A recent study found that 26% of the sheep in the United States are infected with the OPP virus. In some flocks the incidence of infection is much higher and can approach 100%; in others it is much lower. Related retroviruses are found in sheep worldwide with the exception of Australia, Iceland and New Zealand. The incidence of infection increases with ewe/ram age probably due to sheep-to-sheep spread of the virus. The percent of infected sheep that develop clinical disease is variable and is possibly dependent on flock management, strain and dose of virus, and genetics of sheep.

       Sheep and goats are the only species that have proven to be naturally susceptible to the OPP virus. Certain breeds of sheep are more susceptible to infection and to the development of disease. In the United States, the following breeds appear to be more susceptible to the development of disease: Border Leicester, Finnsheep, Finn-crosses, Corriedales, Dorsets, and North Country Cheviots. In Europe, the Texel breed has been found to be very susceptible to the disease, whereas, the Ile de France has been shown to be resistant to infection and disease. These findings may also relate to differences in the pathogenicity of the strains of OPP virus and not solely a function of genotype of the sheep.

       The primary means of transmission is through the ingestion of infected colostrum and milk. This occurs when a lamb nurses from its infected dam or steals milk from another infected ewe. Contact transmission also occurs in closely confined sheep. This type of spread probably occurs at a much lower incidence than the former. It potentially occurs when respiratory droplets from infected sheep directly contact uninfected sheep or contaminate feed and water sources. Upon environmental exposure the virus has a very short life. Therefore transmission from sheep to sheep is probably heavily dependent on environmental conditions: eg. a tightly confined group of sheep in a warm humid barn are probably at a greater risk of infection. Transmission in utero occurs very rarely and has not been found to be a practical problem in control programs. Transmission via semen has not been demonstrated.

       Due to the nature of the virus, signs are seen primarily in mature sheep, usually those over two years of age. The disease progresses slowly. Once an animal starts to show signs, it will ultimately die if not culled beforehand. Two early signs are:

1. progressive weight loss while maintaining a normal appetite, and
2. exercise intolerance which is noticeable in the early stages only when the sheep are forced to exercise further or more vigorously than a brief walk to the feed bunk.

       Some ewes develop a diffusely firm udder within three days post-lambing. Both halves of the udder are equally involved and vary from meaty to very hard in consistency. The affected udder is not inflamed and the scant amounts of milk produced appear normal in color and consistency, eg. not mastitic. This apparently subnormal milk production becomes apparent in the form of hungry twin or triplet lambs. Many of the affected ewes milk well enough to raise one lamb. To the eye these udders appear full of milk, but when the ewe walks one notices that this udder is not pliable. When these udders are examined microscopically, the normal milk secreting glands and ducts are replaced to a variable extent with fibrous/scar tissue and lymphoid follicles with active germinal centers, eg. active lymph node tissue which are not present in the normal lactating udder. Some affected udders soften up during the first few weeks of lactation. Insufficient controlled research has been done to determine whether this condition which has been named "Hardbag" repeats in subsequent lactations.

       Thirdly, some sheep develop a lameness due to knee and/or hock arthritis. The incidence of this clinical manifestation is currently unknown, but the author suspects it to be higher than many realize.

       Lastly, a rare clinical manifestation of the OPP virus is spinal cord or brain involvement which results in a rear limb weakness which may progress to paralysis.

       A diagnosis of infection is made by identifying either virus or antibody in the blood. The presence of antibody only implies infection and does not equal immunity. The presence of antibody does not indicate the extent of the disease process. A presumptive diagnosis of disease is made from clinical signs and correlation with a seropositive test result from the affected animal. A definitive diagnosis of disease is made at necropsy when the characteristic lesions are found grossly and histologically in conjunction with a supportive history of disease and seropositive test results. Characteristic lung lesions are as follows: oversized, heavy, meaty, grayish-blue discolored lungs often with lesions of an active secondary bacterial pneumonia in the antero-ventral areas. Sometimes the lungs are so distended due to the accumulation of fibrous tissue that one can actually see rib impressions on the lung surface. The lymph node which normally lies in between the lungs is often enlarged to the size of a fat cigar.

