Reproductive consequences of infection with bovine viral diarrhea virus (Proceedings)

Bovine viral diarrhea virus (BVDV) has emerged as one of the most important infectious disease agents in cattle. The insidious nature of BVDV has led to substantial economic losses in both the dairy and beef industry on a worldwide level. Contrary to its name, BVDV has been associated with pathology in several physiological systems including the respiratory, hematological, immunological, neurological and reproductive systems. Reproductive losses may be the most economically important consequence associated with BVDV infection and evidence suggests the incidence of BVDV related reproductive losses are increasing in the United States (Evermann & Ridpath, 2002). In addition to reduced reproductive efficiency, BVDV uses the reproductive system to maintain and spread itself in the cattle population by inducing immunotolerance following fetal infection, resulting in birth of calves persistently infected with the virus. Cattle persistently infected with BVDV are the major source of virus spread within and between farms.

Reproductive losses associated with BVDV infection was described by Olafson et al (1946) in the first clinical description of BVDV. In this report, pregnant cows subclinically infected with BVDV often aborted 10 to 90 days later. Since that time, it has become evident that BVDV can cause a wide array of reproductive losses that are largely dependent on the time of gestation which infection occurs and the virus strain.

Infection Prior to Conception Through the Embryonic Stage (-9 to 45 days of gestation)

Field and epidemiological studies suggest that BVDV can have a significant impact on early reproductive performance. In a group of seronegative cattle accidentally exposed to a persistently infected cow, conception rates in groups that seroconverted to BVDV before, during or after breeding were 78.6, 44.4 and 22.2% (Virakul et al., 1988). Those seroconverting to BVDV at breeding or soon after breeding were less likely to conceive than those who had seroconverted prior to breeding. Similarly, McGowan et al. (1993) compared BVDV seropositive heifers to heifers that seroconverted between breeding and pregnancy diagnosis at 51 days post insemination and found the pregnancy rate was significantly reduced in the seroconverting heifers. However, no difference was found with cows under the same circumstances. Houe et al (1993) identified and defined a specific risk period for BVDV infection in dairy herds in which cattle persistently infected with BVDV were found. The risk period was defined during the 2-year study period 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 the post-risk period. In an experimental study looking at BVDV infection around breeding, conception rates in heifers infected intranasally nine 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 four days following insemination was 60%. However, significant embryo loss was experienced in this group resulting in a day 77-pregnancy rate of only 33% 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 BVDV (Grooms et al., 1998a) and noncytopathic virus (Grooms et al., 1998b). Exposure of developing oocytes to BVDV could result in reduced survivability either through direct cell damage or indirectly through changes in the local environment of the oocyte. Following acute infection with BVDV, interstitial oophoritis has been described with lesions lasting up to 60 days (Grooms et al., 1998a). Significant long-term oophoritis could result in ovarian malfunction with subsequent poor conception rates. Limited information is available on ovarian function following BVDV infection. 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 also undergoing superovulation (Kafi et al., 1994). Infection and subsequent viremia during the preovulatory phase can reduce the rate of follicle growth (Grooms et al., 1998c, Fray et al., 1999). Modulation of ovarian hormone secretion has been demonstrated following acute BVDV infection and has been postulated as a potential cause of BVDV induced infertility (Fray et al., 1999, Fray et al., 2000, Fray et al., 2002). Taken together, these studies suggest that ovarian dynamics may be changed in cattle infected with BVDV, and these changes may subsequently lead to transient or long-term decreases in fertility.

