Following an ELISA and culture positive reaction in a steer sold to an export feedlot in early 2010, four co-owned properties were investigated for the presence of Bovine Johne's disease (BJD). The properties were well separated, but had been allocated the same PIC to reduce the property to property transfer requirements caused by multiple movements occurring between them.
The four properties involved were extensively run beef properties with a feedlot present on one property (Property A). Over 3,000 adult cattle were present on the properties. Some movements occurred between the properties.
Several risk factors were identified:
A series of tests was conducted based on the risk assessments outlined to determine the status of the properties. One property was allocated an IN status after one bull was found to be positive on ELISA and follow-up culture. Further testing and risk assessments along with judicious culling of “higher risk” cattle allowed the early return of the properties to non-assessed (NA) status.
The affected enterprise has two operational components on four separate properties (hereafter Properties A-D):
Calving occurs over 4-5 months from June to October. Calves are weaned from 5-10 months of age and are then transported to Property A for backgrounding on a ration for a period of 1-5 months, until they reach 280-300 kg liveweight and are approximately 12 months of age. They are then transported to a feedlot in QLD, where they are kept for the mandatory quarantine period (28 days) before being exported to Japan.
Bulls are used on multiple properties and when not with the cows they were kept in the feedlot (Property A). No other livestock are run on the properties.
The feedlot drains into a sump and therefore grazing land was determined not to be at risk of contamination through drainage.
The steer (index case) originated from Property B (born October 2008); however, it was backgrounded on Property A (November 2009) before being transported to a feedlot in QLD (December 2009). In January 2010, the steer was found to be ELISA and ileal culture positive when at the QLD feedlot despite being negative on faecal culture (BACTEC radiometric culture method) and histopathology of standard test tissues. In July 2010, Property B, Property C and Property D were all classified as Suspect Properties due to their PIC being shared with Property A.
Dairy heifers originating from a known BJD-infected herd were held at Property A from 2005-2010 and were fed in the feedlot between 2005 and2008. Property A is considered to have high risk of contamination with BJD because of the presence of the high-risk dairy heifers on the property during that period.
Additionally, between 2007 and 2010 a number of bulls were introduced to Property A after spending several months from 12 months of age onward at a BJD-infected property. They originated from a CattleMAP property in Tasmania. These bulls were housed in the feedlot when not being joined due to drought conditions. During the eight-week joining periods they could have been a source of pasture contamination. Home-bred bulls (offspring of the nucleus herd) also resided on Property A. These bulls were raised in a grazing area well segregated from the introduced bulls; however, between 2007 and2010 the bulls were held in the feedlot at Property A due to drought conditions. From about 30 months of age onward they were moved to the paddocks for joining and then returned to the feedlot when joining had finished.
The level of infection in this herd was considered very low with one positive in the 3120 adult cattle remaining on the property when testing occurred in mid-2010. There had been no evidence of clinical BJD. The two positive animals that had been detected (the steer at the Queensland feedlot and the home-bred bull on Property A) both had minimal evidence of disease. They were both histopathology negative on testing at standard sites and were culture positive at only one site.
Property A feedlot was considered to be at high risk of contamination from 2005 to 2010 because “dairy-origin” heifers (some originating from known BJD-infected herd) had been custom fed in the feedlot between 2005 and 2008. In addition, during the drought years (between 2007 and early 2010) bulls were held in the feedlot between weaning and joining at around 30 months of age and mature bulls, including AAAA-origin bulls, were held in the feedlot when not being joined.
These bulls were born on a CattleMAP property in Tasmania but did spend some months from around 12 months of age on the infected AAAA property prior to moving to Property A. All AAAA-origin bulls on the property during the quarantine period had two negative ELISA and faecal culture tests three months apart. Nevertheless there was still some suspicion over them because of the high risk of exposure during the time they spent on the AAAA property. A number of AAAA-origin bulls had gone for slaughter prior to detection of the index case, so were not available for further investigation. The AAAA-origin bulls were assessed as medium risk. These animals were culled and sent to slaughter.
The home-bred bulls were born into the nucleus herd, which was considered low risk. In normal seasons these bulls were raised in a grazing area well segregated from where the AAAA-origin bulls were held.
From June 2007 to March 2010 bulls were held in the Property A Feedlot to grow out due to the drought conditions. From about 30 months of age, they were moved to paddocks for joining. When not being joined they were returned to the feedlot and held there.
Risk assessments of these bulls determined that bulls entering feedlot before 12 months of age were high risk (the positive bull was in this group); bulls entering the feedlot at 12-24 months of age were a medium risk; bulls entering feedlot at more than 24 months of age were a low risk.
From this group of home-bred bulls, high- and medium-risk bulls were slaughtered following two negative ELISA and faecal culture tests three months apart. Low-risk bulls were kept and were tested negative using two ELISA and faecal culture tests 12 months apart.
Additionally, some home-bred bulls were grown out on Property B and were not exposed to the contaminated feedlot. However they were exposed to the AAAA bulls as young animals. These bulls were classified as low to medium risk and were slaughtered following two negative ELISA and faecal culture tests three months apart.
