[Biological cycle of radionuclides and dynamics of cattle leukemia in the Orenburg region]

Global technogenic radionuclides pollution of environment, migration of radionuclides from soil into animal feeds stimulate additional contamination doses. Concentration of natural and of technogenic radionuclides in the investigated objects was determined as result of radiochemical and radiometric studies. It was established that maximum concentrations of 40K, 232Th, 226Ra (Bk/kg) radionuclides in the soil of the eastern zone was 391.6 +/- 77.64; 20.8 +/- 0.71; 16.7 +/- 0.8 (respectively) without exceeding permissable levels. It is also found that the density of soil pollution with 137Cs in the western zone is 0.28 +/- 0.075 mBk/m2 (76.8 +/- 20.4 mKi/km2), such soils are to be considered as locally polluted soils. The number of cattle infected with cattle leucosis is obviously higher in ecologically unsafe areas: 19.3% on the western farms, 12.95% in the central areas and 9.70% on the farms of the eastern zone. Long-term exposure to ecologically unsafe conditions may lead to the reduction of populations immune status expressed in lower disease resistance and in incidence of carcinogenic pathology.

[Detection of bovine leukaemia virus (BLV) in tissue samples of naturally and experimentally infected cattle]

Enzootic bovine leukaemia (EBL) which is caused by the bovine leukaemia virus (BLV) still plays a remarkable role despite a significant success in sanitation programmes. In the Federal Republic of Germany it was not possible to eradicate the disease until today. Sporadically during slaughter or necropsy of cattle neoplastic lesions of the lymphatic tissues are observed that need to be clarified with regard to BLV as etiological agent. Due to the fact that in most instances no serological data are available from the respective animals and blood drawings from the original holdings are not easy to obtain the polymerase chain reaction (PCR) opens new avenues as supplementary diagnostic tool to test unfixed lymphatic tissues for the presence of BLV proviral DNA. Lymph node tissues from 10 naturally or experimentally BLV-infected cattle, which have been monitored virologically and serologically, and tissues from 4 negative animals were processed, DNA was extracted and subjected to PCR to amplify BLV env gene specific sequences. The results show that in cattle with BLV-induced leukosis as well as in cattle, which were clinically healthy and unsuspicious at slaughter or at post-mortem, either with persistent lymphocytosis (PL) or without, BLV proviral DNA could be detected easily in samples of lymphatic tissues and in high concordance with serological data. In this article data from the National and OIE reference laboratory for EBL at the Friedrich-Loeffler-Institut (FLI, Germany) are presented. Elaborated laboratory protocols for processing of tissue samples and performing of BLV-PCR are recommended.

The eradication experience of enzootic bovine leukosis from Lithuania

Before 1985 the situation regarding enzootic bovine leukosis (EBL) in Lithuanian cattle was described only haphazardly. In 1986 serological investigations were initiated together with an eradication programme. The EBL bovine leukosis virus (BLV) situation was monitored by the Institute of Immunology Vilnius University, national and regional veterinary laboratories. Starting in 1986 all EBL-positive cattle were separated from negative cattle into BLV-infected and BLV-free herds. To create the latter, calves were fed pasteurized milk. The seroprevalence in 1990 was 7.29%, but it steadily declined to 0.32% in 2006.

Molecular characterization of the env gene from Brazilian field isolates of Bovine Leukemia Virus

Molecular characterization of Bovine leukemia virus (BLV) isolates from Brazil using the env gene sequences revealed a high conservation of this gene. In most cases the substitutions corresponded to silent transitions. In addition, cystein residues, potential glycosylation sites, neutralization domains and other critical residues involved with the envelope structural domains and viral infectivity were conserved. Most of the substitutions found in the aminoacid sequences of the gp51 protein were localized in the G and H epitopes. Using the SIFT software, it was predicted that they should not alter the protein functions. Phylogenetic analyses showed that partial or complete env gene sequences grouped in three or four phylogenetic clusters, respectively. The sequences from the Brazilian isolates had similar mutation rates as compared to samples from other countries, and belonged to at least two phylogenetic clusters.

