Optimization of in-house ELISA based on recombinant major sperm protein (rMSP) of Dictyocaulus viviparus for the detection of lungworm infection in cattle

The aim of this study was to optimize an in-house ELISA based on a recombinant version of the major sperm protein (MSP) of Dictyocaulus viviparus for routine diagnosis of lungworm infection in cattle. A recombinant MSP (rMSP) was cloned into pGEX-6P-1 vector and expressed as a glutathione-S-transferase (GST) fusion protein in Escherichia coli BL21 (DE3) chemically competent cells. The product was then employed as capture antigen in an ELISA, and validated against 304 samples of known status (216 negative and 88 positive) in which the antibody levels in sera had also been measured earlier with a commercial ELISA kit (Ceditest® lungworm ELISA). The receiver operating characteristic (ROC) curve analysis of the ELISA results estimated the optimized diagnostic sensitivity and specificity as 97.7% (95% confidence interval [CI]: 91.9-99.7%) and 98.1% (CI: 95.3-99.5%), respectively. The results from the in-house rMSP-based ELISA were compared with results obtained on both fecal examination and the Ceditest® lungworm ELISA. Rising antibody levels in sera of experimentally infected calves were observed between 21 and 28 days post infection, when patency was also confirmed by the presence of larvae in feces. Notably, using the in-house rMSP-based ELISA infection was confirmed in calves shedding larvae approximately 3-4 weeks post inoculation, while using the Ceditest® lungworm ELISA those animals remained negative. Additionally, 251 sera samples from calves naturally exposed to the parasites on pasture were used to evaluate the test. In in-house rMSP-based ELISA no cross-reactions were observed with sera from calves infected with the gastrointestinal nematodes (Ostertagia ostertagi and Cooperia oncophora), even though the presence of eggs in the feces was confirmed. Overall, the in-house rMSP-based ELISA optimized in this study showed excellent diagnostic performance for detection of lungworm infection in cattle.