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

(Endsley et al., 2003; Ridpath et al., 2003), suggesting that vaccination in the face of maternal antibodies may be effective.

Passive immunity is not fail safe, however. Factors such as dose of challenge virus, poor nutrition, exposure to harsh weather, poor ventilation, transportation stress, and increased stocking density may affect the outcome of infection in the presence of colostral antibodies. In addition, antigenic variation among BVDV may result in incomplete protection, and colostrum may vary in quality and quantity of available antibodies. Nutrition and vaccination status of the dam will also affect the immune quality of colostrum. For example, vaccination of pregnant cows in the last month of pregnancy with killed vaccines will improve the amount of antiviral antibodies in colostrum. Vaccination during pregnancy with live, modified vaccines will also increase the amount of antibody in colostrum, but this type of vaccination is advised only when the dam has preexisting antibodies. Further, the benefits of colostrum are not limited to the action of passive antibodies; immune lymphocytes and macrophages present in colostrum are also of help. In this regard, fresh colostrum is better than frozen colostrums.

CELLULAR IMMUNE RESPONSE

Infection with BVDV results in mild (10–20% decrease) or severe lymphopenia (50–60% decrease) depending upon the strain of the virus (Brodersen and Kelling, 1999; Archambault et al., 2000). Cytotoxic T-lymphocytes (CD8+) are affected more than helper T-lymphocytes (CD4+) with little or no affect on circulating / T-cells (Brodersen and Kelling, 1999; Ellis et al. 1988). Depletion of CD4+ cells increases the period of virus shedding; that of CD8+ and / T-cells does not, indicating that CD4+ helper cells play a pivotal role in coordinating a cellmediated response early in infection. These CD4+ responses are directed primarily at NS3 and E2 proteins (Lambot et al., 1997; Collen and Morrison, 2000; Collen et al., 2000; Collen et al., 2002) and also against the capsid protein (C), glycoprotein Erns, amino-terminal proteinase (Npro), and nonstructural protein 2-3 (NS2-3) (Collen et al., 2002).

Proliferation assays are an indirect measure of CD4+ responses to viral antigens. The cells of Th1 phenotype produce IL-2 and IFN- , but not IL-4 or B-cell stimulatory activity. In contrast, cells of Th2 phenotype express high levels of B-cell growth factor and IL-4 activity with relatively low levels of IL- 2 and IFN- (Rhodes et al., 1999). IL-4 increases the expression of MHC II on B-cells and is also im-

portant for the growth and survival of Th2 responses. In cattle, unlike human and mouse systems, pregnancy has no observable shift in IL-4 pattern (Waldvogel et al., 2000).

The response generated by cp and ncp BVDV is different. For example, ncp viruses tend to shift the immune response toward the Th2 response and avoid the production of high levels of cell-mediated immunity (Lambot et al., 1997; Rhodes et al., 1999). cp BVDV, on the other hand, produces higher CMI response (Th1), along with up-regulation of IL-2 receptor (IL-2R) in response to increased levels of IL- 2. The down-regulation of IFNby acute ncp BVDV infection inhibits the cell-mediated response to Mycobacterium bovis that could result in the failure to identify cattle with tuberculosis (Charleston et al., 2001b). Delayed-type hypersensitivity is a cellular immune response that occurs approximately 18 hours after exposure to an antigen and is used as an assay for cell-mediated immunity. This response is characterized by induration and erythema at the site of antigen injection such as with the Mycobacterium antigen. After exposure to BVDV, the delayed type hypersensitivity to Mycobacterium is inhibited. Thus, BVDV causes general and nonspecific inhibition of cellular immune responses in cattle (Thoen and Waite, 1990) and may interfere with the diagnosis of bovine tuberculosis (Charleston et al., 2001b).

Proliferating CD8+ cytotoxic T-lymphocytes (CTL) produce IL-2 and IFN- , indicating a type 1 memory response in BVDV-seropositive cattle (Howard et al., 1992; Rhodes et al., 1999). Although fine mapping of BVDV CTL epitopes has not been done, computer predictions based on MHC I binding domains indicate that regions in the C, Erns, E2, and NS2-3 are likely BVDV CTL epitopes (Hegde and Srikumaran, 1997).

