Mysterious Calf Deaths

[mstahl]

Greetings,
I have had two highland calves, ages about 6 months, die under mysterious circumstances. They have died in two separate instances shortly after the temperatures rose from about -20F to about +25F over the course of 1 to 2 days. They appear to be bloated but I do not know if bloating occurred shortly before or after death. In both cases they appeared fine, active with bright eyes the evening before and then dead the following morning. Forage, shelter, minerals and water are freely available at all times although I have wondered if they were drinking enough water. The adult cows are healthy and happy. Any ideas?