Pig semen-the Vector of virus Transmission (3)

Pig semen-the Vector of virus Transmission (3)

Porcine semen as a vector for transmission of viral pathogens-Part 3

Dominiek Maesa,*, Ann Van Sooma, Ruth Appeltanta, Ioannis Arsenakisa, Hans Nauwynckb

A University of Ghent, Mellerbeck, Belgium, College of Veterinary Medicine, Animal Health and Obstetrics, Reproduction

A Department of Reproduction, Obstetrics and Herd Health, Faculty of Veterinary Medicine, Ghent University, Merelbeke, Belgium

B Belgium, Mellerbeck, University of Ghent, College of Veterinary Medicine, Virology Laboratory, Immunology and Parasitology, virus discipline

B Department of Virology, Immunology and Parasitology, Laboratory of Virology, Faculty of Veterinary Medicine, Ghent University, Merelbeke, Belgium

Keyword Keywords:

Pig semen, pig, artificial insemination, virus, review / examination

Semen, Pig, Artificial insemination, Virus, Review

Follow up the above.

2.2 viruses present in pig semen but not included in the OIE list

Viruses in porcine semen which are not in the OIE list

2.2.4. Atypical classical swine fever virus

Nonclassical swine fever pestiviruses

Pigs are susceptible to atypical classical swine fever viruses, including 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 spread the virus in large quantities. Terpstra and Wenswoort [35] isolated BVDV from oropharynx, 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 different from classical swine fever virus (CSF), but it can be included in the genus of classical swine fever virus because of its molecular characteristics. In 2003, the virus caused an outbreak of serious diseases in two pig farms in Australia, mainly stillbirths, an increase in mummified fetuses and an increase in pre-weaning mortality [72]. The risk of transmission of the virus through embryos or semen is not yet clear.

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 intestinal small ribonucleic acid virus

Porcine enteric picornaviruses

Porcine enterovirus (PEV) and porcine Jieshen virus are unenveloped RNA viruses and belong to the family Microribonucleic acid viridae. These two types of viruses are very active in the environment. The transmission of the virus between pigs is mainly through oral contact with contaminated feces. Semen may be contaminated during semen collection. Boars infected with PEV may lead to seminal vesiculitis, abnormal sperm and decreased libido [32]. Except for infection during pregnancy, clinical symptoms of sows are usually not observed. Infection during pregnancy can lead to embryonic death, mummified fetuses, stillbirths and weak babies [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 Geshen virus has been isolated from the reproductive tract of boars [32]. The use of virus-carrying semen to inseminate reserve sows will not affect fertilization [74]. However, Dunne et al. [44] proposed that semen infected by PEV and porcine Teschen virus can lead to the death of embryos and newborn piglets. There is no follow-up study to confirm 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. Circovirus

Torque teno virus

Two types of circovirus (TTV) have been found in pigs: circovirus type 1 (TTsuV1) and type 2 (TTsuV2). All of them are DNA viruses, belonging to the newly created family circoviridae and circovirus genus. It is estimated that porcine circovirus (TTsuVs) is widespread in pig herds around the world [75]. At present, there are no clinical symptoms associated with porcine circovirus (TTsuV) infection. However, TTsuV is related to diseases related to porcine circovirus (PCV). Circovirus spreads vertically and horizontally. The virus can be detected in fetal tissue and blood, semen and colostrum [36], which proves the possibility of vertical transmission. Semen samples from 100 boars were collected from artificial insemination center, and TTsuV1 and TTsuV2 were detected by routine PCR. Among them, four samples were TTsuV1 PCR positive and five samples were TTsuV2 positive. After co-infection, both TTsuVs were detected. The route of fetal infection is not clear, but it is suggested that it may be due to semen infection 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 pig semen but not included in the OIE list

