Platelet aggregation responses and virus isolation from platelets in calves experimentally infected with type I or type II bovine viral diarrhea virus.

Altered platelet function has been reported in calves experimentally infected with type II bovine viral diarrhea virus (BVDV). The purpose of the present study was to further evaluate the ability of BVDV isolates to alter platelet function and to examine for the presence of a virus-platelet interaction during BVDV infection. Colostrum-deprived Holstein calves were obtained immediately after birth, housed in isolation, and assigned to 1 of 4 groups (1 control and 3 treatment groups). Control calves (n = 4) were sham inoculated, while calves in the infected groups (n = 4 for each group) were inoculated by intranasal instillation with 10(7) TCID50 of either BVDV 890 (type II), BVDV 7937 (type II), or BVDV TGAN (type I). Whole blood was collected prior to inoculation (day 0) and on days 4, 6, 8, 10, and 12 after inoculation for platelet function testing by optical aggregometry by using adenosine diphosphate and platelet activating factor. The maximum percentage aggregation and the slope of the aggregation curve decreased over time in BVDV-infected calves; however, statistically significant differences (Freidman repeated measures ANOVA on ranks, P < 0.05) were only observed in calves infected with the type II BVDV isolates. Bovine viral diarrhea virus was not isolated from control calves, but was isolated from all calves infected with both type II BVDV isolates from days 4 through 12 after inoculation. In calves infected with type I BVDV, virus was isolated from 1 of 4 calves on days 4 and 12 after inoculation and from all calves on days 6 and 8 after inoculation. Altered platelet function was observed in calves infected with both type II BVDV isolates, but was not observed in calves infected with type I BVDV. Altered platelet function may be important as a difference in virulence between type I and type II BVDV infection.     

Effect of experimentally induced type II bovine viral diarrhea virus infection on platelet function in calves

OBJECTIVE: To evaluate platelet aggregation responses in calves experimentally infected with a thrombocytopenia-inducing type II bovine viral diarrhea virus (BVDV) isolate (BVDV 890). ANIMALS: 9 neonatal male Holstein calves. PROCEDURE: 5 calves were inoculated with BVDV 890, and 4 were used as controls. Platelet aggregation studies and attempts to isolate BVDV from platelets were performed 2 days before, the day of, and every 2 days for 12 days after inoculation. Platelet function was assessed by means of optical aggregometry, using adenosine diphosphate and platelet-activating factor as agonists. Bovine viral diarrhea virus was isolated from purified platelet preparations by use of an immunoperoxidase monolayer assay. RESULTS: Maximum percentage aggregation and slope of the aggregation curve decreased over time in calves infected with BVDV. Bovine viral diarrhea virus was not isolated from platelets from control calves, but it was isolated from infected calves from 4 through 12 days after inoculation. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that platelet function may be depressed in calves infected with type II BVDV. Although the mechanism for altered platelet function was not determined, it likely involved an increase in the percentage of aged platelets in the circulation, a direct virus-platelet interaction, or an indirect virus-platelet interaction. Platelet dysfunction, in addition to thrombocytopenia, may contribute to the hemorrhagic syndrome associated with acute type II BVDV infection in calves.     

Relationship between degree of viremia and disease manifestation in calves with experimentally induced bovine viral diarrhea virus infection.