Sequencing and phylogenetic analysis of gp51 gene of bovine leukaemia virus in Iranian isolates

Analysis of the partial bovine leukaemia virus (BLV) gp51 gene sequences obtained from five BLV strains isolated in different regions of Iran and BLV-FLK strain was carried out. The Iranian BLV gp51 sequences were compared with seven other corresponding sequences of BLV strains isolated in different countries. Nucleotide sequence analysis showed a variability of 0.003-5.1% and the phylogenetic tree constructed revealed three clusters. The first cluster included French, German and FLK-BLV samples; the second cluster included four Iranian samples; and the third cluster included Australian, Korean, Japanese, Brazilian and Belgian reference strains and one Iranian sample. Iranian samples were had significantly similarity to European and Australian samples.

Sequence polymorphisms in the long terminal repeat of bovine leukemia virus: evidence for selection pressures in regulatory sequences

Bovine leukemia virus (BLV) is an oncogenic virus widespread in cattle. It belongs to the genus Deltaretrovirus of the family Retroviridae along with human and simian T-lymphotropic viruses. The BLV transcriptional promoter is located in the proviral 5' long terminal repeat (LTR), composed of U3, R, and U5 regions. BLV LTR contains multiple cis-acting elements important for promoter activity, a short coding sequence (encoding the NH(2) terminus of the G4 regulatory protein), and non-regulatory/non-coding regions. Variation in coding sequences of BLV structural proteins has been studied extensively, but little work has been done on sequence variability of non-coding regions, mostly located in LTR. Here, we report the first study on the natural diversity of the BLV LTR, using viral isolates from 52 cattle in several different areas worldwide. Nucleotide variations from the consensus sequence were observed in most isolates and clustered phylogenetically, corresponding to the geographic distribution of donor cattle. Overall, regulatory regions were significantly more conserved than non-regulatory regions in the BLV LTR, as well as in LTR sub-regions (U3, R, and U5). Evidence of selection pressures in BLV LTR suggests that selection occurs not only in coding sequences, but may also involve regulatory sequences.

Dissemination of bovine leukemia virus-infected cells from a newly infected sheep lymph node

To investigate the early establishment of bovine leukemia virus (BLV) infection, we injected BLV-infected or mock-infected allogeneic cells into the shoulder of sheep in which an efferent lymphatic duct of the draining prescapular lymph node had been cannulated. Rare mononuclear cells acting as centers of BLV infection in culture were present within 4 to 6 days in efferent lymph and within 6 to 10 days in blood. Soon after BLV injection, immunoglobulin M+ (IgM+) and CD8+ cells increased in efferent lymph and oscillated reciprocally in frequency. CD8+ blasts increased on days 4 to 6, when infectious centers increased 100-fold in lymph. On days 6 and 7, both lymph and blood were enriched with CD8+ cells that were labeled late on day 5 with an intravenous pulse of 5-bromo-2'-deoxyuridine (BrdU). Lymph, but not blood, was enriched with BrdU+ B cells on day 7. Capsid-specific antibodies became detectable in efferent lymph on days 6 to 8 and surface glycoprotein-specific antibodies on day 9, preceding their detection in serum by 9 to 14 days. Systemic dissemination of BLV-infected cells was thus accompanied by an increase in proliferating CD8+ cells and the onset of BLV-specific antibodies in lymph. Infectious centers reached maximum frequencies of 0.2% in lymph by days 11 to 13, and then their frequencies increased by 5- to 40-fold in blood cells, suggesting that many infected blood cells do not recirculate back into lymph. Beginning on days 10 to 13, a subpopulation of B cells having high levels of surface IgM increased sharply in peripheral blood. Such cells were not present in lymph. After a day 16 pulse of BrdU, recently proliferated cells that stained intensely for surface IgM appeared in blood within 15 h. Predominantly B lymphocytes contained the viral capsid protein when lymph and blood cells were cultured briefly to allow BLV expression. However, both early in lymph and later in blood, BrdU+ B cells greatly exceeded productively infected cells, indicating that new BLV infections stimulate proliferation of two different populations of B cells.

Development of a PCR to diagnose BLV genome in frozen semen samples

The sanitary and economic impact of BLV infection is associated with the interference in the international movement of cattle and their germ plasm. Although experimental data support the improbability that semen from BLV-positive bulls could infect recipient cows, restriction for commercialization of semen from infected animals is still present. The objective of this work was to standardize a PCR assay to diagnose the presence of BLV genome in frozen semen samples. The developed methodology involves the amplification of an internal fragment of gag gene. The limit of detection of this technique was six viral particles, using gag-PCR followed by hybridization analysis. Frozen semen samples from seropositive bulls were analyzed. It was possible to detect proviral DNA in 9 out of 173 samples. Additionally, a biological test in susceptible sheep was performed in order to evaluate the transmission of BLV genome by semen from seropositive animals. This data strongly suggest that semen from seropositive bulls that resulted negative by PCR can be used for artificial insemination (AI), accompanied by proper collection protocols. The development of this PCR assay constitutes a valuable diagnostic tool to determine the BLV-free status of frozen semen samples used for AI.