Several in vitro studies have been undertaken to elucidate the effects of BVDV on early reproductive function. Ova exposed to BVDV in vitro can have virus particles attached to the zona pellucida (Gillespie et al., 1990). However, in vitro studies have shown that the intact zona pellucida protects the developing embryonic cells from BVDV infection, allowing normal development to continue (Singh et al., 1982, Potter et al., 1984). In morula and blastocyst stage bovine embryos with the zona pellucida intact or damaged, no cytopathic effects were seen for 48 hours following exposure to cytopathic BVDV (Bilenski et al., 1988). Similarly, zona pellucida intact embryos exposed to noncytopathic BVDV infected bovine oviductal epithelial cells for 7 days showed no adverse effects in their rates of development (Zurovac et al., 1994). In contrast, blastocysts hatched from the zona pellucida (day 8 of gestation) have been shown to decrease viability when exposed to cytopathic BVDV in vitro. In the same study, noncytopathic BVDV did not decrease blastocyst survivability (Brock & Stringfellow, 1993). These studies suggest that the zona pellucida protects the developing embryo from direct effects of BVDV. However, following removal of the zona pellucida, cytopathic BVDV may have detrimental effects on survivability of blastocysts. Noncytopathic BVDV has not been shown to have the same effects. As noncytopathic BVDV is the most common virus isolated in acute outbreaks of BVDV, and has been the biotype associated with reported decreases in conception rates, further characterization of the effect of noncytopathic BVDV on the early stages of the developing embryo is necessary.

Infection Following the Embryo Stage (45-175 days of gestation)

Following implantation, transplacental infection of the developing fetus can occur in susceptible cows with either biotype of BVDV. The outcome of the infection is largely dependent on the timing of the infection, the immunocompetence of the developing fetus, the virus biotype involved and the virulence of the virus.

Abortion: Abortions associated with BVDV infection were first described in 1946 (Olafson et al., 1946). Early reports linked abortions to epizootics of disease described as BVD although a definitive cause of the abortion was not established (Dow et al., 1956, Nielson et al., 1955). 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 (Scott et al., 1972) were isolated from aborted fetuses. Under experimental conditions, both cytopathic BVDV (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. In a field investigation where BVDV was introduced into a susceptible herd as a point source, an abortion rate of 21% occurred during the subsequent 6 months (Roeder et al., 1986). In endemically infected herds without BVDV 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 point during gestation, although they are most common during the first trimester (Done et al., 1980, Roeder et al., 1986, Kahrs 1986, Sprecher et al, 1991). However, BVDV should not be ruled out in cases where late term abortions predominate. 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 three months of gestation (Grooms, Unpublished data). Depending on the time of infection, fetal reabsorption, mummification or expulsion can occur (Done et al., 1980, Casaro et al., 1971). Fetal death usually follows 10-27 days post exposure with expulsion of the fetus occurring up to 50 days later. Because of the delay between fetal death and subsequent diagnosis of abortion, fetal and placental lesions seen are usually non-diagnostic and BVDV virus isolation is not always successful. Under experimental conditions or when aborted fetuses are expelled soon after death, lesions observed include conjunctivitis, peribronchiolar and interalveolar pneumonia, and non-specific myocarditis. Placental lesions consist mainly of vasculitis, edema, congestion, and hemorrhage with some degeneration and necrosis (Jubb et al., 1985).

Immunotolerance: Fetuses that survive infection with noncytopathic BVDV between 18 and 125 days of gestation invariably develop immunotolerance to the virus and subsequently become persistently infected with BVDV. This phenomenon was first described in an apparently healthy bull (Coria and McClurkin, 1978) and subsequently reproduced experimentally (McClurkin et al., 1984). Although the exact mechanism of immunotolerance is unclear, it is generally felt that circulation of virus during the period of gestation when immunocompetence is developing (90-120 days) is a prerequisite for persistence. Viral proteins are recognized as self-antigens with resulting negative selection of BVDV specific B and T lymphocytes during their ontogeny. Persistent BVDV infection in cattle appears to arise from specific B- and T-lymphocyte immunotolerance (Coria et al., 1978, McClurkin et al., 1984), that results in an absence of neutralizing and non-neutralizing antibodies to the persistent virus (Donis et al., 1987). It is not clear when the exact stage of fetal genesis is during which infection must occur to cause immunotolerance. Under experimental conditions, persistence occurred in 86% and 100%, of calves derived from cows infected with BVDV at day 18 and 30 of gestation, respectively (Kirkland et al., 1993). Likewise, persistent infections were induced in 100% of fetuses derived from dams challenged at 75 days of gestation (Cortese et al., 1998, Brock and Cortese, 2001, Brock and Chase 2001). Persistent infections are rare following fetal infection after day one hundred, but have been reported up to day one hundred twenty-five of gestation (Baker 1995). Noncytopathic BVDV is the only biotype that has been observed or been able to experimentally produce persistence. Experimental infections with cytopathic BVDV have failed to produce persistently infected calves (Casaro et., 1971).

