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Cache Valley Virus

Next Lambing-Time’s Losses
Caused In Autumn

By Laurie Ball-Gisch

The Lavender Fleece

“Tiny Tim,” deformed by Cache Valley Virus.

Spring 2012 would see our 551st lamb born at The Lavender Fleece and it would be our 13th lambing season. It was also our first season of lambing the newest additions to our farm—our Leicester Longwools. We always eagerly await the year’s first lambs. Still, each lambing season always teaches me something new. In other words: “It’s always something….”

On February 7, I found a stillborn Leicester ewe lamb, born about two weeks earlier than expected. I phoned my vet and Dr. Brenda reminded me that it’s always worse at the very beginning and at the end of lambing season. She assured me our flock was healthy and all would go well. Six days later at first light, I discovered a Leicester ewe with two healthy lambs dried off and with full bellies. Whew! Big sigh of relief. The next Leicester ewe repeated the happy scenario 10 days later and we now had four healthy lambs.

Deformities, Abortions, Stillbirths

A week later another Leicester ewe gave birth in the early morning hours as well: There was a healthy ram lamb, but her ewe lamb was weak and couldn’t get up on her own. She was crying for milk; her dam kept pawing her to try to get her to stand. I gave the lamb Nursemate ASAP® and then held her up to nurse, but she was too weak to suckle. To my dismay, upon looking her over more carefully, I discovered a distinct “S” curve to her spine and a bit of a “hump” to her back across the shoulders. I took her in the house to keep her warm and she died later that afternoon. In all the years I’ve had sheep I had never seen a deformed lamb.

Coincidentally, a few days later a fellow Leicester breeder forwarded to me a video clip she found on the internet of a newborn set of lambs. The lambs were alive, but both were lying on their sides, crying, as they struggled to stand. They were unable to get up. It was heartbreaking to watch the video, and the shepherdess said that she had to put the lambs down. Her veterinarian diagnosed the problem as Cache (pronounced “cash”) Valley Virus (CVV). Since I had never heard of Cache Valley Virus, I immediately searched through my rather large library of sheep books. The only book I have that mentions CVV is Laura Lawson’s Managing Your Ewe and Her Newborn Lambs. Lawson wrote:

“A virus carried by a mosquito or insect bite causes the disease. The disease affects the fetus early in gestation… Ewes with this disease give birth to mummified, stillborn, or weak lambs that die soon after birth. Lambs born alive are weak and deformed. They have a corkscrew-like twist to their spine.” (Lawson, p. 171)

As soon as I read the words “corkscrew-like twist to their spine” I immediately realized that the deformed Leicester ewe lamb was most likely the result of CVV. It was so timely for me, that my sheep colleague had sent me the video, because without that key, I may never have been able to understand what was happening with some of our lambs during this season. When we share information with each other; when we can turn our flock tragedies into learning experiences that help fellow shepherds, we honor the lives of animals we can’t save. This is one reason I’m compelled to write about my sheep and what they’ve taught me through the years.

I found an article published by South Dakota State University titled “Effects of Cache Valley Virus during winter lambing season” (January 26, 2011):

“Cache Valley Virus is a potential cause of the birth of abnormal lambs, and … the most dramatic effects of the virus lead to birth defects in lambs, mostly affecting the brain and central nervous system. The virus also affects the skeletal tissue and muscles … these defects show up as fused joints, curved or twisted spines, unusually thin and underdeveloped muscles, and enlarged skulls.”

Our Icelandic lambing season started off the same way the Leicesters did: One morning I discovered an experienced ewe had aborted a set of lambs, about two weeks prior to term.

That same weekend I heard from a woman who had purchased a bred ewe from me. She had found stillborn twins the day before my own ewe aborted. Because I had recently done the reading about Cache Valley Virus, I now knew that in addition to causing deformities, CVV can also cause stillbirths.

I was sure CVV caused the deformed ewe lamb; now I wondered if it had also caused the first lamb in early February to be stillborn, as well as the loss of now four additional lambs. While there are many reasons lambs can be stillborn, I did find it odd that two ewes from my flock had this problem on the same weekend. It wasn’t the way we usually start lambing here. If I hadn’t had the live Leicester lambs in the meantime, I would have started to worry about a possible abortion vector, either viral or bacterial. While we’ve never had an “abortion storm” on our farm, I had purchased new sheep that previous summer and one never knows what can be brought in, so I became increasingly uneasy.

Then Normal Again…Almost

I was immensely grateful on March 18th to discover a healthy, vigorous set of ram lambs out of an Icelandic ewe.

