Natural transmission of bovine leukemia virus in dairy calves by dehorning.

Gouge dehorning was evaluated as a mode of transmitting bovine leukemia virus in Holstein calves at a commercial dairy. Significantly (p less than 0.05) more calves dehorned by the gouge method developed antibodies to bovine leukemia virus, as measured by agar-gel immunodiffusion, three months after dehorning, than calves not dehorned. The field use of a blood-contaminated dehorning device resulted in transmission of bovine leukemia virus.     

Factors associated with in utero or periparturient transmission of bovine leukemia virus in calves on a California dairy.

A three-year prospective study involving 143 calves born from infected cows was undertaken on a California dairy to evaluate possible factors of the dam associated with bovine leukemia virus infection in utero or during the periparturient period. In utero or periparturient infection occurred at a rate of 4.8% and was more likely in calves born to cows with an average peripheral blood lymphocyte count during pregnancy greater than 12,000 cells/microL (p = 0.043) or in calves born to cows that developed malignant lymphoma (p = 0.00004), but not in calves born to cows with p-24 antibodies (p = 0.675).     

Vertical transmission of bovine leukemia virus and bovine immunodeficiency virus in dairy cattle herds.

Vertical transmission of bovine leukemia virus (BLV) and bovine immunodeficiency virus (BIV) was investigated in five dairy cattle herds in Hokkaido, where 36.1 and 17.0% of cattle were BLV and BIV seropositive, respectively, and 9.9% of dams were co-infected with both BIV and BLV. Twenty six cases of offspring born from dams infected with only BLV (17 cases) or with both BIV and BLV (9 cases) were examined for the presence of BLV and BIV before and after colostrum feeding by polymerase chain reaction (PCR) and syncytium assay. After birth, all calves were separated immediately from their dams. The offspring born from BLV-positive dams were BLV-negative before colostrum feeding, suggesting that no transplacental transmission had occurred. Thereafter, these offspring were fed colostrum or milk from their dams, but still remained BLV-negative. The other offspring born from BLV-positive dams were fed with BLV-negative colostrum, or with pasteurized BLV-positive colostrum. All these calves remained negative for BLV infection, suggesting that in utero transmission of BLV is negligible. In the case of offspring born from dams co-infected with BLV and BIV, calves were BIV-positive before colostrum feeding at 1 day after the birth, indicating in utero transmission of BIV. After colostrum feeding from their dams, newborn calves became BLV-positive. In addition, one calf was BLV-positive even before colostrum feeding. These results suggest that BIV can be transmitted to offspring in utero, and that BLV can be transmitted through colostrum or milk if dams are infected with both BIV and BLV.     

Rectal palpation and transmission of bovine leukemia virus in dairy cattle.

To determine whether rectal palpation, using common obstetrical sleeves, serves as a mode of transmission of bovine leukemia virus in dairy cattle, field studies were conducted at 2 dairies. At a commercial dairy, significant difference was not observed in rate of seroconversion in heifers and cows in which the same sleeve or new sleeves were used for palpations. At a university dairy, where cattle were used to teach dairy husbandry and veterinary procedures, significantly (P less than 0.02) greater rate of seroconversion was observed in heifers and cows palpated with unwashed common sleeves than that observed in heifers and cows palpated with sleeves washed between use. Although rectal transmission of bovine leukemia virus under field conditions was documented, it was related to frequency of palpation and age of cattle.     

Control of bovine leukosis virus in a dairy herd by a change in dehorning.

Following the demonstration that bovine leukosis virus was transmitted in calves by gouge dehorning, electrical dehorning at a younger age was implemented in a commercial Holstein herd. Subsequently, annual testing of the herd revealed a decline in the prevalence of bovine leukosis virus antibodies as older cattle dehorned by the former method were replaced by younger cattle dehorned by the latter method.     

Protection of colostral antibodies against bovine leukemia virus infection in calves on a California dairy.

A three-year prospective study involving 244 calves was undertaken on a California dairy to evaluate the protective role of colostral antibodies against bovine leukemia virus (BLV) infection in calves. Calves were followed from birth to the time they left their individual hutch (TLIH), at about 90 days of age. The probability of being infected at TLIH and the daily risk of infection between birth and TLIH were modelled using the logistic and the Cox models, respectively. Calves with no detectable antibodies during the first week of life were up to 2.00 and 2.75 times more likely to be infected at TLIH compared to calves with low and high concentrations of antibodies during the first week of life, respectively (p = 0.01). When the daily risk was modelled, calves without antibodies at the estimated day of infection were up to 3.4 and 11.6 times more likely to become infected than calves with low and high concentrations of antibodies on that day, respectively (p less than 0.001). Results indicated that calfhood infection may be reduced by about 45% through the feeding of colostrum with BLV antibodies. Further reduction in infection may be possible by feeding calves milk powder, milk replacer, and/or milk from noninfected cows. Results also indicated that quantification of the effect of a time-dependent risk factor, such as colostral antibody concentration, might be affected if treated as a fixed factor.     