Congenital defects: Fetal infection between one hundred and one hundred fifty days of gestation, also referred to as congenital infection (CI), often results in the development of a variety of congenital defects. During this stage of gestation, organogenesis is being completed and the immune system is becoming fully functional. Although not clear, the combination of direct cellular damage by virus and inflammatory responses to virus have been proposed as pathogenic mechanisms (Castrucci et al., 1990). Congenital anomalies involving the central nervous system are most common following fetal infection with BVDV. These include microencephalopathy, hydrocephalus, hydranencephaly, porencephaly, cerebella hypoplasia, and hypomyelination. Cerebella hypoplasia was the first recognized teratogenic effect of BVDV and has been well documented (Kahrs et al., 1970). At birth, calves that have cerebella hypoplasia show extreme difficulty in becoming ambulatory. Those that can stand are ataxic, resulting in tremors, wide based stance and stumbling gait. The defects are usually severe enough that compensation does not occur and the calves either die or are euthanized. The changes in the cerebellum have been characterized as a reduction in the number of molecular layer cells and granular layer cells (Brown et al., 1973). Purkinje cell numbers are also reduced and often displaced. Fetal cerebella effects have been seen following infection as early as the seventy-ninth day and as late as the one hundred fiftieth day of gestation (Brown et al., 1973). Severity of cerebella lesions increase with the age of the fetus when infected (Brown et al., 1973).

Other teratogenic effects that have been associated with BVDV infection include cataracts, microopthalmia, optic neuritis, retinal degeneration, thymic hypoplasia, hypotrichosis/alopecia, curly hair coat, deranged osteogenesis, mandibular brachygnathism, and growth retardation.

Infection Late in Gestation (125 – 285 days of gestation)

In the later stages of gestation, immunocompetence and organogenesis are usually complete. Although abortions and the birth of weak calves have been attributed to infection with BVDV late in gestation, fetuses infected during this time period are normally able to mount an effective immune response to BVDV and effectively clear the virus. These calves are usually normal at birth and have precolostral neutralizing antibodies to BVDV (Braun 1973). However, calves congenitally infected with BVDV may be at more risk for experiencing a serious postnatal health event. In a study attempting to define the impact of congenital BVDV infections on large dairy farms, Munoz-Zanzi et al. (2003) showed that calves born with BVDV neutralizing titers were twice as likely to experience a severe illness during their first 10 months of life compared to calves born free of BVDV neutralizing titers. Further studies are needed to determine if detrimental effects may continue long-term.

Summary

Reproductive efficiency is imperative for the maintenance of profitability in both dairy and cow-calf enterprises. Bovine viral diarrhea virus is an important infectious disease agent of cattle that can potentially have a negative effect on all phases of reproduction. Reduced conception rates, early embryonic deaths, abortions, congenital defects and weak calves have all been associated BVDV infection of susceptible females. In addition, the birth of calves persistently infected with BVDV as a result of in utero fetal exposure is extremely important in the perpetuation of the virus in an infected herd or spread to other susceptible herds. Management practices including elimination of persistently infected cattle, biosecurity measures and strategic use of vaccination can be implemented to reduce the risk of BVDV related reproductive losses. Development of vaccines and vaccine strategies capable of providing better protection against fetal infection would be of benefit.

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