Hook Lake Wood Bison Recovery Project: An Attempt to Eradicate Bovine Tuberculosis and Brucellosis from a Wood Bison Herd in Northern Canada
C. Cormack Gates1,3, PhD; Brett T. Elkin2, DVM; Troy Ellsworth1, RRTP
In 1990, the federally appointed Northern Diseased Bison Environmental Assessment Panel recommended the depopulation of tuberculosis and brucellosis infected free-ranging bison (Bison bison athabascae) herds in and around Wood Buffalo National Park and their replacement with disease-free wood bison. An alternative approach to depopulation was conceived jointly by the Deninu Kúe First Nation and the Government of the Northwest Territories and is being implemented as a pilot project for one infected bison herd. Over three successive calving seasons, 60 newborn calves were captured from the wild and placed into an isolation facility at Fort Resolution, Northwest Territories. The neonatal calves were treated with prophylactic antibiotics to prevent infection with Mycobacterium bovis and Brucella abortus microorganisms potentially present perinatally. Calves were housed in isolated pairs until they reached 12 months of age. Multiple testing with the intradermal tuberculin test for tuberculosis and serology for Brucella antibodies revealed one caudal fold test-positive calf. The reactor calf was slaughtered with its pen mate, but bacterial culture was unable to isolate any Mycobacterium species. The remaining 58 calves have remained test-negative for brucellosis and tuberculosis.
Approximately 2,800 free-ranging bison (Bison bison athabascae) in northern Canada are found in populations known to be infected with Mycobacterium bovis and Brucella abortus. Two thousand and one hundred of these are located in Wood Buffalo National Park. The remainder range in areas adjacent to the park in northeastern Alberta and the Northwest Territories. The ongoing presence of both diseases in the region pose a threat to the conservation and recovery of healthy free-ranging bison populations, the ecologic, cultural and economic value of bison to local communities and the country, and the disease-free status and international reputation of the commercial bison and cattle industries in Canada.
In the absence of management support, infected herds have declined by 80% since 1970. In 1990, the federally appointed Northern Diseased Bison Environmental Assessment Panel6 recommended depopulation of tuberculosis and brucellosis infected free-ranging bison herds in and around Wood Buffalo National Park and their replacement with disease-free wood bison. The recommendation was strongly protested by many sectors of the Canadian public. The major reasons for opposition were impairment of ecosystem integrity and loss of genetic diversity represented in the infected herds.
Recognizing that the establishment of healthy bison populations is a desirable alternative to the status quo, the Deninu Kúe First Nation (Fort Resolution, NWT), supported by the Government of the Northwest Territories, initiated a long-term recovery program for the Hook Lake herd located in the Slave River Lowlands. Elements of the program include habitat renewal using prescribed burning, establishment of a disease-free captive breeding herd based on the capture and prophylactic antibiotic treatment of neonatal calves, the gradual elimination of the infected parent herd largely through aboriginal subsistence hunting, isolation of the depopulated bison range, and reintroduction of disease-free bison.
Newborn calves were captured by helicopter-based net guns during May 1996, 1997 and 1998. In 1997 and 1998, calves were tested for antibodies against Brucella abortus using the Brewer’s card test (BCT) immediately after capture at a field staging site. Test-positive calves were returned to their individual site of capture as determined by global positioning system and released. Test-negative calves were transported to an isolation unit in Fort Resolution, NWT. They were housed in isolated pairs in 1.3×2.5-m boxes for 2 weeks during which they were treated with intramuscular antibiotics. Calves were ear tagged with a numbered plastic disk at the time of capture, and with a tamper-proof metal tag provided by the Canadian Food Inspection Agency (CFIA) at the first disease test.
Calves were fed reconstituted milk replacer (22% fat and 24% protein by weight) (Brown’s Feeds, Airdrie, Alberta). High-quality legume hay was offered ad libitum. Calf starter was provided from 2 weeks after capture until the end of August. After that time, rolled barley and alfalfa pellets were provided at a rate of 1 kg and 0.5 kg/hd/day respectively. Upon completion of injectable antibiotic therapy, calf pairs were released into isolated 13×23-m paddocks.
In the absence of antibiotic sensitivity testing of the two microorganisms, the antibiotic protocol was selected based on information available from the veterinary and medical literature.3-5,7,10,11 Oxytetracycline (Liquamycin LA-200®; Rogar/STB Inc., London, ON) and dihydrostreptomycin (Ethamycin®; Rogar/STB Inc., London, ON) were administered by intramuscular injection at 10 mg/kg every other day for 14 days. Isoniazid (pms-INH7; Pharmascience, Montreal, QC) was administered orally in the daily milk feedings for 5 months at 10 mg/kg UID. Isoniazid therapy was terminated 1 month before the first intradermal TB test was carried out on each calf cohort. Enrofloxacin (Baytril®; Haver, Etobicoke, ON) and rifampin (Rofact®; ICN Canada Ltd., Montreal, QC) were also administered orally in the milk ration at 10 mg/kg and 15 mg/kg UID respectively.
