When we experience an unexplained death loss, especially this close to lambing, we often take the dead animal to the California Animal Health and Food Safety (CAHFS) Lab at the UC Davis School of Veterinary Medicine. Not only is it cheaper to have an autopsy performed on a dead sheep than it is to have the rendering company remove the animal, we often get valuable information. In the case of this particular ewe, we found out that she died of an unusual case of meningitis (a rare condition in mature sheep). While we were reassured that she didn't have a condition that was likely to spread to the rest of the flock, an additional (and unrelated) finding did concern us. In checking her liver for heavy metals, the pathologist at CAHFS found that she had elevated copper levels (she had 260 ppm of copper; the high end of normal is 100). Copper is an essential micronutrient for sheep, but excess copper can be extremely toxic. Sheep store copper in their livers. When sheep undergo some type of stress (like a difficult lambing, harsh weather, or abrupt change in diet), their livers shed this copper into their bloodstream, often with fatal consequences.
When we read the results, we obviously became concerned about the copper levels in the rest of the flock. If this ewe had elevated copper, it was entirely possible that other ewes may have elevated levels as well. If so, we might have problems during lambing. Sami and I, along with our partner Roger, immediately went into Sheepherder CSI detective mode. Roger and I began researching possible sources of copper, as well as potential treatment options. Sami and I made plans to draw blood samples from a subset of the ewes.
Fortunately, I also placed a call to a friend who is on the faculty at the Vet School. Dr. Bret McNabb is a very experienced and knowledgeable large animal clinician and teacher. He told us that blood tests would not be a good indicator of copper status - we'd need more expensive liver biopsies. He also told me that he usually didn't see copper-related deaths until liver levels were above 400 ppm. He encouraged us instead to continue looking for potential sources of copper in our flock's diet, and to send any additional fatalities to the CAHFS lab. We also talked about treatment options, which included providing supplemental molybdenum to bind the copper that the ewes had already stored.
With Dr. McNabb's reassurance, we turned our focus to sources of dietary copper. We have a couple of suspects:
- The annual grasses that feed our sheep this time of year likely have some copper in them. Roger went back through some forage tests from the Sierra Foothill Research and Extension Center and found that at some times of the year, the forage may contain slightly more copper than the sheep need.
- For at least half the year, our sheep graze on land that was once in orchards. Copper sulfate may have been used as a dormant spray on this land; we may have residual copper in the soil. Our next step will be to test the forage for copper during the grazing season (spring, summer and early fall).
- We utilize surface water to irrigate our summer pastures and to provide drinking water to the sheep during most of the year. To control algae in their canals (which can clog our sprinklers), our water agency uses one of several aquatic herbicides in which copper is an active ingredient. While both products are labeled as safe for livestock consumption, this may be something we need to investigate further.
- As Dr. McNabb pointed out, sometimes feed manufacturers make mixing errors. For example, the mineral mix we provide to our sheep does not have any added copper. However, mistakes can happen. This weekend, we sent a sample of our mineral mix to CAHFS for analysis. We should know the results by the end of this week.
- The weed common groundsel (Senecio vulgaris) contains a liver toxin that apparently inhibits the liver's ability to metabolize copper. We know we have common groundsel in areas where the soil has been disturbed, and we know the sheep will graze it on occasion. However, according to Toxic Plants of North America (Burrows and Tyrl 2001), sheep and goats are less susceptible to common groundsel than other livestock - and can actually be used to control it. Not exactly a smoking gun...
For now, the source of copper remains a mystery. The rest of the flock seems healthy, so we're hopeful that this ewe's copper levels were an anomaly. We'll definitely investigate any subsequent death losses - we're fortunate to have the CAHFS lab close by. And we'll continue to investigate the potential sources outlined above. Stay tuned.... And if you've had similar experiences with your sheep, please let us know!