Evaluation of a new antibody-based enzyme-linked immunosorbent assay for the detection of bovine leukemia virus infection in dairy cattle

The objective of this study was to validate a new blocking enzyme-linked immunosorbent assay (ELISA) (designated M108 for milk and S108 for serum samples) for detecting bovine leukemia virus (BLV) infection in dairy cattle. Milk, serum, and ethylenediaminetetraacetic acid-blood samples were collected from 524 adult Holstein cows originating from 6 dairy herds in Central Argentina. The M108 and S108 were compared with agar gel immunodiffusion (AGID), polymerase chain reaction and a commercial ELISA. Because there is currently no reference test capable of serving as a gold standard, the test sensitivity (SE) and specificity (SP) were evaluated by the use of a latent class model. Statistical inference was performed by classical maximum likelihood and by Bayesian techniques. The maximum-likelihood analysis was performed assuming conditional independence of tests, whereas the Bayesian approach allowed for conditional dependence. No clear conclusion could be drawn about conditional dependence of tests. Results with maximum likelihood (under conditional independence) and posterior Bayes (under conditional dependence) were practically the same. Conservative estimates of SE and SP (with 95% confidence intervals) for M108 were 98.6 (96.7; 99.6) and 96.7 (92.9; 98.8) and for S108 99.5 (98.2; 99.9) and 95.4 (90.9; 98.1), respectively. The ELISA 108 using either milk or serum to detect BLV-infected animals had comparable SE and SP with the official AGID and a commercial ELISA test, which are currently the most widely accepted tests for the serological diagnosis of BLV infection. Therefore, ELISA 108 can be used as an alternative test in monitoring and control programs.

Evaluation of risks of viral transmission to recipients of bovine embryos arising from fertilisation with virus-infected semen

This scientific review was prompted by recent legislation to curtail the use of semen from potentially virus-infected bulls to produce embryos for import into the European Union. From studies in laboratory animals, humans and horses, it is apparent that viruses may sometimes attach to, or be integrated into, spermatozoa, although in domestic livestock, including cattle, this seems to be a rare phenomenon, and carriage of virus through the zona pellucida into the oocyte by fertilising sperm has never been described in these species. Four specific viruses; enzootic bovine leukosis (EBLV), bovine herpesvirus-1 (BoHV-1), bovine viral diarrhoea virus (BVDV) and bluetongue virus (BTV), all of which tend to cause subclinical infections in cattle, but which can occur in bovine semen, are examined with regard to the risks that use of infected semen might lead to production of infected embryos. With regard to in vivo-derived embryos, when internationally approved embryo processing protocols are used, the risks from EBLV- and BTV-infected semen are negligible, and the same is almost certainly true for semen infected with BoHV-1 if the embryos are also treated with trypsin. For BVDV, there is insufficient data on how the virus is carried in semen and how different BVDV strains can interact with sperm, oocytes and embryos. There is a potential, at least, that in vivo-derived embryos resulting from infected semen might carry BVDV, although field studies so far suggest that this is very unlikely. With regard to in vitro-produced embryos, use of semen infected with any of the four viruses, with the probable exception of EBLV, will often lead to contaminated embryos, and virus removal from these embryos is difficult even when the internationally approved embryo processing protocols are used. However, it has never been demonstrated that such embryos have resulted in transmission of infection to recipients or offspring.

Detection of bovine leukemia virus by in situ polymerase chain reaction in tissues from a heifer diagnosed with sporadic thymic lymphosarcoma

An 18-month-old bovine heifer was presented for clinical evaluation after a sudden onset of ventral edema. Clinical and pathological evaluations were consistent with thymic lymphosarcoma, a sporadic form of lymphosarcoma in cattle, which is not generally considered to be associated with bovine leukemia virus (BLV). This heifer was seropositive for BLV at 6 and 18 months of age. Tissues obtained at necropsy were evaluated using in situ polymerase chain reaction. The BLV proviral DNA was detected in lymphocytes of the thymus as well as in epithelial cells of the liver and kidney. This report presents evidence that thymic lymphosarcomas can be associated with BLV infection and that BLV may have a broader cellular tropism than was supposed previously.