Lambing then began in earnest. In fact, we ended up with 33 lambs by the end of that first week of lambing, all perfect sizes, perfect health and no assists needed. Lambing season continued to be good and steady and tapered off after the first week in April. One ewe however, had a healthy ram lamb and a mummified second lamb.

Then my first yearling ewe of the year gave birth to a set of twins. It was a sunny morning and I was able to watch her progress. The first lamb was up and nursing while she was having the second lamb. Once he was on the ground, up popped his beautiful moorit-spotted head as he struggled to stand for the first time. I watched and waited; she coaxed and nickered to him. He was sitting up, but his hind legs were flailing and he could not stand up. I went into the paddock and picked the lamb up, moving the family into a clean, straw-bedded pen so I could better assist. Sadly, the lamb was able to sit up with his front legs, but his hind legs were weak and “oddly put together” (the best way I can describe it).

…Lambs carried to near full-term are found to have skeletal deformities and rigid limbs. (Lawson, p. 171)

He was crying for milk, so I held him up and his young dam was sweet enough to let me assist, though she wasn’t really happy with the human interference. I massaged the lamb and stretched out his hind legs, thinking perhaps he’d gotten scrunched up in the uterus and lost blood flow. Or was it loss of muscle tone? It’s very difficult to just let a lamb lie there and die, especially when they’re vigorous enough to cry out for food and trying and trying and trying to get up on their own.

“The deformed lamb has leg defects with immovable joints. Its legs are either flexed or extended in a rigid position.” (Lawson, p. 171)

I worked with the lamb for hours, doing “therapy” with him for two days. He was finally able to stay standing, if I placed him foursquare. Even more exciting: He was able to start taking some steps. I still stubbornly hoped he’d eventually walk on his own, so I kept him fed and did therapy every day. Named “Tiny Tim,” that lamb was such a fighter! We all took turns standing him up so he could walk. I’ve never seen an animal more determined to live. Standing on his own and taking his first tiny steps was a cause of celebration for my family. But his tiny body just wasn’t able to keep up with his spirit.

By the third day Tiny Tim’s breathing was becoming labored. Despite an antibiotic and vitamins to prevent pneumonia, his lungs were filling with fluid. Instead of getting stronger through exercise and eating well, he was actually getting weaker.

Our little heroic lamb hung on for six days trying with all his strength to stand on his own, but he got progressively weaker and his breathing more labored. He slept in my daughter’s arms that night; the next day he expired.

Soon after our experience with Tiny Tim, one morning I found a yearling Leicester ewe with a newborn ram lamb at her side. He was already well fed and she was an attentive and protective first time mother. When I picked him up to dip his navel in iodine, I remember thinking he had an awfully narrow front end, his chest disappointingly narrow.

I put them both in a lambing pen and checked on them several times that first day. At 36 hours old the lamb was found dead. His mouth was warm, his belly full, but when I picked him up fluid ran out of his nostrils and mouth. I realized again he had an incredibly flat rib cage. In fact, instead of a rounded rib cage with room for his lungs to expand, his was shaped like a “V,” essentially almost flat. This lamb also was likely affected by Cache Valley Virus.

“Their ribcage is deformed and unable to expand to allow them to breathe.” (Lawson, p. 171)

Hope: Acquired Immunity

Near the end of lambing season we seemed to have a number of experienced, adult ewes still open and a very low percentage of the one-winter ewes had lambed.

“The virus also may cause infertility with resorption or early abortions in ewes bred during the mosquito season.” (Lawson, p. 171)

Looking back to the previous summer, I remembered it had been exceptionally hot and humid here in mid-Michigan, with a lot of rainfall. Our county’s mosquito control program had, for the first time, not been successful in keeping the mosquito population under control because of the excessive rain.

The South Dakota article quoted Jeff Held, Cooperative Extension Sheep Specialist:

“Mosquitoes cause CVV infection in sheep, and last fall during the early breeding season (August through September) there remained a high population of these pests following the warm, wet summer…. In addition, sheep-flock owners have reported a higher incidence of open ewes and a lower lambing rate this winter, and we often associate lower ewe reproductive efficiency, low lambing rates, and higher percentage of open ewes with ram fertility, nutritional status at breeding, and weather induced embryonic death losses. However, CVV also can contribute to reduced ewe reproductive efficiency.”

Over the past few years, we’ve been trying to have our lambs start arriving earlier in the spring—aiming for March lambing rather than April. So I put the breeding groups together in September and early October. Because Cache Valley Virus is transmitted by mosquitoes, the only prevention is mosquito control. In areas without spray programs, make sure stagnant water is eliminated as much as possible from your farm. Our county has a yearly program where people can drop off all old tires—which often have standing water in them—a perfect breeding ground for mosquitoes.

