The epidemiology of ovine progressive pneumonia (ovine lentivirus): Diagnosis, risk factor analysis, and production effects in sheep at the United States Meat Animal Research Center

Molecular diagnostic tests to detect ovine lentivirus (OLV) infected sheep were developed and validated and then applied to study the epidemiology and production impacts of natural OLV infection in flocks at the U.S. Meat Animal Research Center (MARC) in Nebraska. Three OLV recombinant proteins (transmembrane (TM), major capsid P25, and matrix P16) were produced in E. coli and purified for use as antigen for anti-OLV antibody detection by indirect enzyme-linked immunosorbent assay (ELISA). In a comparison of recombinant TM, P25, and P16 ELISAs, a whole-virus ELISA and the agar gel immunodiffusion test (AGIDT) for antibody detection in 412 field sera, the TM ELISA was highly specific, had the highest relative sensitivity of the 5 tests, and detected 95% of sheep with clinical OLV lung lesions. The AGIDT had a relative sensitivity of only 54% compared to the TM ELISA. The P25 and P16 ELISAs were complementary to the TM ELISA and detected 15% additional positive sheep which were TM ELISA-negative. When serum antibody conversion was compared over time in a cohort of 105 two-year old crossbred ewes, the TM ELISA detected antibodies up to 1.5 years sooner than the AGIDT. Most antibody conversion occurred in the breeding through weaning production period. The TM ELISA also detected anti-OLV antibodies in 83% of colostral and 75% of milk samples from seropositive ewes collected at 1 day and 8 weeks post-parturition. A significant association between subclinical mastitis and ewe TM ELISA seropositivity occurred at 8 weeks post-parturition (weaning). Using the TM and P25 ELISAs, the prevalence of and risk factors for OLV infection among 1466 breeding ewes in 9 MARC flocks and their 328 yearling replacement offspring were determined. Overall, 48% of ewes and 41% of yearlings were ELISA-positive. Based on multiple logistic regression, greater risk of ewe OLV infection was significantly associated with confinement birth and rearing (odds ratio (OR) = 1.6), older weaning ages (OR = 1.1 per week), and increasing age (OR = 1.3 to 2.5 per year beyond age 1). Seroprevalence varied greatly by flock, with Finnsheep and Texel ewes having the highest and Booroola Merino and Suffolk ewes having the lowest prevalence. Among yearling replacements, increased risk of OLV infection was associated with dam seropositivity (OR = 2.0) and difficult birth (OR = 3.0) while decreased risk was associated with artificial (nursery) rearing (OR = 0.44). Yearling seroprevalence varied by flock, with Texels at highest risk. The production impacts of dam TM ELISA seropositivity on ewe and lamb productivity were estimated using the 1466 breeding ewes and their 2452 lambs born in spring 1992 by multiple linear and logistic regression. TM ELISA-negative ewes produced significantly more pounds of weaned lamb per ewe lambing (8.5 lb.) and per ewe exposed (10.9 lb.) compared to their OLV-positive flockmates. (Abstract shortened by UMI.)U of I OnlyETDs are only available to UIUC Users without author permissio