Establishment of a bovine leukemia virus-free dairy herd in Korea

In view of the high prevalence rate of bovine leukemia virus (BLV) infections in cattle over the entire country, a large dairy farm in Chungnam province was chosen and 'test and segregate' program was instituted. On July 1999, ELISA test was performed on 491 animals on the farm and only 163 cattle (139 adult cows, 18 female and 6 male calves)were BLV-seronegative. From February 2000 through April 2004, the seronegative group was placed in barns 1,500 to 2,000 m from seropositive group and thereafter tested at 3-to 5-month intervals by ELISA. Animals seroconverted in consecutive tests were removed from the seronegative group immediately after the detection of anti-BLV antibodies. The changes in management were aimed at preventing iatrogenic transfer of blood between cattle. Replacement heifers imported from other countries and calves born at the farm were repeatedly tested by ELISA, and only seronegative animals were introduced into the group. As of April 2004, there were 311 cattle in the BLV seronegative group of the farm. Twenty four cows of the initial 139 adult cows were seroconverted in 2000, and no seropositive animals were found since February 2001. Follow up of the group, from which all seropositive cattle were moved to a separate location, revealed no recurrence of BLV infection for three years. The approach in the present study might be valuable for Korean producers who would like to move toward a BLV-negative status.

Effects of seropositivity for bovine leukemia virus, bovine viral diarrhoea virus, Mycobacterium avium subspecies paratuberculosis, and Neospora caninum on culling in dairy cattle in four Canadian provinces

The purpose of this research was to determine the effects of seropositivity for exposure to bovine leukemia virus (BLV), bovine viral diarrhoea virus (BVDV), Mycobacterium avium subspecies paratuberculosis (MAP) and Neospora caninum (NC) on overall and reason-specific culling in Canadian dairy cattle. Serum samples from, approximately, 30 randomly selected cows from 134 herds were tested for antibodies against BLV, MAP and NC using commercially available ELISA test kits, while 5 unvaccinated cattle over 6 months of age were tested for antibodies to bovine viral diarrhoea virus (BVDV). For analyzing the time (in days) to culling of cows after the blood testing, a two-step approach was utilized, non-parametric (Kaplan-Meier survival graphs) visualization and then semi-parametric survival modelling (Cox proportional hazards model), while controlling for confounding variables and adjusting for within herd clustering. For all reasons of culling, MAP-seropositive cows had a 1.38 (1.05-1.81, 95% C.I.) times increased hazard of culling compared to MAP-seronegative cows. Seropositivity for the other pathogens was not associated with an increased risk of overall culling. Among cows that were culled because of either decreased reproductive efficiency or decreased milk production or mastitis, MAP-seropositive cows were associated with 1.55 (1.12-2.15, 95% C.I.) times increased hazard compared to MAP-seronegative cows. Among cows that were culled because of reproductive inefficiency, NC-seropositive cows had a 1.43 (1.15-1.79, 95% C.I.) times greater hazard than NC-seronegative cows. Among cows that were culled because of decreased milk production, cows in BVDV-seropositive herds had a 1.86 (1.28-2.70, 95% C.I.) times increased hazard compared to cows in BVDV-seronegative herds. BLV-seropositive cows did not have an increased risk of reason-specific culling as compared to BLV-seronegative cows. No significant interaction on culling among seropositivity for the pathogens was detected, but only a limited number of cows tested seropositive for multiple pathogens. Results from our research will help in better understanding the economic impacts of these pathogens and justification for their control.

A comparison of two ELISAs for the detection of antibodies to bovine leucosis virus in bulk-milk

OBJECTIVE

To estimate the sensitivity, specificity and detection limits for two bulk-milk enzyme-linked immunosorbent assays, the Svanovir BLV-gp51-Ab and the Lactelisa BLV Ab Bi indirect tank 250, for the detection of antibody to bovine leucosis virus in milk.

