Distribution of viral antigen and development of lesions after experimental infection with highly virulent bovine viral diarrhea virus type 2 in calves

OBJECTIVE: To correlate tissue distribution with development of lesions after experimental infection with a virulent strain of noncytopathic bovine viral diarrhea virus (BVDV) type 2 in calves. ANIMALS: Ten 14-day-old and two 2-month-old colostrum-deprived calves. PROCEDURE: Calves were intranasally inoculated with BVDV type-2 strain 1373 from an outbreak of clinically severe bovine viral diarrhea (BVD).Two 14-day-old calves served as noninfected controls. Two calves each were euthanatized on postinoculation days 3, 6, and 12, and 1 each on days 8, 9, 13, and 14. Tissues were collected for immunohistologic and histologic examination. RESULTS: Inoculated calves developed nonspecific clinical signs characterized by high fever and decreased numbers of leukocytes and thrombocytes. Viral antigen was detected focally in lymphoid tissues on day 3. On days 6, 8, 9, 12, and 14, viral antigen became increasingly widespread throughout organs and tissues. Viral antigen in lymphoid tissues was associated with severe depletion of all compartments. Lesions in other tissues were not well correlated with distribution of viral antigen. Depletion of lymphoid tissues was observed in a calf on day 13, but viral antigen had been cleared from most tissues and was detected in vascular walls only. CONCLUSIONS AND CLINICAL RELEVANCE: Infection with a virulent BVDV strain resulted in wide dissemination of viral antigen in host tissues. Severe lymphoid depletion developed in lymphoid tissues, whereas viral antigen was generally not associated with lesions in other tissues. Findings suggest that development of lesions in acute BVD is not solely a function of viral replication and is also attributable to host reaction to infection.     

Distribution of viral antigen and development of lesions after experimental infection of calves with a BVDV 2 strain of low virulence.

To examine the virus-host interaction in subclinical bovine viral diarrhea virus (BVDV) infections, the spread of a BVDV 2 strain of low virulence to different organs and the development of lesions were investigated. Eight colostrum-deprived, clinically healthy, 2-3-month-old calves were intranasally inoculated with 10(6) tissue culture infective dose of the naturally occurring BVDV 2 strain 28508-5 of low virulence, and 2 served as controls. Two calves each were euthanized at days 3, 6, 9, and 13 postinoculation (pi). Representative tissues were processed for histology and immunohistology. Signs of overt clinical disease were absent. However, a mild temperature elevation at days 7 or 8 pi and a moderate decrease of circulating lymphocytes occurred in all inoculated calves. The BVDV antigen was detected at day 3 pi in several lymphoid tissues. At day 6 pi, BVDV antigen was found widespread in lymphoid tissues and multifocally in intestinal epithelial cells but was associated with no or subtle lesions only. At day 9 pi, much less BVDV antigen was detectable, but there was severe depletion of lymphoid tissues. At day 13 pi, BVDV antigen had been cleared from most lymphoid tissues that were at variable phases of depletion and recovery. In conclusion, the BVDV strain of low virulence spread to lymphoid tissues and intestinal epithelial cells but was rapidly eliminated. Transient depletion of lymphoid tissues was followed by recovery.     

Distribution pattern of bovine viral diarrhoea virus type 1 genome in lymphoid tissues of experimentally infected sheep

