Diagnosis of bovine viral diarrhea virus infections

Perfil da infecção pelo vírus da diarreia viral bovina (BVDV) em um rebanho bovino leiteiro de alta produção e com programa de vacinação contra o BVDV

A infecção pelo vírus da diarreia viral bovina (BVDV) foi avaliada em um rebanho bovino leiteiro de alta produção com histórico de problemas reprodutivos e de vacinação regular contra o BVDV. A identificação do vírus foi realizada por RT-PCR em soro sanguíneo e o perfil sorológico por vírus-neutralização. Inicialmente, 100% (n=692) dos animais do rebanho foram avaliados com relação à presença de infecção ativa pelo BVDV por meio da RT-PCR. Quatro meses após, todos os animais positivos (n=29) na primeira avaliação foram avaliados novamente pela RT-PCR, assim como todos os animais que nasceram (n=72) e os que apresentaram problemas reprodutivos (n=36) no intervalo entre a primeira e a segunda colheita de sangue. Os resultados finais do estudo possibilitaram identificar 27 animais transitoriamente infectados e três animais persistentemente infectados (PI). A sorologia, realizada apenas nos animais positivos na primeira avaliação pela RT-PCR e nas vacas que apresentaram problemas reprodutivos entre a primeira e a segunda RT-PCR, demonstrou grande flutuação nos títulos de anticorpos neutralizantes, além de soroconversão na maioria dos animais. Foram identificados aumentos nos títulos de anticorpos neutralizantes que variaram entre 3 e 8 log2, indicando infecção ativa no rebanho. A circulação viral no rebanho avaliado foi responsável pela expressão de sinais clínicos da esfera reprodutiva em animais com baixo título de anticorpos e consequente falha na proteção fetal. Os resultados demonstram que o controle da infecção pelo BVDV apenas por meio da vacinação regular em rebanhos com animais PI pode não ser eficaz na profilaxia dessa virose.

Genetic characterization of bovine viral diarrhoea (BVD) viruses: confirmation of the presence of BVD genotype 2 in Africa

Bovine viral diarrhoea virus (BVDV) has emerged as one of the economically important pathogens in cattle populations, with a worldwide distribution and causing a complex of disease syndromes. Two genotypes, BVDV 1 and 2, exist and are discriminated on the basis of the sequence of the 5' non-coding region (5' NCR) using real-time PCR. Real-time PCR is more sensitive, specific, and less time-consuming than conventional PCR, and it has less risk of cross-contamination of samples. Limited information exists on BVDV genetic subtypes in South Africa. The aim of this study was to determine the genotypes of BVDV currently circulating in South African feedlots. A total of 279 specimens (219 tissue samples, 59 trans-tracheal aspirates and 1 blood sample) were collected from dead and living cattle with lesions or clinical signs compatible with BVDV infection. Pooled homogenates from the same animals were prepared, and total RNA was extracted. A screening test was performed on the pooled samples, and positive pools were investigated individually. A Cador BVDV Type 1/2 RT-PCR Kit (QIAGEN, Hilden, Germany) was used for the real-time PCR assay on a LightCycler(®) V2.0 real-time PCR machine (Roche Diagnostics, Mannheim, Germany). The results were read at 530 and 640 nm for BVDV 1 and 2, respectively. Bovine viral diarrhoea virus was detected in a total of 103 samples that included 91 tissue samples, 1 blood sample and 11 trans-tracheal aspirates. Eighty-five (82.5 %) of the strains were genotype 1 and 18 (17.5 %) were genotype 2. Comparing the sequencing data, genotypes 1 and 2 from the field strains did not cluster with vaccine strains currently used in feedlots in South Africa. The present study revealed the presence of BVDV genotype 2 in cattle in South Africa based on the high sequence similarity between genotype 2 field strains and strain 890 from North America. The presence of genotype 2 viruses that phylogenetically belong to different clusters and coexist in feedlots is consistent with the possibility of multiple virus introductions. These results represent the first documented evidence for the presence of BVDV genotype 2 in African cattle.

