According to a report in the TResearch Teagasc magazine, by researchers Aideen Kennedy, Ríona Sayers, and Noel Byrne, this long-term nature of control should be highlighted to all farmers and veterinarians planning programmes, in order to prevent unrealistic expectations and subsequent distrust of such schemes.
Johne’s disease (JD) is a chronic incurable diarrhoea caused by a mycobacterium called MAP.
Infection most commonly occurs in calves, but clinical signs usually do not become apparent until adulthood.
Infection has been associated with economic losses in dairy enterprises, primarily due to reduced milk production and sub-optimal fertility.
Many international JD control programmes have been established, because of the economic concerns, the impact on animal welfare, and the hypothesised zoonotic link between JD in cows and Crohn’s disease in humans.
Most of these programmes aim to break the cycle of JD transmission by identifying and removing infected animals, and hygienic dry-cow and calf management.
Due to the slow and progressive nature of the disease, identification of infected/infectious individuals is problematic.
Currently, there is no test available with perfect sensitivity and specificity for JD diagnosis in sub-clinical animals (those not showing signs of the disease).
However, due to its speed and relatively low cost, enzyme-linked immunosorbent assay (ELISA) testing remains a popular diagnostic choice for control programmes (including Animal Health Ireland’s pilot national JD control programme).
The Teagasc trial further examined the usefulness of ELISA testing in an Irish context.
Dry-cow management, calf management and culling of high-risk cows were also part of the trial control programme.
Before commencement of the programme, no clinical cases of JD had been reported in the trial herd.
Monthly collection of blood samples for ELISA testing continued from May 2012 to December 2014, and quarterly thereafter. ELISA-positive cows were subject to veterinary examinations and faecal sampling throughout the testing period.
All cows that recorded at least one ELISA-positive result were calved in isolation, and their colostrum was discarded.
Their calves got colostrum from consistently ELISA negative cows.
At the first sampling in May 2012, 10 of the 139 cows had ELISA-positive test results for JD.
The figure peaked at 16 in July, 2013 (excluding three-month periods when results are likely to be influenced by statutory annual testing for bovine TB).
Veterinary examination did not yield any clinical signs of JD. In 2013, a single cow (denoted Cow A) was selected for pathological examination on the basis of repeated high positive ELISA readings.
Cow faecal samples were also positive.
Pathological examination of Cow A revealed severe gross changes consistent with JD, in July, 2013. Other ELISA-positive cows showed no lesions indicative of MAP.
The entire herd remained ELISA-negative from the winter of 2013 to the summer of 2015, when five cows were ELISA-positive. Of these, a single cow (Cow B) recorded a very high ELISA reading.She was subjected to pathological examination in November, 2015, but no gross lesions were identified.
Animals reared using JD management protocols are due to enter the milking herd this spring, which will allow the research team to further study the impact of the control programme.
So far, the programme has shown how serial ELISA sampling identified a cow clinically positive for JD, which could have acted as a source of MAP within the herd.
Taken as a whole, however, results indicate that consistently positive MAP ELISA results are not a definitive indicator of the gross pathological lesions associated with JD. Additional tests (faecal culture or PCR), therefore, should be used to more definitively identify cows for culling.
Culling programmes and appropriate calf management can reduce herd prevalence of MAP ELISA-positive results.
It is likely, however, particularly in an Irish context, that cows may be culled unnecessarily if ELISA readings are used as the only indicator of disease.