In this study, cellular localization and the distribution pattern of BVDV genome in lymphoid tissues during the course of experimental acute BVDV-1 infection of sheep was investigated. Tonsils, mesenteric lymph nodes (MLN) and spleen were collected on 3, 6, 9, 12 and 15 days post infection (dpi) from twenty 4-month-old lambs, experimentally inoculated intra-nasally with 5?×?10⁵ TCID₅₀ of a non-cytopathic (ncp) BVDV-1 isolate, Ind-17555. Tissues collected from ten mock-infected lambs served as controls. In situ hybridization (ISH) was carried out in paraformaldehyde fixed paraffin embedded tissue sections using digoxigenin labelled riboprobe targeting 5′-UTR of BVDV-1. BVDV genome was detected at all the intervals from 3 dpi to 15 dpi in the lymphoid tissues with variations between the intervals and also amongst the infected sheep. During the early phase of acute infection, presence of viral genome was more in tonsils than MLN and spleen, whereas the distribution was higher in MLN during later stages. BVDV-1 genome positive cells included lymphocytes, macrophages, plasma cells, reticular cells and sometimes crypt epithelial cells. Genome distribution was frequently observed in the lymphoid follicles of tonsils, MLN and spleen, besides the crypt epithelium in tonsils, paracortex and medullary sinus and cords of MLN. Most abundant and widespread distribution of BVDV-1 genome was observed on 6 dpi while there was a reduction in number and intensity of positive signals by 15 dpi in most of the infected animals. This is the first attempt made to study the localisation of BVDV-1 in lymphoid tissues of acutely infected sheep by in situ hybridization. The results show that the kinetics of BVDV-1 distribution in lymphoid tissues of experimentally infected non-pregnant sheep follows almost a similar pattern to that demonstrated in BVDV infected cattle.     

Viral antigen distribution in the respiratory tract of cattle persistently infected with bovine viral diarrhea virus subtype 2a

Tissues were obtained at necropsy from the nasal vestibule, turbinates, nasopharynx, trachea, tracheobronchial bifurcation, and lung from each of 10 clinically healthy calves persistently infected (PI) with bovine viral diarrhea virus (BVDV) serotype 2a. Tissues from the nasal vestibule were obtained by biopsy from five additional PI calves. Formalin-fixed tissues were processed for immunohistochemistry to localize the distribution of BVDV throughout the respiratory tract. Antigen distribution and intensity were subjectively evaluated. Throughout the respiratory tract, mononuclear leukocytes, vascular smooth muscle, and endoneural and perineural cells had BVDV immunoreactivity (BVDV-IR). Multifocally, squamous and ciliated columnar epithelium throughout the respiratory tract contained weak to moderate BVDV antigen. Viral antigen was not seen in goblet cells. BVDV-IR in mixed tubuloalveolar glands of the nasal cavity was weak to strong in serous secretory cells and ductular epithelium. Chondrocytes of the concha often contained BVDV antigen diffusely. Nasal mucus-secreting and tracheobronchial glands multifocally contained weak viral signal. In all cases, alveolar macrophages had moderate to strong BVDV-IR, whereas BVDV-IR in alveolar epithelial cells was weak to moderate. BVDV was present in interalveolar leukocytes and mesenchymal cells. Results indicate that serous secretions of the nasal cavity, productive viral infection of epithelium, and infected leukocytes in respiratory secretions are likely major sources of infectious BVDV from PI calves. The presence of BVDV antigen in respiratory epithelium is, at least, indirect support for the notion that this virus predisposes PI cattle to secondary microbial infections.     

Challenge with Bovine viral diarrhea virus by exposure to persistently infected calves: protection by vaccination and negative results of antigen testing in nonvaccinated acutely infected calves

Calves persistently infected (PI) with Bovine viral diarrhea virus (BVDV) represent an important source of infection for susceptible cattle. We evaluated vaccine efficacy using calves PI with noncytopathic BVDV2a for the challenge and compared tests to detect BVDV in acutely or transiently infected calves versus PI calves. Vaccination with 2 doses of modified live virus vaccine containing BVDV1a and BVDV2a protected the calves exposed to the PI calves: neither viremia nor nasal shedding occurred. An immunohistochemistry test on formalin-fixed ear notches and an antigen-capture enzyme-linked immunosorbent assay on fresh notches in phosphate-buffered saline did not detect BVDV antigen in any of the acutely or transiently infected calves, whereas both tests had positive results in all the PI calves.     

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.     

Infection of ovine fetal brain cell cultures with cytopathogenic and non-cytopathogenic bovine viral diarrhoea virus.