OBJECTIVE: To compare degree of viremia and disease manifestations in calves with type-I and -II bovine viral diarrhea virus (BVDV) infection. ANIMALS: 16 calves. PROCEDURE: Colostrum-deprived calves obtained immediately after birth were assigned to 1 control and 3 treatment groups (4 calves/group). Calves in treatment groups were inoculated (day 0) by intranasal instillation of 10(7) median tissue culture infective dose BVDV 890 (type II), BVDV 7937 (type II), or BVDV TGAN (type I). Blood cell counts and virus isolation from serum and leukocytes were performed daily, whereas degree of viremia was determined immediately before and 4, 6, 8, and 12 days after inoculation. Calves were euthanatized on day 12, and pathologic, virologic, and immunohistochemical examinations were performed. RESULTS: Type-II BVDV 890 induced the highest degree of viremia, and type-I BVDV TGAN induced the lowest. Virus was isolated more frequently and for a longer duration in calves inoculated with BVDV 890. A parallel relationship between degree of viremia and rectal temperature and an inverse relationship between degree of viremia and blood cell counts was observed. Pathologic and immunohistochemical examinations revealed more pronounced lesions and more extensive distribution of viral antigen in calves inoculated with type-II BVDV. CONCLUSIONS AND CLINICAL RELEVANCE: Degree of viremia induced during BVDV infection is associated with severity of clinical disease. Isolates of BVDV that induce a high degree of viremia may be more capable of inducing clinical signs of disease. Strategies (eg, vaccination) that reduce viremia may control clinical signs of acute infection with BVDV.     

Thrombocytopenia and hemorrhages in veal calves infected with bovine viral diarrhea virus

The relationship between bovine viral diarrhea virus (BVDV) infection and thrombocytopenia was studied in 18 veal calves experimentally infected with BVDV. All calves were free of BVDV, and 13 calves were free of serum neutralizing antibodies to BVDV before virus inoculation. Calves were inoculated at approximately 10 days of age, and platelet counts were monitored over a period of several weeks. Ten additional calves housed in close proximity were kept as uninoculated controls. A profound decrease in platelet counts by 3 to 11 days after inoculation was seen in all calves that had neutralizing antibody titers less than 1:32 before infection. Severe thrombocytopenia (less than 5,000 platelets/microliter) was seen in 12 calves, 11 of which also developed hemorrhages. Necropsy findings in 3 severely thrombocytopenic calves that died included multiple hemorrhages throughout the body. Calves that recovered had increased platelet counts, and in most instances, a corresponding increase in neutralizing antibody titers to BVDV. At 11 days after inoculation, BVDV was detected on platelets by use of immunofluorescence, but evidence of surface-bound immunoglobulin was not found. The results suggest that a nonimmunoglobulin-mediated method of platelet destruction or sequestration develops as a sequela to BVDV infection.     

Characterization of protection against systemic infection and disease from experimental bovine viral diarrhea virus type 2 infection by use of a modified-live noncytopathic type 1 vaccine in calves

OBJECTIVE: To evaluate protection resulting from use of a modified-live noncytopathic bovine viral diarrhea virus (BVDV) type 1 vaccine against systemic infection and clinical disease in calves challenged with type 2 BVDV. ANIMALS: 10 calves, 5 to 7 months of age. PROCEDURES: Calves were allocated (n = 5/group) to be nonvaccinated or vaccinated SC on day 0 with BVDV 1 (WRL strain). Calves in both groups were challenged intranasally with BVDV type 2 isolate 890 on day 21. Rectal temperatures and clinical signs of disease were recorded daily, and total and differential WBC and platelet counts were performed. Histologic examinations and immunohistochemical analyses to detect lesions and distribution of viral antigens, respectively, were performed. RESULTS: After challenge exposure to BVDV type 2, nonvaccinated calves developed high rectal temperatures, increased respiratory rates, viremia, leukopenia, lymphopenia, and infection of the thymus. Vaccinated calves did not develop high rectal temperatures or clinical signs of respiratory tract disease. Vaccinated calves appeared to be protected against systemic replication of virus in that they did not develop leukopenia, lymphopenia, viremia, or infection of target organs, and infectious virus was not detected in peripheral blood mononuclear cells or the thymus. CONCLUSIONS AND CLINICAL RELEVANCE: The modified-live BVDV type 1 vaccine protected against systemic infection and disease after experimental challenge exposure with BVDV type 2. The vaccine protected calves against infection and viremia and prevented infection of target lymphoid cells.     

