ARCHIVE FILE
This article was published in 2000
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Enzootic Bovine Leucosis
EBL in Victoria, September, 2000
By Ian Holmes, DVO, Echuca
EBL rarely shows as a clinical disease, and so the Animal Health authorities in Victoria were not aware of the extent of its spread in this state. Occasional reactors were detected during testing for various reasons, but the number of herds infected was thought to be very low.
In 1994 a pilot survey of herds in the north-east found that 3% were infected. This was more than was anticipated, and so in late 1994 testing of all dairy herds in the state by Bulk Milk Testing (BMT) commenced, with the aim of initially controlling, then later eradicating the disease from the state's dairy industry. Victoria's 8,200 dairy herds were found to have a 2% herd prevalence of EBL.
Since then, 498 / 8400 herds have returned one or more positive or inconclusive results. (As herds change hands, the total number of herds tested over the Life of the program increases, even though the number of herds still milking is decreasing. In June 1999, there were 7,935 licensed dairy farms in Victoria). About half these herds have undertaken herd testing under the voluntary arrangements that applied to date. Many others have been found negative on further investigation. By September 2000, very few known infected herds had not commenced whole herd testing, but new infected herds continue to be detected.
Current Situation in Victoria, September, 2010
| Infected Herds | 113 |
| Provisionally Free | 17 |
| Suspect Herds | 238 |
| Total | 368 |
In late 1999 the following targets were set by the DNRE / Industry Steering Group:
• All dairy herds to be Bulk Milk Test Negative (or better status) or undertaking herd testing to eradicate EBL by 30 June 2000; and
• No EBL infected herds in Victoria by 30 June 2001.
Some policy changes were also introduced at that time due to the program evolving.
1. It has now progressed to an Eradication Program, from a Control Program.
2. Dairy Factories will now be provided with a list of their suppliers who are IN or SU.
3. The Cattle Compensation Fund will now pay for herd testing to the status of Monitored Free (previously only to Provisionally Free).
4. EBL testing of known IN or SU herds is now compulsory and non-cooperators will he quarantined.
5. Herd testing intervals will be reduced to a minimum 6 months from the removal of the last reactor, (IN —> PF) & a minimum of 4 months after the first negative herd test. (PF —> MF).
6. All herds with a history of an Inconclusive or Positive Bulk Milk Test (BMT) which was not resolved by a herd test will be further investigated.
7. Herd-recording herds larger than 200 cows will be further investigated by pool testing.
Suspect Herds
When BM testing started in Victoria there were teething problems with the test and the identification of the samples and herds. The policy at that time was for herds with Inconclusive or Positive BMTs to be repeat BM tested by the DVO. This involved taking a milk sample directly from the vat and retesting it. If the sample was positive the whole herd was individually tested. If it was negative, then the first result was deemed to be incorrect.
These herds and herds with any other suspicion of EBL infection are being further investigated. Where they are herd-recording, milks are collected from the herd-recording centre and pooled. If the herd is not herd-recording, milks are collected individually on farm and pooled also. If any pool is positive the whole herd is individually tested.
ELISA Tests used in Victoria
Victorian Veterinary Pathology Services (VVPS) is the preferred laboratory for all testing for DNRE. All EBL serological testing is done at this lab.
The standard serological tests for EBL that are currently in use by VVPS are all ELISAs:
| BMT screening of milks | Svanovir kit, Sweden |
| Confirmatory testing of sera positive to Svanovir | Repeat testing with Svanovir |
| Screening of individual sera | Pourquier Institute EBL test, France (DADE) |
| Confirmatory testing of sera positive to Pourquier Institute test | Euro-Diagnostica assay, Netherlands (Bio-Sys) |
Large Herds
The BMT as used in Victoria is claimed to be sensitive enough to detect two infected cows out of 100. Experience has shown that while in some instances it can detect one infected cow in a herd of up to 250 cows, it has also been missing herds where there is a prevalence as high as 1:25 cows in some instances.
>20% of herds in Victoria (1,800 herds) are larger than 200 cows and it was evident that if these herds had a low prevalence of EBL infected cows, then the BMT was likely to miss many of them. Therefore a survey was initiated in 2000 to investigate the prevalence of EBL in these large herds.
~50% of dairy farms in Victoria herd-record and this is independent of herd size. (Victorian Herd Recording Centres (HRCs) also test milks from many interstate herds). A pilot survey was run in South Gippsland Herd Improvement, Leongatha and Dairy Technical Services, Melbourne, followed by Consolidated Herd Improvement Services, Kyabram (CHIS).
Milk samples were collected at the HRCs by the HR lab staff at the time of the routine processing of the milks from that herd. The pool size was determined by the number of milks in a "tray", usually 30. These 30 milks were combined into a single pooled sample by the HR staff, labelled and then sent in trays of up to 100 pools to VVPS where they were tested by the EBL ELISA. Each pool matches one HR sheet which lists the individual cows IDs. The farm details were recorded on the submission sheet (Pools 1-8 = Farm a, Pools 9-20, Farm b,etc.).
(In addition, District herds with a history of a Positive or Inconclusive result that had not been fully investigated by whole herd testing were pool tested if they were herd recording).
In this pilot survey 4.6% of the 673 herds tested were found to be positive for EBL. A more detailed breakdown of the results from CHIS is detailed below.
• Ten positive farms were detected out of the 280 farms tested.
• Two of these farms were known IN farms prior to being sampled at CHIS.
• Three had an earlier indicatinn of EBL by traceforward or BMT Inconclusive.
• The other five had never had any history of EBL.
ie. 5/280 = 2% new herds were detected by sub-sampling herds >200 cows.
(3 others should have been detected by a targeted investigation of herds with a previous history of EBL).
| Herd Test Centre | Herds tested | Pools Tested | Herds Positive | Pools Positive |
| CHIS, Kyabram | 280 | 2633 | 10 | 18 + 2 inconclusive |
All positive and inconclusive pools were found to contain positive cows.
Only one negative pool was found to contain a positive cow, out of 82 negative pools checked by whole herd testing.
We concluded that pool testing via a herd recording centre was a practical and effective method for a survey of large herds or for the investigation of herds with a suspect history. For suspect herds being investigated where a significant proportion of the adult herd is not being herd recorded, then individual blood sampling is required for the non-lactating animals.
The high incidence of previously undetected herds revealed by this survey is a concern as up to 1000 large herds are not herd recording. Options for investigating these herds are being considered.