Biomedical subjects

W O Johnson

Publications and source records attributed to W O Johnson.

25 records · Page 2Linked to original sources

PubMed1998

Evaluation of bluetongue virus diagnostic tests in free-ranging bighorn sheep.

Five bluetongue virus (BTV) diagnostic tests were evaluated for use in free-ranging bighorn sheep. We sampled one bighorn sheep population four times between 1989 and 1995. The tests evaluated included virus isolation (VI), polymerase-chain reaction (PCR), serum neutralization (SN), agar-gel immunodiffusion (AGID), and competitive enzyme-linked immunosorbent assay (c-ELISA). The c-ELISA, AGID and SN tests had high levels of agreement in determining serogroup exposure in bighorn sheep. We used maximum-likelihood algorithms to estimate the parameters of each diagnostic test used. Although the c-ELISA and AGID had high sensitivity and specificity, the SN had perfect specificity but lower apparent sensitivity. Due to the potential of cross-reactions among multiple serotypes, results of the SN must be interpreted with caution when assessing serotype exposure in an area where multiple serotypes are endemic. The PCR assay delineated convalescent antibody titers from more-recent infections, and consequently, was pivotal in distinguishing a different exposure pattern between the bighorn sheep and cattle in an adjacent herd. Based on an increasing seroprevalence (50% to 100%), BTV circulated through this bighorn sheep population between 1989 and 1993. This increase in seroprevalence coincided with a bighorn die-off due to BTV infection in June, 1991. An adjacent cattle herd was sampled in 1995 for comparison. The bighorn sheep and adjacent cattle had different patterns of exposure to BTV between 1994 and 1995. There was no evidence that BTV circulated through the bighorn sheep population from 1994 to 1995. In 1995, seroprevalence to BTV decreased to 72%, none of yearling bighorn was seropositive, and all of the 39 bighorn sheep were PCR-negative. In contrast, all adult cattle were seropositive to BTV by c-ELISA and SN, and 4 of the calves were seropositive; 11 of the 24 cattle were PCR-positive, including all five calves. Overall, the pattern of temporal herd immunity in the bighorn sheep appeared to follow a classic epidemic curve, with the appearance and subsequent disappearance of herd immunity coinciding with the 1991 die-off in this population. As low levels of herd immunity and high proportions of susceptible animals are key factors in the development of epidemics, this population of bighorn sheep may be at increased risk for a BTV epidemic in the future.

Animals↗

PubMed1991

Factors associated with transmission of bovine leukemia virus by contact in cows on a California dairy.

A prospective study was undertaken on a California dairy from 1984 to 1987 to examine factors associated with contact transmission of bovine leukemia virus in cows. Two approaches were used to model the probability of infection. First, the expected number of new infections per pen-month was assumed to follow a binomial distribution. The probability of infection was modeled, using logistic regression, as a function of prevalence of infection in pen, presence of lactating cows, proportion of pregnant cows, presence of an infected bull, and proportion of infected cows with a lymphocyte count above 10,000 cells/microliters of blood and/or with Mr 24,000 protein (p24) antibodies. The probability of infection was significantly associated with the prevalence of infection and at the limit of significance for presence of lactating cows in pen. Second, the Cox model with time-dependent covariates was used to analyze time from first parturition or from the beginning of the study to infection. Factors examined were age, breed, lactation number, whether the cow was pregnant and/or lactating, prevalence of infection, presence of an infected bull, and density of cattle in the pen holding the cow. Non-pregnant cows were 2.9 times more at risk of infection than were pregnant cows, and risk of infection significantly increased as prevalence of infection in a pen increased.

Animals↗

PubMed1991

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).

Animals↗

PubMed1990

Effect of brucellosis vaccination and dehorning on transmission of bovine leukemia virus in heifers on a California dairy.

Brucellosis vaccination and dehorning were examined for an association with bovine leukemia virus (BLV) infection in heifers on a California dairy between April 1984 and June 1987. Between December 1985 and June 1986, weaned heifers were dehorned using the gouge method at the time of brucellosis vaccination. Using logistic regression, the estimated probability for a nondehorned heifer to seroconvert within three months after brucellosis vaccination (0.08) was significantly less than that for heifers dehorned after a noninfected heifer (0.46) or than that for heifers dehorned after an infected heifer (0.85) (p = 0.039 and p less than 0.001, respectively). To evaluate risk of transmission by brucellosis vaccination, which was usually done within one month postweaning, cumulative proportions of heifers remaining uninfected were computed among heifers that did not seroconvert three months after dehorning. Because results of a Cox model analysis indicated that groups of heifers were 6.6 times more at risk of becoming infected if placed in pens holding gouge-dehorned heifers (where prevalence varied between 50 and 70%) (p less than 0.001) than other groups placed in pens without gouge-dehorned heifers (where prevalence varied between 10 and 30%), cumulative proportions of heifers remaining uninfected were computed for each type of group. The cumulative proportion of heifers remaining uninfected from weaning to first calving was 0.60 for the high prevalence group and 0.96 for the low prevalence group. No change in slope of cumulative proportions was observed before and after one month postweaning, suggesting that brucellosis vaccination was not an effective means of transmission.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1989

Regression models for time to seroconversion following experimental bovine leukaemia virus infection.

This paper develops a parametric model for time to seroconversion after experimental bovine leukaemia virus (BLV) infection, and examines the effects of inoculation route, volume of inoculum, type of inoculation material, and antigen status of donor on seroconversion time. We used parametric and nonparametric statistical methodology to analyse interval data on 150 animals from 13 published reports. The log-logistic model fitted the observed times to seroconversion better than the log-normal or Weibull models, which were the considered alternatives.

Animals↗

PubMed1989

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.

Age Factors↗

PubMed1985

Measurement of erythrocyte velocity by use of a periodic differential detector.

An optical velocimeter employing a linear array of photodiodes has been developed and utilized for measuring erythrocyte velocities in the microcirculation. A magnified image of a microvessel is projected and aligned on a one-dimensional array of photodiodes. Photocurrent from odd-ordered diodes is summed, photocurrent from even-ordered diodes is summed, and a signal proportional to the difference between these two currents is produced by a differential amplifier. The center frequency of the output signal of the differential amplifier is proportional to the erythrocyte velocity. After lowpass filtering the output of the differential amplifier, a signal proportional to its frequency and therefore velocity is produced by a frequency-voltage converter. In vitro calibration with a moving dried smear of erythrocytes illustrated a linear relation between the output of the frequency-voltage converter and erythrocyte velocity for a wide range of velocities and magnifications. The system produces a stable zero output at zero velocity and had an estimated frequency response of greater than 40 Hz in vivo. Volumetric flow rates computed from velocity and diameter measurements at arteriolar bifurcations in the rat cremaster muscle were consistent with mass conservation.

Animals↗