I lost a year-old Brangus heifer to what we think was blackleg. A neighbor also lost a heifer recently. My heifer had a 7-Way at 4 months and a second shot at 6 months. The shots were applied by two different vets. I wonder if both shots were improperly put into the muscle or if both doses were out of date when applied. I've also heard there is a new strain of blackleg that's immune to the 7-Way. Do you have any other ideas?
You don't mention this as a possibility, but I'm going to add to your list and say it was likely not blackleg but some other disease that killed these heifers.
Let's look at your question in more detail. First, clostridial vaccines, or more properly bacterins, should work even if given in the muscle. Reactions to these bacterins when given in the muscles of the hip and rear leg were costing packers huge sums of money, which was one of the driving forces in the development of the Beef Quality Assurance program and the focus on subcutaneous and low-reaction bacterins.
Second, I seriously doubt a veterinarian (much less two veterinarians) would use outdated products. Third, I have not heard of any new strains. Bacterins to all clostridia are very effective, and the diseases have shown little antigenic shift over time.
A factor that must be considered is maternal immunity. Calves get protection in the colostrum, or first milk, to protect them from diseases until their own immune system takes over. That protection interferes with vaccines or bacterins. Maternal immunity may persist for more than six months to some viral diseases, but most calves respond to clostridial bacterins by 2 or 3 months of age. So this is unlikely a factor in your case given the age you report the 7-Way was given.
Now back to my theory this wasn't blackleg. I can't tell you exactly what happened, never having been to your operation. But I'd be interested to see what a necropsy would reveal should this ever happen again.
In cases like these, it pays to have an accurate diagnosis, and nothing is better for that than a necropsy. You will not always get the definitive answer, but it provides the best chance to take appropriate actions to prevent more disease or death loss in the future. I have often been surprised by the answers I found during a necropsy or from samples I submitted to the lab for analysis.
Hopefully you and your neighbor will not have any more problems, but if you do, call your veterinarian and get to the bottom of this. No more guessing.
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