Материал: Bovine Viral Diarrhea Virus Diagnosis, Management, and Control

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INTRODUCTION AND OVERVIEW

Bovine viral diarrhea virus (BVDV) is recognized as one of the most important infectious diseases of cattle (Baker, 1987; Duffell and Harkness, 1985). The insidious nature of BVDV contributes to substantial economic losses in both the dairy and the beef cattle industries on a worldwide basis (Duffell et al., 1986; Houe et al., 1993a). The most common consequences of BVDV infection in cattle are respiratory and reproductive disorders. Losses due to BVDV-related reproductive disorders are probably the most economically important consequence, and there is evidence to suggest that such losses are increasing in the United States (Evermann and Ridpath, 2002). Although difficult to measure, the economic consequences of reproductive infections can be devastating due to negative effects on the individual animal and on the net return per cow. In addition to reduced reproductive efficiency, BVDV has adapted to the bovine reproductive system, thereby maintaining itself in the cattle population by inducing immunotolerant fetal infections; this results in the birth of calves persistently infected with BVDV. The primary consequences of reproductive infection are due to the direct infection of the fetus. In turn, persistently infected (PI) animals act as a source of BVDV within the cattle populations.

Reproductive losses associated with BVDV infection were initially described by Olafson and associates in the first clinical description of bovine viral diarrhea (Olafson et al., 1946). Pregnant cows that were subclinically infected with BVDV, often aborted 10–90 days following infection. It has been observed that BVDV can cause a wide array of reproductive losses manifested in many different clin-

ical pictures. The outcome of BVDV infection depends primarily on the stage of the reproductive cycle or gestation.

IMPACT OF TESTICULAR INFECTION ON BULL FERTILITY

The influence of BVDV infection on bull fertility and reproduction is often overlooked. Both acute and persistent infections can affect the reproductive soundness of bulls by reducing semen quality (Coria and McClurkin, 1978; Barlow et al., 1986; Meyling and Jensen, 1988; Revell et al., 1988; Kirkland et al., 1994; Whitmore and Archbald, 1977; Kirkland et al., 1991; Kommisrud et al., 1996). The primary effect of infection is the subsequent potential for venereal transmission during breeding and the shedding of BVDV in semen. Virus has been isolated from the semen of PI bulls ranging from 104 to 107 CCID50/ ml of semen (Kirkland et al., 1991). Following acute experimental infection of bulls with noncytopathic BVDV, virus was isolated from the semen between 7 and 14 days postinfection at titers ranging from 5–75 CCID50/ml of semen (Kirkland et al., 1991). The ability to transmit BVDV via semen is supported by the demonstration that susceptible cows can become infected following artificial insemination (Kirkland et al., 1994; Paton et al., 1990) or natural service (McClurkin et al., 1979; Wentink et al., 1989).

Persistently infected bulls can successfully sire offspring but usually with lowered conception rates (Meyling and Jensen, 1988; Kirkland et al., 1994; Paton et al., 1990). Poor reproductive efficiency following the use of PI bulls is likely attributable to a combination of several factors including low quality semen, ill thrift, and the effects of the virus on the re-

productive tract and conceptus of infected females. The quality of semen from PI bulls may range from acceptable (Barlow et al., 1986; Kirkland et al., 1991) to abnormal with various defects predominantly involving the head of the spermatozoa (Barlow et al., 1986; Revell et al., 1988; McClurkin et al., 1979). Prior to the development of standardized BVDV testing protocols in semen collection/production facilities, semen was unknowingly collected and distributed from PI bulls. A retrospective analysis of breeding records in 97 dairy farms in which semen from a PI bull was used, indicated a first service conception rate of 38% in cows inseminated with this semen as compared to 66% for cows bred during the same period on the same farms with semen from a different bull (Kirkland et al., 1994).