Ocorrência de animais persistentemente infectados pelo vírus da diarréia viral bovina em rebanhos bovinos nos Estados de Minas Gerais e São Paulo

A pesquisa de animais persistentemente infectados (PI) pelo vírus da diarréia viral bovina (BVDV) foi realizada em 26 rebanhos bovinos, não vacinados contra o BVDV, localizados nos Estados de Minas Gerais e São Paulo, Brasil. Utilizando uma estratégia de amostragem, de cada rebanho foram obtidas cinco amostras de sangue de bezerros, entre 6 e 12 meses de idade, e os soros sanguíneos foram submetidos ao teste de virusneutralização (VN) para o BVDV-1 e o BVDV-2. Os rebanhos que apresentaram pelo menos três das cinco amostras reagentes a um dos genótipos do BVDV, e com títulos de anticorpos superiores a 128, foram selecionados para a pesquisa de animais PI. Em três rebanhos que apresentaram tal condição, foram colhidas amostras pareadas de sangue de todos os bovinos do rebanho, com intervalo de 30 dias entre as colheitas, e o soro sanguíneo foi submetido ao teste de VN para o BVDV-1 e o BVDV-2. Nas amostras não reagentes a pelo menos um dos genótipos do BVDV e naquelas provenientes de bovinos com menos de seis meses de idade, realizou-se a pesquisa do BVDV pela reação em cadeia da polimerase precedida pela transcrição reversa (RT-PCR). Dos rebanhos analisados, foram detectados dois animais PI a partir de amostras obtidas nas colheitas pareadas provenientes de um rebanho localizado no Estado de Minas Gerais.

Double immunization strategy with a BoHV-4-vectorialized secreted chimeric peptide BVDV-E2/BoHV-1-gD

A bovine herpesvirus 4 was isolated from the milk cell fraction of a healthy cow and his full genome cloned as a bacterial artificial chromosome. So cloned viral genome was used as a vector platform to deliver in vitro and in vivo an optimized secreted chimeric peptide obtained by the fusion of the bovine viral diarrhoea virus glycoprotein E2 ectodomain with the bovine herpesvirus 1 glycoprotein D ectodomain. Recombinant virus infected cells robustly expressed and secreted the chimeric peptide into the culture medium and inoculated animals with the recombinant virus successfully responded toward antigens, gE2 and gD. Thus, this work has implications for the development of safe and effective polyvalent vaccines.

Development and application of an enzyme-linked immunosorbent assay for detection of bovine viral diarrhea antibody based on Erns glycoprotein expressed in a baculovirus system

The BVDV envelope glycoprotein E(rns)/gp48 and the C terminal 79 amino acids of the capsid protein coding region were expressed in a baculovirus system and antigenically characterized. Western blot assay was used to detect recombinant E(rns) (r-E(rns)) in infected insect cells using specific monoclonal antibodies. The r-E(rns) was then used in an indirect ELISA to detect BVDV specific antibodies in a panel of 540 well-characterized sera. Results of the r-E(rns) ELISA were compared to those obtained with a commercially available competitive ELISA targeting anti-NS2/3 antibodies. A good correlation was observed between the 2 ELISA (kappa = 0.916, 95% C.I.: 0.876, 0.956). Using the commercial NS2/3 ELISA as the reference test, the relative sensitivity of r-E(rns) ELISA was 97.5% (95% C.I.: 94.3%, 99.1%) and the relative specificity was 93.9% (95% C.I.: 89.4%, 96.9%), while relative specificity was 100% (95% C.I.: 97%, 100%) using true negative sera (derived from a negative herd). All but 1 antigen positive animals (n = 36) tested negative in the r-E(rns) ELISA; among them all 22 confirmed PI animals were negative by r-E(rns) ELISA. The ability of r-E(rns) ELISA to identify cattle immunized with inactivated vaccine was also demonstrated in a small group of cattle, compared to an NS2/3 antibody ELISA. Results suggest that r-E(rns) ELISA represents an alternative test for antibody generated by natural infection or BVDV vaccination.

