin [78]. In pigs, three PERV subtypes have been identified: PERV-A, PERV-B, and PERV-C. Subtypes A and B are ubiquitous in all pig breeds, whereas PERV-C is variably distributed between and within breeds [38].
No retroviruses that spread horizontally as exogenous agents have yet been found in pigs. However, recombination between PERV-A and PERV-C has been identified. PERV A/C recombinants do not exist as proviruses in the host germline, and are therefore considered potential exogenous viruses [79]. Complete A/C proviral genomes are present in the somatic cells of some pigs, indicating that they are capable of replication. The possibility of PERV transmission between pigs is not yet fully understood. However, because PERVs are embedded in the genome, this indicates that they are transmitted via semen.
Exogenous horizontally transmissible retroviruses have not been found in pigs. However, a recombination between PERV-A and PERV-C has been found. The PERV A/C recombinant is not present in proviral form within the host germ line and is therefore considered a candidate exogenous virus [79]. The presence of the full genomic A/C provirus in the somatic cells of some pigs indicates its potential for replication. Little is known regarding the potential for transmission of PERVs between pigs. However, as PERVs are embedded in the genome, they are transmitted via semen.
2.2.9. Rubulavirus (blue eye disease)
Rubulavirus (blue eye disease)
Porcine rubulavirus is an enveloped RNA virus belonging to the family Paramyxoviridae, genus Rubulavirus. This virus can cause blue eye disease [80]. This virus is prevalent in Mexico and causesbreeding stockreproductive disorders. After infection, as with other adult animals, boars suffer from severe epididymitis and orchitis, and reduced libido, usually without showing clinical signs. Transmission of the virus through semen has not yet been experimentally demonstrated; however, the virus has been detected in semen, testes and other tissues of the reproductive tract for up to 49 days after inoculation [39]. Recent data indicate that the virus can be isolated from semen 5 to 48 days after inoculation, and from the testes and epididymis 64 to 142 days after inoculation. Viral RNA can be detected in serum 2 to 64 days after inoculation, and the virus can still be detected in semen 142 days after inoculation [40]. This demonstrates that porcine rubulavirus RNA persists in semen and that the reproductive tract is infected for a long time. Semen examination showed that in virus-infected herds, about 30% of boars showed temporary or permanent infertility, decreased sperm concentration, increased morphological abnormalities, and reduced sperm motility and viability. Some boars developed azoospermia [40].
Porcine rubulavirus is an enveloped RNA virus that belongs to the genus Rubulavirus in the family paramyxoviridae. The virus is associated with blue eye disease in pigs [80]. It is an important pathogen in Mexico and causes reproductive problems in breeding pigs. Boars, like other adult animals, generally do not show clinical signs, except for epididymitis and orchitis and in severe cases, loss of libido. Transmission of the virus through semen has not been proven experimentally; however, virus has been recovered from semen, the testis, and other tissues of the reproductive tract for up to 49 days after inoculation [39]. More recently, isolation of the virus from semen was achieved between 5 and 48 days post inoculation (DPI) and from the testicles and epididymides between 64 and 142 DPI. Viral RNA was detected in the serum between 2 and 64 DPI and in the semen until 142 DPI [40]. These results confirm that the RNA of the porcine rubulavirus persists in the semen and that this virus remains in the reproductive tract for prolonged periods of infection. Semen evaluation demonstrated that about 30% of boars in herds infected with the virus showed temporary or permanent infertility, with a decrease in concentration, an increase in abnormalities in morphology, and a decrease in motility and viability of spermatozoa. In some boars, there was azoospermia [40].
3. Risk of virus transmission to sows through artificial insemination
Risk for transmission of viruses by AI to the recipient sow
Generally, the likelihood of virus being present in semen is highest during the period of disease onset. However, in practice, semen is not collected from clinically affected boars, which can reduce the risk of virus transmission to sows. Shedding begins before clinical signs appear or when only mild clinical signs are present, and in addition, severely infected pigs may go undetected; under these circumstances, if special control measures are not taken, the risk of virus transmission increases.
In general, the risk of virus to be present in semen is the highest during the stage of clinical disease. Under practical circumstances, however, no sperm collection will take place from clinically affected boars, and consequently, the risk of pathogen transmission to the sow is low in this case. Although virus shedding can start before the development of clinical signs, which can be mild or absent, acutely infected boars can remain unnoticed. In these situations, the risk of virus transmission is much higher as no special control measures will be taken.