North Dakota sheep producers had also been affected by Cache Valley Virus, some flocks reporting up to 80 percent lamb losses. Their solution:

“We had a rash of Cache Valley Virus outbreaks across North Dakota in 2010,” said Agriculture Commissioner Doug Goehring. “I have therefore issued a special local needs (SLN) registration to Y-TEX Corp., allowing sheep producers to use PYthon™ Insecticide Cattle Ear Tags to better control mosquitoes that carry the virus.

“The PYthon tags are only registered for use on cattle to repel flies, lice and ticks. The SLN registration allows use of the tags on sheep to repel biting mosquitoes.

“No federally registered products are available that adequately manage mosquitoes and prevent transmission of the virus,” Goehring said. “I am confident the situation meets the criteria of being a special local need.

“The SLN labeling allows one tag per head of sheep. Use is prohibited on sheep less than three months of age. The product labeling provides detailed instructions on securing the tags to ears, as well as a requirement to remove tags before slaughter.”

In mid-April I spoke with some close friends in the area who also raise sheep. The first thing they said when I asked how their lambing was going was, “This has been the strangest lambing season we have ever had.”

They too had seen deformed, non-viable lambs, stillbirths and difficult lambings. A neighbor had a lamb born (with great difficulty) whose neck was fused to the shoulder. I spoke with another breeder in West Virginia who also mentioned Cache Valley Virus going through her flock. A great number of shepherds told me they had unusually difficult deliveries, as well as ewes not able to give birth without veterinarian assistance; several ewes even had to be put down because of the abnormal lamb presentations.

“Upon necropsy, lesions are found in the central nervous system and usually a fluid filled space in the brain. If the lamb is carried to full term, the various deformities make it difficult if not impossible for the ewe to deliver the lamb.

“If the lamb is full term and has immovable joints, call your veterinarian. Often the limbs of full-term lambs must be amputated before the lamb can be removed from the uterus because of rigid lambs. … Be sure the ewe doesn’t develop postpartum depression, postpartum shock or sheep stress syndrome as a result of the abortions or premature birth. Treatments for these disorders are listed elsewhere in the book.” (Lawson, p. 171)

As horrible as Cache Valley Virus is, it’s not the fault of the shepherd; it’s a mosquito-borne virus.

Once a ewe is bitten, she becomes immune. CVV only affects lambs if the ewe is bitten by a mosquito during pregnancy. How it affects the lambs depends on the degree of advancement of fetal development during gestation. Cache Valley Virus is not transmissible from sheep to sheep; it is not contagious.

The South Dakota article added:

“…Infections early in gestation (up to day 28, generally) result in fetal reabsorption, …the most critical period is between days 28 and 45 of gestation.

“Infection at this stage of pregnancy has the highest risk of CVV-related neonatal developmental abnormalities …After day 45 of gestation, a CVV infection is not expected to cause abnormalities in lambs.”

Held said that ewes bred later in the fall after the mosquito activity had declined are expected to have lower risk to deliver lambs with clinical CVV induced abnormalities and return to normal flock lambing rates. Daly added that the virus is not contagious.

“We want to remind producers that the virus is not spread from ewe to ewe, even during the lambing season,” said Daly. “Unfortunately, there is no vaccine for CVV. Since the cause is a virus, there are no treatments available, either.”

Daly and Held both said that CVV is constantly present in sheep populations in the U.S. Clinical manifestations of the disease tend to occur in cycles, as the sheep population seems to gain some natural immunity after infection. As this immunity wanes over a period of years, the clinical effects become more prevalent. Past research with this virus has shown that a high percentage of the ewes in a flock will develop immunity including ewes that delivered clinical CVV and normal lambs.

I like to think, after our season of Cache Valley Virus, that we will now enter a period of immunity from the disease for our flock.


Lawson, Laura. Managing Your Ewe and Her Newborn Lambs, LDF Publications, Culpeper, Va. Fifth Printing June, 1997, pp. 171-172

S. Dakota State University “Effects of Cache Valley Virus during winter lambing season,” January 26, 2011. http://phys.org/news/2011-01-effects-cache-valley-virus-winter.html

North Dakota Department of Agriculture: “Goehring OKs ear tags to protect sheep from mosquitoes” Submitted December 13, 2011 www.nd.gov/ndda/news/goehring-oks-ear-tags-protect-sheep-mosquitoes

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