The in vitro cell tropism of non-cytopathogenic (ncp) and cytopathogenic (cp) bovine viral diarrhoea virus (BVDV) was studied in primary dissociated brain cell cultures derived from ovine fetuses of different gestational ages. The cell types infected were identified by double immunofluorescence using antibodies against BVDV and cell type-specific markers. In cultures infected with ncp BVDV viral antigen was present in neurofilament (NF 200 kDa)-positive neurons, glial fibrillary acidic protein (GFAP)-positive astrocytes and fibronectin-expressing cells. Estimation of the percentages of individual cell types infected with ncp BVDV indicated a tropism for NF 200-positive neurons. In cultures infected with cp BVD virus cytopathic changes were observed beginning at 40 hours post infection. Viral antigen was present in vacuolated NF 200-, GFAP- and fibronectin-positive cells. In comparison with non-infected control cultures a considerable reduction of the number of the different cell types was seen.     

Transmission of Bovine viral diarrhea virus 1b to susceptible and vaccinated calves by exposure to persistently infected calves

Bovine viral diarrhea virus (BVDV) persistently infected (PI) calves represent significant sources of infection to susceptible cattle. The objectives of this study were to determine if PI calves transmitted infection to vaccinated and unvaccinated calves, to determine if BVDV vaccine strains could be differentiated from the PI field strains by subtyping molecular techniques, and if there were different rates of recovery from peripheral blood leukocytes (PBL) versus serums for acutely infected calves. Calves PI with BVDV1b were placed in pens with nonvaccinated and vaccinated calves for 35 d. Peripheral blood leukocytes, serums, and nasal swabs were collected for viral isolation and serology. In addition, transmission of Bovine herpes virus 1 (BHV-1), Parainfluenza-3 virus (PI-3V), and Bovine respiratory syncytial virus (BRSV) was monitored during the 35 d observation period. Bovine viral diarrhea virus subtype 1b was transmitted to both vaccinated and nonvaccinated calves, including BVDV1b seronegative and seropositive calves, after exposure to PI calves. There was evidence of transmission by viral isolation from PBL, nasal swabs, or both, and seroconversions to BVDV1b. For the unvaccinated calves, 83.2% seroconverted to BVDV1b. The high level of transmission by PI calves is illustrated by seroconversion rates of nonvaccinated calves in individual pens: 70% to 100% seroconversion to the BVDV1b. Bovine viral diarrhea virus was isolated from 45 out of 202 calves in this study. These included BVDV1b in ranch and order buyer (OB) calves, plus BVDV strains identified as vaccinal strains that were in modified live virus (MLV) vaccines given to half the OB calves 3 d prior to the study. The BVDV1b isolates in exposed calves were detected between collection days 7 and 21 after exposure to PI calves. Bovine viral diarrhea virus was recovered more frequently from PBL than serum in acutely infected calves. Bovine viral diarrhea virus was also isolated from the lungs of 2 of 7 calves that were dying with pulmonary lesions. Two of the calves dying with pneumonic lesions in the study had been BVDV1b viremic prior to death. Bovine viral diarrhea virus 1b was isolated from both calves that received the killed or MLV vaccines. There were cytopathic (CP) strains isolated from MLV vaccinated calves during the same time frame as the BVDV1b isolations. These viruses were typed by polymerase chain reaction (PCR) and genetic sequencing, and most CP were confirmed as vaccinal origin. A BVDV2 NCP strain was found in only 1 OB calf, on multiple collections, and the calf seroconverted to BVDV2. This virus was not identical to the BVDV2 CP 296 vaccine strain. The use of subtyping is required to differentiate vaccinal strains from the field strains. This study detected 2 different vaccine strains, the BVDV1b in PI calves and infected contact calves, and a heterologous BVDV2 subtype brought in as an acutely infected calf. The MLV vaccination, with BVDV1a and BVDV2 components, administered 3 d prior to exposure to PI calves did not protect 100% against BVDV1b viremias or nasal shedding. There were other agents associated with the bovine respiratory disease signs and lesions in this study including Mannheimia haemolytica, Mycoplasma spp., PI-3V, BRSV, and BHV-1.     

