Control and eradication methods for bovine viral diarrhea virus

Abstract

The objectives of this research were 1) to determine the efficacy of an antigen-capture (AC)-ELISA and a micro-titer virus isolation (MTVI)-ELISA in identifying persistently infected (PI) animals from pooled samples; 2) evaluate the economic benefit of pooling samples for PI animal identification; 3) examine the merit of a diagnostic assay able to identify PI animals in utero; and 4) to develop an indirect capture ELISA capable of identifying PI animals in utero. To reduce the cost of whole herd screening for BVDV PI animals, the sensitivity and specificity of an AC-ELISA and a MTVI-ELISA using saline from ear notch samples or pooled serum was determined. Pooled saline from ear notch samples, assayed by ACELISA, gave a sensitivity and specificity of 98% and 94%, respectively for pools containing two samples and 72% and 100%, for pools of five. The sensitivity of pooled ear notch or serum samples for bovine viral diarrhea virus detection by MTVI-ELISA (sensitivity <5%) or serum samples for detection by AC-ELISA (sensitivity <15%) was found to be too low for use in whole herd screening. Pooling saline from ear notch samples from two animals tested by antigen-capture ELISA, however, could provide a less expensive, reliable method for whole herd screening for bovine viral diarrhea virus. To assess the economic benefit of using pooled saline from ear notch samples for AC-ELISA, a simulation model (BTMSim$) was used to determine the cost per cow for whole herd screening and time to BVDV eradication. Identification and removal of PI animals was simulated for years 1, 2 or 3 after BVDV introduction. Simulation results indicate the time to BVDV eradication could increase by one year when using pools of 2 or 3 and may never be achieved using pools of 4 or 5. Simulation herds where BVDV infection was becoming endemic, i.e. 3 years after initial introduction of BVDV, resulted in significantly lower testing costs when pooling samples than when testing individual animals. Persistently infected animals can transmit BVDV as soon as they are born. Therefore, detection of PI animals in utero could greatly benefit cattle producers. To evaluate the impact of PI animal identification in utero a BVDV transmission model, BTMSim, was modified to examine the efficacy of a diagnostic test able to identify PI animals in utero. Simulation results from BTMSim_inUtero indicate that identification of PI animals in utero does not decrease the number of years required to test for PI animals in order to eliminate BVDV from a herd, nor does earlier detection decrease the median number of PI animals born. While use of an in utero diagnostic assay may not result in elimination of BVDV transmission sooner than using assay methods currently available, identification of replacement heifers carrying PI calves, prior to introduction to a herd would still be useful in preventing the spread of BVDV. Currently, there are no diagnostic assays able to identify PI animals in utero. An indirect capture ELISA to detect anti-BVDV IgA in the nasal secretions of cows was developed in an attempt to identify BVDV PI fetuses in utero. The concentration of IgA, based on optical density (OD) values, in nasal secretion samples from BVDV unexposed and BVDV exposed animals was determined. Using ROC curve analysis, a threshold OD of 0.12 gave an optimal sensitivity of 65% and a specificity of 80%. To determine if IgA levels in the nasal secretions of cows carrying BVDV persistently infected (PI) fetuses were greater than those of cows not carrying PI fetuses, the concentration of anti-BVDV IgA present in the nasal secretions of 143 cows at 7 to 8 months gestation was obtained. Of the 143 cows, 19 had IgA concentrations 2-fold greater than the positive control. Included in those 19, was a cow carrying a PI fetus. This research shows that there are varying methods for identifying PI animals to eradicate BVDV from a herd. In addition to current methods of identification, the research identifies opportunities for future development of more efficacious BVDV diagnostic tools.