Multiple recombinant ELISA for the detection of bovine viral diarrhoea virus antibodies in cattle sera

The most immunogenic proteins (E0, E2 and NS3) of bovine viral diarrhoea virus (BVDV) (NADL strain) were expressed in the baculovirus/insect cells system. Recombinant antigens were applied to the design of enzyme immunoabsorbent assays (ELISAs) for the detection of specific antibodies in cattle sera. The assays developed were shown to be highly sensitive and specific in comparison with the viral neutralization test, which is the reference test for the serological diagnosis of BVDV. The present results demonstrate the contribution of each recombinant antigen to determine clearly the pattern of anti-BVDV antibodies in bovine serum samples.

Oesophagoscopy and detection of viral nucleic acids in oesophageal biopsies--A contribution to BVDV diagnosis

The endoscopic appearance of the oesophagus of animals infected with bovine virus diarrhoea virus (BVDV) but without signs of acute mucosal disease (MD) was investigated for any common or 'early warning' lesions. Thirty-seven BVDV-infected animals [36 persistently infected (PI) and one transiently infected] were examined clinically and endoscopically for typical erosions of the oronasal and oesophageal mucosa, respectively. During oesophagoscopy, mucosal biopsies were taken and tested for pestivirus-specific nucleic acids by reverse transcription polymerase chain reaction (RT-PCR). The results were compared with those of RT-PCR with various sample materials (blood, nasal and conjunctival swabs and faecal samples) usually used for routine diagnosis of BVDV infection. The average age of the 36 PI animals was 13.5 months, eight animals were 24 months or older. None of the PI animals had clinically visible erosions of the oronasal mucous membranes. During oesophagoscopy, all animals showed a reddening but unexpectedly no erosions or ulcerations of the oesophageal mucosa. Although in 21 of 36 PI animals all samples tested positive, the oesophageal biopsies were with no exception constantly RT-PCR positive in contrast to all other materials investigated. Remarkably, in the transiently infected animal all samples tested negative except for the oesophageal biopsy. Hence, this infection would have been missed by conventional diagnostic sampling.

Factors affecting sensitivity and specificity of pooled-sample testing for diagnosis of low prevalence infections

Testing of pooled samples has been proposed as a low-cost alternative for diagnostic screening and surveillance for infectious agents in situations where the prevalence of infection is low and most samples can be expected to test negative. The present study extends our previous work in pooled-sample testing (PST) to evaluate effects of the following factors on the overall PST sensitivity (SE(k)) and specificity (SP(k)): dilution (pool size), cross-contamination, and cross-reaction. A probabilistic model, in conjunction with Monte Carlo simulations, was used to calculate SE(k) and SP(k), as applied to detection of bovine viral diarrhea virus (BVDV) persistently infected (PI) animals using RT-PCR. For an average prevalence of BVDV PI of 0.01 and viremia in each animal between 10(2) and 10(7)virusparticles/mL, the pool size associated with the lowest number of tests, and lowest cost, corresponded to eight samples/pool. However, the least-cost pool size (lowest number of tests) was associated with a SE(k) of 0.90 (0.75-1), which corresponded to a decrease of 0.04, relative to the assay sensitivity for a single sample. The SP(k) for the same pool size, considering the effect of detection of BVDV acutely infected animals and cross-contamination as source of false positive results, was 0.90 (0.85-0.95). The effect of a hypothetical cross-reacting agent was to markedly decrease SP(k), especially as the prevalence of the cross-reacting agent increased. For a pool size of eight samples and a prevalence of the cross-reacting agent of 0.3, SP(k) ranged from 0.67 to 0.86, depending on the probability that the assay would detect the cross-reacting agent. The methods presented offer a means of evaluating and understanding the various factors that can influence overall accuracy of PST procedures.