PROCEDURE

Milk samples from 27 cows known to have enzootic bovine leucosis (EBL) were serially diluted with milk from a herd known to be free from the disease. The dilution at which antibodies could no longer be detected by each test was determined. A total of 1959 bulk-milk samples submitted to a laboratory for the Victorian (EBL) eradication program were tested with both the Svanovir and the Lactelisa assays. A Bayesian approach was used to calculate maximum-likelihood estimates of test sensitivity and specificity. An additional 660 bulk-milk samples were tested with both the Svanovir and the Lactelisa assays. Herds that had positive results on either or both of the assays were subjected to blood or milk testing of individual cattle.

RESULTS

The dilution of milk at which the Svanovir assay failed to detect enzootic bovine leucosis antibody in half of the samples was 1 in 40, whereas the comparable value for the Lactelisa was 1 in 200. Computer modeling of the operating characteristics of the Svanovir assay indicated that the sensitivity of that assay would be considerably lower than that for the Lactelisa, and the specificity was estimated to be higher. Evaluation of the assays using 660 bulk-milk samples showed that the Lactelisa assay detected four infected herds that were not detected by the Svanovir test. No false positive results were recorded for either assay.

CONCLUSION

Use of the Lactelisa assay in the Victorian EBL eradication program will enhance disease detection and eradication, but may also result in an increased frequency of false positive bulk-milk test results.

Genetic diversity and spread of Bovine leukaemia virus isolates in Argentine dairy cattle

Effective tools for use in control programmes against bovine leukaemia virus (BLV) infections require insight into the relationship between the variant structure of the bovine leukaemia virus and the spatial-temporal interaction of isolates and hosts. Our study showed the presence of two types of BLV isolates - Australian and Argentine - in dairy herds from various parts of Central Argentina; these isolates were characterised by RFLP on PCR amplicons, and some of them were confirmed by sequencing. One genotype (Argentine) was present in all herds, and the Australian genotype was found in two herds. Phylogenetic analysis indicated four clusters. The first cluster was composed of the Argentine isolates and one from Brazil; the second was composed of several isolates found in European countries and one from Brazil; the third cluster was composed of BLV isolates found in Japan and Germany; the fourth cluster included American and Australian isolates and those from other countries. The comparison of a number of synonymous and non-synonymous nucleotide substitutions using various BLV genes revealed purifying selection, suggesting that molecular evolution occurred under some functional constraint.

Circulating immune complexes as indicators of environmental contamination

The aim of this study was to examine the effect of environmental contamination on the concentration of circulating immune complexes (CIC) in bovine blood. We determined that in a conventionally ecologically clear district, the CIC concentration in healthy bovine blood serum was within the limits of 0.18-0.52 mg/mL, (mean = 0.31 mg/mL, V = 30.14%). In a conventionally ecologically contaminated district, the CIC concentration in healthy bovine blood serum was detected within the limits of 0.48-0.99 mg/mL (mean = 0.65 mg/mL, V = 17.97%; P < 0.05). The CIC concentration in serum of bovine leukemia virus (BLV)-infected bovines from different districts was within the limits of 0.29-1.76 mg/mL (mean = 0.81 mg/mL, V = 36.67%; P < 0.05). According to these preliminary results of the investigation, it may be supposed that the increased concentration of CIC is closely related to the environmental contamination. Therefore, studies of CIC concentration in healthy mammals are of great importance for the monitoring of anthropogenic factors and can be exploited as an indicator of environmental contamination.

Evidence for bovine immunodeficiency virus infection in cattle in Zambia

We report herein on the first evidence for the presence of bovine immunodeficiency virus (BIV) in Zambia. Serological surveillance of BIV and bovine leukemia virus (BLV) was conducted in traditional cattle herds in Zambia. Out of a total of 262 sera analyzed, 11.4% were found positive for anti-BIV p26 antibodies as determined by Western blot analysis, while 5.0% were positive for anti-BLV gp51 antibodies as detected by immunodiffusion test. Peripheral blood mononuclear cells from BIV seropositive cattle were found to have BIV-provirus DNA, as detected by nested polymerase chain reaction. A nucleotide sequence corresponding to a 298 bp fragment of the BIV pol gene was also analyzed. Amino acid sequences of these Zambian pol gene products showed 98.0 to 100% homology to the American strain BIV R29, 97.0 to 99.0% to Japanese BIV isolates, and divergence ranged from 0.0 to 2.0% among Zambian BIV isolates.