Experiments have demonstrated that CSFV [10] and PRRSV [61] can be transmitted to sows through semen, and likewise after adding PPV [45], ADV [52], PRRSV [18,21] and PCV2 [41] to semen. Habu et al. [13] and Guérin and Pozzi [57] proposed that, although no scientific article has been found to confirm it, if insemination is performed with semen containing Japanese encephalitis virus, gilts are highly susceptible to infection. If virus-contaminated semen is used for artificial insemination, sows and even sow herds are at risk of virus transmission. Nathues et al. [81] illustrated this with the example of a PRRSV outbreak caused by boar semen introduced into Switzerland.
Transmission of viral pathogens by semen to the sow has been clearly proven for CSF virus [10] and PRRSV [61] on experimental infection of boars and for PPV [45], ADV [52], PRRSV [18,21], and PCV2 [41] after inoculation of the virus in the semen. Habu et al. [13] and Guérin and Pozzi [57] reported that Japanese encephalitis virus is easily transmitted to gilts if they are inseminated with contaminated semen, although we did not find a scientific article confirming these statements. Semen contaminated with viral pathogens used for AI does indeed poses a risk for transmission to the recipient sow and by extension the sow herd. This was illustrated by Nathues et al. [81] who described an outbreak of PRRSV in Switzerland after import of boar semen.
However, viruses do not always manage to establish transmission (for example, PRRSV) [18,61]. The conditions required for a sow to become infected are complex, and the failure to transmit may be due to sow immunity, the characteristics of the virus, and failure to reach the minimum infectious dose. Therefore, more research has been carried out on the risk of PRRSV transmission through semen and the minimum dose required for sows to become infected with the virus. So then, compared with conventional cervical insemination, deep insemination with a small dose of semen (40–50 mL versus 80–90 mL) can reduce the risk of virus transmission to sows [82].
However, transmission will not always be successful (e.g., PRRSV) [18,61]. The conditions required for establishment of infection in the sow are complex, and lack of transmission might be explained by sow immunity, virus characteristics, and failure to reach the minimum infectious dose. In this regard, much research has been directed toward the risk of transmission of PRRSV by semen and the minimum dose necessary to establish infection in the sow. In this regard, it can be expected that postcervical insemination with smaller semen doses than conventional cervical insemination (40–50 mL compared to 80–90 mL) is associated with a lower risk for pathogen transmission to the sow [82].
At the population level, the number of sows mated with virus-carrying semen also determines the likelihood of transmission being established. The greater the number of such sows, the higher the likelihood of virus transmission. Other animals, such as cattle, mainly use frozen semen rather than fresh semen for artificial insemination. In this way, there is more time and opportunity to test whether the bull and/or the semen contains a specific pathogen, thereby reducing the risk of pathogen transmission through artificial insemination.
At a population level, also the number of sows inseminated with contaminated semen determines the likelihood of successful transmission. The more sows are inseminated with contaminated semen, the higher the likelihood of successful transmission of the virus. In other animal species such as cattle, AI is mostly practiced with frozen semen and not with fresh semen as in pigs. This allows more time and opportunities to verify whether the bulls and/or semen are free from specific pathogens, and consequently, the risk for pathogen transmission via AI is lower.
4. Impact of viruses on sows through artificial insemination
Impact of viruses by AI to the recipient sow
Artificial insemination of sows with virus-carrying semen can produce a variety of outcomes. Decreased sperm quality, early embryonic death and/or endometritis, clinical disease in the sow herd, and/or infection with harmful pathogens leading to reduced sow health, hindering pathogen elimination or interfering with regulatory measures, can all lead to reduced conception rates [57]. Pathogens can cross the zona pellucida and directly invade the embryo, and/or pathogens (for example, PPV infection) cause lesions of the uterine epithelium, resulting in early embryonic death [83]. From 6 to 7 days after pregnancy, the zona pellucida acts as an impermeable barrier, surrounding and protecting the embryo, helping it avoid invasion by pathogens such as PPV, PCV2, ADV and PRRSV [65,84,85]. However, pathogens (such as ADV) can cross the zona pellucida at the blastocyst stage, making the embryo more susceptible to infection [84].
