North Coast private veterinarians carry out work under the Market Assurance Program (MAP) for Bovine Johnes disease (BJD). If an infected animal is detected in this program a district veterinarian from the North Coast LHPA undertakes activities according to NSW BJD policy and procedure.
In September 2011 BJD was diagnosed by a private veterinarian in a single cow in a North Coast dairy herd that was enrolled in the Market Assurance Program (MAP) for BJD. The herd had reached a Monitored Negative 2 (MN2) status.
The infected cow was a positive ELISA reactor in a routine MN2 herd bleed and was subsequently found to be faecal culture positive. It was an 8 year old homebred Guernsey cow.
The cow had over its life been generally healthy with limited illness. Just prior to the faecal culture results being known the cow was examined by a private veterinarian and suspected to have some acidosis. Some antibiotics had been prescribed at that time.
The dairy herd was established in 19961997 by the purchase of dairy cattle of various ages from multiple sources. The majority of cattle came from local herds and no cattle were purchased from Victoria. The herd enrolled in the MAP as soon as it was established. Results of testing are summarised below:
|Year||Cattle no. and test||Results|
|1997||158 BJD ELISA||One ELISA reactor, negative faecal culture|
|1998||165 BJD ELISA||Seven ELISA reactors, negative faecal culture|
|2001||50 BJD ELISA||All negative|
|2003||50 BJD ELISA||All negative|
|2005||50 BJD ELISA||All negative|
|2007||50 BJD ELISA||All negative|
|2009||50 BJD ELISA||One ELISA reactor with negative faecal culture|
|2011||50 BJD ELISA||Two ELISA reactors with one POSITIVE faecal culture|
The infected cow had been removed from its mother at birth and reared in a calf shed (uphill from dairy and adult paddocks). The dam of the infected animal is no longer in the herd but arrived when the herd was set up in 1997. She was approximately 5 years old at the time of her arrival.
Cattle have been run on this property for decades with no break. Originally the property had a Guernsey herd milked by the current owner's father. The property then had a Hereford Stud after ceasing dairy production. The current farmer started milking again in 1996. His herd was first tested in 1997 and has remained in the MAP program since then. The herd has largely been a closed MAP herd with only introduction of bulls. The environment is very wet.
Following the positive faecal culture results, the North Coast LHPA visited the farm to examine, euthanase and necropsy the cow.
The cow was of low but not emaciated body condition. She was producing normal formed faeces. Body temperature was 38.5°C and there was a normal heart rate, lung sounds and rumen sounds. A slight systolic heart murmur was detected.
There were no obvious gross findings of BJD. There was slight thickening of the lining of the proximal ileum. The mucosa of the jejunum was inflamed in some areas. There was a mild area of inflammation on the ileocaecal valve. The liver was grossly enlarged with fibrosis and evidence of significant past liver fluke damage. No live liver fluke were observed. There was a moderate burden of adult stomach fluke.
Histopathology was requested on samples taken from the ileocaecal valve, caecum and proximal ileum. The pathology report indicated diffuse, lymphoplasmacytic and eosinophilic, chronic, moderate to marked, enteritis. It also described multifocal
to coalescing, histiocytic, chronic, moderate, enteritis, with giant cells. Acid-fast organisms were observed within macrophages of the ileocacal valve, caecum and proximal ileum. The lesions were reported as being consistent with chronic parasitism and Johnes Disease.
The diagnosis of BJD in a herd that has been undergoing monitoring for many years is always of interest. Infection has only been found in a single animal so the herd is considered a ‘low prevalence herd’. Only further testing, over a number of years, can assist to determine how many other cattle are infected in the herd. Knowledge from other herds suggests that this may range from 0 to 5. Because the producer has carried out segregated calf rearing for many years there may be no other infected animals. It seems unlikely that the producer will observe clinical disease from BJD in his herd.
The origin of the infection will be difficult to establish. Most cattle become infected from their mother. It is likely infection in this case originated from the dairy herd the mother came from. The producer has no records that can show this origin. Alternatively any other cattle that he purchased in 1996-98 may have been infected and infection spread somehow. It is also possible but less likely that BJD may have been present in the original dairy herd in the 1960's and has survived in low quantity for many decades. If the dam is assumed to be the source of infection the following can be established:
The infected animal was not old enough to be tested till the 2005 sample test. It was not selected that year but was tested in 2007 and was negative. It was not selected for testing in 2009. Antibodies were detected when it was selected for testing in 2011.
Whether or not this producer should actually have been allocated an MN2 status is open to question. It appears from the records that the status was acquired after two whole herd bleeds 12 months apart rather than 2 years apart. Although a herd test a year later may not have detected infection any earlier than that in 2011 it is of scientific interest that since 1998 the herd has only had to test 50 cows a year to stay at a relatively high status. With a low prevalence and a test of low sensitivity (depending on usage approximately 5-60%) it is not surprising that a sample size of 50 from 250 failed to detect infection earlier. A whole herd bleed at some stage during that time would have had much higher chance of detecting infection.
The producer in this case has still not fully accepted the diagnosis. He has requested tissue culture on all six gastrointestinal sites sampled from the infected animal. These tests will be undertaken at the producer's cost.
However, the producer is keen to take necessary steps to improve his status. For general BJD control the North Coast LHPA has advised him to consider some or all of:
The producer has been advised that for the purposes of clearing his BJD status dairy herds would need three full herd tests two years apart. The first full herd test cannot begin till 12 months after the infected cow has left the herd.