Sensitive and specific detection of proviral bovine leukemia virus by 5' Taq nuclease PCR using a 3' minor groove binder fluorogenic probe

Sensitive assays are required to detect proviral bovine leukemia virus (BLV) in donor cattle used for the in vivo preparation of Australian tick fever vaccines. 5' Taq nuclease assays using 3' minor groove binder DNA probes (TaqManMGB) were developed and compared to conventional PCR assays for sensitive detection of Australian BLV. Seven beef and dairy herds were screened using DNA prepared by a variety of protocols to evaluate these tests. Comparative sensitivities of PCR tests were determined by testing log(10) dilutions of plasmids with inserted BLV sequences. Animals were also screened by the BLV standard agar-gel immunodiffusion test (AGID) and commercial enzyme linked immunosorbent assays (ELISA) for antibodies, and an ELISA for detecting viral antigens expressed (VAE) in lymphocyte cultures. The TaqMan MGB assay based on the pol region was the most sensitive and specific for the detection of BLV. This is the first report of a sensitive BLV 5' Taq nuclease assay.

In vivo transcription of bovine leukemia virus and bovine immunodeficiency-like virus

Cellular tropism and transcription of bovine leukemia virus (BLV) and bovine immunodeficiency-like virus (BIV) were investigated using peripheral blood mononuclear cells (PBMC) collected from a cow infected with both viruses. Each PBMC subset, purified by magnetic cell sorting, was subjected to PCR and RT-PCR for detection of their integrated proviruses and transcript mRNAs. Both BLV and BIV genomes were detected by nested PCR in CD3(+), CD4(+), CD8(+) and gammadelta T cells, B cells and monocytes. However, BLV tax transcription was only detected in B cells, and only B cells also formed BLV syncytia in CC81 cells. On the other hand, BIV transcript was detected in each subpopulation of PBMC. These results indicated that BLV can infect T cells and monocytes as well as B cells, but can be expressed by transcription only in B cells. In contrast, BIV can express its transcripts in all infected cells.

Association between bovine-leukosis virus seroprevalence and herd-level productivity on US dairy farms

Bovine-leukosis virus (BLV; also termed 'bovine-leukemia virus') is a retrovirus that primarily affects lymphoid tissue of dairy and beef cattle. Our objective was to investigate the association between BLV infection and annual value of production (AVP) on dairy herds within the United States, as part of the USDA National Animal Health Monitoring System's 1996 dairy study. 1006 herds (in 20 states) with at least 30 dairy cows were interviewed during 1996. The agar-gel immunodiffusion test was used to detect serum antibodies to BLV. 10-40 cows from each herd were tested and each tested cow was classified as negative or positive based on results of a single test. A multivariable regression model was used with the 976 herds with complete data for analysis. When compared to herds with no test-positive cows, herds with test-positive cows produced 218 kg per cow (i.e. 3%) less milk. The average reduction in AVP was $59 per cow for test-positive herds relative to test-negative herds. For the dairy industry as a whole, BLV seropositivity was associated with loss to producers of $285 million and $240 million for consumers. Most of this $525 million industry loss was due to reduced milk production in test-positive herds.

Economic implications of bovine leukemia virus infection in mid-Atlantic dairy herds

OBJECTIVE

To determine the baseline costs of bovine leukemia virus (BLV) infection, including costs of clinical disease and subclinical infection, in a dairy herd representative of the mid-Atlantic region and compare these costs with the cost of a test-and-manage BLV control program.

DESIGN

Stochastic spreadsheet model.

SAMPLE POPULATION

A commercial Holstein dairy herd with 100 milking cows.

PROCEDURES

A spreadsheet model was developed. The overall cost of infection included the cost of clinical disease (ie, lymphosarcoma [LS]) and the effects of subclinical infection on milk production and premature culling. Model input values and distributions were designed to reflect economic conditions in the mid-Atlantic region. Relative costs of infection and control were calculated for infection prevalences of 20, 50, and 80%.

RESULTS

Estimated mean cost to the producer per case of LS was 412 dollars; for a herd with a 50% prevalence of BLV infection, annual incidence of LS was 0.66. Mean annual cost of subclinical infection at a 50% prevalence of infection was 6,406 dollars. Mean annual cost of a test-and-manage control program was 1,765 dollars. The cost of clinical disease and subclinical infection varied substantially with the prevalence of infection, whereas the cost of control varied with herd size.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that a basic BLV control program may be economically beneficial in herds in which the prevalence of BLV infection is > or = 12.5%. Farm-specific considerations may factor prominently when weighing the costs and benefits of an individual BLV control program.

