Imagine this clinical scenario: you are a veterinary surgeon working for a practice specialising in farm animals. You receive a call out to a dairy farm that has noticed their herd is producing less milk than expected. After arriving on-site, you suspect there may be a Fasciola hepatica – liver fluke – outbreak among the herd and consider methods of diagnosis. You typically use a faecal egg sedimentation test because it is generally considered affordable and effective, but you have heard a colleague mention the coproantigen enzyme-linked immunosorbent assay (coproantigen ELISA) as an alternative. You decide to consult the evidence to learn more.
The evidence
Three papers were critically reviewed: two cross-sectional diagnostic accuracy studies (Charlier et al., 2008; Mazeri et al., 2016) and one case-control diagnostic accuracy study (Palmer et al., 2014).
Charlier et al. (2008) studied 200 cattle over two sampling periods, with no more than three cattle selected from a single farm. The cattle were of mixed beef and dairy breeds, all were designated for slaughter and all were under 24 months of age. Samples of faeces, blood, whole livers and gall bladders were obtained from each animal following slaughter. Coproantigen ELISAs were carried out on the blood and faecal samples, while two faecal egg sedimentation tests were carried out on the faecal samples. The outcomes studied were positive and negative results of each diagnostic test for the presence of F. hepatica, calculated sensitivity of each diagnostic test when compared with liver necropsy and calculated likelihood ratios for each diagnostic test.
Mazeri et al. (2016) studied 619 mixed-breed cattle over three sampling periods. All cattle were between 369 and 1,121 days old. Systematic sampling was conducted with 1 of every 10 cattle selected for inclusion. Samples of faeces, blood, whole livers and gall bladders were obtained from each animal following slaughter. Gall-bladder egg counts were performed via sedimentation and microscopic analysis. Coproantigen ELISAs were carried out on blood and faecal samples, while faecal egg sedimentation tests were carried out. The outcomes studied were the positive and negative results of each diagnostic test for the presence of F. hepatica.
Palmer et al. (2014) studied 250 cattle faecal samples obtained from cattle of previously known F. hepatica status, 156 of which were negative and 94 of which were positive (via faecal sedimentation testing). All samples were subjected to coproantigen ELISA testing, and test sensitivities were calculated from the known positive and negative faecal samples. The outcomes studied were positive and negative results for coproantigen ELISA testing, calculated sensitivity of coproantigen ELISA test kits and estimated sensitivity of coproantigen ELISA test kits.
Limitations of the evidence
The overall strength of the evidence is moderate, owing to various limitations
The overall strength of the evidence is moderate, owing to various limitations. Charlier et al. (2008) and Mazeri et al. (2016) both had weaknesses in their sample populations. Both studied clinically healthy cattle that were being presented for slaughter. These cattle were therefore likely to be well-conditioned, and not representative of the cattle that would be tested in clinical scenarios. Other limitations in Charlier et al. included the possibility for false positives, unknown flukicide treatment history, small sample size and variations in geographic location, management practices and grazing strategies within sampled cattle.
Mazeri et al. (2016) also had the following limitations: the sampling was limited to one year, which does not address variances due to climatic factors or variances in fluke cycle; sampled cattle may vary in geographic location, management practices and grazing strategies; and there was a possibility of false positive results.
Palmer et al. (2014) provided a weak body of evidence and did not entirely answer the research question. The quality of evidence is reduced by the case control sampling method, a relatively small positive sample size, failure to include relevant inclusion criteria of submitted faecal samples, failure to include confidence intervals when reporting test accuracy, and modification of the test protocol halfway through the study.
Summary of findings
All studies reported a coproantigen ELISA test specificity equal to or greater than that of the faecal egg sedimentation test
All studies reported a coproantigen ELISA test specificity equal to or greater than that of the faecal egg sedimentation test. Despite this, sensitivity was significantly increased in only one study – Charlier et al. (2008). This study found that the coproantigen ELISA test had a sensitivity of 94 percent, while the 4g and 10g faecal egg sedimentation tests had sensitivities of 43 percent and 64 percent, respectively.
Mazeri et al. (2016) exhibited an equivalent sensitivity in one out of three sampling periods, where both faecal egg sedimentation and coproantigen ELISA had a sensitivity of 77 percent, and an increased sensitivity in another period where faecal egg sedimentation had a sensitivity of 58 percent and coproantigen ELISA had a sensitivity of 77 percent.
Palmer et al. (2014) demonstrated an increase in test sensitivity of 80 percent compared to the reference value of 70 percent. However, there was a 20 percent false negative rate when recommended test cut-off values were applied.
Conclusion
The literature reviewed provides moderate evidence to support the use of coproantigen ELISA to diagnose F. hepatica in cattle;however, the evidence is not strong enough to recommend a total replacement of the faecal egg sedimentation test. This is due to the relatively small body of evidence and the limitations of existing studies, which are outlined above. Nonetheless, it is important to note that the coproantigen ELISA test can still play a significant role in the diagnosis of F. hepatica in cattle when used to supplement, rather than replace, the faecal egg sedimentation test.
it is important to note that the coproantigen ELISA test can still play a significant role in the diagnosis of F. hepatica in cattle when used to supplement, rather than replace, the faecal egg sedimentation test
As F. hepatica is an economically important disease within the livestock industry, there is a need for further studies to draw clearer conclusions on the efficacy of the two diagnostic methods.
| The full Knowledge Summary can be read in RCVS Knowledge’s open access journal Veterinary Evidence. |
Disclaimer
The application of evidence into practice should take into account multiple factors, not limited to: individual clinical expertise; patient’s circumstances and owner’s values; country, location or clinic where you work; the individual case in front of you; and the availability of therapies and resources.
