Location: Animal Disease Research2009 Annual Report
1a. Objectives (from AD-416)
(1) Determine the genetic organization and expression of Ovine MHC Class I, IIa, and IIb loci, (2) Determine and validate that specific MHC Class I and/or MCH Class II alleles associate with OPPV clinical disease phenotypes and, (3) Determine the functional significance and subsequent immune responses of MHC alleles that associate with OPPV clinical disease phenotypes.
1b. Approach (from AD-416)
Chronic persistent infections such as ovine progressive pneumonia virus (OPPV) cause significant economic losses because of the current inability to predict transmission risk and which infected animals will progressive to clinical disease. Current methods of diagnosis are often not used because many of the most economically important animals are infected and their long-term health and production characteristics are unknown. Serological diagnostic tests which test for the presence of anti-OPPV antibodies have provided a highly sensitive and specific means of testing but have not shown statistical correlations with pathology or other clinical markers of disease. Therefore, an OPPV test that predicts or determines which infected sheep will proceed to clinical disease progression is highly sought. Our laboratory is currently evaluating two different tests for the prediction or determination of OPPV clinical disease. One test is a quantitative PCR test utilizing real time technology, which targets a conserved region of the transmembrane protein of OPP provirus, and the second test is an immunogenetics test for MHC Class II DRB1. With one or both of these tests, we hope to provide a diagnostic OPPV test that determines or predicts whether the sheep will progress to OPPV clinical signs. In addition, these types of tests will significantly reduce the number of other tests necessary for determining infection and possibly lower the transmission potential in a flock. Therefore, these new tests offer significant long-term economic advantages for the producer over conventional serological diagnostic tests. Replacing 5348-32000-025-00D, April 2007.
3. Progress Report
Ovine progressive pneumonia virus (OPPV) is a lentivirus disease of sheep, which causes breathing problems, hard-udder, lower milk production, swollen and/or arthritic joints, wasting syndrome, and in rare cases, the loss of full control of bodily movements. Once an adult animal has been infected with OPPV, the animal cannot clear the infection, and remains persistently infected for life. Earlier reports have indicated that some sheep have a higher incidence of OPPV infection; therefore, it is believed that control of OPPV is highly dependent upon sheep genetics. Since the recommended bi-annual testing of sheep for OPPV infection is cost prohibitive for most sheep producers, our main focus is to evaluate whether genetic markers can be utilized as predictive tools of disease severity in naturally OPPV infected sheep. Since our last progress report, OPPV levels in the blood were found to quantitatively correlate with the severity of disease as measured by tissue changes in the lungs, udder, joints, and brain of naturally infected sheep. Furthermore, OPPV levels in the blood were monitored in three sheep breeds, and interestingly, the Rambouillet breed at five and six years of age showed significantly lower OPPV levels than the Polypay or Columbia breeds at the same age. Two different immune response genetic markers, CCR5 and DRB1 were also evaluated in respect to OPPV levels in the blood. Sheep with a particular genetic marker of CCR5 (CCR5 is a genetic marker of the sheep immune system) maintained two-fold less OPPV levels in the blood as compared to other sheep without this CCR5 marker. In addition, sheep with genetic markers DRB1*0403 and DRB1*07012 showed two-fold and ten-fold lower OPPV levels in the blood, respectively, as compared to sheep without these DRB1 markers. Since DRB1 is also thought to participate as part of the receptor for OPPV, DRB1*07012 and DRB1*1202, a common DRB1 marker, have been separately transfected into cells along with their other co-receptor counterpart, and these transfected cells will be evaluated for their ability to support OPPV growth. In the future, these genetic markers for OPPV will need to be confirmed in a number of different sheep flocks. In addition, production traits, other diseases, and other genetic markers will be further evaluated in respect to the immune response genetic markers to ensure that selection for these genetic markers does not impose any detrimental effects. The long term outcome of this research is to provide tools which predict the clinical course of OPPV infection in sheep thereby greatly decreasing the economic impact of this infection. This work also serves as guidance for other chronic persistent infections for which the economic losses are derived later in the life of the infected animal.
1. Prediction of disease outcome in sheep infected with Ovine Progressive Pneumonia Virus (OPPV). The inability to predict which OPPV infected sheep will progress to clinical disease and economic loss is a severe burden on the producer. ARS scientists at Pullman, WA showed that levels of OPPV in the blood of chronically infected sheep predicted disease outcome in lung, mammary gland, joints and brain. Use of easily obtained OPPV blood levels will allow for rapid validation of genetic markers capable of predicting the outcome of OPPV infection. This will signicantly reduce the cost of biannual testing of sheep for OPPV infection.
2. Blood levels of Ovine Progressive Pneumonia Virus (OPPV) associate with breed and DRB1 gene, an immune response marker. Cost effective methods which predict the outcome of chronic persistent infections such as OPPV must be developed to decrease the economic impact of such infections. ARS scientists at Pullman, WA showed that Rambouillet sheep exhibited lower OPPV levels than Polypay or Columbia sheep at the same age. The markers DRB1*0403 or DRB1*07012, found in Rambouillet sheep, correlated with lower OPPV levels than sheep with other DRB1 markers. Validation of the ability of these markers to predict OPPV progressive in sheep is a first step in developing genetic tools for producers to manage losses from chronic persistent infections such as OPPV.
3. Control of persistent Ovine Progressive Pneumonia Virus (OPPV) infection is not linked to selection of scrapie resistant genotypes. A large component of the current scrapie (prion disease in sheep) control program is selection of sheep possessing genetic markers which predict resistance to infection and disease. It is unknown whether the widespread genetic selection of sheep resistance to scrapie was impacting control of other persistent infections such as OPPV. ARS scientists at Pullman, WA showed that selection of scrapie resistance genotypes didn’t impact control of OPPV levels in blood of infected sheep. This result allows sheep producers to continue toward scrapie eradication with the knowledge that they are not impacting the genetics of OPPV control.
5. Significant Activities that Support Special Target Populations
We continue to monitor the performance of the molecular diagnostic test, OPPV levels in the blood, against a serological diagnostic test, a competitive enzyme-linked immunosorbent assay, within differing production and U.S. state sheep populations. We collaborate with a number of small sheep producer operations in the U.S. states of New York, Montana, North Dakota, Iowa, Colorado, Minnesota, and Oregon. In these collaborations, we provide OPPV diagnostic test results to the producers while the producers allow us to genetically test their sheep.
Hoesing, L.M., White, S.N., Mousel, M.R., Lewis, G.S., Knowles Jr, D.P. 2008. Ovine progressive pneumonia provirus levels associate with breed and Ovar-DRB1. Immunogenetics. 60(12):749-758.