Following acute infection, bulls can shed BVDV in semen (5–75 CCID50/ml of semen). In contrast to semen from PI bulls, the semen of acutely infected animals (based on the criteria of concentration, motility, and morphology of spermatozoa) will pass breeding soundness examination (Kirkland et al., 1997; Kirkland et al., 1991). It is generally accepted that the ability to isolate virus from semen ceases when serum antibodies become detectable (Kirkland et al., 1991; Paton et al., 1989). BVDV isolation from raw semen may be less successful than from extended semen (Revell et al., 1988). This is presumably due to the documented viricidal effects of semen. Using semen that was collected prior to seroconversion (12 days postinoculation), approximately 5% of inseminated heifers became infected as evidenced by seroconversion (Kirkland et al., 1997). It is interesting to note that in this report the primary impact of BVDV was the horizontal transmission of BVDV to penmates and the subsequent birth of PI calves (Kirkland et al., 1997).

As is true for most biological systems, there are always exceptions to the rule. In addition to classical acute and persistent infections, the replication of BVDV in a “privileged” site has been described. Thus, persistent BVDV infection localized in the testes of an immunocompetent, seropositive, nonviremic bull has been documented (Voges et al., 1998). The concentration of BVDV in the semen of this bull was lower (<2 103 CCID50/ml) than that observed in PI bulls (104–107 CCID50/ml), but higher than that observed in acutely infected bulls (5–75 CCID50/ml) (Kirkland et al., 1991; Voges et al., 1998). The insemination of seronegative heifers with semen collected from this bull resulted in BVDV infection and subsequent seroconversion

(Niskanen et al., 2002). In addition, during the reported period of collection, the bull continued to have consistent, high levels of BVDV neutralizing antibody against the viral strain isolated from the semen. Bovine viral diarrhea virus could not be isolated from white blood cells but was continually isolated from semen samples. At postmortem, BVDV was isolated only from the testicular tissue. It is hypothesized that this bull was acutely infected with BVDV near the time of puberty when the bloodtestes barrier forms, thus trapping the virus in gonadal cells away from the animal’s immune response. These findings suggest that screening bulls for persistent infection with BVDV using serum or white blood cells may not be adequate in assuring BVDV-free semen.

Persistent, localized testicular BVDV infections in experimentally infected, post-pubertal, nonviremic bulls have been characterized (Givens et al., 2003a). Following experimental infection, BVDV persisted within the testicular tissues of some bulls for at least 7 months (Givens et al., 2003a). Experimental results have indicated that an epididymal infection may progress to a testicular infection. Due to the blood-testes barrier, BVDV is protected from elimination by the immune system and testicular infection can persist (Paton et al., 1989; Kirkland et al., 1991). The prevalence of bulls that are nonviremic (based on failure to isolate virus from serum or buffy coat sample) but shed BVDV in semen is probably extremely low (Givens et al., 2003b; Niskanen et al., 2002). Further studies are required to determine whether persistent-testicular BVDV infections contribute to transmission of BVDV by semen to susceptible cows.

Currently, BVDV contamination of distributed semen is prevented by practicing standardized testing and quarantine procedures in AI semen collection facilities. Certified Semen Services, Inc. (CSS) is a cooperative organization, and membership in CSS ensures that the standardized procedures are used appropriately (CSS guidelines: http://www. naab-css.org/about_css/disease_control-2002.html). Therefore, use of semen from CSS-certified collections is recommended to prevent introduction of BVDV via semen. Current CSS prevention measures include screening of all bulls 30 days prior to entry by virus isolation or ELISA to detect persistent infections. The collection and distribution of semen from bulls with persistent testicular infections is prevented by requiring that specimens of semen be negative by virus isolation.

IMPACT OF OVARIAN INFECTION ON REPRODUCTIVE CAPACITY

BVDV infections prior to breeding often go unnoticed in production operations, and thus field data to gauge its impact are sparse. Limited information is available on ovarian function following BVDV infection. Reproductive capacity of bulls and cows/heifers can be affected directly by BVDV infection. Bovine viral diarrhea virus has been identified by virus isolation and immunohistochemistry in ovarian tissues and oviductal cells following acute infection. The identification of BVDV and viral antigen has been associated with chronic oophoritis and salpingitis of cattle. Clinical examination of cattle experimentally infected with BVDV revealed reduced number of corpora lutea and recovered embryos. Between 6 and 60 days following experimental acute infection, viral antigen (Erns) was detected by immunohistochemistry in interstitial stromal cells and macrophage-like cells that were associated with primary follicles, secondary follicles, antral follicles, corpus luteum, and corpus albicans (Grooms et al., 1998a). Viral antigen (Erns) was detected by immunohistochemical techniques in ovarian sections taken on days 10, 20, and 30 after immunization with a modified-live BVDV vaccine (Grooms et al., 1998b). Experimental acute BVDV infection near estrus has resulted in reduced follicular development following infection (Grooms et al., 1998c).