Comparative virulence of isolates of bovine viral diarrhea virus type II in experimentally inoculated six- to nine-month-old calves

OBJECTIVE: To determine the comparative virulence of 5 isolates of bovine viral diarrhea virus (BVDV) type II by inoculating 6- to 9-month-old beef calves with isolates originating from the tissues of cattle affected with naturally occurring, transient, acute, nonfatal infections or naturally occurring, peracute, fatal infections. ANIMALS: 22 calves that were 6 to 9 months old. PROCEDURE: The study used BVDV isolates 17011, 713, and 5521 that originated from fetuses aborted from cows with transient, nonfatal, acute BVDV infections and isolates 23025 and 17583 that originated from the tissues of cattle with peracute, fatal BVDV infections. Calves were allotted to 6 groups (1, mock-infected control calves [n = 2]; 2, inoculated with BVDV 17011 [4]; 3, inoculated with BVDV 713 [4]; 4, inoculated with BVDV 5521 [4]; 5, inoculated with BVDV 23025 [4]; and 6, inoculated with BVDV 17583 [41]. Rectal temperatures and clinical signs of disease were recorded daily. Total and differential WBC and platelet counts were performed. Histologic examination and immunohistochemical analysis were conducted to detect lesions and distribution of viral antigens, respectively. RESULTS: Calves inoculated with BVDV 23025 or 17583 developed more severe clinical signs of disease (fever and diarrhea), more severe lymphopenia, and more severe lesions (alimentary epithelial necrosis, lymphoid depletion, and BVDV antigen deposition in lymphatic tissues), compared with calves inoculated with BVDV 713, 5521, or 17011. CONCLUSIONS AND CLINICAL RELEVANCE: Relative severity of experimentally induced infections corresponded to severity of clinical signs of naturally occurring infections with respective BVDV isolates.     

Effects of modified-live bovine viral diarrhea virus vaccines containing either type 1 or types 1 and 2 BVDV on heifers and their offspring after challenge with noncytopathic type 2 BVDV during gestation

OBJECTIVE: To compare the efficacy of modified-live virus (MLV) vaccines containing either type 1 bovine viral diarrhea virus (BVDV) or types 1 and 2 BVDV in protecting heifers and their offspring against infection associated with heterologous noncytopathic type 2 BVDV challenge during gestation. DESIGN: Randomized controlled study. ANIMALS: 160 heifers and their offspring. PROCEDURES: After inoculation with a placebo vaccine, 1 or 2 doses of an MLV vaccine containing type 1 BVDV, or 1 dose of an MLV vaccine containing both types 1 and 2 BVDV, heifers were bred naturally and challenge exposed with a type 2 BVDV field isolate between 62 and 104 days of gestation. Pregnancies were monitored; after parturition, virus isolation and immunohistochemical analyses of ear-notch specimens were used to determine whether calves were persistently infected. Blood samples were collected at intervals from heifers for serologic evaluation and virus isolation. RESULTS: Persistent infection was detected in 18 of 19 calves from heifers in the control group and in 6 of 18 calves and 7 of 19 calves from heifers that received 1 or 2 doses of the type 1 BVDV vaccine, respectively. None of the 18 calves from heifers that received the type 1-type 2 BVDV vaccine were persistently infected. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that the incidence of persistent BVDV infection among offspring from dams inoculated with 1 dose of the MLV vaccine containing types 1 and 2 BVDV was decreased, compared with 1 or 2 doses of the MLV vaccine containing only type 1 BVDV.     

Characterization of protection from systemic infection and disease by use of a modified-live noncytopathic bovine viral diarrhea virus type 1 vaccine in experimentally infected calves