The consequences of AI with semen contaminated with viral pathogens for the sow can be quite variable. It can result in reduced conception rates because of reduced semen quality, early embryonic death and/or endometritis, clinical disease in the sow herds, and/or infections with unwanted pathogens leading to reduced health status, stamping out, or regulatory measures [57]. Early embryonic death may result from direct invasion of the embryo by the pathogen after it has hatched from the zona pellucida, and/or by uterine epithelial alterations in response to the pathogen (e.g., with PPV) [83]. Until 6 to 7 days after conception, embryos are surrounded and protected by the zona pellucida, an impervious barrier, which helps the embryo to avoid invasion of pathogens such as PPV, PCV2, ADV, and PRRSV [65,84,85]. After hatching, however, blastocyst stage embryos may become susceptible to the infection as is the case, e.g., with ADV [84].
5. Diagnostics Diagnostics
Different diagnostic methods, such as virus activity detection, viral nucleic acid detection or viral antibody detection, can be used to determine whether boars at artificial insemination centers are infected with a virus and/or whether the semen carries virus. From the early 1990s to the present, major progress has been made in improving sensitivity and specificity, as well as in simultaneously detecting different pathogens and in detection speed.
The presence of viral infections in boars of AI centers and/or the presence of viral pathogens in semen can be assessed using different diagnostic methods such as demonstration of viable virus, nucleic acid of the virus, or antibodies against the virus. From the early 1990s onward, major improvements have been made in terms of increased sensitivity and specificity, simultaneous testing of different pathogens and speed of testing.
5.1. Virus activity detection
Detection of viable virus
Usually, boar semen is diluted and used within a few days after collection, so test results should be obtained within a short time. Conventional methods for detecting viruses in semen, such as virus isolation and identification or biological assays, are not highly sensitive, are time-consuming and are expensive. Due to cytotoxicity, bacterial contamination, and interference with cell culture systems, as well as antiviral factors that non-specifically neutralize the virus, virus isolation is now rarely used in sementechnology[59]. Based on practical conditions and animal welfare constraints, it is impossible to perform sample inoculation testing on a large number of animals [64]. Limited by the above factors, virus isolation and pig bioassays are not practical for routine clinical diagnosis.
As diluted boar semen is mostly used within a few days after collection, the outcome of analysis should be available within a very short time. Conventional methods for virus detection in semen, such as virus isolation or conducting bioassays, are not very sensitive; they are time-consuming and very expensive. The application of the virus isolation technique is markedly reduced for semen because of cell toxicity, bacterial contamination, resulting in interference with cell culture systems, and antiviral factors that nonspecifically neutralize the virus [59]. Animal inoculations cannot be performed for testing large numbers of samples because of practical reasons and also because of animal welfare reasons [64]. Given these limitations, virus isolation and swine bioassays are not useful techniques for routine diagnosis.
5.2. Viral nucleic acid detection
Detection of viral nucleic acid
PCR is a highly sensitive, highly specific detection method capable of rapidly detecting genomic sequences (such as viruses)technology. A variety of PCRtechnologiescan be used to detect viruses present in boar semen. Van Rijn et al. [86] developed a real-time fluorescence quantitative PCR that can detect 5 viruses in semen that can severely affect economic benefits (ADV, CSFV, FMDV, SVDV, PRRSV). Real-time PCRtechnologycombines amplification and detection of the amplification products into one. As a result, contamination from the environment can be greatly reduced. Experiments have found that, compared with virus isolation, using real-time PCR to detect virus-carrying semen can detect the virus earlier and faster after infection. Rovira et al. [87] studied the sensitivity of reverse transcriptase PCR for different biological samples, with these samples pooled as 3 samples per pool and 5 samples per pool. Twenty-nine boars were inoculated with a low-virulence PRRSV isolate. Within 2 weeks, serum, blood swab and semen samples were obtained from each boar every 2 to 3 days. The data obtained showed that during acute infection, serum is the best sample for detecting PRRSV, and blood swabs are also suitable. However, in most semen samples, PRRSV infection could not be detected. Pooling 3 and 5 samples caused a decrease in the sensitivity of reverse transcriptase PCR. When serum or blood swabs were tested with 5 samples per pool, their sensitivity decreased by 6% and 8% respectively.
