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

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INTRODUCTION

The management and control of bovine viral diarrhea virus (BVDV) infection in cattle herds must take into consideration two methods of virus transmission: postnatal horizontal transmission and gestational vertical transmission from a viremic dam to her fetus (Meyling et al., 1990). Postnatal horizontal transmission results in a transient infection that is usually mild or subclinical, but can result in severe disease if susceptible cattle are exposed to a virulent strain of the virus (Kelling et al., 2002; Hamers et al., 2000a). In addition, postnatal horizontal transmission can lead to vertical transmission of BVDV, because postnatal horizontal infection is the primary method by which a pregnant dam becomes viremic and subsequently infects her fetus.

PRINCIPAL RESERVOIRS OF BVDV

The primary source of BVDV in postnatal transmission is cattle persistently infected (PI) with BVDV. Persistently infected animals are much more efficient transmitters of BVDV than transiently infected animals because they shed large amounts of virus for a long period of time. Transiently infected animals experience a short period of viremia and shed virus in their body secretions and excretions from day 4–15 postinfection (Brownlie et al., 1987; Duffell and Harkness, 1985). In contrast, PI animals usually have a very high and persistent viremia, and BVDV is shed throughout life from virtually all secretions and excretions, including nasal discharge, saliva, semen, urine, tears, milk, and to a lesser extent, feces (Rae et al., 1987; Brock et al., 1991; Bezek et al., 1995; Brock et al., 1998). Horizontal transmission of BVDV to seronegative cattle has been shown to occur after only 1 hour of direct contact with a PI an-

imal (Traven et al., 1991). Over-the-fence contact with a PI animal from a neighboring herd can also introduce BVDV into a susceptible herd (Roeder and Harkness, 1986; Miller et al., 2002).

Although transiently infected cattle are far less efficient at transmitting the virus to susceptible incontact animals (Meyling and Jensen, 1988; Niskanen et al., 2000; Niskanen et al., 2002), the occurrence of seroconversion among assembled cattle without the presence of PI animals indicates that transmission from transiently infected animals does occur (Meyling et al., 1990). In a field study by Moerman et al. (1993) it was found that BVDV infections circulated within a herd over a period of 30 months in the absence of PI cattle and BVDV vaccination, but in the presence of transiently viremic cattle. Horizontal transmission of the virus from either persistently or transiently infected animals to susceptible cattle may be via inhalation or ingestion of virus-containing body fluids (Duffell and Harkness, 1985). Although aerosol transmission over short distances seems likely; the spread of infection is slow or absent when cattle are housed at long distances from PI animals (Wentink et al., 1991).

REPRODUCTIVE EFFECTS OF BVDV

Even mild or subclinical infections of pregnant and susceptible seronegative breeding females may result in conception failure, abortion, or vertical fetal infection. The immune status of the dam, the stage of gestation, and the viral biotype are important factors in determining the result of vertical infection. Transplacental infection occurs with high efficiency in pregnant, seronegative dams (Done, et al., 1980; McClurkin et al., 1984). However, naturally acquired immunity can prevent later fetal infection (Orban et

al., 1983). Infection of fetuses can lead to early embryonic death, abortion, congenital defects, stunting, and the birth of either PI or normal calves (Figure 14.1) (Baker, 1987). Fetal infection of susceptible dams with a cytopathic biotype of BVDV early in gestation will usually result in abortion. However, fetal infection with a noncytopathic biotype of BVDV will result either in abortion or, in a certain proportion of animals, the birth of a calf that is immunotolerant to and persistently infected with that particular noncytopathic strain of BVDV. Thus, PI cattle are the result of in utero exposure to the noncytopathic biotype of BVDV prior to the development of a competent fetal immune system (Casaro et al., 1971; McClurkin et al., 1984). Age at immune competence in the face of BVDV exposure is variable and has been reported to range from 90–125 days (Casaro et al., 1971; McClurkin et al., 1984; Roeder et al., 1986). If PI fetuses survive to term, they are continually viremic, but immunotolerant to the homologous BVDV (Duffell and Harkness, 1985; Roeder et al., 1986).

In addition to BVDV causing conception failure and abortion, reproductive efficiency can be decreased due to fatal congenital defects following fetal infection between 100 and 150 days of gestation (Duffell and Harkness, 1985). The teratogenic lesions associated with fetal infection with BVDV include microencephaly, cerebellar hypoplasia, hydranencephaly, hydrocephalus, hypomyelination of the spinal cord, cataracts, retinal degeneration, optic neuritis, microphthalmia, thymic aplasia, hypotrichosis, alopecia, brachygnathism, growth retardation, and pulmonary hypoplasia (Baker, 1987).

EXPOSURE OF HERDS TO PI CATTLE

Suckling calves are commonly in contact with the breeding herd and thus come in contact with dams in early stages of pregnancy. As a result, PI suckling calves may be a source of BVDV infection in breed-

ing herds, causing decreased pregnancy percentage, pregnancy loss, preweaning mortality and the induction of PI calves in the next generation (Wittum et al., 2001; Duffell and Harkness, 1985; McClurkin et al., 1984).

Although mortality of PI calves prior to weaning has been reported to be very high due to fatal congenital defects and secondary infections that cause enteritis, pneumonia, and arthritis (McClurkin et al., 1979; McClurkin et al., 1984), in some situations 17–50% of PI calves may reach breeding age (Barber et al., 1985; Binkhorst et al., 1983; Houe, 1993). Persistently infected females of breeding age are not only a source of horizontal transfer of BVDV, but also will produce a PI calf themselves, which if it survives, creates a familial clustering of PI animals (Houe et al., 1995; McLurkin et al., 1979).

The consequence of introducing a PI animal into a beef herd (confined breeding and calving seasons) depends on the timing of the introduction relative to the breeding season and the subsequent immunologic status of the herd during early gestation (Table 14.1). A likely scenario for a BVDV-exposed herd is to experience an initial peak of disease followed by low-level chronic reproductive losses in subsequent months and years. If a PI animal enters the herd either by birth or by purchase near the start of the breeding season, a high percentage of the herd may not be immunologically protected to the degree necessary to prevent viremia, conception failure, abortion, or fetal infection. If the PI animal is in contact with the breeding herd for a long enough period of time, the majority of the herd should become infected and seroconvert. Seropositive animals are less likely to have conception failures, abortions, or infected fetuses compared to seronegative animals. If no intervention is applied to the herd, the number of susceptible females the following year should be greatly decreased and the number of abortions and

infected fetuses (both PI and immunocompetent) should decrease. Thus, even in the absence of vaccination and culling, the number of PI animals and BVDV infections in a closed herd may be selflimiting over time (Houe, 1995). A model developed by Cherry et. al. (1998) indicates that in continuous calving situations, the proportion of PI animals in a dairy herd will reach an equilibrium of about

0.9–1.2% in closed herds with no BVDV control procedures (Cherry et al., 1998). This model does not hold true if the herd is not closed (replacement females and bulls are added). Prevalence of PI animals and the resulting problems would not be expected to reach equilibrium in BVDV-infected herds with no control measures that are purchasing heifers, particularly bred heifers.