In the past few months CAHFS has seen submissions of dairy cows where the herd reports an increase in sudden death or cows that go down and die within 24 hours. These cows are unresponsive to calcium treatment and show little or no other clinical signs. In a few herds, low magnesium was diagnosed and analysis of the total mixed ration indicated that magnesium levels were generally adequate, but the potassium was too high resulting in a potassium to magnesium (K:Mg) ratio of 6-10:1, while the recommended ratio is 3-4:1.
These dairies have experienced 1 to 2 deaths per day with some cows found dead and others showing signs of tetany prior to death. Affected cows have reportedly been between 20 and 100 days in milk. Suspected sources of high potassium should include alfalfa hay and citrus products though other sources may also be implicated.
The clinical signs of low magnesium in some cows can be confused with tetanus. Cows with low magnesium may also have low calcium and look more like a milk fever cow but fail to respond to calcium therapy. 
On one dairy, analysis of serum from several cows showing knuckling in the rear legs with rapid decline and death within 24 hours revealed low potassium and phosphorus with moderately low magnesium and calcium. Low potassium is an uncommon finding and our laboratory did not learn what the source of the problem was on this dairy.
Low potassium can cause profound weakness with animals unable to raise their head. Usually low potassium is associated with animals that are off feed and receiving isoflupredone acetate. The compound results in potassium loss via the kidneys and when cows are not eating they are not taking in potassium. Whenever there is an increased incidence of down cows, CAHFS strongly recommends serum samples (in red stopper tube) be collected prior to treatment and before animals are euthanized on the farm. The blood can then be submitted with the whole cow for necropsy.
The serum is often the most valuable sample we receive in cases of electrolyte problems as cows do not typically have any postmortem lesions. If the serum cannot be submitted the same day of collection, the clear portion which separates from the blood clot should be removed into a separate container. The sample should never be frozen with the blood clot still in the tube.
On another dairy, a cluster of 1 to 2 week postpartum cows developed bloody diarrhea and died within 24 hours. The one cow submitted had red intestinal content and heart lesions and was diagnosed with Oleander poisoning. Oleander causes heart lesions and can be an irritant to the intestinal tract, causing intestine bleeding. The intestine lesion is not common in dairy cattle where sudden death or weakness is most often reported. The
most common cause of bloody diarrhea in postpartum cows is Salmonella infection.
Several dairies reported cows that died suddenly or after a very short period of abdominal pain. The cows submitted had massively enlarged spleens that had ruptured and bled out into the abdomen. The spleen
enlargement was from lymphocytic leukemia. Other cows died suddenly due to lymphosarcoma which had infiltrated the heart but also involved the uterus, lymph nodes and abomasum. All these cows were positive for
bovine leukemia virus (BLV). 
Whenever a dairy is experiencing increased death losses among the cow herd, consulting your veterinarian and submitting freshly dead or euthanized cows for necropsy may provide additional information that can prevent further losses.