IMMUNE RESPONSE OF PERSISTENTLY

INFECTED ANIMALS

Persistent BVDV infections can arise in animals infected as fetuses. Persistently infected cattle are immunotolerant to the infecting BVDV isolate but may mount an immune response to heterologous BVDV. It has been demonstrated that heifers carrying PI calves developed BVDV antibody titers 5–10 times higher than those carrying non-PI calves (Brownlie et al., 1998). A number of studies have been done to understand the immunological defects in PI animals (Chase, 2004). The inability of ncp BVDV to induce IFNin the fetus is certainly one of the major immune evasion mechanisms that allow BVDV to establish persistence (Charleston et al., 2002).

ASSESSMENT OF IMMUNITY

HERD IMMUNITY

When assessing herd immunity, it is important to sample several animals or use a pooling strategy. The exact parameters of cellular and humoral immune responses that indicate protection against BVDV infection have not been determined. Veterinary practitioners routinely screen cattle herds for humoral immune response against BVDV to assess the effectiveness of vaccination programs. However, it is difficult to interpret serological profiles when only a single serum sample is tested. Because antibody titers resulting from natural infections usually exceed those generated from vaccination, uniformally high antibody titers within a herd or jointly housed group may indicate a recent BVDV outbreak. In addition, the presence of antibodies in fetal thoracic fluid or of precolostral antibodies in neonates is suggestive of fetal BVDV infection. Cattle that are not immunotolerent to BVDV antigens and are not sufficiently immunized can then be immunized with a BVDV vaccine containing a slightly different antigen. Enough individual variation exists among outbred cattle that some animals may need booster injections to maintain high levels of herd immunity.

The serum neutralization (SN) test has been extensively used as a correlate of protective immunity. Since the SN test measures the total neutralizing activity of a serum sample or a body fluid, it represents a composite of neutralizing activity due to all classes of antibodies that have activity against surface proteins of BVDV. Some regions of viral proteins are highly conserved between members of pestiviruses, and others are variable between different members of the family. Most of the highly conserved regions are found in the inner part of the virion. Typespecific variable regions are present on the outer surface of the BVDV protein. Members of pestiviruses, such as BVDV, classical swine fever virus, and border disease virus often cross-react in SN tests (de Smit et al., 1999).

There are two major problems with BVDV serology as it is currently used: a lack of test standardization and the existence of antigenic variation among BVDV isolates. In an interlaboratory study of 14 U.S. diagnostic laboratories, SN titers for a single serum sample varied from 1:8 to 1:3642 (Vaugn, 1997). This variability may be due to technical expertise of the person performing the test and to virus strain, cell type and passage, and amount of virus used in the test. In one study, the SN titers varied by

5–10-fold with the use of different BVDV strains (Fort Dodge, 1999b) while in another study they varied from 2–250 fold (Fulton et al., 1997). Although ELISA tests hold the promise of negating many of the above variables, they have not been widely used because they suffer from a lack of correlation with SN titers, which appear to correlate relatively well with protective immunity against BVDV (Bolin and Saliki, 1996). At present, there are two recognized genotypes of BVDV (1 and 2) and several proposed subgenotypes. Although there is cross-reactivity between BVDV 1 and BVDV 2, cross-reactivity is significantly higher within a genotype than between genotypes (Jones et al., 2001). Most of the veterinary diagnostic laboratories in the U.S. now perform both type 1 and type 2 SN tests.

PROTECTIVE IMMUNITY

A large number of BVDV vaccines are available in the U.S. They are usually approved on the basis of their safety, their ability to induce serological response against the virus, and protection achieved in challenge protection experiments. In view of antigenic diversity of BVDV, the panel of challenge viruses should include both homologous and heterologous BVDV isolates from geographical areas where vaccines are to be administered. Inclusion of novel, antigenically different variants of BVDV in the challenge panel would also be of value.

BVDV has at least two genotypes (genotype 1 and 2) and two biotypes (cytopathic and noncytopathic). Despite widespread vaccination and the availability of a wide range of vaccines (Van Oirschot et al., 1999), BVDV remains a problem in most areas of the United States, raising the concern that evolution of the virus may occur under immunological pressure leading to the continued emergence of antigenic variation among BVDV isolates. With the recent recognition of genotype 2 BVDV, most vaccine manufacturers have now started to include both BVDV 1 and BVDV 2 in their vaccines. Although type 1 BVDV vaccines provide some protection against type 2 BVDV (Cortese et al., 1998; Fairbanks et al., 2003; Makoschey et al., 2001), a number of challenge studies have indicated that the best protection rates are obtained with the use of homologous BVDV vaccines (Potgieter, 1995; Fulton et al., 2003).

