The gateway disease
by Sally Colby
Dr. Jesse Goff began a presentation about subclinical hypocalcemia (SCH), the milk fever a dairy farmer might miss, with a take-home message: it’s controllable. Goff, senior researcher and professor emeritus at Iowa State University Veterinary College, said as many as 50% of older cows in the national herd are affected by SCH.
“The cutoff we were using is 8 mg calcium/deciliter of blood,” said Goff, adding that 8.5 might be a better cutoff but difficult to attain. “As many as 25% of heifers included in the survey were in the subclinical milk fever zone.”
Goff said affected animals look normal and are hard to pick out in the herd. The deeper problem is that SCH is a leading factor for a variety of metabolic issues. “Hypocalcemia is a gateway disease,” said Goff. “If a cow starts to become hypocalcemic, you can expect to see many other diseases follow. We know hypocalcemia directly affects the immune cells – animals are not able to respond to antigens.”
Hypocalcemia reduces muscle tone, which can lead to mastitis because the teat sphincter doesn’t close adequately. Muscle tone also affects proper abomasum contraction, which can result in a DA. “True milk fever affects dry matter intake, which exacerbates negative energy balance,” said Goff, “and we know negative energy balance is also a factor that affects immunity.”
Garrett Oetzel, DVM, professor at the University of Wisconsin-Madison School of Veterinary Medicine, said the name “milk fever” is inaccurate – no fever is involved. Other names for the condition include parturient paresis, paresis puerperalis and parturient apoplexy.
SCH cases are usually non-progressive with minimal clinical signs, and usually occur in older cows. It’s most harmful if it persists more than 48 hours post-calving. “Calving is a difficult time metabolically for the cow,” said Oetzel. “It’s because milk is really, really high in calcium. Cows are pumping all this calcium to support the skeletal growth of their calf, but put themselves at great risk to secrete that much calcium.”
Colostrum contains twice the amount of calcium compared to milk. While milk contains 0.5 grams of calcium/pound, colostrum contains 0.8 to 1.0 grams of calcium/pound. “This is a real shock to the cow’s system,” he said. “She’s been developing the fetal skeleton which takes a little bit [of calcium], but suddenly, the day before calving, she could easily put 15 grams of calcium into the first colostrum.” Oetzel said the cow repeats that output 12 hours later, which means about 20 to 30 grams of calcium output in the first 24 hours post-calving.
“Cows are going to struggle with calcium balance post-calving just like they struggle with negative energy balance,” said Oetzel. “But what makes a cow drop calcium levels post calving?” Oetzel cited a study in which cows on an ov-synch program had a distinct drop in calcium in week 11 post-calving. “Most of the cows were around estrus. Estrus decreases appetite, estrogen hormone impairs bone mobilization of calcium, and now you see a drop in fertility from calcium imbalance in early lactation cows.”
The calcium drop occurs due to lactational osteoporosis – cows are drawing calcium from their own bones. “They lose 9% to 13% of skeletal calcium in the first 30 days,” said Oetzel. “That’s about 2.5 pounds of elemental calcium lost from the 21 pounds of elemental calcium in the bone.” The cow’s skeletal store of calcium is sufficient, but the post-calving challenge is to get the mechanisms activated.
Dairy farmers should be aware that the risk of SCH is high, and for the average cow, the outcomes are bad. Oetzel suggested risk assessment for the herd, and to account for higher risk in older cows. “If not done that way,” he said, “you will overestimate the problem in older herds and underestimate the problem in younger herds.”
Oetzel explained how metritis is directly related to SCH. The serum total calcium levels in cows that did not get metritis dropped but not significantly. Cows that did get it had drastic drops in serum total calcium, and those levels persisted after calving. Oetzel said there are two scenarios: low calcium caused the metritis because calcium is integral to immune function, or cows were “working on” metritis and had inflammatory problems. He said both are probably happening.
Learning when to look for SCH can be tricky. “Day zero and day one are the low point (nadir),” said Oetzel. “However, some depression in blood calcium is necessary to trigger a homeostatic response and mobilize bone calcium. We can use that time period to monitor subclinical hypocalcemia. Testing at later days in milk would better predict cows that are in trouble.” Oetzel added that delayed hypocalcemia is more detrimental than hypocalcemia that occurs around calving, even if the initial hypocalcemia nadir was quite low.
“Are the pre-fresh cow diets doing their job?” said Oetzel. “Are we doing the right thing with nutrition in the pre-fresh period? If you want to answer that question, test around the nadir. If you test after that, you are getting hypocalcemia that is secondary – it isn’t diet.” Oetzel suggested random sampling, and because timing isn’t critical, samples can be sent to a lab.
“This approach can work,” he said. “Subclinical hypocalcemia for the average cow is detrimental. I don’t want people to say that low blood calcium around calving means nothing because that isn’t true. We have way too much data that say it is very detrimental to cows, even if they aren’t clinical.”
For some cows, oral boluses are a good solution. “Lame and higher previous lactation cows responded best to oral calcium supplements,” said Oetzel, referencing a bolus study. “Lame cows had fewer health events if they were given at least two boluses around calving. The test group gave 7.2% more milk at first test if given boluses.” Boluses help shunt calcium to milk, allowing cows to continue give milk without compromising calcium levels in milk.
Oetzel cautioned that a true calcium deficiency must be created to activate calcium homeostatic mechanisms. “We have to be under 15 grams of elemental calcium per cow per day,” he said. “That is nearly impossible. If you only get partway there and cows are not deficient, the mechanisms aren’t turned on and not much help at all.”