Investigation of bovine viral diarrhea virus infections in a range beef cattle herd

Bovine viral diarrhea virus (BVDV) infections resulting in clinical disease developed in calves, despite vaccination of dams and high maternal BVDV antibody titers in calves. Eight persistently infected (PI) calves born to immunocompetent dams were identified in the herd. Neutralizing BVDV antibody titers of PI calves had decreased greatly by the time the calves were 1 to 2 months old. Antibody titers of PI calves decreased more rapidly than antibody titers of calves that were not PI. Reduced antibody titers in PI calves allowed detection of BVDV in serum specimens of all PI calves by the time they were 8 weeks old. Persistent infection in suspect calves was detectable serologically and was confirmed by virologic examination of serum specimens 4 months after weaning, when the calves were 9 months old. Growth rates were reduced in viremic calves.     

Prevalence of bovine viral diarrhea virus (BVDV) in persistently infected cattle and BVDV subtypes in affected cattle in beef herds in south central United States

The prevalence of bovine viral diarrhea virus (BVDV) in persistently infected (PI) cattle in beef breeding herds was determined using 30 herds with 4530 calves. The samples were collected by ear notches and tested for BVDV antigens using immunohistochemistry (IHC) and antigen capture enzyme-linked immunosorbent assay (ACE). Animals with initial positives on both IHC and ACE were sampled again using both tests and serums were collected for viral propagation and sequencing of a viral genomic region, 5′-untranslated region (5′-UTR) for viral subtyping. Samples were also collected from the dams of PI calves. There were 25 PI calves from 4530 samples (0.55%) and these PI calves were from 5 of the 30 herds (16.7%). Two herds had multiple PI calves and 3 herds had only 1 PI calf. Only 1 of the 25 dams with a PI calf was also PI (4.0%). The subtype of all the PI isolates was BVDV1b. Histories of the ranches indicated 23 out of 30 had herd additions of untested breeding females. Twenty-four of the 30 herds had adult cowherd vaccinations against BVDV, primarily using killed BVDV vaccines at pregnancy examination.     

Losses over a 2-year period associated with fetal infection with the bovine viral diarrhea virus in a beef cow-calf herd in Saskatchewan.

In 1992, significant calf losses occurred between birth and weaning in a 650-cow Saskatchewan beef herd. These losses occurred subsequent to ill-thrift and disease, and every calf necropsied was found to be persistently infected with bovine viral diarrhea virus (BVDV). The objectives of this study were to describe the losses associated with fetal infection with BVDV in this herd and to determine why they occurred. For investigative purposes, blood samples were collected from the entire cow herd and the surviving calves at pregnancy testing in 1992, and tested by virus isolation for BVDV. Between 51 and 71 persistently infected calves were born in 1992. Bovine viral diarrhea virus was only isolated from calves. The only confirmed fetal infections with BVDV were recorded as the birth of persistently infected calves. However, abortions, reduced pregnancy rates, and delayed calvings were also recorded in the cow herd and may have been the result of fetal infections. The herd was monitored again in 1993. Fetal infections with BVDV were recorded as the birth of stunted, deformed, and persistently infected calves. The greatest losses due to fetal infection with BVDV in the 2 years of this study occurred in cows that were 3-years-old at calving (second calves). Bovine viral diarrhea virus appears to have remained endemic in this herd by transmission from persistently infected calves on young 3- and 4-year-old cows to naive calved 2-year-old cows that were mingled with them annually for rebreeding. Significant numbers of the 2-year-old cows remained naive to BVDV, because they were segregated from persistently infected calves at weaning, preventing cross-infection with BVDV.     

Primary bovine viral diarrhoea virus infection in calves following direct contact with a persistently viraemic calf.