Seroconversion of calves following intravenous injection with embryos exposed to bovine viral diarrhea virus (BVDV) in vitro

Two recent studies demonstrated that a high-affinity isolate of BVDV (SD-1), remained associated with a small percentage of in vivo-derived bovine embryos following artificial exposure to the virus and either washing or trypsin treatment. Further, the embryo-associated virus was infective in an in vitro environment. Therefore, the objective of this study was to determine if the quantity of a high-affinity isolate of BVDV associated with single-washed or trypsin-treated embryos could cause infection in vivo. Twenty zona-pellucida-intact morulae and blastocysts (MB) were collected on day 7 from superovulated cows. After collection, all MB were washed according to International Embryo Transfer Society (IETS) standards, and all but 4 MB (negative controls) were exposed for 2 h to 10(5)-10(6) cell culture infective doses (50% endpoint) per milliliter (CCID(50)/mL) of viral strain SD-1. Following exposure, according to IETS standards, one half of the MB were washed and one half were trypsin treated. All MB were then individually sonicated, and sonicate fluids were injected intravenously into calves on day 0. Blood was drawn to monitor for viremia and(or) seroconversion. Seroconversion of calves injected with sonicate fluids from washed and trypsin-treated embryos occurred 38% and 13% of the time, respectively. Therefore, the quantity of a high-affinity isolate of BVDV associated with single-washed or trypsin-treated embryos was infective in vivo.

Infectivity of bovine viral diarrhea virus associated with in vivo-derived bovine embryos

Early research indicated that bovine viral diarrhea virus (BVDV) would not adhere to zona pellucida-intact (ZP-I), in vivo-derived bovine embryos. However, in a recent study, viral association of BVDV and in vivo-derived embryos was demonstrated. These findings raised questions regarding the infectivity of the embryo-associated virus. The objectives of this study were to evaluate the infectivity of BVDV associated with in vivo-derived bovine embryos through utilization of primary cultures of uterine tubal cells (UTC) as an in vitro model of the uterine environment and to determine if washing procedures, including trypsin treatment, were adequate to remove virus from in vivo-derived embryos. One hundred and nine ZP-I morulae and blastocysts (MB) and 77 non-fertile and degenerated (NFD) ova were collected on day 7 from 34, BVDV-negative, superovulated cows. After collection, all MB and NFD ova were washed according to International Embryo Transfer Society (IETS) standards and exposed for 2h to approximately 10(6) cell culture infective doses (50% endpoint) per milliliter of viral strain SD-1. Following exposure, some groups of <10 MB or NFD ova were washed in accordance with IETS standards. In addition, an equivalent number of MB and NFD ova were subjected to IETS standards for trypsin treatment. Subsequently, NFD ova were immediately sonicated and sonicate fluids were assayed for presence of virus, while individual and groups of MB were placed in microdrops containing primary cultures of UTCs and incubated. After 3 days, embryos, media, and UTCs were harvested from each microdrop and assayed for BVDV. Virus was detected in the sonicate fluids of 56 and 43% of the groups of NFD ova that were washed and trypsin-treated, respectively. After 3 days of microdrop culture, virus was not detected in media or sonicate fluids from any individual or groups of MB, regardless of treatment. However, virus was detected in a proportion of UTC that were co-cultured with washed groups of MB (30%), washed individual MB (9%) and trypsin treated individual MB (9%), but no virus was detected in the UTC associated with groups of trypsin-treated embryos. In conclusion, virus associated with developing embryos was infective for permissive cells. Further, the quantity of virus associated with a proportion of individual embryos (both washed and trypsin treated) was sufficient to infect the UTC. In light of these results, an attempt should be made to determine if the quantity of a high-affinity isolate of BVDV associated with an individual embryo would infect recipients via the intrauterine route.

