Lasalocid Toxicosis in Dairy Calves

In February 1990, a 1,500 milking cow dairy in the California Central Valley changed from feed containing monensin to feed containing lasalocid to 350 head of 2-4-monthold calves of both sexes. The lasalocid was mixed at the feed mill with corn and barley and pelleted. The feed was delivered bulk to the farm and a 1 -gallon bucket of feed was given to each calf once/day. The feed comprised 2/3 of their diet, whereas the remainder was hay. Calves were housed in individual crates. After the first day on the new feed, calves became anorexic, but feed intakes gradually increased over the 5 subsequent days. One calf died suddenly on day 4 after starting the new feed. On day 5, additional calves developed sudden weakness, increased respiratory rate, mild nasal discharge, and reddening of the nose without a fever. At the time, the feed was replaced with a corn/barley mix without additives. Eleven calves were found dead on day 6. Field necropsies, performed on 6 of the 11 calves, revealed pleural and peritoneal effusions and pulmonary edema. One calf was necropsied at the California Veterinary Diagnostic Laboratory System (CVDLS) at Tulare 2 hours after it died. This calf had a rumen pH of 5.0, blunting and swelling of rumen papillae, gas accumulation, and large quantities of grain in the rumen. The urine pH was 5.0. No other gross findings were present. Histologic examination revealed suppurative rumenitis compatible with lactic acidosis. No heart lesions were observed. The next day (day 7) 1 female and 5 male calves, which were found dead, were submitted to CVDLS. At that time, all grain feeding was discontinued and replaced with alfalfa hay. All 6 calves were in good condition and the hearts were contracted. One calf had epicardial hemorrhages. Two calves had yellow mottling of the heart muscle that was most noticeable in the interventricular septum on cut section. Three calves had congestion and hemorrhage of the left papillary