A study of the sensitivity and specificity of an immunoperoxidase technique for the diagnosis of maedi-visna virus infection in sheep

Abstract

In this study the sensitivity and specificity of an immunoperoxidase technique for the detection of lentiviral protein in formalin-fixed lung tissues of sheep were assessed. Formalin-fixed specimens of lungs of 52 sheep originating from two different infected flocks and formalin-fixed lung specimens of 20 sheep from a third, non-infected flock were selected. Multiple sections from each animal were stained with a monoclonal antibody against Maedi-Visna Virus (MVV) antigen. Control sections were stained with an irrelevant antibody, and without antibody, respectively in both the positive and negative groups. Histologic lesions of Maedi were graded according to severity and the number of sections staining positive. The average count of infected cells was determined for all animals which exhibited positive staining against viral protein. It was determined that the IMP technique had a sensitivity of 51,9% and a specificity of 100% if three sections of each animal were examined in both infected flocks. The sensitivity was greatly influenced by the presence or absence of typical histological lesions and the average cell counts seem to be correlated with the severity of the histological lesions. Animals without histological lesions were mostly negative with the IMP technique. The sensitivity was also influenced by the number of sections examined per animal and decreased if fewer sections were examined. Statistical analysis confirmed that the proportions of sections which were positive by means of IMP staining differed significantly between the four histological categories of normal, mild, moderate and severe. The more severe the lesions, the higher the proportion of sections were that could be diagnosed as infected by means of the IMP technique. It also confirmed that the ability to detect MVV infection in sections decreases significantly with a smaller sample size. In conclusion it would seem that this technique may be used in confirming infection in individual animals selected on the presence of typical pulmonary lesions. It is, however, not suitable as a routine screening test for the infection.

Die sensitiwiteit en spesifisiteit van die immunoperoksidase tegniek (IMP) vir die diagnose van lentivirus in formalien-gefikseerde longweefsel in skape was ondersoek. Formalien-gefikseerde long monsters van 52 skape , afkomstig van twee verskillende besmette kuddes, en formaliengefikseerde long monsters van 20 skape afkomstig van ‘n derde, nie-besmette kudde, was ondersoek. Veelvuldige histologiese snitte van elke dier was gekleur met ‘n monoklonale teenliggaam teen die Maedi-Visna Virus (MVV) antigeen. Kontrole snitte was gekleur met ‘n irrelevante teenliggaam, en sonder teenliggaam, in alle diere. Histologiese letsels was gegradeer ooreenkomstig die ergheidsgraad van die letsels; en die aantal snitte wat positief gekleur het per dier. Die gemiddelde aantal selle wat positief gekleur het vir antigeen was ook bepaal. Die IMP tegniek het ‘n sensitiwiteit van 51,9% en ‘n spesifisiteit van 100% indien drie long snitte per dier in beide die besmette kuddes ondersoek word. Die sensitiwiteit was grootliks beïnvloed deur die teenwoordigheid of afwesigheid van tipiese letsels en die aantal selle wat virus antigeen bevat blyk om te korreleer met die graad van die letsels. Diere sonder tipiese histologiese letsels was meestal negatief vir virus antigeen met die IMP tegniek. Die aantal snitte ondersoek het die sensitiwiteit beïnvloed en ‘n afname in die sensitiwiteit was opgemerk indien minder snitte per dier ondersoek was. Statistiese ontleding het getoon dat dat die aantal snitte wat positief toets met die IMP tegniek betekenisvol verskil tussen die histologiese kategorieë van erg, matig, lig en normaal. ‘n Groter persentasie van die snitte het positief gekleur in gevalle met meer gevorderde letsels. Dit het ook getoon dat die vermoë om MVV infeksie te diagnoseer betekenisvol afneem met vermindering in die aantal snitte wat ondersoek word. Die gevolgtrekking is dus dat die IMP tegniek van diagnostiese waarde is in individuele diere met tipiese histologiese letsels, en dat dit nie geskik is nie as ‘n roetine siftings-toets op kudde basis.