Different strains of noncytopathic bovine viral diarrhea virus (BVDV) vary in their affinity for in vivo-derived bovine embryos

Washing procedures (without trypsin treatment) recommended by the International Embryo Transfer Society (IETS) for use on in vivo-derived embryos effectively removed a cytopathic strain (NADL) of bovine viral diarrhea virus (BVDV) after artificial exposure. However, these washing procedures have not been evaluated using other isolates of BVDV, including representative non-cytopathic strains. Thus, the objective of this study was to evaluate the efficacy of the IETS procedures following artificial exposure of in vivo-derived bovine embryos to two different strains and biotypes of BVDV. One hundred and twenty-nine zona pellucida-intact (ZP-I) morulae and blastocysts (MB) and 56 non-fertile and degenerated (NFD) ova were collected 7 days following exposure to bulls from 32, BVDV-negative, superovulated cows. After collection, all MB and NFD ova were washed according to IETS standards. Subsequently, half of the MB and NFD ova were exposed for 1h to approximately 10(6)-cell culture infective doses (50% endpoint) per milliliter of viral strain SD-1, and the other half were exposed to the same concentration of CD-87. After exposure, groups of > or =3 and < or = 10 MB or NFD ova were washed using methods that met or exceeded IETS standards. Then, the washed groups were sonicated, and sonicate fluids were assayed for presence of virus using virus isolation and a reverse transcription nested polymerase chain reaction. No virus was detected in any group of MB or NFD ova that had been exposed to the CD-87 isolate. However, virus was detected in association with 50% of the groups of MB and 33% of the groups of NFD ova that had been exposed to the SD-1 isolate. Therefore, standard embryo-washing procedures recommended by the IETS are more effective for removal of some isolates of BVDV than for others. It remains to be determined if the quantity of a high-affinity isolate of BVDV associated with individual washed embryos would infect recipients via the intrauterine route. Further, it should be determined if an alternative embryo processing procedure, washing and trypsin treatment, would be more effective for removal of high-affinity isolates.

Biosecurity and biocontainment of bovine viral diarrhea virus

Infection of cattle with BVDV results in a variety of clinical illnesses costly to the cattle industry worldwide. The reservoir and primary source of transmission is cattle born PI with BVDV after transplacental infection in early gestation. It is a challenge to determine with certainty whether or not BVDV is circulating among a herd of cattle. If the virus is present in a herd,then biocontainment strategies are used to eliminate the virus by testing to removing PI cattle, preventing exposure of pregnant cattle to the virus, and increasing resistance to infection using vaccination. If it is clear that the virus is not present in a herd then, biosecurity actions must be taken to prevent introducing the virus into the herd.

The persistence of bovine viral diarrhea virus

Bovine viral diarrhoea virus (BVDV) has a unique capacity to cause persistent infections of foetuses exposed within the first 150 days of gestation. Preventing foetal BVDV infection will aid in improved control. This unique ability gives BVDV a selective advantage allowing continual mutation and antigenic variation within cattle populations. Therefore, BVDV has become widespread and causes economic losses due to respiratory, reproductive and enteric disease. Vaccination (modified-live or killed) can provide some protection from acute disease and the development of persistently infected foetuses. However, vaccination programmes alone cannot control or eliminate BVDV. In naturally exposed and vaccinated herds, BVDV infections are not self-limiting and may persistent over time. This underscores the ability of the BVDV genome to remain fluid and adapt under selective pressures. Factors influencing persistence of BVDV infections in cattle populations include: non-lytic infections; evasion of host immune responses; foetal infections; acute infections; management practices; contaminated biologics; secondary hosts; defective replicated intermediates; antigenic variation; and replication in privileged anatomical sites.

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.

Prevention of persistent infection in calves by vaccination of dams with noncytopathic type-1 modified-live bovine viral diarrhea virus prior to breeding

OBJECTIVE

To determine the ability of a modified-live virus (MLV) bovine viral diarrhea virus (BVDV) type 1 (BVDV1) vaccine administered to heifers prior to breeding to stimulate protective immunity that would block transmission of virulent heterologous BVDV during gestation, thus preventing persistent infection of a fetus.

ANIMAL

40 crossbred Angus heifers that were 15 to 18 months old and seronegative for BVDV and 36 calves born to those heifers.

