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Anaplasmosis in cattle, caused by Anaplasma marginale, is a federally reportable disease and has been since December 17, 1969 after Canada’s first outbreak in Manitoba the previous year. It is widespread globally and is an endemic non-regulated disease in the United States. Anaplasma marginale poses no direct human health or food safety risk. It is a disease of ruminants, although only clinically apparent in cattle. Transmission is either biologically by competent ticks or mechanically (by transferring of blood from an infected animal via biting flies or use of hypodermic needles on multiple animals). In Canada, Dermacentor andersoni and Dermacentor variabilis ticks have been shown to be competent biological vectors for A. marginale (1). Cattle surviving infection remain carriers for life, undergoing cycles of parasitemia whereby the organism uses antigenic modulation to escape the host’s immune system.
Anaplasma marginale is an obligate intraerythrocytic rickettsial gram-negative bacterium which can cause fever, anemia, jaundice, weakness, and/or respiratory distress in infected animals. Clinically affected dairy cattle may also have a rapid decline in milk production. The severity of clinical signs varies considerably, depending on the species and age of the infected animal. Cattle infected as adults are usually the most severely affected. Carrier animals do not display clinical signs of infection (2). No antibiotics are approved by Health Canada for the treatment or prevention of bovine anaplasmosis in Canada.
Only 5 occurrences of anaplasmosis have been recorded previously in Canada: Manitoba in 1968, Quebec in 1979, Saskatchewan in 1983, Ontario in 1996, and Saskatchewan in 2000. All cases, with the exception of the 2000 outbreak that occurred in bison, involved beef cow-calf operations. In 1968 and 1983, spread of anaplasmosis occurred in Canada, whereas in 1979, 1996, and 2000, transmission did not occur beyond the index premises where infection was associated with imported cattle. Anaplasmosis was eradicated in each of these earlier outbreaks by applying the federal policy of testing and slaughtering positive animals as well as tracing of animal movement and conducting surveillance in surrounding herds. These actions resulted in Canada being able to regain its anaplasmosis-free status. Stakeholder consultations were held in January 2007 regarding the maintenance of anaplasmosis as a reportable disease under the Health of Animals Act (3). Most respondents favored retention of the current federal anaplasmosis domestic disease control program (4).
Surveillance for anaplasmosis in Canadian cattle is conducted every 3 to 5 years via the national bovine serological survey (BSS). Surveys undertaken in 1998–1999 and 2002–2003 of approximately 15 000 cattle each confirmed that Canada was free from A. marginale infection (at 0.02% design prevalence with 95% confidence). Follow-up investigations of sero-reactors from the most recent survey in 2007–2008 identified 3 outbreaks of anaplasmosis in cow-calf operations, which were reported to the World Organisation for Animal Health (OIE): Saskatchewan in October 2008, investigation concluded December 2008; Manitoba in January 2009, ongoing; and British Columbia in July 2009, investigation terminated in March 2010 (Figure 1).
Internationally recognized standard tests and methodology are used for the detection of A. marginale in Canada (5). The screening test for A. marginale in cattle is a commercially available competitive enzyme-linked immunosorbent assay (cELISA) that detects serum antibodies that recognize an epitope of the major surface protein 5 (MSP5). Estimated test sensitivity and specificity using a 42% inhibition cut-off are 95.6% and 98.6%, respectively (6). The confirmatory test is a polymerase chain reaction (PCR) test on whole blood. The sensitivity and specificity of the PCR are unknown but the detection limit of the assay is estimated to be > 30 infected erythrocytes/mL of blood (7). The case definition for a confirmed case of anaplasmosis requires either that the animal be positive on both cELISA and PCR testing, or have clinical signs compatible with anaplasmosis and be positive on a direct diagnostic method (either identification of organisms in a blood smear or PCR).
One cow meeting the case definition for a confirmed positive was identified in Saskatchewan (positive on cELISA and PCR) in September 2008 as a result of follow-up herd investigation and testing due to a reactor identified from the BSS. This cow originated from a herd located in the same geographic area as the 1983 outbreak, near St. Victor, Saskatchewan. Over 3600 neighboring and other epidemiologically linked animals were tested with no additional positive animals being identified. The quarantine was terminated on December 5, 2008 and the investigation concluded. It has not been possible to confirm the source of infection for this case.