       The most commonly used live-animal test to identify virus-infected animals is the Agar Gel Immunodiffusion Test or AGID. This test detects circulating antibody to the virus. Since colostrally-derived antibody is present until 6 months of age, animals should not be tested before reaching this age. A positive test result from a sheep older than six months of age means that animal is infected for life with the OPP virus. Be aware that a negative test result from a sheep older than six months can mean that either the animal was not infected at the time of drawing blood or the animal is actually infected, but has not produced detectable levels of antibodies. For presently unknown reasons, a small percentage of infected animals never produce detectable levels of antibodies. Some seropositive sheep or goats become temporarily seronegative right after lambing/kidding due to losses of large quantities of OPP virus antibodies in colostrum. The AGID test has been successfully employed in OPP flock eradication programs for years.

       The Enzyme Linked Immunosorbent Assay or ELISA test for OPP is a more sensitive test than the AGID test, because it can detect an infected animal two weeks after it has become infected. This test is being used in research in the United States.

       There is no effective treatment or vaccine for OPP. The future holds promise for the development of a vaccine.

       Flock owners may be interested in eradicating this infection from their sheep flocks when OPP has been demonstrated to result in a significant proportion of

1. ewe losses via forced culling or deaths and/or
2. milk production resulting in poor lamb growth and high rates of starvation or a high percentage of milk replacer-raised lambs.

1. Bleed all sheep in the flock over six months of age and test serologically for antibodies to the OPP virus.
2. Cull all seropositive sheep and any of their progeny that are less than 1 year old. The offspring born to positive ewes are very likely to have been infected with the virus transmitted from the ewe to the lambs during the natural rearing process.
3. Isolate the clean flock from infected sheep and from people and equipment in very recent contact with infected sheep. Fortunately the virus is very sensitive to environmental exposure and will only survive for minutes outside of the sheep's body.
4. Add only seronegative animals to the flock. Such sheep should originate from other seronegative flocks. A riskier alternative is to obtain sheep from seronegative parents from an infected flock and isolate the new sheep for one year and re-test them prior to mixing them with the resident seronegative flock. Only add them to the seronegative flock if they have tested negative.
5. Test annually until there are at least two to three consecutive 100% negative flock tests to be reasonably sure that the flock is free of the virus.
6. Monitor the flock regularly for signs of the disease and blood test and necropsy any suspected cases whenever practical.

This method usually takes at least three or more years to achieve a negative status, and can only be advised in flocks which on initial testing are already less than 50% infected.

1. Remove lambs from dams prior to licking and nursing and rear them in isolation. Do not feed these lambs colostrum from ewes which test positive for the virus. Either feed them colostrum from ewes known to be negative, or clean (heat-treated) cow's colostrum, or an antibody-rich colostrum substitute. Where there is any doubt as to whether or not the lamb nursed, leave the lamb in question with the ewe assuming that it nursed and ingested the virus. In the author's experience, the practice of taping teats as a means of preventing suckling in unobserved lambings has not been a reliable method of preventing nursing by aggressive lambs. Secondly, lambs fed goat colostrum that contains the Caprine Arthritis Encephalitis (CAE) virus are likely to later test positive on the OPP/CAE AGID test. In an OPP flock eradication program, artificially reared lambs should not receive goat colostrum that contains CAE virus.
2. Proceed with steps 3 and 4 as described above to assure an OPP virus-free flock. Serologically test all animals on an annual basis until two consecutive negative flock tests have been achieved.

       This method requires 24-hour lambing supervision in order to be successful, thus when considering this method one should manage the flocks' breeding program accordingly. © 1991 by Cynthia B. Wolf, DVM

       The mission of the OPP Concerned Sheep Breeders Society is to build awareness while, at the same time, providing education and support for those working on eradication and control of the Ovine Progressive Pneumonia (OPP) virus. To that end, the Society has prepared the following voluntary guidelines for OPP testing. It is important to note that, while many of our members continue to lobby for a national sheep health certification program, the OPP Society does not intend to certify OPP status either now or in the future.

       Note: The adoption of these guidelines is not meant to imply certification of OPP-free status. The OPP Concerned Sheep Breeders Society cannot guarantee any flock to be free of the Ovine Progressive Pneumonia (OPP) virus, nor can the Society confirm the validity of laboratory submissions or the accuracy of test results. Recommended testing guidelines are provided solely for the assistance of those who wish to control the spread of OPP. In addition to requesting test reports, buyers should also inquire about flock management practices.

