IMMUNOHISTOCHEMICAL DETECTION OF HAEMOPHILUS-PARASUIS SEROVAR-5 IN FORMALIN-FIXED, PARAFFIN-EMBEDDED TISSUES OF EXPERIMENTALLY INFECTED SWINE

An avidin-biotin complex immunohistochemistry technique was developed to detect Haemophilus parasuis serovar 5 in experimentally infected 18 -21-day-old conventional pigs, using a rabbit polyclonal antiserum. Se ven of 10 intratracheally inoculated animals developed a low to medium degree of fibrinous polyserositis; meninges and pleura were the most severely affected areas. Haemophilus parasuis was recovered from 9 of 10 pigs; in 2 of them H. parasuis was isolated from tracheal swabs onl y. Positive immunohistochemistry results, mainly observed as free bact eria or bacteria within inflammatory cell cytoplasm in the fibrinopuru lent exudate, were observed in 8 of 10 animals. Cross-reactivity with Actinobacillus pleuropneumoniae was detected but not with other gram-p ositive and gram-negative bacteria tested. This immunohistochemistry t echnique seemed to be at least as sensitive as microbiologic cultures and could be useful in studies of pathogenesis and retrospective diagn osis. However, cross-reactivity with A. pleuropneumoniae means that po sitive immunohistochemistry results in lung tissue from field cases wo uld be dubious.