In a study of cattle being superovulated while undergoing experimental challenge with BVDV, the number of palpable corpora lutea and recovered embryos was significantly reduced when compared to noninfected cows undergoing superovulation (Kafi et al., 1994). BVDV infection resulting in viremia during the preovulatory phase can reduce the rate of follicle growth (Grooms et al., 1998a; Fray et al., 1999). Cows that are persistently infected with BVDV have ovaries that are often hypoplastic with a significantly reduced number of ovarian antral follicles when compared to ovaries from cattle not persistently infected with BVDV (Grooms et al., 1996). Alteration of ovarian hormone secretions has been demonstrated following acute BVDV infection and has been postulated to contribute to BVDV-induced infertility (Fray et al., 1999; Fray et al., 2000a; 2000b; 2002). The changes in follicular development and ovarian hormonal dynamics associated with BVDV infection may subsequently lead to a transient and/or longterm reduction in fertility.

IMPACT OF STAGE OF GESTATION ON THE OUTCOME OF REPRODUCTIVE DISEASE

As stated previously, the outcome of reproductive disease depends on the stage of gestation at which fetal infection occurs. Abortions associated with BVDV infection were first described in 1946 (Olafson et al., 1946). Initial reports linked abortions to epizootics of disease described as bovine viral diarrhea although definitive causes of abortions were not identified (Dow et al., 1956; Nielson et al., 1955; Swope and Luedke, 1956). Early studies involving experimental infection with BVDV resulted in abortion although virus was not isolated from the fetus (Baker et al., 1954; Huck, 1957). Subsequently noncytopathic (Gillespie et al., 1967) and cytopathic BVDV (Shope, 1968; Scott et al., 1972) were isolated from aborted fetuses. Experimental, transplacental infection of fetus with BVDV was first demonstrated in 1969 (Ward et al., 1969). Under experimental conditions, both cytopathic (Brownlie et al., 1989) and noncytopathic BVDV (Done et al., 1980; Duffell et al., 1984; Liess et al., 1984) can cause fetal death following infection of seronegative dams. An abortion rate of 21% in a 6-month period has been documented to have occurred in a susceptible herd following the introduction of BVDV in the herd (Roeder et al., 1986). In endemically infected herds without BVDV prevention and control programs (vaccination, biosecurity, test, and removal), it has been estimated that 7% of fetal deaths may be attributable to infection with BVDV (Rufenacht et al., 2001).

Fetal death following BVDV infection of susceptible dams can occur at any stage during gestation, although abortions are most common during the first trimester (Duffell and Harkness, 1985; Done et al., 1980; Roeder et al., 1986; Casaro et al., 1971; Kahrs, 1968; Kendrick, 1971; Sprecher et al., 1991; McGowan et al., 1993). However, BVDV can also be associated with late-term abortions. In a field investigation of an abortion outbreak in a large dairy operation, BVDV was isolated from 18 fetuses, 14 of which were aborted during the last trimester of gestation (Grooms, unpublished data). Depending on the age of the fetus, fetal reabsorption, mummification, or expulsion can occur following infection with BVDV (Done et al., 1980; Casaro et al., 1971).