To date, calves captured in 1996 have been tested five times for brucellosis and tuberculosis between November 1996 and February 1998. The intradermal caudal fold test using M. bovis PPD tuberculin was run in November 1996; February, April and November 1997; and February 1998. The 1997 calves have been tested two times during the last two test periods. The Blood Tuberculosis Test (BTB) was run on whole blood and sera collected in March 1997 from 20 calves. The assay series was performed at the BTB Diagnostic Laboratory at Texas A&M University. Serum from each of the five test periods was tested for B. abortus antibodies at the isolation unit using the Brewer’s card test. Additional serum was sent to the CFIA’s Animal Disease Research Institute in Lethbridge, Alberta, where it was tested using two or three of the following assays: buffered plate antigen test, standard tube agglutination test, and compliment fixation test.
Fourteen female and six male newborn calves were captured in 1996. In 1997, 26 calves were captured. Five calves tested positive on field screening for B. abortus antibodies and were released at their respective capture sites. Sixteen female and four male test-negative calves were transported to the captive breeding facility at Fort Resolution.
All tests conducted in November 1996 were negative. In February 1997, one female calf tested positive on the caudal fold test for tuberculosis. A swelling >10 mm was observed on visual inspection and palpation. Due to logistic constraints a comparative cervical skin test could not be conducted within the required 10-day interval following tuberculin injection. The test-positive calf and its pen mate were slaughtered on March 5, 1997. No lesions were grossly visible on postmortem examination. Histopathology revealed acid-fast bacilli in a minute mineralized granulomatous lesion in a mediastinal lymph node. Bacterial culture failed to isolate any Mycobacterium spp. All BTB tests performed on blood collected on March 4 were negative, including samples from the two slaughtered calves.
Part of the protocol employed during this study (orphaning neonatal calves and elimination of test reactor pairs) was adapted from a successful approach used previously to eradicate tuberculosis and brucellosis from captive wood bison.2 In addition to orphaning and depopulation of isolated pairs, the Hook Lake project employed the prophylactic use of antibiotics. Antituberculosis drugs have been used in deer, kudus, oryxes, camels, monkeys, and great apes in captivity.8, 12 The antibiotic sensitivity of the strains of B. abortus and M. bovis found in bison in northern Canada has not been evaluated but is the subject of a current study. Therefore, antibiotics were selected based on published findings from previous studies in which antibiotics were used to combat infections with M. bovis or B. abortus in other captive ungulate species. In the absence of controls, the efficacy of individual components of the protocol used in our study cannot be fully assessed. Success will be determined by the ability of the protocol to establish a disease-free herd.
It was unfortunate that the single positive reactor to the caudal fold tuberculin test was not subsequently tested with the comparative cervical test to distinguish a possible infection by the most likely atypical mycobacterium, M. avium, before slaughtering the calf and its pen mate. Potential avian hosts of M. avium were abundant around the bison pens, including ravens (Corpus corax), common grackles (Quiscalus quiscala), and snow buntings (Plectrophenax nivalis). The incidence of atypical mycobacterial infections in people in the Northwest Territories (53 cases since 1990)8 provides an indication of the ubiquity of these organisms in the environment. Although there is a possibility that the calf was infected with an atypical mycobacterium, the location of the single detected lesion in a mediastinal lymph node is consistent with the pathogenesis of M. bovis. Identification of the acid-fast bacilli detected in tissues from the test-positive calf was unsuccessful, as bacterial culture did not isolate any Mycobacterium species.
We suggest that the risk associated with latent infections with either organism is minimized by the protocol applied in this study which includes field screening of neonatal calves for Brucella antibodies, administration of appropriate antibiotics, and regular disease testing. Multiple testing of the captive population will continue at a frequency of at least twice per year over the long term. One critical period remains for expression of latent B. abortus infections. Bovine calves infected by B. abortus in utero or after ingestion of infected milk may maintain a latent infection with seroconversion following their first calving or abortion in the case of females,13 or later in life in the case of males.9 Hence, it will be important to physically isolate primiparous cows during pregnancy beginning at 5 months of gestation prior to the third trimester when brucellosis-induced abortions may occur, and to monitor for anti-Brucella antibodies for several weeks following parturition.
The approach being attempted in this study to eradicate M. bovis and B. abortus through salvage and creation of a healthy captive breeding herd may offer a possible alternative to the mass slaughter of wild bison populations and the attendant loss of genetic diversity that was proposed previously.1 However, latent undetected infections may yet compromise the project. Even if successful, establishing a clean captive herd represents only one step towards reestablishing a disease-free wild population. Additional measures, including a range of options identified in the planning of the program, will be required to ensure complete eradication of reservoirs of the microorganisms and protection of healthy bison from reinfection once they are restored to the Hook Lake bison range.
Planning a project such as this involved the support of many organizations and individuals. The patience and indulgence of many Aboriginal Elders in Fort Resolution in accepting unfamiliar technology is recognized. The Deninu Kúe First Nation and Aboriginal Wildlife Harvesters Committee were strongly supportive of the project. Financial support for the operation was provided by the Government of the Northwest Territories Department of Resources, Wildlife and Economic Development.
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