The positive steer spent five weeks in Property A Feedlot from approximately 13 months of age and could have been exposed there from contamination by any of the sources listed above. It is assumed that the steer became infected in the feedlot and was reacting to the initial exposure when found to be ELISA and ileal culture positive at the QLD feedlot (NB this testing was within two months of the initial likely exposure).
Nucleus breeding herd
The nucleus breeding herd consisted of around 85 pure-bred cows that were the dams of the home-bred bulls used in the commercial herds. 23 of these cows were born on other properties and introduced when over 12 months of age. The others were either home-bred in the nucleus herd following AI using a mop-up bull, or were born via embryo transfer into the “brown” mob. The “brown” mob is a mob of cross-bred cows introduced from low risk sources used as embryo recipients - they are kept as a separate mob and never have a bull running with them. They were all ELISA negative on 2 August 2010. The mop-up bulls used were Bull 5160 (a bull introduced from a Bowral property) in 2008 and 2009 and Bulls 112 and 114 in 2010. Bulls 112 and 114 were born to embryo recipients and raised there to 12 months of age. These cows were assessed as low risk. All cows over two years old in the nucleus herd were sampled with a single faecal culture and ELISA - all tests were negative.
These approximately 3000 cows were run on four separate properties. They were all introduced from low-risk sources as adult cattle. All commercial cows were ELISA tested. Two cows returned positive ELISA results. These cows were then autopsied and sampled for BJD testing. Both samples were positive for sheep-strain Johne's disease. (The mob of origin of the index case was known but its dam was not known.) The commercial cows were assessed as low risk.
The origin of sheep-strain JD in the two cows needed to be investigated further but did not impact on the BJD status of Property C.
Separate PICs were allocated to the four properties once it was determined that Property A was IN and that the other properties could be classified as NA for the testing outlined above and slaughter of assessed high- and medium-risk animals. This process allowed some trading to continue.
The origin of the BJD in these cattle was not conclusively determined, but rather a series of detailed risk assessments, backed by herd and individual tests, allowed the risks to be removed from the herd and the status to be returned to NA. The ability to do this in a relatively short period was enabled largely by the controlled purchase of adult breeders with only a small number maintained for breeding purposes, and those largely bred by artificial breeding. The property manager also kept exceptional records, allowing easy tracing of cattle.
This case and the assumptions made as part of the investigation and eradication challenge some of the accepted paradigms relating to BJD. The detection of the steer with a positive ELISA within two months or less of its exposure to Mycobacterium avium subspecies paratuberculosis (MAP) does not fit with our understanding of the development of humoral immunity to the disease. Further, in general it is considered most likely that animals will contract the disease at a younger age, most commonly at less than 30 days of age when nursing on an infected dam .
The mean age of diagnosis for BJD using ELISA is between four and five years old, depending on the initial dose, the lifetime history and any stressors (pers. comm. Dan Salmon). The ability of ELISA to detect an individual animal is related to the level of individual animal shedding. It seems most likely, given the risk assessments and testing, that the steer picked the infection up from the feedlot area from earlier contamination (likely months before his introduction) at the age of approximately 13 months. Two months after this time the steer was ELISA and ileal culture positive when tested at a feedlot. Experimentally challenged calves have not been found to show ELISA positive results until at least 134 days/4 months post-challenge with MAP suggesting that the steer's positive ELISA at 5 weeks post-infection is highly unusual, and indeed possibly unlikely [1,2].
It is not likely that the area containing the source of infection was highly contaminated. The feedlot was not well shaded and the months that the steer appeared to have picked up the disease were some of our hottest. At the time the steer was put into the feedlot, it had been at least 12 months since the dairy heifers had been there, although they were run on Property A until 2010. As they left the property as heifers, it would seem unlikely that the level of MAP shedding by these animals was high when they left the feedlot area. CattleMAP-origin bulls that had spent time on an AAAA IN property from 12 months of age had also been held in the feedlot during this time. Not all bulls from this source were tested as some were sold prior to the diagnosis. Of those tested, no AAAA bulls returned a positive ELISA or faecal culture test for BJD. Overall the prevalence was very low on the properties with only two animals being detected with the disease, and none shown to be shedding MAP.
Although the actual dam was not known, the dam group was considered negative following two ELISA negative tests. All cows were purchased from low-risk areas and brought onto the property as adults, making them a low-risk source of pathogen. No positive tests were returned on the property where the steer was born and reared until being moved to the feedlot at 13 months of age. The likely age of infection in the steer (index case) was considered to be 13 months. Although recent work has provided evidence of horizontal spread of MAP between adult cattle  and at any age, and varying dose rates in calves up to 12 months of age , it has until recently been determined that calves are most susceptible to MAP at less than 30 days of age and that the most likely route of infection is from nursing on an infected dam that is shedding the bacteria in her faeces .
Calf Accreditation programs (such as JDCAP) are based on keeping calves less than 12 months of age away from land (or fodder) potentially grazed or contaminated by cattle over 2 years of age . If the likely age of infection determined in this case is correct, in line with other recent experiments, then the efficacy of these programs in protecting cattle born on infected properties is questionable.