PROCEDURE

Heifers were randomly assigned to control (n = 13) or vaccinated (27) groups. The control group was administered a multivalent vaccine where-in the BVDV component had been omitted. The vaccinated heifers were administered a single dose of vaccine (IM or SC) containing MLV BVDV1 (WRL strain). All vaccinated and control heifers were maintained in pastures and exposed to BVDV-negative bulls 21 days later. Thirty-five heifers were confirmed pregnant and were challenge exposed at 55 to 100 days of gestation by IV administration of virulent BVDV1 (7443 strain).

RESULTS

All control heifers were viremic following challenge exposure, and calves born to control heifers were persistently infected with BVDV. Viremia was not detected in the vaccinated heifers, and 92% of calves born to vaccinated heifers were not persistently infected with BVDV.

CONCLUSIONS AND CLINICAL RELEVANCE

These results document that vaccination with BVDV1 strain WRL protects fetuses from infection with heterologous virulent BVDV1.

Bovine viral diarrhoea virus infections and its control. A review

Infections with bovine viral diarrhoea virus continue to plague the cattle industry worldwide. The wish to control the negative effects of the virus has lead to the development of numerous vaccines, but also of eradication schemes. In this paper, a comprehensive overview on BVDV is given: the virus and its clinical manifestations, its occurrence and economic impact, the different routes of transmission, as well as diagnostic methods and objectives. Furthermore, the two major options for BVDV control--eradication and vaccination--are discussed as well as the risk for reintroduction of BVDV after eradication.

Screening of neonatal calves for persistent infection with bovine viral diarrhea virus by immunohistochemistry on skin biopsy samples

Detection and elimination of cattle that are persistently infected with bovine viral diarrhea virus (BVDV) is important for controlling the transmission of this virus. Colostrum-derived antibodies make the detection of persistently BVDV-infected neonatal calves cumbersome and expensive. The objective of this study was to evaluate the use of immunohistochemical staining of skin biopsy samples from neonatal calves as a method for the early detection of persistent BVDV infection. Three hundred thirty-two 1- to 4-week-old dairy calves were screened for BVDV as part of a routine control program. Formalin-fixed skin biopsy samples were stained for BVDV antigen by immunohistochemistry (IHC), and the results were compared to those of virus isolation (VI) from white blood cell preparations. Six calves were positive by both IHC and VI and remained positive for BVDV upon subsequent follow-up testing; thus, they were classified as persistently infected with BVDV. One calf was positive by VI but negative by IHC. On subsequent testing, the calf was negative by VI, suggesting that the initial VI result was due to an acute BVDV infection. One calf was positive by IHC but negative by VI. This calf remained negative by VI on follow-up testing. Immunohistochemical staining of skin biopsy samples is a reliable method for screening neonatal calves for persistent BVDV infection and would be a useful management tool as an aid for controlling and preventing BVDV infection.

Serologic evaluation of five unvaccinated heifers to detect herds that have cattle persistently infected with bovine viral diarrhea virus

OBJECTIVE

To determine whether serologic evaluation of 5 unvaccinated 6- to 12-month-old heifers is a valid method for identifying herds that contain cattle persistently infected (PI) with bovine viral diarrhea virus (BVDV).

ANIMALS

14 dairy herds with a history of BVDV infection, with health problems consistent with BVDV infection, or at risk for contracting BVDV infection.

PROCEDURE

5 unvaccinated 6- to 12-month-old heifers were randomly selected from each herd. Neutralizing antibody titers for type-I and -II BVDV were determined. A herd was classified as likely to contain PI cattle when at least 3/5 heifers had antibody titers > or = 128. Virus isolation was performed on all cattle to identify PI cattle. Genotype of isolated viruses was determined by nested multiplex polymerase chain reaction.

RESULTS

6 of 14 herds contained PI cattle. Sensitivity and specificity of serologic evaluation of 5 heifers for identifying these herds were 66 and 100%, respectively. In herds that contained PI cattle, the predominant BVDV titer in the tested heifers corresponded to the genotype of the isolated virus.

