Ovine progressive pneumonia (OPP) is a chronic, viral disease of sheep caused by a lentivirus but new research is showing that genetics may play a bigger part in the disease than previously thought.
Some of the most recent research on the disease was showcased at the Sheep Day program March 16 at the University of Wisconsin's Arlington Agriculture Research Station. Kreg Leymaster, with the U.S. Department of Agriculture's Agricultural Research Service told the shepherds attending the event about a study he was involved looking at the link between genetics and OPP.
A 2009 project, led by the International Sheep Genome Consortium, located a gene (TMEM154), which encodes membrane proteins and looks to be the key mechanism for the virus to enter a cell. The project found that some versions of this gene appear to be associated with greater susceptibility to infection.
"This is the easy part of the research," he said. "What's difficult is turning it into something that can be used by the sheep industry."
The disease is costly to producers when sheep exhibit the classical signs of the disease - labored breathing or pneumonia, weight loss, paralysis or lameness and hardened, unproductive udders. Some sheep don't show clinical symptoms but may be infected and will be carriers for life.
Leymaster said the lentivirus that causes OPP is in the same class as HIV - the virus that causes AIDS in humans - and both of them are proving difficult to find a cure or a vaccine for.
That's because the virus works its way into the host's cells and becomes part of its genetic makeup. Once it is in the blood cells and can travel all over the body, the disease can affect the lungs, central nervous system, lymph nodes, joints and mammary glands. That's when clinical symptoms are seen.
Leymaster, who works at the U.S. Meat Animal Research Center in Clay Center, NE, said few producers believe they have OPP in their flocks, but when research trials drew blood and tested sheep, they found that 36 percent are infected.
Unlike in other viral diseases, when antibodies are found it doesn't mean the animal has immunity to the disease. "When sheep are infected they are infected for life and there is no treatment or vaccine," he said. Yet infected sheep often show no clinical signs.
Those sheep are costly to the flock operator, however, because these carriers are less likely to produce lambs; overall they wean 8 percent fewer lambs. An infected ewe will fall out of the flock earlier and sell for less than she would have if she had been healthy.
When that happens she must be replaced with a ewe lamb, which is also a cost to the operation.
Recent research done at Clay Center builds on the discovery of that gene. The researchers took samples from several thousand sheep and sequenced the DNA in that gene from all those sheep. From this they discovered three common forms of the gene.
These three "haplotypes" as they are called, encode a specific amino acid sequence and were found to influence a sheep's risk of OPP infection, Leymaster said.
Over 2,700 sheep in Nebraska, Idaho and Iowa (in a number of breeds) were tested to confirm the hypothesis that sheep with haplotypes 2 and 3 were much more likely to get infected than were sheep with haplotype 1.
"Overall, the OPP infection rate of sheep with at least one copy of haplotype 2 or 3 was 2.8 times greater than sheep with two copies of haplotype 1."
Leymaster told the shepherds that other studies have also looked at the predicted susceptibility to OPP infection in 74 different breeds of sheep and found there are huge breed variations. They ranged from 100 percent susceptibility in some breeds to nearly zero.
Leymaster's group also tested how sheep get OPP. "We think there are two primary routes of OPP virus exposure: maternal - meaning they get the virus from the colostrum and milk from the infected dam; and non-maternal - meaning the virus is being exhaled from infected sheep and other flock mates are picking it up that way."
In this experiment they took 20 "sentinel" lambs from uninfected dams and 187 lambs from heavily infected dams and allowed the ewes and lambs to commingle.
Researchers took blood samples from all the lambs one week after weaning and every five weeks thereafter until about the lambs were nine months of age. Each of those blood samples was used to test their OPP virus status by ELISA assay.
In the sentinel lambs, there was no exposure to the virus from their dams, so their only exposure was running with infected flock mates, Leymaster said. The blood tests showed, however, that they didn't readily pick up the infection that way either.
Their blood antibody levels stayed below a benchmark cutoff point, he said.
In their genetics, half of the sentinel lambs had two copies haplotype 1 and the other half had one copy of haplotype 1 and the other was a 3.
Those lambs that came from the infected ewes had haplotypes of three different mixes. There were 56 lambs with two copies of haplotype 1; 71 lambs with a 1,3 "diplotype" and 60 lambs with a 3,3 diplotype.
This research confirmed that lambs with one or two copies of haplotype 3 had a 3.2 time greater OPP infection rate than lambs with two copies of haplotype 1. But lambs with a 1,3 diplotype were also at risk of infection.
"You have to have two copies of haplotype 1," he said. "One copy doesn't do you any good."
The study was important in that it confirmed the association of the TMEM154 haplotypes with susceptibility to OPP and established that haplotype 1 is recessive to haplotype 3.
"Also, we found that non-maternal exposure caused little, if any, OPP virus infection to nine months of age."
Maternal exposure during the period when lambs were nursing, infected at most 11 percent of the genetically less-susceptible lambs and only 35 percent of the genetically more-susceptible lambs.
Leymaster concludes that the primary cause of infection in a flock of mature ewes must be due to non-maternal exposure that occurs after young ewes join the infected breeding flock.
The key management strategy is isolation of young ewes to prevent subsequent non-maternal exposure, he added.
The conventional approach to managing for OPP-free flocks involves periodically blood testing all sheep and culling those that test positive. Leymaster recommended if testing annually, that shepherds test a month before lambing and retain as flock replacements lambs from seronegative ewes - especially those that are older.
If a ewe is older it means she has probably got some genetic ability to withstand the disease.
Another strategy is to artificially rear lambs and isolate them from infected sheep. In theory an OPP-free flock could be created by depopulating and then repopulating with sheep from OPP-free flocks.
Even with these strategies, he said, the sheep would remain genetically susceptible to the virus and will become infected if they are ever exposed to infected sheep.
An alternative approach to reduce OPP prevalence in a highly infected flock would be to put all ewes into breeding but try to use rams with haplotype 1 - the one that seems to confer some immunity.
Then blood test the resulting ewe lambs at seven months of age or older to determine their serological status. Testing at this age helps assure that any maternal antibody effect from nursing will have faded away.
Leymaster said that under this strategy, the seronegative ewes should then be isolated from the infected flock to remove them from the virus. These seronegative ewe lambs should then be bred to rams that will increase the frequency of haplotype 1.
This approach has not been tried at the U.S. Meat Animal Research Center but some commercial producers are implementing it, he said. "That way you're not throwing away good genetics because of one disease."
Leymaster told shepherds that they could use the information to supplement, not replace, their current selection and culling procedures. They might want to sample older ewes to estimate the infection rate in their flocks and he urged them to not automatically cull lambs born to infected ewes.
What the research has shown is that it's important to try to get genetics that may help confer some immunity to OPP. "Know the TMEM154 haplotype of the breeding rams you're using. Start paying attention to this," he said.
These kinds of DNA tests can be done on blood or hair samples and cost about $12. The serology tests to determine if a sheep is infected cost about $6, he said.
"It's early in the research and there's still a lot to learn and we know that adverse environmental conditions can cause high rates of OPP virus infection regardless of TMEM154 haplotypes."
Poor ventilation, high humidity and high flock density can be factors in OPP infection rates.
Viruses are also notorious for a high mutation rate and they may find a way to adapt as management seeks ways to reduce infection.