Causative agents:
The following stomach and intestinal parasites have been identified in bison (42, 43, 47, 48, 50, 51,52,53):
Cooperia spp.
Haemonchus contortus
Monezia benedeni
Nematodirella alcidis
Nematodirus spp
Oesophagostumum radiatum
Ostertagia spp
Seratia
Trichostrongylus spp
Trichuris ovis, and other unidentified Trichurus spp.
This array of intestinal parasites causes a wide variety of disease syndromes in cattle, ranging from very mild or nonexistent, to very severe and life threatening. In cattle, many of these parasites are also associated with unthriftiness, reduced rates of growth or reduced productivity. Parasitic disease in cattle is often associated with overcrowded or wet conditions especially during the summer, when parasite survival on the ground is highest.
Neither reduced growth rate, loss of productivity, lack of vitality, unthriftiness, nor any other disease syndrome, has been associated with the infection of bison by any of these parasites, other than Ostertagii.
In bison, ostertagia has been the only species reported to cause clinical disease and mortalities. Ostertagii larva can penetrate the glands of the abomasum and there they can enter into a dormant, hypobiotic, state. While they are in this state they cause no damage to the abomasum. If large numbers of larvae emerge from this state at the same time they cause serious damage to the abomasum, resulting in clinical disease and death. This form of ostertagiosis called Type II ostertagiosis has been reported in 3 bison herds in New York State (53).
In ranched bison in Northern Alberta that were tested by fecal analysis, Cooperia was the predominant fecal parasite identified, accounting for 96% of all parasites found in calves and 92% of all parasites found in cows (51).
Clinical signs:
The clinical signs associated with Type II ostertagiosis in bison include: anorexia, weight loss, weakness, dull hair coat, severe diarrhea, anemia, hypoproteinemia, lymphocytopenia, neutorphilia and death (53).
Postmortem findings:
Pathological findings associated with Type II ostertagiosis in bison include: emaciation, and irregular thickening of the abomasal mucosa, giving it a morocco-leather appearance (53).
Diagnosis:
Fecal samples may be examined to identify the presence of parasite eggs and larvae. The identification of parasite eggs or larvae in the feces of bison should not be considered an indication that parasites are the cause of disease. In cattle, the number of eggs found in fecal samples is not related to the total number of parasites in the hosts stomach or intestinal tract (9).
Fecal analysis should only be used to identify the presence of the parasite in the herd, and to estimate the number of bison that are infected.
Fecal samples should be collected in the spring from April to June. If fecal samples are collected in the late fall or winter, they may be unrewarding since parasites may reduce egg or larva production during this time.
Treatment:
The treatment of bison with parisiticides should vary with the geographic location, the species of parasites that are infecting the bison, as well as the housing and pasturing practices on each ranch. In New York state where Ostertagii was present and bison were housed in close quarters, the burden of Ostertagii in bison increased to the point where mortalities occurred (53). In Northern Alberta where bison were maintained on large, spacious ranges in both the summer and winter, Cooperia was the predominantly identified parasite, even though Ostertagii was present in small numbers. Clinical disease was not identified evan after 18 months without parasiticide treatment (51).
In cattle, these parasites have been found to be susceptible to a wide variety of parasiticides, including bezimidazoles, probenzimidazoles, levamisole and ivermectins (9). None of these products has been approved for use in bison.
In bison, ivermectin in a 0.5% pour-on formulation has been found to be effective against Ostertagii at a dose of 1ml/10kg of body weight (54).
Control:
Control programs will also vary with the geographic location of the bison and the parasite populations that are endemic to bison in that location. Control programs should include routine fecal egg counts, pasture rotation and judicious use of parasiticides.