Imagine you work all year to prepare a ewe for lambing, and she produces a lamb.
A short time later, you notice her lamb is not getting enough milk, and you check the ewe’s udder. It is not hot and doesn’t show signs of mastitis. You might describe the udder as feeling like a “hard bag.”
It’s possible this ewe has contracted the ovine progressive pneumonia virus (OPPV). Signs of OPPV include hard bag, lameness, decreased productivity, respiratory problems and a lack of thriftiness. OPPV causes persistent retroviral infections and affects multiple tissues.
The virus was first reported in Texel sheep in a Dutch ag journal in 1862 and in the U.S. in 1923, but OPPV continues to cause problems today.
This slow-acting, wasting disease affects millions of sheep. Once sheep are infected with OPPV, they remain carriers, and there is no treatment or vaccine except culling.
In the past, researchers have thought that the primary route of infection was lambs exposed to OPPV through milk from infected ewes. Now, researchers think that aerosol particles play a larger role in spreading OPPV.
Scientists at the USDA Meat Animal Research Center (USMARC) in Clay Center, Neb. are among those working to reduce or eliminate the challenges of this very difficult disease. They include Scientists Mike Heaton, Ph.D. and Kreg Leymaster, Ph.D. at the USMARC.
“It is a significant problem in the U.S. sheep industry,” said Leymaster. “A lot of producers would just describe it as the sheep is not doing well. Loss of appetite, or starting to go downhill, but the main thing that concerns producers the most would be the lack of milk production. That affects lamb survival and growth.”
In a recent phone interview, the two scientists said there is no easy answer for controlling OPPV. Good production practices – providing plenty of room for ewes and lambs, using good ventilation when sheep are housed indoors and keeping the climate dry – may reduce virus transmission.
The scientists are also studying the potential of using genetic information to help cull sheep with OPPV from a flock, and keep lambs from getting the disease.
A recent breakthrough in OPPV research occurred when the International Sheep Genomics Consortium Ovine SNP50 BeadChip became available.
The BeadChip offers gene expression and genotyping information.
Heaton and his colleagues had collected tissues from OPPV sheep in 2003, but did not have a genome-wide tool to study any genetic links until the BeadChip became available in 2009.
Using this technology, scientists discovered that sheep have a gene (TMEM154) that could provide information about the susceptibility of sheep to OPPV infection.
“The BeadChip was instrumental in identifying regions of the chromosome that were affecting the susceptibility to the disease,” said Leymaster. “It looks to us that this gene is one of the critical players that the virus exploits to make the animal sick.”
They determined there were three major variants (called haplotypes 1,2 and 3) of the gene TMEM154 that were primarily associated with susceptibility to OPPV.
“Haplotypes” are a combination of DNA sequences at adjacent locations on a chromosome that are transmitted together.
Haplotypes 2 and 3 were strongly associated with OPPV infection and considered “susceptibility alleles.”
Sheep that had two copies of haplotype 1 were much less likely to be infected with OPPV.
The scientists are now studying to see if OPPV strains can adapt to sheep with the TMEM154 haplotype 1.
The USMARC has worked with GeneSeek®, a Neogen Corporation based in Lincoln, Neb. to come up with a TMEM154 genotyping test for commercial use. The test determines the TMEM154 haplotypes.
A commercial genetic test became available in April 2012.
“The general strategy is if a producer wants to make their flock less susceptible to the disease – they would want to lower the frequencies of haplotype 2 and 3 in their flock, and by doing that they could increase the frequency of haplotype 1,” said Heaton.
For producers who want to eradicate or lessen the amount of OPPV in their flock, the scientists suggest first submitting blood samples from older ewes in the flock to find out the prevalence of OPPV. Several labs have the capability of testing for OPPV via a blood sample.
“They may find out they don’t have OPPV in their flock, and that’s a totally different picture of how to move forward than if they find half of their mature ewes were infected,” he said. “You have to customize your protocol going forward once you know where you are at.”
After producers have discovered the prevalence of OPPV, and ewes that have the disease, they may want to run a genotyping test.
The genotyping test is a separate test that can use blood or ear hair follicles that contain DNA.
“If a flock is infected – whether it’s registered or commercial – OPPV will reduce the productivity of the flock,” said Leymaster. “Many producers would be interested in decreasing the incidence of the disease.
“If you are a purebred breeder and could offer sheep for sale that are also the genetically least susceptible to it, then you have that potential to increase the value of your seed stock.”
The scientists are continuing their work on OPPV. They are confident there are more solutions that will help sheep producers increase the number of lambs that survive by reducing the impact of this virus.