OBJECTIVE: To evaluate protection against systemic infection and clinical disease provided by use of a modified-live noncytopathic bovine viral diarrhea virus (BVDV) type 1 vaccine in calves challenged with NY-1 BVDV. ANIMALS: 10 calves, 5 to 7 months of age. PROCEDURES: Calves were allocated (n = 5/group) to be nonvaccinated or vaccinated SC on day 0 with BVDV type 1 (WRL strain). Calves in both groups were challenged intranasally with NY-1 BVDV on day 21. Calves' rectal temperatures and clinical signs of disease were recorded daily, total and differential WBC and platelet counts were performed, and serum neutralizing antibody titers against NY-1 BVDV were determined. Histologic examinations and immunohistochemical analyses to detect gross lesions and distribution of viral antigens, respectively, were performed. RESULTS: After challenge exposure to NY-1 BVDV, nonvaccinated calves developed high rectal temperatures, increased respiratory rates, viremia, leukopenia, lymphopenia, and infection of the thymus. Vaccinated calves did not develop high rectal temperatures or clinical signs of respiratory tract disease. Vaccinated calves appeared to be protected against systemic replication of virus in that they did not develop leukopenia, lymphopenia, viremia, or infection of target organs, and infectious virus was not detected in peripheral blood mononuclear cells or the thymus. CONCLUSIONS AND CLINICAL RELEVANCE: The modified-live BVDV vaccine protected calves against systemic infection and disease after experimental challenge exposure with NY-1 BVDV. The vaccine protected calves against infection and viremia and prevented infection of target lymphoid cells.     

Effect of infection with bovine viral diarrhea virus alone,bovine rotavirus alone,or concurrent infection with both on enteric disease in gnotobiotic neonatal calves

OBJECTIVE: To compare experimentally induced concurrent infection with bovine viral diarrhea virus (BVDV) and bovine rotavirus (BRV) with infection of either virus alone in calves. ANIMALS: Seventeen 1-day-old gnotobiotic calves. PROCEDURE: Calves were allotted to 8 treatments as follows: group 1, mock-infected control calves (n = 2); group 2, inoculated with BVDV on day 1 (2); groups 3, 5, and 7, inoculated with BRV on days 1 (2), 4 (1), or 7 (2), respectively; and groups 4, 6, and 8, inoculated with BVDV on day 1 and with BRV on days 1 (2), 4 (2), or 7 (4), respectively. Concentrations of BVDV in serum and ileal tissues were measured, and BRV shedding in feces was determined. Histologic examination and immunohistochemical analysis were conducted to detect lesions and viral antigens. RESULTS: Neonatal calves inoculated with BVDV alone or with BVDV on day 1 and BRV on day 7 developed villus atrophy and submucosal inflammation of the intestines. Concurrent BVDV and BRV infections acted synergistically in the intestinal tract, causing more severe enteric disease than infection with either virus alone. Severe lymphoid depletion was associated with BVDV infection in calves regardlesss of concurrent BRV infection. CONCLUSIONS AND CLINICAL RELEVANCE: Infection with BVDV played direct and indirect roles in enteritis in neonatal calves, causing villus atrophy in the duodenum and submucosal inflammation of the intestines. Also, BVDV potentiated effects of BRV. Concurrent infection with BVDV and BRV resulted in more severe enteric disease in neonatal calves than infection with BRV or BVDV alone.     

Efficacy of bovine viral diarrhea vaccine used in Japan against bovine viral diarrhea virus type 2 strain 890

Bovine viral diarrhea virus (BVDV) has been segregated into two genotypes, type 1 and type 2. To determine the efficacy of the commercially available bovine viral diarrhea type 1 vaccine used in Japan against BVDV type 2, calves were infected with BVDV type 2 strain 890 4 weeks after administration of the vaccine. The vaccinated calves did not develop any clinical signs and hematological changes such as observed in unvaccinated calves after the challenge. Furthermore, the challenge virus was not recovered from the vaccinated calves throughout the duration of the experiment, whereas it was recovered from all unvaccinated calves. The bovine viral diarrhea vaccine used in Japan is efficacious against infection with BVDV type 2 strain 890.     

Comparison of type I and type II bovine viral diarrhea virus infection in swine.