CONCLUSIONS AND CLINICAL RELEVANCE

Serologic evaluation of unvaccinated 6- to 12- month-old heifers is an accurate method for identifying herds containing PI cattle. Both type-I and -II BVDV antibody titers should be determined to prevent herd misclassification. The genotype of BVDV found in PI cattle can be predicted by the predominant neutralizing antibody titers found in tested heifers. Serologic evaluation of 5 unvaccinated heifers can be used to determine whether a herd is likely to contain PI cattle.

Biosecurity and bovine respiratory disease

Although biosecurity practices play a role in minimizing respiratory disease in cattle, they must be used in combination with other management strategies that address the many other risk factors. Because the pathogens involved in bovine respiratory disease are enzootic in the general cattle population, biosecurity practices aimed at the complete elimination of exposure are currently impractical. Several animal husbandry and production management practices can be used to minimize pathogen shedding, exposure, and transmission within a given population, however. Various combinations of these control measures can be applied to individual farms to help decrease the morbidity and mortality attributed to respiratory disease.

Estimating transfer of bovine virus-diarrhoea virus in Danish cattle by use of register data

To study how routinely recorded data (also called "register data") might be used in disease monitoring on a regional or national level, a database for bovine virus-diarrhoea virus (BVDV) was made from existing databases, covering the period January 1995-November 1999. This paper includes a general description of the database, including basic statistics for selected variables. Information was largely complete for cattle herds in the milk-recording scheme (MRS), but only partly available for other herds. A methodology was developed to identify when and how a herd initially was infected. For most herds, it was possible to determine when and how BVDV first was introduced. Among the infected herds, most were already infected by the start of the study. BVDV had been present in 40% of the MRS herds and in 9% of the non-MRS herds. In the MRS herds, most new infections were associated with a dam that had been present in the same herd during gestation. Among the non-MRS herds, most new infections were associated with movement of a persistently infected animal. The monthly number of newly infected herds is presented; it is seen that the incidence declined substantially during the study period.

Study of cattle persistently infected with bovine viral diarrhea virus that lack detectable virus in serum

OBJECTIVE

To determine whether cattle persistently infected with bovine viral diarrhea virus (BVDV) that lack virus detectable in serum by use of the immunoperoxidase microtiter assay (IPMA) can transmit the virus to susceptible herdmates and determine prevalence of these cattle.

DESIGN

Clinical trial and serologic survey.

SAMPLE POPULATION

2 cattle and 1,952 blood samples.

PROCEDURE

A persistently infected cow in which virus could not be detected in serum was housed with a BVDV-seronegative steer. Blood and nasal swab specimens were tested via virus isolation and serum virus neutralization. Parallel WBC preparations and sera from blood samples of 1,952 adult cows were screened for BVDV by use of IPMA.

RESULTS

The steer seroconverted to BVDV within 4 weeks of contact with the cow. Virus was detected in sera and WBC of 5 adult cows that were verified as persistently infected by retest 3 weeks later. Cattle persistently infected with BVDV in which virus could not be detected in both serum and WBC by use of IPMA were not found.

CONCLUSION AND CLINICAL RELEVANCE

Cattle persistently infected with BVDV in which virus cannot be detected in serum by use of IPMA may serve as virus reservoirs for infecting susceptible cattle. Persistent infection was detected at a prevalence of 0.26%. Screening adult cattle by use of IPMA on serum samples appears to be a reliable means of detecting persistent infection with BVDV. Prevalence of cattle persistently infected with BVDV that have negative results of IPMA on serum is extremely low.