Effect of calfhood vaccination on transmission of bovine viral diarrhea virus under typical drylot dairy conditions

OBJECTIVE: To estimate transmission of bovine viral diarrhea virus (BVDV) and crude morbidity and mortality ratios in BVDV-vaccinated and unvaccinated dairy heifer calves managed under typical dairy drylot conditions. DESIGN: Randomized clinical trial. ANIMALS: 106 female Holstein calves. PROCEDURE: Seroconversion rates for BVDV types I and II and proportional morbidity and mortality ratios were compared between calves given a killed BVDV type-I vaccine at 15 days of age and a modified-live BVDV type-I vaccine at 40 to 45 days of age (n = 53) and calves given no BVDV vaccines (53). Sera were collected at 45-day intervals as calves moved from individual hutches to corrals holding increasingly larger numbers of calves. Seroconversion was used as evidence of exposure to BVDV. RESULTS: Crude proportional morbidity (0.16) and mortality (0.17) ratios for control calves did not differ significantly from those of vaccinated calves (0.28 and 0.12, respectively). The proportion of control calves that seroconverted to BVDV type I through 9 months of age (0.629) was significantly higher than that of vaccinated calves that seroconverted, unrelated to vaccination, during the same period (0.536). Estimated overall protective effect of vaccination against BVDV type I through 4 to 9 months of age was 48%. The proportion of control calves that seroconverted to BVDV type II (0.356) was not different from that of vaccinated calves (0.470). CONCLUSIONS AND CLINICAL RELEVANCE: Findings suggest that calfhood vaccination may be an appropriate strategy to help reduce short-term transmission of some but not necessarily all strains of BVDV.     

Seroprevalence of bovine immunodeficiency-like virus and bovine leukemia virus in a dairy cattle herd.

To determine the prevalence of single vs. dual infection with bovine immunodeficiency virus (BIV) and bovine leukemia virus (BLV), sera (n = 95) from a dairy cattle herd were analyzed for anti-BIV and anti-BLV antibodies by an enzyme linked immunosorbent assay. Twenty-one percent (20/95) of samples were BIV-seropositive, while 52% (49/95) of the same samples were BLV-seropositive. A significantly greater percentage of BIV-seronegative samples were BLV-seropositive, 57% (43/75), than were BIV-seropositive samples, 30% (6/20). There was no significant correlation between data ranked from least to greatest amount of anti-viral antibody. Five cattle had persistent lymphocytosis (PL); all five were BLV-seropositive and two were BIV-positive. The mean anti-BLV titer was significantly greater in PL cattle, as compared at non-PL cattle, whereas there was no significant difference between the mean anti-BIV titer in PL cattle, as compared with non-PL cattle. These results provide additional information on the seroprevalence of naturally occurring BIV infection, and indicate that BIV can exist independent of other common infectious agents, such as BLV. Further, the results suggest that infection with BIV is not associated with an increased rate of infection with other infectious agents such as BLV.     

Evidence for transmission of bovine leukemia virus by rectal palpation in a commercial dairy herd

The risk of Bovine Leukemia Virus (BLV) transmission by rectal examination was determined over 22 months in a commercial dairy herd. All 167 BLV seronegative cattle, of breeding age or greater, were divided randomly into two groups and identified by neck-chain color. In the treatment group, routine rectal palpation occurred after a BLV infected animal and without a change of sleeve, while in the other group, palpation occurred in a similar manner with the exception that sleeves were changed between animals. When BLV seronegative cattle in either group were palpated after BLV infected cattle, the event and identification of the cattle involved were recorded. Serologic testing was performed eight times during the 22 month study to determine the number of animals that became infected following a palpation (an event). Thirty-one animals seroconverted during the study; 24 in the treatment (no sleeve change) group and seven in the sleeve change group. Sixteen of the animals in the treatment group that seroconverted had been palpated prior to their seroconversion. A hazard ratio (relative risk) for BLV seroconversion was determined between the two groups. Cows palpated with no sleeve change had a 2.8-fold increase in risk (confidence interval 1.1–6.8) of BLV infection. The increased risk of BLV infection associated with rectal palpation may have been affected by the presence of some highly infectious cows in the herd. This study confirms that rectal palpation without a change of sleeve may be a significant risk factor in some herds, and if efforts are made to decrease the spread of BLV in a herd, the potential for rectal sleeve transmission must be considered.     