Some isolates of type II bovine viral diarrhea virus (BVDV) are capable of causing severe clinical disease in cattle. Bovine viral diarrhea virus infection has been reported in pigs, but the ability of these more virulent isolates of type II BVDV to induce severe clinical disease in pigs is unknown. It was our objective to compare clinical, virologic, and pathologic findings between type I and type II BVDV infection in pigs. Noninfected control and BVDV-infected 2-month-old pigs were used. A noncytopathic type I and a noncytopathic type II BVDV isolate were chosen for evaluation in feeder age swine based upon preliminary in vitro and in vivo experiments. A dose titration study was performed using 4 groups of 4 pigs for each viral isolate. The groups were inoculated intranasally with either sham (control), 10(3), 10(5), or 10(7) TCID50 of virus. The pigs were examined daily and clinical findings were recorded. Antemortem and postmortem samples were collected for virus isolation. Neither the type I nor type II BVDV isolates resulted in clinical signs of disease in pigs. Bovine viral diarrhea virus was isolated from antemortem and postmortem samples from groups of pigs receiving the 10(5) and the 10(7) TCID50 dose of the type I BVDV isolate. In contrast, BVDV was only isolated from postmortem samples in the group of pigs receiving the 10(7) TCID50 dose of the type II BVDV isolate. Type I BVDV was able to establish infection in pigs at lower doses by intranasal instillation than type II BVDV. Infection of pigs with a type II isolate of BVDV known to cause severe disease in calves did not result in clinically apparent disease in pigs.     

Evaluation of the efficacy of a modified-live combination vaccine against bovine viral diarrhea virus types 1 and 2 challenge exposures in a one-year duration-of-immunity fetal protection study

This study demonstrated that the modified-live bovine viral diarrhea virus (BVDV) type 1 and 2 fractions of a multivalent vaccine protected pregnant heifers and their fetuses against virulent BVDV types 1 and 2 challenge exposures at 370 days after vaccination. All BVDV vaccinated heifers inoculated with either BVDV type 1 or 2 at approximately 62 to 94 days of gestation delivered fetuses or calves that were negative for BVDV by ear-notch immunohistochemistry and virus isolation and serum neutralization on a prenursing serum sample. In comparison, eight of nine and 10 of 10 fetuses or calves from non-BVDV-vaccinated heifers were considered persistently infected following exposure to BVDV type 1 and type 2, respectively.     

Clinical response and immunomodulation following experimental challenge of calves with type 2 noncytopathogenic bovine viral diarrhea virus

Eight calves between 16 and 18 weeks of age that were seronegative to bovine viral diarrhea virus (BVDV), bovine leucosis virus and bovine immunodeficiency-like virus were infected (day 0) intranasally with the type 2 noncytopathogenic Canadian 24515 field isolate of BVDV in order to evaluate the effect of BVDV infection on certain clinical, hematological and immunological parameters. All virus-exposed animals developed fever and showed a significant (P < 0.05, 0.01 or 0.001) drop in the number of circulating leucocytes (neutrophils, lymphocytes and monocytes) by day 3 or 5 post-exposure (PE), which continued to the end of the experiment at day 12 PE. BVDV was consistently isolated from the peripheral blood buffy coat cells from day 5 PE, and also from selected tissues (spleen, thymus, mesenteric and submaxillary lymph nodes, small intestine, lungs and thyroid gland) that were collected at the time of euthanasia of the animals at day 12 PE. Diminished significant (P < 0.05) percentages of peripheral blood mononuclear cells (PBMCs) expressing at their surface either B7 and MHC II molecules were observed in virus-exposed calves at days 7, 10 and/or 12 PE, when compared to virus-nonexposed control calves (n = 5). However, no changes in the percentages of PBMCs expressing either B4 or MHC I molecules were observed throughout the experiment. Finally, a significant (P < 0.05 or 0.01) enhanced phagocytic capability of the PBMCs, as analyzed by flow cytometry, was observed in virus-exposed animals at days 3, 5, 7, 10 and 12 PE, when compared to control calves. These results demonstrated the virulence of the 24515 isolate of BVDV in 4 to 4.5 month-old calves, and suggest that type 2 BVDV infection in calves is associated with dysregulation of certain immunological functions.     

Lesions and distribution of viral antigen following an experimental infection of young seronegative calves with virulent bovine virus diarrhea virus-type II.