IMMUNOSUPPRESSION

Diverse clinical manifestations are associated with BVDV infection in cattle. Although a majority of

BVDV infections in immunocompetent cattle are transient and self-limiting, it is apparent that when infection occurs in the presence of other microorganisms, BVDV can contribute to a disease that becomes clinically evident (Baker, 1995). Both natural and experimental studies have demonstrated the relationship between BVDV and other infectious agents, suggesting that BVDV has the ability to induce immunosuppression in immunocompetent animals. Impairment of lymphocyte and neutrophil functions decreases in the number of circulating and tissue immune cells, and environmental and/or management stressors have been cited as contributors to immunosuppression (Potgieter, 1995).

BVDV AND SECONDARY INFECTIONS

Increased susceptibility to secondary infections is a consequence of BVDV-induced immunosuppression. BVDV increases susceptibility to secondary infections because lymphocytes from BVDVinfected animals have impaired memory responses to other antigens and BVDV (Lamontage et al., 1989). Some pathogens may induce disease alone, but the disease is enhanced in the presence of BVDV. In some cases, BVDV-induced disease can be enhanced by opportunistic organisms. Substantial data indicate that BVDV infection is important in multiple-etiology diseases. Bovine respiratory disease complex in feedlot animals and in intensively housed calves is an example of this type of multipleetiology disease process.

It is debatable whether BVDV-induced immunosuppression or primary infection of the respiratory tract plays a major role in the bovine respiratory disease complex. Little experimental evidence exists establishing a primary role of BVDV in the bovine respiratory disease complex (Potgieter, 1997). To support a primary role, BVDV is frequently isolated from pneumonic lungs of cattle (Greig et al., 1981). In a study on respiratory disease outbreaks with multiple virus infections, BVDV was isolated from pneumonic cattle more frequently than any other virus (Richer et al., 1988). Suggestive evidence exists for certain BVDV isolates to be pneumotropic (Jewett et al. 1990; Potgieter et al., 1985). In an experimental infection study, endobronchial inoculation of two different isolates of BVDV resulted in interstitial pneumonia. However, when calves were challenged with Mannheimia hemolytica, more severe disease resulted with only one of the two isolates (Potgieter et al., 1985). In another study, certain BVDV isolates were shown to be able to induce severe respi-

ratory disease in gnotobiotic lambs (Jewett et al., 1990).

Although BVDV is frequently isolated from cattle with pneumonia, it is often present with other infectious agents, including bovine herpes virus-1 (BHV- 1) (Biuk-Rudan et al., 1999; Greig et al., 1981), parainfluenza-3 virus (PI-3) (Dinter and Bakos, 1961; Fulton et al., 2000), bovine respiratory coronavirus (BCV), bovine respiratory syncytial virus (BRSV) (Brodersen and Kelling, 1998, 1999),

Mannheimia hemolytica (Fulton et al., 2002), Pasteurella multocida (Fulton et al., 2002), Mycoplasma bovis (Martin et al., 1990; Shahriar et al., 2002; Haines et al., 2001), and Hemophilus somnus.

In addition to causing immunosuppression, BVDV may interact directly with pathogens to make them more virulent. Combined infection of bovine alveolar macrophages with BRSV and BVDV produces synergistic depression of alveolar macrophage functions (Liu et al., 1999). Concurrent infection with BVDV and BRSV in cattle causes more severe enteric and respiratory disease, and these animals shed higher concentrations of BVDV in their nasal secretions (Brodersen and Kelling, 1998). Reports from disease outbreaks and experimental studies have supported a role for synergism with BHV-1 also (Greig et al., 1981; Potgieter et al., 1984). An experimental infection study was performed for evaluating this synergism. Calves inoculated with BVDV 7 days prior to inoculation with the Cooper strain of BHV-1 developed severe clinical disease, with dissemination of BHV-1 into nonrespiratory tissues, including intestinal and ocular tissues, as compared to calves inoculated with BHV- 1 alone (Greig et al., 1981; Potgieter et al., 1984). These observations suggested that initial BVDV infection impaired the ability of calves to clear BHV- 1 from the lungs and to contain BHV-1 at the local infection site.

The ability of BVDV to synergistically interact with pathogens does not appear to be confined to the respiratory tract either, since synergism between BVDV and enteric pathogens has also been reported (Kelling et al., 2002a; Woods et al., 1999). From experimental infection studies in calves, it has been observed that BVDV and rotavirus may interact in a synergistic manner to induce more severe clinical disease (Kelling et al., 2002a). Synergism between BVDV and transmissible gastroenteritis virus was demonstrated by generalized lymphocyte depletion throughout the lymphatic system and villous atrophy in the intestinal tract of experimentally infected pigs (Woods et al., 1999).

