**Artificial Insemination and Its Role in Transmission of Swine Viruses**

Tanja Opriessnig, Luis G. Giménez-Lirola and Patrick G. Halbur *Department of Veterinary Diagnostic and Production Animal Medicine, College of Veterinary Medicine, Iowa State University, Ames, Iowa, USA* 

#### **1. Introduction**

254 A Bird's-Eye View of Veterinary Medicine

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Wilkins, Baltimore, USA

Artificial insemination (AI) in swine is not a new technique and reports as early as the 1930s (Lush, 1925) describe collecting semen for AI. However, because of farm structure changes, increasing farm sizes and separation of production stages, interest in intensive pig production is growing and AI has become a critical component in modern pig production. In 2001, nearly 60% of North American swine herds utilized AI (Singleton, 2001), a drastic increase from the estimated 5% in the 1990's (Flowers & Esbenshade, 1993). This is still relatively low compared to the 90% or greater use of AI in Western Europe (Madsen, 2005; Maes et al., 2008). The extensive use of AI in pig reproduction in the last decade has facilitated the exchange of desirable genetic characteristics at an international level, allowing producers to make greater use of superior genetics at a lower cost than some natural-service systems (Gerrits et al., 2005). However, the growth in use of AI has increased the risk of quick and widespread transmission of venereally transmissible pathogens (Thacker et al., 1984). It has been reported that the porcine male reproductive tract is highly susceptible to viral infections (Phillips et al., 1972; Spradbrow, 1968). This, coupled with the ability of boars to produce tens to thousands of insemination doses per week and the widespread distribution of the processed semen (both nationally and internationally), further increases the risk of wide transmission of viral pathogens by semen.

Many viruses have been reported to be present in boar semen (Yaeger et al., 1993; Lucas at al., 1974, Madson et al., 2008) and have the potential of being transmitted to susceptible breeding animals during AI, particularly when the boar is viremic or clinically sick. Viral shedding may also continue long after clinical signs have abated. In addition, infertility or reduced reproductive performance have been reported in boars with detectable virus in their semen (Guerin & Pozzi, 2005; Larsen et al., 1980). The potential economic impact and liability associated with transmission of diseases through semen has created great interest and investment in testing boars for viral diseases prior to entry and while at AI centers.

In many countries, foot-and-mouth disease virus (FMDV), porcine reproductive and respiratory syndrome virus (PRRSV), Japanese B encephalitis virus (JBEV), pseudorabies virus (PRV), classical swine fever virus (CSFV) and African swine fever virus (ASFV) are of particular importance. Accurate monitoring of boars in AI stations is essential to reduce the

Artificial Insemination and Its Role in Transmission of Swine Viruses 257

infection. Early embryonic death may result from direct invasion of the embryo by the pathogen or by uterine epithelial alterations in response to the pathogen (Wrathall & Mengeling, 1979). Until six to seven days after conception, an embryo is surrounded and protected by an impervious barrier called the zona pellucida, which helps the embryo avoid pathogen invasion (Mateusen et al., 2004). However, after entering the blastocyst stage, embryos may become susceptible to viral infections. Fetuses that are infected prior to 70 days of gestation usually die (mummies) and contribute to smaller litter sizes or early termination of pregnancy. Fetuses that are infected around or after 70 days of gestation are able to mount an active immune response against the pathogen and survive

As the risk of rapid virus spread via semen is well known, sanitation protocols in AI centers are constantly being reviewed to assure customers that current best practices are in place. A new categorization of viruses has been proposed to further address different risk levels (Guerin & Pozzi, 2005): Category 1 includes viruses for which there is scientific proof of transmission by semen, but without any risks for AI due to official national eradication programs such as FMDV, PRV, CSFV, and ASFV. Category 2 includes viruses for which there is scientific proof of transmission by semen and which can be commonly encountered such as PRRSV. Category 3 includes viruses for which additional scientific proof is needed to better assess the risk of transmission by semen (PCV2, porcine rubulavirus). In addition, some viruses have not been shown to be present in semen and thus are considered non-

Porcine reproductive and respiratory syndrome virus (PRRSV) is a single-stranded, positive-sense, enveloped RNA virus that belongs to the family *Arteriviridae*, genus *Arterivirus* (Cavanagh, 1997). PRRSV was first recognized in the late 1980's (Cavanagh, 1997; Meulenberg et al., 1993) and today is found globally in swine producing countries. As the name of the virus implies, PRRSV infection is associated with reproductive failure in pregnant sows and respiratory disease in pigs of all ages. It is less commonly associated with neonatal diarrhea (Albina et al., 1994; Bierk et al., 2001; Neumann et al., 2005; Rossow, 1998; Wensvoort et al., 1991). Although clinical disease associated with PRRSV in growing pigs can be quite severe, no or mild symptoms are typically seen in boars (Wensvoort et al., 1991). One of the main characteristics of PRRSV is its high transmissibility, making it difficult to maintain pig populations free of PRRSV (Prieto & Castro, 2005). The 50% tissue culture infective dose (TCID50) for exposure via oral, intranasal and AI routes was determined to be 105.3, 104.0 and 104.5, respectively (Benfield et al., 2000). Experimental infection in boars has demonstrated seminal shedding of PRRSV (Prieto et al., 1996b; Swenson et al., 1994a), and epidemiological evidence confirms that transmission of PRRSV from fresh semen of acutely infected boars into breeding herds is possible (Robertson, 1992; Yaeger et al., 1993). However, successful transmission is not always achieved (Swenson et al., 1994b; Yaeger et al., 1993) and appears to depend largely on the amount of PRRSV present in the semen (Benfield et al., 2000). In semen of adult boars, PRRSV can persist for variable periods (Christopher-Hennings et al., 1995a; Swenson et al., 1994) suggesting that the virus continues to replicates in the one or more tissues of the reproductive tract. Several

**3. Swine viruses that can be present in boar semen 3.1 Porcine Reproductive and Respiratory Syndrome Virus** 

intrauterine infection.

hazardous to AI.

risk of virus transmission. Monitoring for the presence of other ubiquitous viruses that can be found in semen, such as porcine circovirus type 2 (PCV2), torque teno sus virus (TTSuV), porcine parvovirus (PPV) and porcine adenovirus (PAV) is commonly not a high priority in most countries despite the fact that some of these viruses have been associated with sporadic reproductive failure in breeding animals.

Due to differences in pathogenicity, economic consequences, geographic localization, and epidemiological parameters, different viruses are commonly assigned different levels of importance. The aims of this review are to summarize the information on swine viruses that can be transmitted via AI, their shedding characteristics in boars, effects of these viruses on naïve breeding animals, testing that is currently done to reduce or prevent the risk of transmission, and current best practices being used to reduce or eliminate virus shedding in semen.