During the past several years, acute infections with bovine viral diarrhea virus (BVDV) have been causally linked to hemorrhagic and acute mucosal disease-like syndromes with high mortality. The majority of BVDVs isolated in such cases have been classified as type II on the basis of genetic and antigenic characteristics. It was our objective to examine clinical disease, lesions and potential sites of viral replication, following experimental BVDV type II infection in young calves. On approximately day 35 after birth, calves that had received BVDV-antibody-negative colostrum were infected by intranasal inoculation of 5 x 10(5) TCID50 of BVDV type II isolate 24,515 in 5 mL of tissue culture fluid (2.5 mL/nostril). Calves were monitored twice daily for signs of clinical disease. Approximately 48-72 h after infection, all calves developed transient pyrexia (39.4-40.5 degrees C) and leukopenia. Beginning on approximately day 7 after infection, all calves developed watery diarrhea, pyrexia (40.5-41.6 degrees C), marked leukopenia (> or = 75% drop from preinoculation values), variable thrombocytopenia, and moderate to severe depression. Calves were euthanized on days 10, 11, or 12 after infection due to severe disease. Gross and histological lesions consisted of multifocal bronchointerstitial pneumonia (involving 10%-25% of affected lungs), bone marrow hypoplasia and necrosis, and minimal erosive lesions in the alimentary tract. Immunohistochemical staining for BVDV revealed widespread viral antigen usually within epithelial cells, smooth muscle cells and mononuclear phagocytes in multiple organs, including lung, Peyer's patches, gastric mucosa, thymus, adrenal gland, spleen, lymph nodes, bone marrow, and skin. This BVDV type II isolate caused rapidly progressive, severe multisystemic disease in seronegative calves that was associated with widespread distribution of viral antigen and few gross or histological inflammatory lesions.     

Fetal protection following exposure to calves persistently infected with bovine viral diarrhea virus type 2 sixteen months after primary vaccination of the dams

This study demonstrated that the bovine viral diarrhea virus (BVDV; types 1 and 2) fractions of a multivalent vaccine protected pregnant heifers and their fetuses at 149 to 217 days of gestation against exposure to calves persistently infected with BVDV type 2a. Eighty percent (eight of 10) of the control heifers were viremic at least 1 day following challenge, whereas all (20 of 20) BVDV-vaccinated heifers were virus isolation-negative on all postchallenge assessment days. Ninety percent (nine of 10) of the calves born to control heifers but only 5% (one of 20) of calves born to BVDV-vaccinated heifers seroconverted to BVDV type 2 before ingesting colostrum. One calf born to a control heifer was persistently infected. No calves from BVDV-vaccinated heifers were persistently infected.     

Failure of foetal protection after vaccination against an experimental infection with bovine virus diarrhea virus

A protocol is described to measure the protection of the bovine fetus against an experimental bovine virus diarrhea virus (BVDV) infection after vaccination. Two inactivated experimental vaccines were applied twice with a 3 week interval. A mixture of three different Dutch field strains was used as challenge on mainly the 82nd day of gestation to vaccinated and unvaccinated control animals. The challenge was applied 5 months after completion of the two-fold vaccinations. All calves born from unvaccinated control animals were persistently infected. The calves born from dams vaccinated with the two different inactivated BVDV vaccines were persistently infected in 78 and 60%, respectively.     

Effect on hematopoietic tissue of experimental infection of calves with noncytopathic type 2 bovine viral diarrhea virus

To investigate the hematologic abnormalities observed with noncytopathic type 2 bovine viral diarrhea virus (ncpBVDV-2), calves 6 to 8 mo old were inoculated with an isolate of either high virulence (HV24515) or low virulence (LV11Q); control animals received the same volume of uninfected cell-culture supernatant. Peripheral blood neutrophil, lymphocyte, and platelet counts decreased in all the virus-inoculated calves but were significantly lower and remained decreased longer in the calves given HV24515. For each isolate, a decrease in the number of mature myeloid cells in the bone marrow coincided with the development of neutropenia, but the depletion persisted significantly longer (4 to 6 d) in the calves given HV24515. In the bone marrow of calves given LV11Q, the number of proliferating myeloid cells increased in proportion to the decrease in the number of mature myeloid cells. In the calves inoculated with HV24515, BVDV antigen was observed in bone marrow cells when the peripheral blood counts were lowest. Megakaryocytes were the predominant cell type exhibiting positive BVDV staining; myeloid cells rarely stained positively. Viral antigen was not observed in the bone marrow of calves given LV11Q. These experiments demonstrated that ncpBVDV-2 isolates of both high and low virulence caused decreased leukocyte and platelet counts, but only the high-virulence HV24515 isolate caused a delay in the production of myeloid proliferating cells. The delay may contribute to the ability of certain ncpBVDV-2 isolates to induce severe disease.     

