Variation in clinical assessment between farmers is well recognised in veterinary practice and has also been reported in the scientific literature . Differences between farmers and scientists have previously been reported for the assessment of locomotion scores in sows , while non-veterinarians showed good diagnostic performance in the assessment of injuries in horses .
However, a small number of publications indicate that farmers’ diagnostic performance and intra- and inter-observer agreement appear not to have been quantitatively evaluated to the same extent as the differences between veterinarians.
The results of our study demonstrated that the farmers’ ability to classify correctly pictures of faecal pools is not different to that of veterinarians. This suggests that the farmers’ diagnostic ability to detect clinical diarrhoea may be similar to that of veterinarians. This further suggests that average farmers and veterinarians will apply antibiotics for treatment of diarrhoea to the same pigs. However, the observed imperfect diagnostic performance of both farmers and veterinarians suggests that the treatment of healthy animals and the non-treatment of sick animals will occur in practice. The occurrence of these in-correct treatments will depend on the true diarrhoea prevalence and the medication strategy. In relation to individual treatments (e.g. treatment by injection of single animals), it is the clinical examination and classification of the individual pigs that will determine whether the pigs will be subjected to antibiotic treatment. In a population with a high prevalence of diarrhoea (e.g. 80%), the observed diagnostic sensitivity for both farmers and veterinarians will result in a large number of diarrhoeic pigs being falsely classified as diarrhoea-negative (low negative predictive value). However in a population with a medium to low prevalence of diarrhoea (e.g. <30%), the observed diagnostic specificity for both farmers and veterinarians will result in a large number of non-diarrhoeic pigs being falsely classified as diarrhoea-positive (low positive predictive value). There are currently no published data on diarrhoea prevalence in Danish nursery pigs. However, low diarrhoea prevalence has previously been reported in Danish finishers . This implies that under Danish conditions false positive classification of pigs as diarrhoeic is a larger problem than false negative classification of pigs as non-diarrhoeic. This could potentially result in overconsumption of antibiotics used for individual treatments.
In relation to the application of batch medication, it is likely the prevalence of diarrhoea in the group of pigs that will be part of the process to determine whether the pigs will be subjected to antibiotic treatment. The observed diagnostic sensitivity and specificity will also result in biased estimation of the diarrhoea prevalence in a group of pigs. In a population with a high true diarrhoea prevalence (e.g. 80%), the apparent diarrhoea prevalence will be marginally underestimated, equivalent to the apparent diarrhoea prevalence of 0.48 observed in the faecal consistency test (true diarrhoea prevalence = 0.50). In contrast, the prevalence will be overestimated in populations with a medium to low true diarrhoea prevalence (e.g. <30%). This implies an overuse of batch medication for diarrhoea under Danish conditions similar to the application of individual treatments.
The variation in diagnostic performance between farmers demonstrated that some farmers have a poor diagnostic ability. Studies of factors explaining such differences between farmers could form a basis for identifying or predicting farmers who need increased supervision or clinical training. In the current study, the number of years working with pigs was the only co-variable associated with the farmers’ diagnostic performance, and the effect was low. The less experienced farmers had a diagnostic performance comparable to that of veterinarians, while the more experienced farmers actually had a poorer diagnostic ability and were more reluctant to assess faecal pools as diarrhoeic. A similar effect of increasing experience has previously been reported to be associated with diagnostic performance of lameness in cattle . A possible explanation could be that less experienced personnel have been trained more recently by veterinarians in making clinical diagnoses.
A potential factor not investigated in the current study is the effect of farm prevalence of clinical signs. Farmers may adapt to the level of a disease. In that way, farmers from high diarrhoea prevalence farms may be more reluctant to classify a pig as being diarrhoeic. However, this aspect needs to be confirmed.
Another aspect that needs further consideration is that it has previously been demonstrated that decisions relating to the treatment of animals are dependent on factors other than the simple assessment of clinical signs .
On-farm examination of faecal pools would be preferable to the pictures used in this study. However, faecal pools are difficult to preserve over longer periods of time, and therefore the inclusion of larger numbers of observers would be impossible. We used digital pictures of faecal pools in order to include a larger number of observers so the study would be more representative. The digital pictures were used as a proxy for real faecal pools similar to the evaluation of inter-observer agreement in relation to the assessment of faeces in human medicine . The use of digital pictures has been evaluated most intensively in dermatology. In dermatology, the examination of digital pictures has a high level of agreement compared with the physical examination of dermatological lesions . Furthermore, the application of digital pictures has been reported to reduce bias more than physical examination in dermatological studies .
All faecal samples were collected from the same herd. Theoretically, this could have increased or decreased the observed agreements if faecal samples from the selected herd had a unique consistency or appearance. This bias is highly unlikely, since inter-observer agreement and observer accuracy for the assessment of faecal consistency in nursery pigs have been demonstrated to be consistent between herds [9, 19].