Six calves, aged 24 to 58 days and not previously exposed to bovine viral diarrhoea virus (BVDV), were infected with this agent by nose-to-nose contact with a persistently BVDV viraemic calf. The study was conducted in two trials, using 3 calves in each. All 6 calves showed a peak interferon level in serum at 4 days post infection (dpi), and they seroconverted to BVDV at 16-21 dpi. The calves in trial 1 had diarrhoea for 2 or 3 days between 2 and 6 dpi and one calf again from 9 to 11 dpi. During the periods of fever, the calves were slightly depressed. Those in trial 2 were more depressed and their oral and nasal mucous membranes were reddened but they never had diarrhoea. In both trials, fever (up to 41.3 degrees C) was a prominent symptom at 8 to 9 dpi and 2 calves showed a diphasic fever course. Respiratory affection was mild and no medical treatment was required. Haematological assessment demonstrated a transient but significant leukopenia and lymphopenia at 4 dpi (P less than 0.01 and P less than 0.05 respectively) and 11 dpi (P less than 0.05 and P less than 0.01 respectively). A significant decrease in thrombocyte count was seen at 4 dpi (P less than 0.05, n = 3). This study has demonstrated that nose-to-nose contact is an effective way of transmitting BVDV from persistently infected to susceptible cattle.     

Reproductive consequences of infection with bovine viral diarrhea virus.

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 PI 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. Bulls acutely or PI with BVDV may bea source of viral spread through either natural service or semen used in artificial insemination. Management practices including elimination of PI 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.     

Pathogenicity of an Indian isolate of bovine viral diarrhea virus 1b in experimentally infected calves

The aim of this study was to determine the pathogenicity of an Indian bovine viral diarrhea virus (BVDV) 1b isolate in 7-9-months-old male calves. Infected (four) and control (two) calves were bled at three days interval for hematological, virological and serological studies until day 27. All infected calves developed respiratory illness, biphasic pyrexia, mild diarrhea, leucopenia and mild thrombocytopenia. Viraemia was demonstrated between 3 and 15dpi and the infected calves seroconverted by 15dpi. Prominent kidney lesions were endothelial cell swelling, proliferation of mesangial cells and podocytes leading to glomerular space obliteration. Degeneration and desquamation of cells lining seminiferous tubules were observed in two infected calves. Consolidation of lungs with interstitial pneumonia, mild gastroenteritis and systemic spread were also evident. It was concluded that Indian BVDV isolate induced moderate clinical disease in calves and glomerulonephritis resulting from acute BVDV infection was observed for the first time.     

Experimental infection of calves with bovine viral diarrhea virus genotype II (NY-93).

To ascertain the virulence of bovine viral diarrhea virus (BVDV) genotype II, isolate NY-93 was inoculated intranasally into 3 calves, 2 of which were treated with a synthetic glucocorticoid prior to and after virus inoculation. Anorexia, fever (up to 42 C), dyspnea, and hemorrhagic diarrhea developed 6 days after intranasal inoculation with BVDV NY-93. The condition of all calves deteriorated further until the end of the study on day 14 postinoculation. The most significant postmortem macroscopic changes in all calves were limited to the gastrointestinal tract and consisted of moderate to severe congestion of the mucosa with multifocal hemorrhages. Microscopic lesions found in the gastrointestinal tract were similar to those observed in mucosal disease, including degeneration and necrosis of crypt epithelium and necrosis of lymphoid tissue throughout the ileum, colon, and rectum. The basal stratum of the epithelium of tongue, esophagus, and rumen had scattered individual necrotic cells. Spleen and lymph nodes had lymphocytolysis and severe lymphoid depletion. Severe acute fibrinous bronchopneumonia was present in dexamethasone-treated calves. Abundant viral antigen was detected by immunohistochemistry in the squamous epithelium of tongue, esophagus, and forestomachs. BVDV antigen was prominent in cells of the media of small arteries and endothelial cells. The presence of infectious virus in tissues correlated with an absence of circulating neutralizing antibodies. These findings highlight the potential of BVDV genotype II to cause severe disease in normal and stressed cattle.     

Evaluation of the reverse transcription-polymerase chain reaction/probe test of serum samples and immunohistochemistry of skin sections for detection of acute bovine viral diarrhea infections.

