Detection by two enzyme-linked immunosorbent assays of antibodies to Ehrlichia ruminantium in field sera collected from sheep and cattle in Ghana

Abstract

Two serological tests for detection of antibodies to Ehrlichia (previously Cowdria) ruminantium, the causative agent of heartwater, were compared by using field sera collected from sheep and cattle as part of serosurveys in Ghana. Sera selected as either negative or positive by a new polyclonal competitive enzyme-linked immunosorbent assay (PC-ELISA) were tested by the indirect MAP1-B ELISA. Cutoff values of 14 percent positivity (14 PP) for both ruminant species were obtained for the MAP1-B ELISA by using preseroconversion Ghanaian sera and were compared with previously recommended cutoff values of 29 PP for sheep and 38 PP for cattle. With the 14-PP cutoff, of 151 sheep sera which tested negative by PC-ELISA, 89% were also negative by MAP1-B ELISA, while of 419 sheep sera positive by PC-ELISA, 98% were also positive by MAP1-B ELISA. Of 261 bovine sera negative by PC-ELISA, 82% were also negative by MAP1-B ELISA. Of 511 bovine sera positive by PC-ELISA, only 47% were positive by MAP1-B ELISA; these included 168 sera collected from cattle following first seroconversion as detected by both tests, with 125 of these sera positive by PC-ELISA but only 59 and 5 positive by MAP1-B ELISA with the 14- and 38-PP cutoff levels, respectively. These results indicate that both assays are highly sensitive and specific for detection of E. ruminantium exposure in sheep but that the MAP1-B ELISA lacks sensitivity for postseroconversion bovine sera in comparison to the PC-ELISA. Both tests confirm E. ruminantium seroprevalence of at least 70% in Ghanaian sheep; levels of exposure among Amblyomma variegatum-infested Ghanaian cattle are likely to be higher than the seroprevalence value of 66% obtained with the PC-ELISA.

FIG. 1.

(A) Scatter plot of paired PI values (y axis; solid line indicates cutoff of 85 PI) obtained with PC-ELISA and PP values (x axis; dotted line on left indicates cutoff of 14 PP, and broken line on right indicates cutoff of 29 PP) obtained with MAP1-B ELISA for sera (n = 437) from longitudinal survey sheep. (B) Scatter plot of the same paired data shown in panel A but after transformation of the percentages by arcsine and fitting of a linear regression trend line.

FIG. 2.

Scatter plot of paired PI values (y axis; solid line indicates cutoff of 85 PI) obtained with PC-ELISA and PP values (x axis; dotted line on left indicates cutoff of 14 PP, and broken line on right indicates cutoff of 29 PP) obtained with MAP1-B ELISA for sera (n = 133) from point prevalence survey adult sheep.

FIG. 3.

Scatter plot of paired PI values (y axis; solid line indicates cutoff of 70 PI) obtained with PC-ELISA and PP values (x axis; dotted line on left indicates cutoff of 14 PP, and broken line on right indicates cutoff of 38 PP) obtained with MAP1-B ELISA for sera (n = 94) from longitudinal survey calves up to and including first seroconversion.

FIG. 4.

Scatter plot of paired PI values (y axis; solid line indicates cutoff of 70 PI) obtained with PC-ELISA and PP values (x axis; dotted line on left indicates cutoff of 14 PP, and broken line on right indicates cutoff of 38 PP) obtained with MAP1-B ELISA for sera (n = 168) from longitudinal survey cattle after first seroconversion.

FIG. 5.

Scatter plot of paired PI values (y axis; solid line indicates cutoff of 70 PI) obtained with PC-ELISA and PP values (x axis; dotted line on left indicates cutoff of 14 PP, and broken line on right indicates cutoff of 38 PP) obtained with MAP1-B ELISA for sera (n = 510) from point prevalence calves and adult cattle.