Recent developments in vaccination against bovine brucellosis.

Recent research in vaccination against bovine brucellosis has been directed towards reducing the serological response to vaccination and to developing serological tests better able to distinguish vaccinal titres from those resulting from field infection. In the case of strain 19 vaccination, developments such as reducing the dose of vaccine and improving serological tests have so reduced the serological response as to allow adult vaccination to be carried out in combination with eradiction by test and slaughter. B. abortus 45/20 vaccine is being increasingly used as a combined diagnostic aid and protective agent. Progress has been made in identifying antibodies produced in response to this vaccine.     

Prevaccination bovine viral diarrhea virus titers and subsequent reproductive performance in dairy heifers.

The study was conducted to determine if there was a relationship between prevaccination bovine viral diarrhea virus (BVDV) titers in 14 month-old dairy heifers and time open during first lactation, length of the conception interval, fetal survival during the second pregnancy, and age at culling. Possible associations were analyzed using nonparametric survival methods and survival distributions were described using the product-limit (Kaplan Meier) methods. Regression analysis was used to estimate the correlation between pre- and postvaccination BVDV titers three years after vaccination. Evidence of exposure to BVDV prior to 14 months of age was demonstrated by serum-virus-neutralization titers greater than 1:4 for 97% and greater than or equal to 1:128 for 67% of the 72 heifers followed. No correlation was found for 38 pairs of prevaccination and postvaccination titers (p = 0.76). The risk of being culled was high for heifers (n = 41) with very low (1:2-1:8) or very high (> or = 1:256) titers, but low for heifers (n = 31) with moderate (1:16-1:128) prevaccination titers (p = 0.098). Risk of subsequent fetal loss was high for heifers (n = 30) with very low (1:2-1:16) or very high (> or = 1:256) prevaccination titers, compared to heifers (n = 24) with moderate (1:32-1:128) titers (p = 0.084). These findings suggest that prevaccination exposure to BVDV eliciting either a very weak or a very strong serological response may contribute to subsequent reproductive inefficiency and an increased risk of culling.     

Embryo transfer and transmission of bovine leukosis virus in a dairy herd

Transmission of bovine leukosis virus (BLV) by embryo transfer was investigated in a large commercial Holstein herd. One hundred and sixteen calves, transplanted as embryos from BLV-positive cows into BLV-negative heifers, were serologically negative, as were recipients, whereas 5 of 29 (17%) calves transplanted as embryos into BLV-positive recipients were infected with BLV, as evidenced by antibodies in the agar gel immunodiffusion test.     

Effects of bovine leukemia virus infection on production and reproduction in dairy cattle.

The purpose of this study was to determine the effects of bovine leukemia virus (BLV) infection on production, reproduction and longevity in dairy cattle. The study population was a commercial Holstein dairy herd of approximately 400 milking cows. Cattle were tested for antibodies to BLV at least annually for three years and when culled. Four groups of culled cows were compared: seronegative cows (n = 79), seropositive cows without lymphocytosis (n = 176), seropositive cows with lymphocytosis (> or = 9,000 lymphocytes/microliter) (n = 74), and seropositive cows with lymphosarcoma (n = 29). Seropositive groups of cows were bred more times and had longer calving intervals than seronegative cows. The seropositive groups had greater 305-day ME (mature equivalent) FCM (3.5% fat-corrected milk) per lactation and were older when culled than seronegative cows. However, the percent fat per lactation was greater in seronegative cows. In the last complete lactation, differences in 305-day ME FCM, days open and cull age between groups were reduced and none were significant (p > 0.05). In the cull lactation, only cows with lymphocytosis had reduced milk production relative to seronegative cows, although this difference was not significant. After adjustment for initial production and reproductive values, only seropositive nonlymphocytotic cows were culled at a significantly older age than seronegative cattle. Lymphocytotic cows were culled four months younger on average than nonlymphocytotic seropositive cows. Hence, BLV infected cows had greater milk production on average than uninfected cows. Adverse effects of BLV infection were primarily limited to lymphocytotic cows which were culled earlier and had reduced milk production in the cull lactation.     

In utero transmission of bovine leukemia virus

In an initial study, 18 calves born to cows persistently infected with bovine leukemia virus (BLV) were tested for infective virus and antibodies at birth, and no infected or seropositive animals were found. Four of these calves were maintained in quarters where infected animals were housed, and 3 of the 4 subsequently became infected. These were probably contact infections acquired during, or at some time after, birth. The remaining 14 calves were kept in isolation pens in a building housing no infected cattle. None of this group was found to be BLV infected during 1 year of observation. In further studies, 15 pregnant cows inoculated with BLV became infected. One abortion, considered to be unrelated to the BLV inoculation, occurred 38 days later. The remaining 14 cows gave birth to 1 dead and 14 live calves. The dead calf and its live twin were seropositive for BLV at birth, indicating that they had been infected in utero. The remaining 13 calves were negative for BLV antibodies at birth and remained so during 1 year of observation.     