CONSEQUENCES OF BVDV INFECTION

PRIOR TO IMPLANTATION (30–45 DAYS)

Understanding the effects of BVDV infection during the early stages of embryo development has proven

to be difficult. Characterization of the complex events that influence conception is difficult and contributes to the lack of understanding of the effects of BVDV. From epidemiological and clinical observations it is apparent that BVDV infection reduces not only the conception rates but also the viability of the early stage conceptus. Conception rates were reduced in cattle exposed to BVDV during breeding when compared to cattle seropositive to BVDV (Houe et al., 1993a, 1993b; McGowan et al., 1993; McGowan et al. 1995; Virakul et al., 1988). The mechanism(s) for reduced conception is not directly known. However, it has been demonstrated that BVDV infection results in the replication of BVDV in ovarian tissues (Grooms et al., 1998a, 1998b). The most common consequence of BVDV infection during the early stages of gestation is infertility. In vitro experimental studies have demonstrated that the zona pellucida provides a protective effect to the early developing embryo.

When a group of seronegative cattle was accidentally exposed to a persistently infected cow during breeding, the conception rates in cattle that seroconverted to BVDV before, during, or after breeding were 78.6%, 44.4%, and 22.2%, respectively (Virakul et al., 1988). Cattle that seroconverted to BVDV at breeding or soon after breeding were less likely to conceive than cattle that had seroconverted prior to breeding (Virakul et al., 1988). McGowan (1993) compared BVDV seropositive heifers to heifers that seroconverted between breeding and pregnancy diagnosis at 51 days postinsemination and found that the pregnancy rate was significantly lower in heifers seroconverting following breeding. Houe et al. (1993b) identified and defined a specific risk period for BVDV infection in dairy herds in which cattle persistently infected with BVDV were present. The risk period was defined as the period of time previous to when the oldest PI animal was 6 months old. In all herds studied, conception rates were significantly lower during the defined risk period than during the post-risk period (Houe et al., 1993b). In an experimental study examining BVDV infection around breeding, conception rates in heifers infected intranasally 9 days before insemination was 44% compared to 79% for the control group (McGowan et al., 1993). The reduction in conception rates was attributed to either failure of fertilization or early embryonic death. In the same report, the conception rate in heifers exposed to a persistently infected cow and calf 4 days following insemination was 60% (McGowan et al., 1993). However, significant embryo loss was experienced

in this group, resulting in a 77-day pregnancy rate of only 33% as compared to 79% for the control group.

The mechanism for decreased conception rates is not clear but may depend on the time of infection with respect to the stage of early reproductive events. Virus has been localized in ovarian tissue for prolonged periods of time following acute infection with cytopathic (Ssentongo et al., 1980; Grooms et al., 1998b) and noncytopathic BVDV (Grooms et al., 1998a). BVDV has also been isolated from follicular fluid collected from slaughterhouse ovaries (Bielanski et al., 1993). Exposure of developing oocytes to BVDV could result in reduced survivability either through direct cell damage or indirectly through changes in the oocyte at the cellular level. Following acute infection with cytopathic BVDV, interstitial oophoritis has been described with lesions lasting up to 60 days (Ssentongo et al., 1980; Grooms et al., 1998a). Significant long-term oophoritis could result in ovarian malfunction with subsequent poor conception rates.

Because of its essential role in fertilization, changes in the oviductal environment could have a detrimental effect on the conception rate. BVDV has been detected in oviductal cells (Bielanski et al., 1993; Booth et al., 1995). Archbald et al. (1973) isolated BVDV from oviductal tissue and detected evidence of salpingitis for up to 21 days following experimental intrauterine infusion with cytopathic BVDV. Similar findings have not been reported with noncytopathic BVDV.

Studies have suggested that the interruption of normal fertilization or embryonic death may be the mechanism for a reduction in conception rates associated with acute BVDV infection. This conclusion was drawn from the observation that infusion of cytopathic BVDV into the uterus at insemination of superovulated cows resulted in a significant reduction in the number of fertilized ova found at slaughter 3 and 13 days later (Grahn et al., 1984). Archbald et al. (1979) provided evidence that BVDV may interfere with early embryonic development. In superovulated cattle in which BVDV had been infused into one uterine horn, the quality of the embryos collected from the infected horn was significantly reduced compared to those collected from the noninfected horn (Archbald et al., 1979). In a similar study, the conception rate in seronegative heifers infused with BVDV 2 hours following breeding was 27% and was significantly reduced (67%) as compared to sham-inoculated cows (Whitmore et al., 1981). However, in the same study, conception rates