CLINICOPATHOLOGICAL ASSESSMENT OF

IMMUNOSUPPRESSION

Although a majority of BVDV infections in immunocompetent, seronegative cattle are subclinical (Baker, 1995), pyrexia, mild inappetance, and a decrease in milk production can be detected on close examination. In clinical infections in cattle, hematologic abnormalities such as thrombocytopenia and leukopenia are frequently reported (Archambault et al., 2000; Bolin et al., 1985; Ellis et al., 1998; Kelling et al., 2002a, 2002b: Walz et al., 2001). Decreases in white blood cells in the peripheral circulation (quantitative disorder or leukopenia) as well as alterations in function of these cells (qualitative disorder) in the peripheral circulation or in tissues are the basis for BVDV-induced immunosuppression. Although both qualitative and quantitative white blood cell disorders may be observed in acute infections, only qualitative disorders are observed in PI cattle (Brown et al., 1991; Muscoplat et al., 1973; Roth et al., 1986).

A transient leukopenia occurs in most cattle acutely infected with BVDV. The very first description of BVDV by Olafson et al. (1946) demonstrated leukopenia as a finding in both naturally and experimentally infected cattle. White blood cell concentrations decrease to various degrees during BVDV infection. White blood cell count for healthy cattle is 4,000–12,000 leukocytes/µl with a mean of 8,000 leukocytes/µl (Kramer, 2000), and the leukocyte concentration is higher in neonatal calves (Tennant et al., 1974). By definition, leukopenia is a decrease in white blood cell concentration below the normal level. Early reports have identified mild reductions in white blood cell concentration in calves with minimal evidence of clinical disease. There seems to be no difference between cytopathic and noncytopathic biotypes as far as production of leukopenia is concerned. After intravenous challenge with a cytopathic BVDV, the mean white blood cell concentration decreased from 7,850 leukocytes/µl to 5,050 leukocytes/µl at 4 days after infection (Bolin et al., 1985). In an experimental infection with a noncytopathic isolate of BVDV, a mild leukopenia was observed on days 3, 5, and 7 after infection (Ellis et al., 1988) with leukocyte concentrations of <5,000 leukocytes/µl. The typical time frame for depression in white blood cell concentrations is between 3 and 12 days after infection (Bolin and Ridpath, 1992; Ellis et al., 1998; Kelling et al., 2002a, 2002b; Walz et al., 1999).

Differences in the degree of leukopenia are associated with the virulence of the virus. Although severe acute disease has been associated only with type 2 BVDV, genotype is not a determinant for virulence. Though some type 2 strains cause severe acute disease, infection with other BVDV 2 strains results in subclinical to mild disease under natural and experimental conditions (Marshall et al., 1996; Walz et al., 2001; Liebler-Tenorio et al., 2003). Kelling et al. (2002b) evaluated five different isolates of BVDV 2 and found that although all five induced leukopenia, the highly virulent isolates induced a significantly more severe lymphopenia than the less virulent isolates. Further study revealed that virulence correlated with depression in lymphocyte counts. In an experimental infection, a highly virulent strain of BVDV 2–induced depression in lymphocyte counts of greater than 80% from preinoculation levels; a less virulent BVDV strain induced depressions of less than 50% (Liebler-Tenorio et al., 2003).

Not only are absolute counts affected by virus virulence, but duration of leukocyte depressions also correlates with virulence (Liebler-Tenorio et al., 2003; Walz et al., 2001). In a study involving two type 2 isolates and one type 1 isolate, the more virulent type 2 isolate induced a more severe leukopenia of a longer duration than the less virulent type 2 isolate and the type 1 isolate (Walz et al., 2001). Differences have also been reported in the type of affected leukocytes following experimental challenge. For example, neutropenia was observed as the major hematologic abnormality with some BVDV isolates (Walz et al., 2001; Hamers et al., 2000; Archambault et al., 2000), and lymphopenia was observed with some others (Bolin et al., 1985; Kelling et al., 2002b). In yet other studies, reduction in both lymphocytes and neutrophils has been documented (Ellis et al., 1988, 1998).

Unlike acutely infected animals, the PI calves have normal numbers of leukocytes and lymphocytes (Bolin et al., 1985; Larsson et al., 1988) but the proportion of lymphocyte subpopulation might change. For example, PI calves have an increased proportion of B-cells and diminished numbers of lymphocytes not identified as B-cells or T-cells (null cells) (Larsson et al., 1988). The mechanism of BVDV-induced leukopenia is currently unknown. Several possibilities exist, including immune system removal of BVDV-infected immune cells, destruction of immune cells by BVDV, and increased trafficking of immune cells into tissue sites of viral replication.