Viruses in porcine semen which are not in the OIE list

2.2.7. Ebola virus

Ebolavirus

Reston Ebola virus is a kind of enveloped RNA virus, which belongs to single molecule negative strand RNA virus family and filamentous virus family. In 2008, a serious disease broke out in pigs in the Philippines, which is reported to be caused by blue ear virus (PRRSV). However, after detailed diagnosis, it was not PRRSV but Reston Ebola virus (REBOV) that caused the disease, and specific antibodies to REBOV were detected in pig farm workers [30]. The far-reaching impact of the REBOV outbreak has yet to be determined. Subsequent experiments showed that although the virus replicated in pigs after infection with REBOV, it may not show symptoms. The transmission mechanism of REBOV among pig herds is not clear. Nor is it clear how REBOV is spread. In non-human primate models, REBOV is dispersed in all body fluids, including semen [29]. It is not clear whether REBOV can be transmitted to sows through semen. Compared with REBOV, it has been confirmed that Zaire Ebola virus can cause severe lung disease and dyspnea in pigs aged 5 to 6 weeks [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 a kind of enveloped RNA virus, which belongs to the family Retroviridae and type C retrovirus. Up to 8% of mammalian genomic DNA is derived from retroviruses [78]. Three subtypes of retrovirus (PERV) have been found in pigs: PERV-A,PERV-B and PERV-C. PERV-An and PERV-B are common in pigs, while the distribution of PERV-C is different between species and within species [38].

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 An and B are ubiquitous in all pig breeds, whereas PERV-C is variably distributed between and within breeds [38].

Retroviruses with exogenous horizontal transmission have not been found in pigs. However, a recombination between PERV-An and PERV-C has been found. The recombinant PERV A _ (A) C does not exist in the host germline as a protovirus, so it is considered to be a potential foreign virus [79]. In some pigs, there is a complete genome of A hand C protovirus in somatic cells, which indicates that it can be replicated. The possibility of PERVs transmission between pigs has not been fully understood. However, because PERVs is embedded in the genome, it shows that they are transmitted through semen.

Exogenous horizontally transmissible retroviruses have not been found in pigs. However, a recombination between PERV-An 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. Rubella virus (blue eye disease)

Rubulavirus (blue eye disease)

Porcine rubella virus is a kind of enveloped RNA virus, which belongs to paramyxoviridae and mumps virus. The virus can cause blue eye disease [80]. The virus is prevalent in Mexico and can cause reproductive obstacles in breeding pigs. After infection, like other adult animals, boars suffer from severe epididymitis and orchitis, decreased libido, and usually show no clinical symptoms. It has not been experimentally proved that the virus is transmitted through semen; however, the virus has been detected in semen, testicles and other tissues of the reproductive tract within 49 days after vaccination [39]. Recent data show that the virus can be isolated from semen 5 to 48 days after inoculation and from testis and epididymis 64 to 142 days after inoculation. Virus RNA could be detected in serum from 2 to 64 days after inoculation, and still in semen at 142 days after inoculation [40]. It is proved that porcine rubella virus RNA persists in semen and infects the reproductive tract for a long time. Semen tests showed that about 30% of the pigs infected with the virus had temporary or permanent infertility, decreased sperm concentration, abnormal morphology, and decreased sperm motility and viability. Some boars have 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 artificial insemination transmitting virus to sows

Risk for transmission of viruses by AI to the recipient sow

Usually, the virus is most likely to be present in semen during the onset of the disease. However, in reality, semen will not be collected from clinically affected boars, which can reduce the risk of virus transmission to sows. Without clinical symptoms and mild clinical symptoms, detoxification has already begun, coupled with the fact that seriously infected pigs have not been detected, in these cases, if special control measures are not taken, the risk of virus transmission will be increased.

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.

The results showed that CSFV [10] PRRSV [61] could infect sows through semen, or after adding PPV [45], ADV [52], PRRSV [18J.21] and PCV2 [41] to semen. Habu et al. [13], Guy é rin and Pozzi [57] proposed that although no scientific articles have been found to confirm it, if semen containing Japanese encephalitis virus is used for insemination, the reserve sows are extremely susceptible to infection. If the semen contaminated by the virus is used in artificial insemination, sows and even sow herds will be at risk of virus transmission. Nathues et al. [81] cited the outbreak of PRRSV caused by the introduction of boar semen into Switzerland as an example.

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 Guerrin 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 form and spread (for example, PRRSV) [18561]. The conditions required for sows to be infected are very complex, and the failure to spread may be due to the immunity of sows, the characteristics of the virus and not reaching the minimum infection dose. As a result, more research has been carried out on the risk of semen transmission of PRRSV and the minimum dose required for sows to be infected with the virus. Then, compared with routine cervical insemination, deep insemination of low-dose semen (40 mL 50 mL vs. 80 mL 90 min) can reduce the risk of virus transmission in sows [82].

However, transmission will not always be successful (e.g., PRRSV) [18Pol 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 mL compared to 50 mL compared to 80 mL 90 mL) is associated with a lower risk for pathogen transmission to the sow [82].

