Safety and efficacy of vaccination of seronegative bulls with modified-live, cytopathic bovine viral diarrhea viruses
Introduction
Bovine viral diarrhea virus (BVDV) is a pestivirus in the Flaviviridae family that causes gastrointestinal, respiratory and reproductive disease in cattle [1]. The clinical importance of BVDV as a pathogen is manifested by the large number (>180) of available licensed vaccines [2], [3] and the efforts in some European and Scandinavian countries to eradicate the virus without vaccination [4]. Bovine viral diarrhea virus can be shed in the semen of persistently and acutely infected bulls, and survives cryopreservation and processing of semen for artificial insemination [5], [6], [7], [8]. Virus in semen collected from persistently infected bulls consistently infects susceptible, inseminated cows [9]. Sequelae to these infections include reduced pregnancy rates, early embryonic death, abortion, and birth of persistently infected offspring [5], [6], [9].
Acute infection with BVDV occurs when an animal with a fully developed immune system is exposed to a cytopathic or noncytopathic strain of BVDV. Whereas acute infections are often subclinical, signs such as depression, inappetance, oral erosions and ulcerations, decreased milk production, diarrhea and death might be observed [10]. An acutely infected bull can shed virus in semen that is apparently normal relative to motility, morphology and concentration of spermatozoa [11]. While BVDV can be isolated from semen of some acutely infected bulls after viremia subsides, the ability to isolate virus from semen using standard techniques (direct inoculation onto susceptible cells without ultracentrifugation or additional passage) ceases upon seroconversion (14–28 d post-infection) [7], [12]. Virus in semen collected from an acutely infected bull prior to seroconversion (12 d post-inoculation) infected 5% of inseminated heifers in previous research [11]. Subsequently, secondary transmission of virus from these infected heifers to pregnant animals resulted in the production of persistently infected fetuses [11]. Epidemiological investigations also indicated that 1.8–15.4% of seronegative heifers developed antibodies to BVDV after insemination with semen from acutely infected bulls [8].
Intranasal inoculation and subcutaneous vaccination of seronegative, post-pubertal, non-viremic bulls with noncytopathic strains of BVDV have produced long-standing, localized testicular infections [13], [14]. After acute infection of these bulls, virus persisted within testicular tissue of some bulls for >7 mo [13]. Four months after acute infection, BVDV was detected in semen by reverse transcription-nested PCR (RT-nPCR) but could not be isolated using standard techniques. Despite this inability to isolate virus using standard techniques, BVDV in semen collected 5 mo after viral challenge proved to be infectious when administered by the intravenous route to a seronegative calf [13]. Notably, the quantity of BVDV necessary to constitute an infective dose differs depending on the route of entry into a susceptible cow [11], [15]; thus the minimum infective dose via natural breeding or artificial insemination remains to be determined. Although the prevalence of bulls that shed BVDV in semen as determined by RT-nPCR appearred to be low [16], the potential for prolonged testicular infections that result from acute infections has been identified under controlled experimental conditions [13] and demonstrated in a field study in Germany (Wolfgang Gaede, European Commission, Thematic Network on BVDV Control in Europe, Porto, Portugal, 20 October 2004).
In previous research, immunization of peri-pubertal bulls with a single dose of modified-live vaccine consisting of cytopathic BVDV strains NADL and 53637 (Bovi-Shield Gold® 5, Pfizer Animal Health, Kalamazoo, MI, USA) prevented development of prolonged testicular infections secondary to viral challenge with a noncytopathic type 1a strain of BVDV [17]. In that study, no semen samples were obtained immediately after vaccination but before viral challenge to assess transient shedding of cytopathic, modified-live virus in semen. Subsequent research demonstrated that vaccination of peri-pubertal bulls with a single dose of modified-live vaccine consisting of a noncytopathic, type 1a strain of BVDV (Arsenal® 4.1, Novartis Animal Health, U.S. Inc., Larchwood, IA, USA) caused prolonged testicular infections of at least 4.5 mo duration in some bulls, but prevented subsequent testicular infections upon viral challenge [14]. In that study, noncytopathic, modified-live BVDV vaccine strain was isolated (using standard techniques) from semen up to 10 d after vaccination and from testicular tissue 134 d after vaccination. To evaluate a modified-live vaccine comprised of two different cytopathic strains, the objectives of this current research were to vaccinate peri-pubertal bulls with a modified-live vaccine consisting of cytopathic BVDV strains Singer (type 1a) and 296 (type 2; Express® FP 5, Boehringer Ingelheim Vetmedica Inc., St. Joseph, MO, USA) and evaluate the resulting: (a) transient shed of modified-live, cytopathic BVDV in semen; (b) risk of prolonged testicular infection; and (c) protection against subsequent testicular infection due to viral challenge.
Section snippets
Animals
The study was reviewed and approved by the Auburn University Institutional Animal Care and Use Committee (AUIACUC No. 2007-1230). This study involved 22 peri-pubertal, beef bulls weighing 274–361 kg (602–795 lbs) at 9–13 mo of age. Bulls were acclimated for 21 d prior to initiation of the research. Bulls were seronegative to types 1 and 2 BVDV, and BVDV could not be isolated from serum at the initiation of the study. Bulls were stratified by weight and randomly assigned to one of two treatment
Results
No clinical signs of BVDV infection were exhibited by bulls after vaccination or viral challenge. After vaccination, challenge and sample collection (Table 1), individual animal response varied as described in Table 2.
A brief viremia was detected in some bulls after vaccination or challenge of control bulls (Table 3). Results of virus neutralization assays indicated immunization of vaccinated bulls and a notable seroconversion of control bulls after viral challenge (Fig. 1). On Days 28 and 44,
Discussion
In this study, a modified-live vaccine consisting of cytopathic strains of BVDV resulted in a transient shed of virus in semen for up to 10 d after vaccination of seronegative bulls, but did not cause prolonged testicular infection. Administration of this modified-live vaccine did prevent development of prolonged testicular infections after subsequent exposure to a field strain of BVDV. This is the first research to our knowledge that evaluates the transient shed of modified-live, cytopathic
Acknowledgements
This research was funded by Boehringer Ingelheim Vetmedica Inc. and Boehringer Ingelheim (Canada) Ltd.
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Cited by (3)
Pestiviruses: How to outmaneuver your hosts
2010, Veterinary MicrobiologyCitation Excerpt :In summary, except in mucosal disease, viral cytopathicity as defined in cultured cells is largely unrelated to disease. This is corroborated by the fact that many of the BVD viral strains used in live vaccines are cytopathic (Moennig and Plagemann, 1992; Fulton et al., 2002; Givens et al., 2009). In CSF, a change from non-cytopathic to cytopathic has recently been shown to lead to attenuation (Gallei et al., 2008).
Cytopathic bovine viral diarrhea viruses (BVDV): Emerging pestiviruses doomed to extinction
2010, Veterinary Research