Seroprevalence of bovine immunodeficiency virus and bovine leukemia virus in dairy and beef cattle in hokkaido

Serological survey of bovine immunodeficiency virus (BIV) and bovine leukemia virus (BLV) infection was conducted in dairy cattle from 10 different regions of Hokkaido, Japan. Among 390 cattle, 11.0% of cattle were BIV-seropositive and 3.3% were BLV-seropositive. Moreover, in two dairy farms, where bovine leukosis has been reported, prevalence of BIV infections were 6.4 and 9.1%, respectively. In contrast, among 150 beef cattle, 16.6% were BIV-seropositive while none was BLV-seropositive. Dual infections with BLV and BIV in dairy cattle were tested by using 107 BLV-seropositive sera, and 20 sera were found BIV-positive (18.7%). These results indicate that BIV infection was widespread in Hokkaido.     

Lack of evidence of transmission of bovine leukemia virus by rectal palpation of dairy cows

A study was undertaken in a California dairy to estimate the risk of transmission of bovine leukemia virus associated with rectal palpation of cows. The probability of seroconversion within 3 months after rectal palpation was examined as a function of the prevalence of infection in cows at the time of palpation. It was hypothesized that the probability of seroconversion within 3 months after palpation would be higher for cows in a group with high prevalence of infection than for cows in a group with low prevalence. The probability of seroconversion after rectal palpation was equal to 0.034, and results of logistic regression, using data from 1,116 noninfected and 1,047 infected cows, did not indicate any significant association between probability of seroconversion after rectal palpation and prevalence of infection (P = 0.189). Results of this study suggest that risk of bovine leukemia virus transmission by rectal palpation typically used in reproductive tract examination of cows either does not occur or is uncommon.     

Descriptive epidemiology of postnatal bovine viral diarrhea virus infection in intensively managed dairy heifers.

OBJECTIVE: To evaluate risk of bovine viral diarrhea virus (BVDV) infection between birth and 9 months of age for dairy replacement heifers raised under typical dry-lot management conditions. DESIGN: Longitudinal observational study. ANIMALS: 446 calves. PROCEDURE: Calves were randomly selected from 2 dairies that used killed and modified-live BVDV vaccines. Repeated serologic and BVDV polymerase chain reaction assays were used to estimate risk of BVDV infection in calves of various ages (1 to 60 days; 61 to 100 days; 101 days to 9 months) and to estimate overall infection rate by 9 months of age. RESULTS: Risk of BVDV infection increased with age (maximum risk, 150 to 260 days). Proportion of calves infected with BVDV by 9 months of age was higher for dairy A (0.665), compared with dairy B (0.357). Percentage infected with BVDV type I did not differ between dairy A (18.2%) and dairy B (15.2%), whereas percentage infected with BVDV type II for dairy A (50%) was twice that for dairy B (21%). Between 210 and 220 days of age, infection with BVDV regardless of type was > 1.3%/d on dairy A and 0.5%/d on dairy B. CONCLUSIONS AND CLINICAL RELEVANCE: Under dry-lot conditions, a considerable amount of BVDV infection may occur before 9 months of age. Risk of infection increases with age. Although dairies may appear to have similar management practices, there can be considerably different risks of BVDV infection among dairies.     

Possible effect of altered management practices on seroprevalence of bovine leukemia virus in heifers of a dairy herd with history of high prevalence of infection.

A bovine leukemia virus (BLV) control program was instituted at a high-prevalence, purebred Holstein dairy. The specific control program required single use of needles and obstetrical sleeves, disinfection of tattoo equipment before use, dehorning by use of electrical burning devices, replacement of whole milk feeding with high-quality milk replacer, and heat treatment of colostrum before feeding. The effect of the program was estimated by comparing the pre- (1987) and postintervention (1989) prevalences of BLV-agar gel immunodiffusion titers within particular heifer age cohorts (0 to 90, 91 to 180, 181 to 360 days, and 361 days to parturition). Random serologic testing was conducted quarterly within each cohort, but was analyzed on a combined yearly basis. Seroprevalence of BLV decreased in all age cohorts greater than 180 days old, with significant decreases in the 361 days to parturition (P = 0.003) and the combined 181 days to parturition cohorts (P = 0.0004). Prevalence within the latter cohort decreased from 0.44 in 1987 to 0.17 in 1989. These results were obtained without either culling or segregation of BLV-agar gel immunodiffusion-positive cattle.