BVDV-INDUCED IMMUNE ORGAN

DYSFUNCTION

Specific changes in immune system function occur with acute BVDV infection. As described above, all types of immune cells are infected by BVDV and their functions affected.

Effect of BVDV on bone marrow

Viral antigen is present in megakaryocytes and myeloid cells of BVDV-infected cattle (Spagnuolo et al., 1997). Bone marrow–derived macrophages can be infected in vitro with BVDV, and after infection, they release type I interferon (Adler et al., 1997). It has been suggested that type I interferon might prime the oral cavity and gastrointestinal tract to decrease NO production and to induce apoptosis in response to LPS (Alder et al., 1997). This biochemical pathway may be the basis of mucosal disease caused by BVDV. In acute BVDV infection, the development of lesions in lymphoid tissue is a function of BVDV replication and host reaction to infection (Liebler-Tenorio et al., 2002).

Effect of BVDV on thymus

Thymus plays an important role in the maturation of CD4+ and CD8+ lymphocytes. Thymus is a central lymphoid organ and all lymphocytes that enter thymus get selected based on molecules expressed on their surface. Lymphocytes that strongly recognize self-antigens are negatively selected in the thymus. Only 1–2% of lymphocytes that enter the thymus mature. The remaining lymphocytes undergo clonal deletion during the selection process. It is from these selected cells that BVDV-specific T-cells are generated. Infection with BVDV significantly decreases thymocyte function (Marshall et al., 1994). Like all lymphoid tissues, there are lesions (such as depletion of lymphocytes) but viral antigen is not detected except in vascular walls only (Liebler-Tenorio et al., 2002).

Effect of BVDV on Peyer’s patches

BVDV significantly alters T-cells in Peyer’s patches. After BVDV infection, there is significant depletion of lymphocytes in Peyer’s patches, and cattle in terminal stages of mucosal disease have extensive loss of lymphocytes in the gut-associated lymphoid tissues (Figure 9.1). The tips of the domes are depleted of lymphocytes and the epithelia of the follicles are invaginated. The number of CD4+ cells decreases in the follicles and the number of lymphocytes in interfollicular areas is also reduced (Liebler et al., 1995). B-lymphocytes are also depleted in lymphoid folli-

cles leading to decreased size of the follicles (Liebler et al., 1995).

Effect of BVDV on spleen

Small arterioles in the spleen are surrounded by periarteriolar lymphoid sheaths that contain CD4+ and CD8+ T-cells. Masses of B-cells called lymphoid follicles are attached to T-cell sheaths. Both cytopathic and non cytopathic biotypes of BVDV infect spleen cells and the virus has been isolated from spleen of cattle that die from BVDV (McClurkin et al., 1985 and Bolin et al., 1987). In the initial phase of BVDV infection, virus is present but no lesions occur. However, lesions develop later in infection and antigen disappears in about 2 weeks.

IMMUNOLOGICALLY PRIVILEGED SITES

In immunologically privileged sites, the immune response is inaccessible or suppressed. In these organs, the immune response is contained to prevent inflammation that could be destructive to the virusinfected tissues.

TESTES

BVDV infection of breeding bulls leads to a decrease in semen quality, although the virus may or may not be detectable in the seminal ejaculate (Fray et al., 2000). In one study, BVDV was isolated from raw, unprocessed, and diluted extended semen (Kirkland et al., 1991) with titers ranging from 5–75 TCID50/ml. Testes can be a site of persistent BVDV infection in non-viremic bulls (Niskanen et al., 2002; Givens et al., 2002). In calves infected with BVDV, seminiferous tubules are lined with sertoli cells and intratubular giant cells (Binkhorst et al., 1983). Studies done in other species (such as humans) indicate that testicular and epidymal lymphocytes express T-cell markers (Yakirevich et al., 2002). Studies to investigate immune responses to BVDV in testes of cattle are sorely needed.

OVARIES

Cattle persistently infected with BVDV contain viral RNA and antigen in ovarian tissues (Booth et al., 1995). In cattle acutely infected with ncp BVDV, the levels of gonadotrophins and sex strands are modulated (Fray et al., 2002). Bovine follicular cells and oocytes are permissive to BVDV infection, and acute infection decreases estradiol secretion (Fray et al., 2000). The replication of BVDV in ovaries causes ovarian dysfunction and reduced fertility (Shin and Aclaud, 2001).