EPIDEMIOLOGY AND ECONOMICS OF CLINICAL LISTERIOSIS IN A SHEEP FLOCK

A 1990 outbreak of listeriosis in silage fed sheep occurred in souther n Illinois, USA with three observed clinical syndromes of Listeria mon ocytogenes mortality (LM). Overall, 3.1% of the ewes (29/936) and 1.3% of the lambs died (17/1262). Since exposure to silage occurred in all ewes, risk factors for mortality and the financial impact of the dise ase could be evaluated. Yearling ewes were at increased risk of LM rel ative to older ewes (OR=4.1). Rambouillet ewes which did not receive b acterin were at increased risk of LM relative to other ewes (OR=4.6). Use of bacterin did not decrease the risk of LM in Rambouillets (OR = 0.8) but did among ewes of other breeds (OR = 0.1). There were no diff erences in lamb's risk for LM between genders, breed types of lambs, o r breeds of dams. Lambs born in smaller barns were at increased risk f or confirmed LM (OR = 5.0). This was the only significant risk factor for LM cases confirmed by necropsy, but bacterin use in the dam approa ched statistical significance, and increased a lamb's risk for LM (OR = 4.1, P = 0.06). With a broader lamb case definition, dam's poor udde r scores and a shed lambing location increased the risk of LM in lambs (OR = 1.7 and OR = 2.1, respectively). The overall cost of an outbrea k in a 1000-ewe flock (without Rambouillets) would be $3145 with no ba cterin protection. Most of the losses would occur in the ewes ($2803) with relatively minor lamb losses ($342). If a bacterin with 50% effic acy was used, the overall costs would drop to $1831 ($450 due to ewe l osses and $1380 in the lambs). If it was a Rambouillet flock (with no bacterin use) the costs incurred in an outbreak would drastically rise to $10895, largely due to the 12% mortality in the ewes.