The PCR technique is known as a sensitive, specific, and rapid tool for the detection of genomic sequences of, e.g., viruses. Different PCR techniques have become available for detecting viral pathogens present in boar semen. Van Rijn et al. [86] developed quantitative real-time PCR tests for detecting five economically important viruses in semen (ADV, CSFV, FMDV, SVDV, PRRSV). The real-time PCR technique combines amplification and detection of amplified products in one closed tube. Therefore, possible contamination from the environment is strongly reduced. In semen of experimentally infected boars, viruses were detected much earlier after infection and more frequently by real-time PCR tests than by virus isolation. Rovira et al. [87] investigated the sensitivity of reverse-transcriptase PCR on different biological samples run individually, in pools of 3 and in pools of 5. Twenty-nine boars were inoculated with a low-virulent PRRSV isolate. Serum, blood swab, and semen samples were obtained from each boar every 2–3 days for 2 weeks. Data showed that serum was the best sample to detect PRRSV during acute infection, with the blood swab sample performing almost as well. Semen samples failed to detect PRRSV infection in most of the cases. Pooling samples at pool sizes of 3 and 5 resulted in a decrease in the sensitivity of reverse-transcriptase PCR. Sensitivity was reduced by 6% and 8%, respectively, when serum or blood swab samples were run in pools of 5.
5.3. Antibody detection
Detection of antibodies
Serological methods are commonly used to investigate whether serum antibodies against different pathogens are present in boar studs. This is a simple and economical detection method. The test results are suitable for explaining the health level of a population or boar stud, rather than that of an individual pig. However, an important disadvantage of this detection method is the time interval between pathogen exposure and detection of antibody levels. Many serological tests generally have an interval of one to several weeks, which means that during this period, boars not identified as infected may also be shedding virus. Jugular venipuncture in adult boars has certain limitations with regard to operator safety. Therefore, blood swabs are more commonly used. To collect a blood swab, the ear is pricked with a needle during semen collection, and a cotton swab is used to collect the blood drop. When detecting PRRSV, compared with semen samples, blood swabs contain a greater amount of virus and can detect PRRSV more quickly [88].
Serology is frequently used to investigate the presence or absence of serum antibodies against different pathogens in boar studs. It is an easy and rather inexpensive method that is suitable for monitoring. The results should be interpreted at group or stud level, not at the individual animal level. An important disadvantage is the time period between pathogen exposure and detectable levels of antibodies. For many serologic tests, this time period ranges from one to several weeks, which implies that during that period, boars may shed the virus although they are not identified as being infected. Puncture of the jugular vein in adult boars has some practical limitations in terms of safety of the handler. Therefore, blood swabs have become popular. For blood swab collection, the ear is pricked with a needle during semen collection and a cotton swab is used to absorb the blood droplet. For PRRSV, increased amounts of PRRSV were shown in the blood swab as compared to semen samples, and infections could be detected earlier [88].
5.4. Notes on clinical diagnosis
Interpretation of diagnostic testing
Obviously, semen cannot be directly classified as virus-free or virus-carrying. For many viruses, such as PRRSV, ADV and PCV2, there is temporal inconsistency between viremia, the presence of virus in semen, and antibodies in serum. In the case of PRRSV, for example, adult boars have a very short duration of viremia, and the disease ends earlier than semen shedding [59]. In addition, in the early stage of infection, although the semen is already shedding virus, the serological result is still negative. Finally, long after the semen stops shedding virus, the boar may still be seropositive. Because semen usually sheds virus intermittently, especially in the later stage of infection, the virus may continue to be shed subsequently even if a semen sample tests negative. A negative result only means that the semen sample tested does not contain virus, and only means that the semen from a single ejaculation may not contain virus. It cannot guarantee that the semen subsequently ejaculated by the boar is free of virus. Therefore, boars that are seropositive but test negative in semen should be treated with greater caution.
It is clear that categorizing semen as either virus free or virus contaminated is not straightforward. For many viruses, e.g., PRRSV, ADV, PCV2, temporal inconsistencies exist among viremia, the presence of virus in semen, and antibodies in serum. In the case of PRRSV, e.g., viremia in adult boars may be of short duration and end before virus shedding in semen ends [59]. Furthermore, in the initial phases of the infection, serologic results will be negative although the virus can be shed in the semen. Finally, boars may remain serologically positive long after the virus is no longer shed in the semen. Because shedding of virus in semen is often intermittent, especially in the later phases of the infection, a negative test result on a semen sample does not preclude subsequent shedding of the virus. A negative result only means that the tested sample of the ejaculate does not contain virus and that the particular ejaculate is likely to be virus free. It does not guarantee absence of contamination in subsequent ejaculates of a boar. Consequently, a negative semen test from a serologically positive boar should be interpreted with caution.
To be continued...
To be continued…