Comparison of culling rates among dairy cows grouped on the basis of serologic status for bovine leukemia virus

OBJECTIVE

To determine the association between serologic status for bovine leukemia virus (BLV) and culling rates by use of survival times in a commercial Holstein dairy herd.

DESIGN

Longitudinal study.

ANIMALS

593 milking cows.

PROCEDURE

Cattle were tested for antibodies against BLV by use of agar gel immunodiffusion or ELISA 4 times each year from 1989 to 1993 and then annually through 1999. Dates of birth, first calving, and culling or death were obtained from Dairy Herd Improvement Association records. Most cows were enrolled in the study on the date of first calving. Survival times were compared among seropositive, seronegative, and seroconverted cows with the Kaplan-Meier method and a Cox regression model stratified on the basis of year of birth.

RESULTS

Complete records were available for 593 of 685 (87%) cattle in the dairy herd during the study period. Median survival time for all cows was 31.7 months. Survival times, which correspond to cull rates, did not differ significantly between seropositive and seronegative cattle, whereas cattle that seroconverted during the study had a significantly longer survival time. Year of birth was positively and significantly associated with survival time.

CONCLUSIONS AND CLINICAL RELEVANCE

BLV serologic status was not associated with cull rate as measured by survival time in this dairy herd. This finding is in contrast to results of studies that used survival analysis techniques; our results may influence management decisions concerning BLV.

Eradication of enzootic bovine leukosis from Finland

Enzootic bovine leukosis (EBL) was recognized among Finnish cattle in 1966. Administrative decisions specifying and refining official control measures were given in 1966, 1976, 1980, and 1993. The measures' key principle always has been 'test and slaughter'. The EBL/bovine leukosis virus (BLV) infection situation was monitored at meat inspection, and hematologically between 1970 and 1977 and serologically between 1978 and 1989. Annual surveys including all dairy herds and samples from beef animals were conducted in 1990-2001. Bulk-tank milk samples represented the dairy herds in the surveys; the beef animals were sampled individually at slaughter. The maximum positive herd-level percentage in the surveys was 0.03%. EBL/BLV infection was evenly dispersed in the southern part of the country and nonexistent in the northern part. We conclude that herd-level prevalence of EBL/BLV infection never exceeded 5%. It nevertheless took 30 years to eradicate the disease and the infection. EBL was eradicated from mainland Finland in 1996 and from the island district of Ahvenanmaa in 1999. Annual monitoring of the EBL situation continues.

Evidence of bovine immunodeficiency virus in cattle in Turkey

A seroepidemiological study of bovine immunodeficiency virus (BIV) and bovine leukemia virus (BLV) infections was conducted in four different cattle herds in Turkey. A total of 300 blood samples were analyzed and 12.3% were found to be positive for anti-BIV p26 antibodies by Western blot analysis and 1.6% positive for anti-BLV gp51 antibodies by an immunodiffusion test. BIV infection was confirmed with the detection of BIV-provirus DNA using the nested polymerase chain reaction. This is the first evidence for the presence of BIV in cattle in Turkey.

Use of a polymerase chain reaction assay to detect bovine leukosis virus in dairy cattle

OBJECTIVE

To evaluate the use of a polymerase chain reaction (PCR) assay in detecting bovine leukosis virus (BLV) in adult dairy cows.

DESIGN

Prospective study.

ANIMALS

223 adult dairy cows.

PROCEDURE

Cows were tested for BLV status by use of an ELISA and a PCR assay. Sensitivity, specificity, predictive values of positive and negative tests, and the percentage of cows correctly classified by PCR assay were calculated. Ninety-five percent confidence intervals were calculated for sensitivity and specificity.

RESULTS

Sensitivity and specificity were 0.672 and 1.00, respectively. Prevalence of BLV in this herd was 0.807. Predictive value of a positive test was 1.00, and predictive value of a negative test was 0.421. The percentage of cows correctly classified by PCR assay was 73.5%.

CONCLUSIONS AND CLINICAL RELEVANCE

A positive PCR assay result provided definitive evidence that a cow was infected with BLV. Sensitivity and negative predictive value for PCR assay were low. Consequently, PCR assay alone is unreliable for routine detection of BLV in herds with high prevalence of the disease.