Bovine viral diarrhea viruses (BVDV) cause both acute and persistent infections. While diagnostic tests have been designed to detect animals persistently infected (PI) with BVDV, the reliability of these tests in detecting acute BVDV infections is not known. It is also possible that acute BVDV infections may be confused with persistent infections in surveys for PI animals. In this study, 2 tests presently in use in diagnostic laboratories to test for PI animals, polymerase chain reaction amplification followed by probe hybridization (RT-PCR/probe) of serum samples and immunohistochemical detection of viral antigen in skin biopsies (IHC), were evaluated for their ability to detect acute BVDV infections. Sixteen colostrum-deprived, BVDV-free, and BVDV-antibody-free calves were infected with 6 different BVDV strains. Clinical signs, seroconversion, and virus isolation indicated that inoculated animals did replicate virus. Virus could be detected in 19% (3/16) of acutely infected animals by the RT-PCR/probe technique. No acutely infected animals were positive by IHC.     

Persistent bovine viral diarrhoea virus infection in US beef herds

In the summer of 1996, we screened 18,931 calves in 128 beef herds located in five US states for persistent bovine viral diarrhea virus (BVDV) infection. Of these, 76 herds were randomly selected from the client database of collaborating veterinary practices, and 52 herds were suspected by the collaborating veterinarians to have BVDV infection based on history or clinical signs. Serum was obtained from each calf in the cooperating herds prior to 4 months of age and tested for the presence of BVDV by microtiter virus isolation. Information about each of the herds (including management practices, vaccination history, and breeding- and calving-season production measures) were collected by the collaborating veterinarians using standardized questionnaires. A total of 56 BVDV-positive calves in 13 herds were identified on initial screening. Ten (19%) of the BVDV-suspect herds and three (4%) of the randomly selected herds had > or = 1 BVDV-positive calf at initial screening. Multiple BVDV-positive calves were identified in 10 of those 13 herds. Follow-up information was obtained for 54 of the 56 positive calves. Ten out of 54 (18%) died prior to weaning, and 1 (2%) was sold because of unusually poor growth. Thirty-three out of 54 (61%) of the initially positive calves remained BVDV positive at 6 months of age - confirming persistent-infection (PI) status. Dams of 45 of the 56 positive calves were tested, with 3 (7%) identified as positive - indicating most PI calves were products of acute dam infection during gestation. The proportion of cows that were pregnant at the fall 1995 pregnancy examination was 5% lower in herds with PI calves born during the 1996 calving season than in randomly selected herds without PI calves. Most of the calves we identified with persistent BVDV infections survived to weaning, and could provide a constant source of virus to the herd throughout the breeding season and early gestation.     

Bovine viral diarrhoea: Pathogenesis and diagnosis

Bovine viral diarrhoea virus (BVDV) is the most prevalent infectious disease of cattle. It causes financial losses from a variety of clinical manifestations and is the subject of a number of mitigation and eradication schemes around the world. The pathogenesis of BVDV infection is complex, with infection pre- and post-gestation leading to different outcomes. Infection of the dam during gestation results in fetal infection, which may lead to embryonic death, teratogenic effects or the birth of persistently infected (PI) calves. PI animals shed BVDV in their excretions and secretions throughout life and are the primary route of transmission of the virus. These animals can usually be readily detected by virus or viral antigen detection assays (RT-PCR, ELISA), except in the immediate post-natal period where colostral antibodies may mask virus presence. PI calves in utero (the ‘Trojan cow’ scenario) currently defy detection with available diagnostic tests, although dams carrying PI calves have been shown to have higher antibody levels than seropositive cows carrying non-PI calves.Acute infection with BVDV results in transient viraemia prior to seroconversion and can lead to reproductive dysfunction and immunosuppression leading to an increased incidence of secondary disease. Antibody assays readily detect virus exposure at the individual level and can also be used in pooled samples (serum and milk) to determine herd exposure or immunity. Diagnostic tests can be used to diagnose clinical cases, establish disease prevalence in groups and detect apparently normal but persistently infected animals. This review outlines the pathogenesis and pathology of BVD viral infection and uses this knowledge to select the best diagnostic tests for clinical diagnosis, monitoring, control and eradication efforts. Test methods, types of samples and problems areas of BVDV diagnosis are discussed.