Artificial insemination centers need to produce the appropriate quantity of quality-controlled semen in a cost-effective, efficient, and biosecure process. The best way to prevent disease transmission via semen is to purchase boars with a preferred health status, to maintain very strict biosecurity measures, and to monitor the health of the animals.
Because new boar entry poses a great risk for disease, and because typically 60% of the boars in AI centers are replaced on a yearly basis, it is of paramount importance for incoming boars to implement proper isolation and acclimation procedures, with adequate time for observation and testing. First, the health status of the source herd should be known. The health status should be similar but preferably higher than the health status of the AI center. It could be evaluated through vet-to-vet communications of the diagnostics and laboratory examinations, vaccination programs, and veterinary involvement of the source herd. Second, replacement boars should be housed for 30 to 60 days in a separate isolation facility. Such a quarantine unit should be located preferably off-site. Isolation populations must always be flowed all-in/all-out. During this quarantine period, the health status should be evaluated, blood sampling could be performed to detect possible viremia or the presence of serum antibodies, and vaccinations can be applied against diseases present in resident boars of the AI center. Animals that are infected with pathogens of concern should not enter the AI center. The isolation period, the testing protocols, and the vaccination schemes vary between AI centers and depend on different factors such as the frequency of boar introduction and the health status of the animals of the origin herds .
The most important biosecurity measures for AI centers relate to the following items [3,88]: (1) location of the AI center and ventilation -- the AI center should be located as far from pigs as feasible. Filter systems for incoming air could be used in pig-dense areas to safeguard AI centers against entry of airborne pathogens . The facilities should be oriented so that exhaust air from the boars does not exhaust toward the laboratory. If the laboratory is connected to the animal facility, there should be positive pressure ventilation to minimize air infiltration from the animals into the laboratory. (2) Employees and visitors -- strict regulations should be in place for persons entering AI centers. A downtime of 1 to 3 days may be required for persons entering the AI center. During this downtime period, persons are not allowed to have contact with pigs. In addition, it is recommended to have a shower-in/shower-out policy for all visitors and employees. (3) Semen pickup point and deliveries -- the semen pickup point for the courier delivery personnel should be located at the outside of the AI center. This pickup point should be in a separate room, or as a minimum, a separate room from both the laboratory and the stud population. By having a separate pickup point, cross-contamination of semen coolers, boots, clothing, and so forth is minimized. Drop-off points at each of the sow units should be located outside of the office. Any deliveries (e.g., supplies, equipment) on the AI center should not have been exposed to pigs previously. They should be delivered directly to a designated receiving chamber and properly disinfected before entering the AI center. (4) Rodent and insect control - rodents and insects are capable of carrying and transmitting several pig viruses, and therefore, proper prevention measures should be in place. (5) Water quality - water sources for boars should be analyzed regularly. If the quality is insufficient, the water source should be changed or the water should be treated properly. (6) Implementation of strict cleaning and disinfection procedures.
The most general method is to monitor the health of the boars and the presence of clinical signs. In case of clinical problems, no semen should be collected until the animal has recovered. Feed and drinking water intake of the boars may be measured in an automated way, allowing to identify diseased boars more easily and at an early stage. Monitoring the health and performance of the boars, even if applied accurately and more than once a day, remains a very basic approach that is insufficient as a stand-alone measure to detect viral infections and/or the presence of viral pathogens in semen. Therefore, it is recommended to develop standard protocols for vaccinations, deworming, and sampling for specific diseases and to review these on a regular basis. Vaccination of boars can be highly effective in eliminating or decreasing shedding of viruses and decreasing the risk of virus transmission by AI. First of all, AI centers need to adhere to the animal health regulations of the country, e.g., vaccination against ADV may be mandatory because it is part of an official eradication program of the virus (e.g., Eastern Europe), whereas in other countries, vaccination may be forbidden because the country is officially free of ADV (e.g., Western Europe, North America). The combination of highly efficacious marker vaccines and accurate differential ELISAs has made eradication of ADV from large areas of the world practical and feasible (e.g., many European countries, North America) . According to requirements of the European Pharmacopoeia, ADV vaccines for parenteral use must not be transmitted by semen . For other diseases, no legal regulations are in place and the AI center can decide which vaccines to use. Vaccination against PPV and PCV2 may help to reduce shedding of the virus after infection . In the case of PRRSV, the use of an attenuated vaccine shortened or eliminated virus shedding in boars challenged with wild-type virus for 50 days after vaccination [92,93]. Apart from the fact that only partial protection is conferred against infection, vaccination with an attenuated vaccine led to semen shedding of the vaccine virus for up to 39 days [17,92]. In contrast, an inactivated vaccine did not clearly reduce subsequent shedding of wild-type virus in semen .
Health monitoring programs are critically important in AI centers, as possible infection with pathogens (e.g., PRRSV) should be detected as soon as possible. Mostly, both blood and semen samples are tested for the presence of virus and/or antibodies. Monitoring programs can very between AI centers. They can be complex because blood and semen tests can sometimes generate different information, and uncertainties exist regarding optimal testing frequency, numbers of animals to test, diagnostic test to choose, and how to interpret the test results .
Semen processing and addition of antimicrobials may have a possible dilution effect and decrease bacterial contamination, but these measures do not eliminate viruses [4,5]. Effective antiviral agents to render semen virus-free are currently not (yet) used in practice. Many different proteins found in semen along with steroid hormones, enzymes, and other substances may provide some protection against viral infections, but at the same time, these substances could damage sperm. Therefore, a delicate balance of these substances needs to be maintained .
Colloid centrifugation of boar semen has been reported sporadically for at least 2 decades, beginning with density gradient centrifugation and progressing more recently to single-layer centrifugation (SLC) . The latter method is easier to use and less time-consuming than density gradient centrifugation. The technique can be used to remove pathogens (bacteria and viruses), thus improving biosecurity of semen doses and potentially reducing the use of antimicrobials. In a study by Blomqvist et al. , it was possible to remove more than 99% of PCV2 from semen with an initial infectious virus titer of 3.25 to 3.82 TCID50/50μL.
Martinez-Alborcia et al.  reported that SLC of semen samples using a pig-specific colloid called Androcoll-P before freezing enhances sperm cryosurvival rates and modifies the functionality of the cryosurviving sperm. A subsequent study showed that SLC processing of boar ejaculates using Androcoll-P is able to process the entire sperm-rich fraction of a boar ejaculate, improving both the sperm cryosurvival and the fertilizing ability of thawed spermatozoa . The removal of pathogens coupled with improved cryopreservation are two important advantages of the technique allowing easier international trade of frozen–thawed semen doses from genetically valuable boars. However, further research is necessary to improve SLC, e.g., by making the technique less time-consuming and the results less variable.
Viral contamination of porcine semen poses a risk for breeding herds because AI may lead to fast introduction of viruses into sow populations and to loss of disease-free status for these pathogens. Pre-entrance testing of boars, routine health monitoring of boars during their stay in the AI center, regular testing of semen before delivery, and enforcement of strict biosecurity protocols at the AI center provide most producers with a high level of confidence that AI is a relatively safe means to offer unlimited exchange of desirable genetic characteristics. Prevention of viral contamination of semen should be a primary focus because it is easier to prevent contamination than to eliminate viruses once they are present in semen. Nevertheless, research and development of novel semen processing treatments such as SLC may even further instill confidence in the use of AI as a safe reproductive technology for pig production.