Effects of reactor retention on the spread of brucellosis in strain 19 adult vaccinated herds

Large herds of cattle are ranged on the marsh lands of southwestern Louisiana. These cattle are heavily infected with brucellosis. Cattlemen work their herds twice a year as they come from or go to the marsh ranges. This form of marsh range cattle management is not compatible with the frequent retesting schedule utilized in clearing conventional brucella-infected herds. Producers are further hampered by their inability to purchase replacement cattle to replace the reactor cattle removed from their herds. In response to these concerns, a 5 year field study was initiated and is currently in progress to determine and compare the spread of brucellosis in strain 19 adult-vaccinated herds in which either reactor cows were removed from or allowed to remain within the herds. The study will also attempt to compare the cost effectiveness of these two programs as they relate to both the producers and to regulatory agencies. The rate of spread of brucellosis in the conventional whole herd vaccination program over a period of 17 to 29 months was 4.8% as compared to a 3.6% rate of spread in 24 months in the vaccinated herds with retained reactors. While these differences are slight, the modified program demonstrated that testing twice a year tests was as successful in limiting disease spread as 60 or 90 day tests in the conventional program. It also demonstrates that there is little additional risk in keeping reproductively active reactor cows within the herds. Reactor cow retention thus eliminated the producers need to purchase replacement animals to maintain acceptable stocking rates. © 1984