Detection of viral antigen in placenta and fetus of cattle acutely infected with bovine viral diarrhea virus

The reproductive organs and fetuses of seven Norwegian Red heifers were investigated for the presence of bovine viral diarrhea virus (BVDV) antigen during the time of initial transplacental transmission of the virus. The heifers were inoculated with a noncytopathogenic BVDV at day 85/86 of gestation and were slaughtered at day 7, 10, 14, 18, or 22 postinoculation (pi). Cryostat sections of uterus, ovaries, placentomes, intercotyledonary fetal membranes, and fetal organs were examined using immunohistochemical techniques. A double immunofluorescence technique was used to identify cells that showed staining with antibodies against the leukocyte common antigen CD45 or the intermediate filament vimentin and BVDV antigens. The earliest stage of infection at which BVDV antigen could be detected in the fetuses was 14 days pi. At this stage, BVDV antigen was detected in cells of mesenchymal origin in the lungs and in large cells that morphologically resembled immature megakaryocytes in the liver. In the intercotyledonary fetal membranes and in the placentomes, BVDV antigen was not detected until 18 and 22 days pi, respectively. BVDV antigen was not detected in maternal tissue from any of the heifers. The present results indicate that fetal infection with BVDV can take place without preceding or simultaneous high concentrations of BVDV in uterus or placenta of acutely infected heifers.

Typing of bovine viral diarrhea viruses directly from blood of persistently infected cattle by multiplex PCR

A nested multiplex PCR was developed for genotyping of bovine viral diarrhea viruses (BVDVs). The assay could detect as little as 3 50% tissue culture infective doses of BVDV per ml and typed 42 out of 42 cell culture isolates. BVDV was also successfully typed, with or without RNA extraction, from all 27 whole-blood samples examined from 22 carriers or probable carriers and 5 experimentally infected cattle.

Level and duration of serum antibodies in cattle infected experimentally and naturally with bovine virus diarrhoea virus

Neutralising serum antibodies against bovine virus diarrhoea virus (BVDV) were monitored for three years in 35 cattle that were infected with the virus as calves; 24 of the calves were inoculated intramuscularly or intranasally, and 11 contracted the infection naturally. All the experimentally infected calves seroconverted within 14 to 28 days after inoculation, and all the animals still had high serum levels of antibodies to BVDV three years after infection. Determinations of antibody levels in milk and blood samples excluded the possibility that the calves had been reinfected with BVDV during the study.

Bovine virus diarrhoea (BVD) control programme in an area in the Rome province (Italy)

A BVD control programme based on the identification and removal of persistently infected (PI) animals is being undertaken in an area in the Rome province, where BVD outbreaks had been previously detected. It involves 174 mainly dairy herds, from which blood samples of all bovines older than 1 year are obtained through the national brucellosis and leukosis eradication programme. Samples sufficient to detect the presence of seropositive animals at a prevalence of 5% or more are initially screened for antibodies against BVD virus (BVDV) using an immunoenzymatic assay. Upon identification of seroreagents additional blood samples are tested from the 6-12-month age category not included in the initial samples. Animals are considered immunotolerant if BVDV is demonstrated twice at a minimum 30-day interval. When no seropositive animals are detected during the first serological screening the herd is declared BVD-free if a second testing, preferably carried on the same animals previously tested, confirms the seronegative status of the herd. At present 147 farms have been tested, of which 63 (42.9%) are negative with respect to antibodies against BVDV. Of the 84 remaining herds in which one or more seropositives are detected, 13 are classified as recently infected. In eight of these recently infected herds, 22 PI animals have been identified.

Laboratory diagnostic investigations for bovine viral diarrhoea virus infections in cattle

There are no pathognomonic clinical signs of infection with bovine viral diarrhoea virus (BVDV) in cattle. Diagnostic investigations therefore rely on laboratory-based detection of the virus, or of virus-induced antigens or antibodies in submitted samples. In unvaccinated dairy herds, serological testing of bulk milk is a convenient method for BVDV prevalence screening. Alternatively, serological testing of young stock may indicate if BVDV is present in a herd. In BVDV positive herds, animals persistently infected (PI) with BVDV can be identified by combined use of serological and virological tests for examination of blood samples. ELISAs have been used for rapid detection of both BVDV antibodies and antigens in blood, but should preferably be backed up by other methods such as virus neutralization, virus isolation in cell cultures or amplification of viral nucleic acid. Detailed knowledge of the performance of the diagnostic tests in use, as well as of the epidemiology of bovine virus diarrhoea is essential for identification of viremic animals in affected herds.