Two geographically separate outbreaks, each with multiple infected herds identified, have been reported in Manitoba. The first confirmed positive animal (positive on cELISA and PCR) was identified in January 2009 as a result of a follow-up investigation to the BSS. Subsequent investigations revealed a total of 8 positive herds associated with the rural municipalities of Lac du Bonnet and Alexander. A second outbreak in Manitoba was identified in October 2009 as a result of enhanced passive surveillance and reporting to the CFIA of clinical signs suggestive of anaplasmosis by a private veterinarian. Animals were found to be positive for anaplasmosis by cELISA, PCR, and a few also on blood smear. Only this 1 herd in Manitoba of all the herds tested in western Canada presented animals with clinical signs of anaplasmosis. However, due to the extensive management of most herds during the vector season it is possible that clinical animals would not have been observed. This outbreak was located in southeastern Manitoba where 15 infected farms were identified from October 2009 to April 2010. No epidemiological link between these 2 outbreaks in Manitoba has been identified to date. During the course of these investigations, over 13 000 animals were tested in Manitoba, with 590 animals identified as meeting the case definition for a confirmed case of anaplasmosis. Within herd apparent prevalence for A. marginale identified to date ranged between 0.04% and 65.96%. Genetic sequencing data from Manitoba has confirmed the presence of A. marginale. Investigations related to the 2 outbreaks in Manitoba are on-going.
British Columbia exhibited a significantly higher sero-prevalence for anaplasmosis than elsewhere in Canada during the 2007–2008 BSS. Follow-up investigation of 36 premises and over 10 000 animals resulted in the quarantine of 9 premises, with 18 animals meeting the case definition for a confirmed case (positive on cELISA and PCR). During the course of the investigation, it became apparent from epidemiological analyses that the observed pattern of test results in British Columbia was dissimilar from the results observed in Manitoba.
Specifically, a large proportion of animals positive on the cELISA test in British Columbia were negative on the confirmatory PCR. This was in contrast to the situation in Manitoba where the vast majority of animals positive on the cELISA test were confirmed positive on PCR, which is what was anticipated based on the assumed specificity of the cELISA.
Laboratory research was initiated in January 2010 at the Centre for Food-borne and Animal Parasitology, CFIA Laboratory, Saskatoon to clarify the disease status of the cattle population in British Columbia and to determine potential causes of the unusual pattern of test results. Disease control activities in British Columbia were suspended while the research was conducted. Samples obtained from a subset of animals identified in British Columbia meeting the case definition (positive on cELISA and PCR) were used in molecular and serological analyses, and in bioassay transmission studies. By March 2010, there was no evidence of A. marginale, and there was DNA evidence of a closely related but previously unrecognized rickettsial hemoparasite of the genus Ehrlichia (8). At the time of writing, studies are on-going to further characterize this organism and determine the possibility that this organism was responsible for the atypical observations in British Columbia. Due to the lack of sufficient scientific evidence to support the presence of A. marginale, a decision was made to remove the remaining quarantines in British Columbia.
The situation in British Columbia and the large numbers of cases identified in Manitoba resulted in a further examination of the status of anaplasmosis as a federally reportable disease in Canada. An Anaplasmosis Steering Committee made up of representatives of the federal and provincial governments, industry, academia and non-government organizations was formed in September 2009. Two working groups reporting to the Anaplasmosis Steering Committee were established to examine the science related to anaplasmosis and review the potential economic impacts of a change in reportable status. As per the CFIA’s mandate, potential impacts on public health (such as, consequences of off-label antimicrobial use) and animal welfare are also being examined. The results of the working groups’ reports are expected in late 2010. A broader stakeholder consultation is anticipated to be undertaken at that time. The current objective is to minimize disruptions to the industry while respecting the CFIA’s mandate to protect animal health and public health in Canada.
The authors thank Drs. Brian Evans, Francine Lord, Jim Clark, Connie Argue, Jag Dhanda, Lynn Bates, Primal Silva, Shane Renwick, and Cyril Lutze-Wallace for their review of the manuscript. The authors acknowledge the diagnostic and research work completed at the CFIA Centre for Food-borne and Animal Parasitology, Saskatoon, the CFIA Lethbridge Laboratory and the Western College of Veterinary Medicine. The CFIA Operations, Programs and Science Branch staff involved in the disease outbreak investigations and response are acknowledged for their hard work and dedication to working towards a greater understanding of this disease in Canada.
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