Failure to spread bovine virus diarrhoea virus infection from primarily infected calves despite concurrent infection with bovine coronavirus

Previous reports on the spread of bovine virus diarrhoea virus (BVDV) from animals primarily infected with the agent are contradictory. In this study, the possibility of transmission of BVDV from calves simultaneously subjected to acute BVDV and bovine coronavirus (BCV) infection was investigated. Ten calves were inoculated intranasally with BVDV Type 1. Each of the 10 calves was then randomly allocated to one of two groups. In each group there were four additional calves, resulting in five infected and four susceptible calves per group. Virulent BCV was actively introduced in one of the groups by means of a transmitter calf. Two calves, susceptible to both BVDV and BCV, were kept in a separate group, as controls. All ten calves actively inoculated with BVDV became infected as shown by seroconversions, and six of them also shed the virus in nasal secretions. However, none of the other eight calves in the two groups (four in each) seroconverted to this agent. In contrast, it proved impossible to prevent the spread of BCV infection between the experimental groups and consequently all 20 study calves became infected with the virus. Following infection, BCV was detected in nasal secretions and in faeces of the calves and, after three weeks in the study, all had seroconverted to this virus. All calves, including the controls, showed at least one of the following clinical signs during days 3-15 after the trial started: fever (> or =40 degrees C), depressed general condition, diarrhoea, and cough. The study showed that BVDV primarily infected cattle, even when co-infected with an enteric and respiratory pathogen, are inefficient transmitters of BVDV. This finding supports the principle of the Scandinavian BVDV control programmes that elimination of BVDV infection from cattle populations can be achieved by identifying and removing persistently infected (PI) animals, i.e. that long-term circulation of the virus without the presence of PI animals is highly unlikely.     

Evaluation of antiviral activity and toxicity of recombinant human interferon alfa-2a in calves persistently infected with type 1 bovine viral diarrhea virus

OBJECTIVE: To evaluate antiviral activity and toxicity of recombinant human interferon alfa-2a in calves persistently infected with noncytopathic type 1 bovine viral diarrhea virus (BVDV). ANIMALS: 5 Holstein heifers, 4 to 12 months of age. PROCEDURES: Calves persistently infected with noncytopathic type 1 BVDV were treated with recombinant human interferon alfa-2a every other day for 12 weeks. Viral loads were measured during the treatment period and compared with pre- and post-treatment values. Complete physical examinations were performed weekly, and calves were observed daily for signs of systemic illness. Complete blood counts and serum biochemical analyses were performed before, during, and after the treatment period. Because calves developed anemia during the treatment period, bone marrow biopsy specimens were collected. Antirecombinant human interferon alfa-2a antibody concentrations in serum samples obtained before, during, and after the treatment period were measured by use of an ELISA. RESULTS: Recombinant human interferon alfa-2a had no antiviral activity against noncytopathic type 1 BVDV in persistently infected calves. All calves developed microcytic anemia during the treatment period that persisted for up to 13 weeks after cessation of treatment. Anti-interferon antibodies were detected during the treatment period and persisted for at least 2 weeks after cessation of treatment. CONCLUSIONS AND CLINICAL RELEVANCE: Because of lack of in vivo antiviral activity against BVDV, recombinant human interferon alfa-2a has little promise as a therapeutic agent for the treatment of BVDV infection, at least in persistently infected cattle. Furthermore, treatment was associated with adverse immunologic and hematologic effects.