As far as the population is concerned, the number of sows mated with toxic semen also determines the possibility of transmission. 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 during artificial insemination. This allows more time and opportunities to detect whether bulls and / or semen contain specific pathogens, 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. Effect of virus on sows by artificial insemination

Impact of viruses by AI to the recipient sow

Artificial insemination of sows with toxic semen will produce a variety of results. Decreased sperm quality, early embryonic death and / or endometritis, clinical diseases in sows, and / or infection with harmful pathogens lead to a decline in the health of sows, hinder pathogen purification or interfere with regulatory measures, all of which may lead to a reduction in the conception rate [57]. Pathogens can invade embryos directly through Zona pellucida, and / or pathogens (for example, infected with PPV) cause uterine epithelial lesions, resulting in early embryonic death [83]. 6 to 7 days after pregnancy, Zona pellucida acts as an impermeable barrier that surrounds and protects the embryo from pathogens such as PPV,PCV2,ADV and PRRSV. However, pathogens such as ADV can pass through the Zona pellucida during the blastocyst stage, making embryos 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. Diagnosing Diagnostics

Different diagnostic methods, such as viral activity test, viral nucleic acid test or viral antibody test, can be used to determine whether boars in artificial insemination centers are infected with the virus and / or whether there is virus in the semen. Since the early 1990s, great progress has been made in improving sensitivity and specificity, as well as simultaneous detection of different pathogens and 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 will be diluted within a few days after collection, and test results should be obtained in a short period of time. Conventional methods for detecting viruses in semen, such as virus isolation and identification or biological identification, are not sensitive, time-consuming and expensive. Virus isolation techniques are rarely used in semen because of cytotoxicity, bacterial contamination, interference with cell culture systems and non-specific neutralization of viruses by antiviral factors [59]. Based on realistic conditions and animal welfare restrictions, it is impossible to vaccinate a large number of animals [64]. Limited to the above factors, virus isolation and porcine biological identification 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. Virus nucleic acid detection

Detection of viral nucleic acid

PCR is a highly sensitive and specific detection technique that can rapidly detect genomic sequences such as viruses. A variety of PCR techniques are available to detect viruses present in boar semen. Van Rijn et al.[86] developed real-time fluorescence quantitative PCR to detect five viruses (ADV, CSFV, FMDV, SVDV, PRRSV) that can seriously affect economic efficiency in semen. Real-time PCR technology combines amplification and detection of amplified products into one. Thus pollution from the environment may be greatly reduced. It was found that the virus could be detected earlier and faster after infection by real-time PCR than virus isolation. Rovira et al.[87] investigated the sensitivity of reverse transcriptase PCR for different biological samples, 3 samples in a pool and 5 samples in a pool. 29 boars were inoculated with low toxicity PRRSV isolates. Serum, blood swabs and semen samples were obtained from each boar every 2 to 3 days for 2 weeks. The data obtained show that serum is the best sample for detecting PRRSV during acute infection, and blood swabs are also available. However, PRRSV infection could not be detected in most semen samples. The sensitivity of reverse transcriptase PCR was decreased when 3 and 5 samples were pooled. When serum or blood swabs were detected with 5 samples as a pool, the 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 the presence of serum antibodies against different pathogens in boar stations. This is a simple and economical method of detection. The test results apply to explaining the health level of a population or boar station, not an individual pig. However, an important disadvantage of this test is the time interval between exposure to pathogens and detection of antibody levels. Many serological tests are typically conducted at intervals of one to several weeks, which means that uninfected boars may also shed virus during this period. Jugular venipuncture in adult boars has certain limitations for operator safety. Therefore, blood swabs are more commonly used. To collect blood swabs, prick the ears at the time of semen collection and collect blood drops using cotton swabs. When detecting PRRSV, blood swabs contain more virus than semen samples and can detect PRRSV faster [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. Description of clinical diagnosis

Interpretation of diagnostic testing

Obviously, semen cannot be classified directly as virus-free or virus-carrying. For many viruses, such as PRRSV, ADV, PCV2, there is a temporal inconsistency between the presence of the virus in viral blood, semen, and antibodies in serum. In the case of PRRSV, viremia, e.g. in adult boars, is of short duration and ends earlier than semen shedding [59]. In addition, serological results were negative in the early stages of infection, despite semen shedding. Finally, boars may still be seropositive long after semen has stopped shedding. Because semen usually sheds intermittently, especially late in infection, even if semen samples test negative, the virus may continue to shed later. A negative result only indicates that the semen sample tested does not contain virus, and only indicates that a single ejaculation semen may not contain virus. There is no guarantee that the boar's subsequent ejaculation of semen is free of poison. Therefore, boars that are seropositive but semen negative 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…