Bovine Serological Survey (2007-2008)
The national Bovine Serological Survey (BSS) was conducted to confirm that the prevalence of brucellosis, bluetongue, and anaplasmosis does not exceed 0.02% (95% confidence) in live cattle in Canada.
Samples were stratified by province, based on the provincial fraction of on-farm Canadian cattle. The allotted provincial quotas were assigned to 21 of 63 federally inspected meat establishments, representing more than 93% of the mature cattle slaughter in each province. Sampling was also done in provincial abattoirs in Newfoundland. We used a systematic random sampling procedure. A sample size of 15,000 was determined to be adequate for detecting disease at a 0.02% prevalence considering the cattle population at risk. Sampling was initiated in November 2007 and completed in April 2008. For each animal sampled, the approved Canadian Cattle Identification Program ear tag number was recorded. This information was used to trace the province of origin, the animal use (dairy or beef), and the date of birth when available.
Samples were tested to detect antibodies for brucellosis, bluetongue, and anaplasmosis, using internationally recognized standard methodology. All testing was performed by Canadian Food Inspection Agency (CFIA) laboratories, and sample submissions and results were tracked using the Laboratory Sample Tracking System (LSTS). Overall, 15,482 samples were collected and analyzed, with survey testing concluded in October 2008. Follow-up testing is ongoing for anaplasmosis.
Of the 15,482 samples analyzed by fluorescence polarization assay (FPA) for brucellosis, 7 were positive. The confirmatory competitive enzyme-linked immunosorbent assay (cELISA) was applied in series and all 7 samples were found negative. Based on these results, we conclude with a confidence level of 95% that the Canadian cattle population remains free of brucellosis at or above a prevalence of 0.02% (2/10,000).
Of the 15,482 samples analyzed by cELISA for bluetongue, 3 were positive. The confirmatory serum neutralization assay (SN) was applied in series, and all 3 samples were found positive to bluetongue virus (BTV)-17, with 1 sample also testing positive to epizootic hemorrhagic disease of deer virus (EHDV) type 2.
The first BTV reactor was sampled in Alberta and had no tag recorded. This animal had a 1/320 SN titer to both BTV-17 and EHDV-2. It was traced to a dealer in the Okanagan Valley in British Columbia; however, it was not possible to confirm the source herd.
The second BTV reactor was sampled in Alberta and had a tag incorrectly recorded. This animal, which had a titer of 1/80 to BTV-17, was traced back to the same dealer in the Okanagan Valley, British Columbia. Again, because of lack of identification, it was impossible to confirm the source herd. It is unknown whether the BTV vector found in the Okanagan Valley has the capacity to transmit the virus as sporadic incursions of bluetongue over the past 30 years were believed to be due to windborne introduction from the U.S. There is no evidence that bluetongue virus has the ability to overwinter in Canada.
The third BTV reactor was traced to a closed commercial dairy herd in Eastern Ontario where two additional reactors were discovered upon complete herd testing. The Culicoides species able to transmit BTV is not known to exist in Canada outside the Okanagan and southern prairies of Manitoba, Saskatchewan, and Alberta at the northern limit of its range.
Reactor #1 had a low SN titer (1/10), and with serial testing eventually became negative on cELISA and SN. Reactor #2 was low positive on cELISA and negative on SN. It, too, eventually became negative on cELISA. These results were considered atypical. All herds with susceptible species within a 5 KM radius tested negative. Based on these results, we conclude with a confidence level of 95% CI that the Canadian cattle population, to the exclusion of the Okanagan Valley, remains free of bluetongue at or above a prevalence of 0.02% (2/10,000).
Of the 15,482 samples analyzed by cELISA for anaplasmosis, 244 were positive. Given the sensitivity and specificity of the test, 232 reactors were expected out of 15,482 samples, at a disease prevalence level of 0.02%. The use of two anaplasma cELISA kit lots that appeared to perform differently prevented a statistically valid estimation of prevalence of anaplasmosis in Canada.
In accordance with Canadian policy, for each herd harbouring a reactor, a risk triage was done that takes into account the laboratory result, the clinical history, the proximity to restricted feedlots, the history of live U.S. imports, and the geographic location. On the basis of this risk triage, requirements for any further testing of the herd are determined. Identifying a confirmed positive animal leads to slaughter of this animal (modified stamping-out). In-depth investigations have revealed infection that was confirmed by polymerase chain reaction (PCR) analysis in 3 western provinces:
- Saskatchewan: One reactor led to the identification of a single infected herd in October 2008. The outbreak was resolved in December 2008.
- Manitoba: One reactor led to the identification of an outbreak, involving 8 farms in Dugald and Lac du Bonnet areas, in January 2009. Moreover, in October 2009, enhanced passive surveillance revealed another outbreak in the south eastern corner of Manitoba, where 14 infected farms were identified. The CFIA anaplasmosis policy continues to be applied.
- British Columbia: From the survey results, BC exhibited a significantly higher seroprevalence for anaplasmosis than elsewhere in Canada. Two BSS reactors initially led to the identification of an infected premises in the Nicola Valley, in July 2009. Follow-up investigations indicated a pattern dissimilar to that of Manitoba cases. In BC, a large proportion of seropositive animals were not confirmed by PCR testing. Additional testing and research did not confirm the presence of anaplasmosis in a subset of cattle that initially tested positive for the disease. Further analysis indicates that the test results may have been due to the presence of an Ehrlichia organism, which is closely related to anaplasmosis. In March 2010, quarantines were removed in BC and the outbreak was considered resolved.
Canada is no longer considered free of anaplasmosis until eradication is completed. A stakeholder consultation is underway to review the regulatory status of anaplasmosis in light of these recent events. The anaplasmosis stakeholder consultation of 2007 resulted in the maintenance of anaplasmosis as a reportable disease and of an eradication policy.
A copy of the Bovine Serological Survey is available upon request.
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