CASE NOTES
BJD IN A MARKET ASSURANCE PROGRAM (MAP) HERD
Matthew Ball — Senior District Veterinarian Lismore
Posted Flock & Herd September 2012
Introduction
In the Tweed-lismore region of NSW an Australian Illawarra Shorthorn (AIS) herd was formed in 1890 from cattle brought to the North Coast from Kiama. In 2012 the herd has 225 cattle over 24 months of age with approximately 150 cows being milked. The herd has consistently had good production figures and maintained excellent records for production and livestock health. There have been limited health issues in either adult cows or calves. Lameness and mastitis rates are low as is calf mortality. A more traditional approach to dairying has continued on this farm with calves left on mothers for approximately one week and cows not reared as intensively as in some other modern herds. For at least the last 15 years the herd has been closed. The herd entered the Market Assurance Program (MAP) and was assigned a herd status of MN1 in 1998. This status has been maintained, through biennial maintenance tests and property audits, until the recent diagnosis of Bovine Johne's Disease (BJD).
BJD history
An AIS homebred cow with ID tag 910 was born on 3rd May 2003. This cow had reduced milk production in her 2011 lactation cycle. The cow had a negative BJD ELISA as part of MAP testing on 16th June 2011. The cow showed no signs of clinical disease until May 2012 when weight loss and a scour was observed by the farmer. This was investigated by a private veterinarian and a positive ELISA of 233 was identified on 7th June 2012. Stomach fluke eggs and previous exposure to pestivirus were also identified by the same laboratory report. By 26th June 2012 the cow had deteriorated to lateral recumbency and the North Coast LHPA was tasked by the private veterinarian to perform a euthanasia and necropsy. At necropsy significant thickening of the ileum was observed in an emaciated cow with copious watery diarrhoea. A large number of adult stomach fluke were also observed. A field diagnosis of BJD was made. This was later confirmed by laboratory testing.
The mother of the infected cow was also homebred and was born on 10th May 1999. She was culled from the herd on 8th August 2008 because of low production. There are still two other daughters of this cow remaining in the herd. It is of interest that they are both starting to lose weight and their production figures have dropped. There are two daughters of cow 910 in the herd and they are currently well with good production.
Summary of BJD laboratory testing
2012: Diagnosis of BJD with histopathology positive and positive BJD ELISA in a clinical cow.
2011: Blood testing of 50 animals above 2 years of age all negative including the cow diagnosed in 2012. Testing was biased to the oldest cows in the herd.
2009: Blood testing of 50 animals above 2 years of age all negative but not including the cow diagnosed in 2012. Testing was biased to the oldest cows in the herd.
2007: Blood testing of 50 animals above 2 years of age all negative but not including the cow diagnosed in 2012. Testing was biased to the oldest cows in the herd.
2005: Blood testing of 50 animals above 2 years of age all negative but not including the cow diagnosed in 2012. Also investigation of two scouring cows was negative for BJD on blood and faecal culture.
2003: Blood testing of 50 animals above 2 years of age all negative. Testing was biased to the oldest cows in the herd.
2001: Blood testing of 50 animals above 2 years of age all negative. Testing was biased to the oldest cows in the herd.
1999: Blood testing of 148 cows (whole herd bleed) with two low positives BJD ELISA (3.3 and 2.0). These cows had negative faecal cultures.
Origin of infection
The infected animal was homebred. The herd has only had 12 animals introduced in the last 25 years. Small groups of heifers were purchased about 15 years ago from two herds in the Casino district. Infection may have been brought in from these purchases or may have been present from when the herd was set up over 100 years ago.
Discussion
The herd is likely to have been endemically infected at a low rate for a long time. The negative testing over many years suggests a low rate of spread within the herd. The failure of the MAP program to detect infection on this farm emphasises the weaknesses of BJD ELISA testing especially when the herd prevalence is low. The sensitivity of the BJD ELISA used by the NSW State Diagnostic Laboratory has been estimated to be 48.8% (EMAI, 2012). The diagnosis of BJD is not considered particularly significant by the dairy farmer because he is principally in the business of producing milk and it is very unlikely he will see a significant rate of clinical BJD in his herd. His production levels are excellent. The dairy farmer has historically sold male calves and surplus heifers through the local saleyards. These cattle are of good quality and are purchased by producers to grow out as vealers, steers or retain as breeders. The diagnosis of BJD will have an impact on the trading of the farmer. Although dairy farms in NSW are not quarantined for BJD the requirement to declare his disease status, through the Dairy Assurance Score (DAS) declaration form, will stop the purchase of his cattle by many beef producers and most dairy farmers. The farmer will now mainly sell his cattle direct to slaughter. He will continue to privately sell very young calves to local buyers who will rear them to 80kg before slaughter. Significant tracing of movements from the dairy and further actions have been undertaken to assist the control of BJD in Australia.
Reference
EMAI (2012): Phone conversation with veterinary pathologist, Steven Hum, from NSW State Diagnostic Laboratory, Elizabeth Macarthur Agricultural Institute in February 2012.
Comment from Evan Sergeant (October 2012):
"Based on Nielsen SS and Toft N (2008) I consider that a more realistic sensitivity for field use of the BJD ELISA is 20-30% at the most."
Reference
- Nielson and Toft (2008) Ante mortem diagnosis of paratuberculosis: a review of accuracies of ELISA, interferon gamma assay and faecal culture techniques. Vet. Microbiology 129: 217-235