       Beginning at the age of six months, all animals believed to be OPP negative are blood-tested at regular intervals of no more than twelve months (six month intervals preferred), and breeder is willing to provide copies of original laboratory reports, showing complete flock test results, for prospective buyers.

       Or *if a flock is believed to have achieved OPP-free status and is no longer being tested regularly* breeder is willing to provide lab reports to verify (a minimum of 2, and preferably 3, consecutive annual tests during which all animals in the flock were determined to be negative.) It is further recommended that breeders continue to monitor OPP status by testing at least 10-20% of the flock each year (preferably ewes 2 years of age or older that have been in the flock for at least 12 months.)

       In addition to following one of the above testing plans, breeders who advertise their flocks as "OPP Tested According to OPPCSB Guidelines" agree to allow prospective buyers to have bled, at the buyer's expense, any animal being offered for sale as well as the dam and/or sire of that animal (assuming that the dam and/or sire are still in seller's possession.) © OPPCSBS - February 1997

       Accurate tests, while only a part of the big picture, are crucial to any program to control or eradicate OPP. Conflicting information, and the resulting confusion, have driven some to throw up their hands in surrender, deciding to "just live with it" (OPP).

       We have been fortunate to have had a lot of experience with both AGID and ELISA tests. Our small flock has been tested at six different labs (3 AGID, 3 ELISA) in as many states over the past eight years. While I don't have all the answers, I have been able to gather information that should be of interest to others battling the Ovine Progressive Pneumonia virus. Following is a brief overview:

        AGID (agar gel immunodiffusion)
First made available in the U.S. during the 1980s through an experimental USDA program headed by Dr. Randall C. Cutlip. The AGID had been refined by Dr. Cutlip and his colleagues at the National Animal Disease Center in Ames, Iowa. Dr. Cutlip's program ended when the AGID test was commercialized in 1990. This important diagnostic tool then became widely available to producers nationwide through state laboratories and schools of veterinary medicine.

       The beauty of this very reliable test is its simplicity. The AGID is "standard" in that most labs use test kits, including OPP antigen (a whole cell preparation encompassing the entire virus), from the same manufacturer. The only drawback is that this test does not always detect positive animals as early as we might like. While a positive AGID means a sheep really is positive, a negative reading may mean that the sheep is negative *or* it might mean that the animal is infected with the virus but not yet producing antibodies recognized by the AGID.

       ELISA (enzyme linked immunosorbent assay)
First used in the U.S. at Cornell University. In 1985 Animal Scientist, Brian Magee, initiated a program to eradicate OPP from Cornell's purebred Dorset and Finnsheep flocks. As part of this project, researchers at Cornell perfected an ELISA test. Their records indicated that, when run side by side with the AGID, the ELISA could detect antibodies earlier (an average of 12 months earlier than AGID.) After the study was completed, the ELISA was discontinued at Cornell due to its high cost.

       In early 1993, through a contact made possible by the OPP Concerned Sheep Breeders Society, our flock became part of a PhD research project being carried out by Jim Keen. Dr. Keen was working under the mentorship of Dr. Jimmy Kwang at the USDA Meat Animal Research Center in Clay Center, Nebraska. Dr. Kwang had developed a "new ELISA", using highly sophisticated techniques common to AIDS research. (Clay Center is now regarded as the center-of-excellence for OPP test research in the U.S.)

       AN ELISA BY ANY OTHER NAME:
Naively, I thought I had finally accessed the test that would allow me to sort out, with absolute certainty, the positives from the negatives. But my education was just beginning! I learned that there are many different kinds of ELISA tests. And each comes with its own quirks. None of the ELISAs, when used alone, is 100% foolproof in all situations. Selected ELISAs are briefy summarized here:

1. WHOLE VIRUS ELISA
   This was the ELISA first done in the U.S. Antigen used for the test is produced by tissue culture, which is very difficult and time consuming (read expensive.) While increased sensitivity (the ability to detect infection very early) is the attraction of any ELISA test, the Whole Virus (WV) ELISA is sometimes overly sensitive, giving results that may be falsely positive.
   
   
2. CORE PROTEIN ELISA
   A more sophisticated ELISA. Antigen is produced in the molecular biology lab, isolating the protein found in the center of the virus particle. The core protein ELISA used by Drs. Kwang and Keen is "p25" (number refers to molecule size.) The p25 detects earliest OPP antibodies produced after infection but loses sensitivity (won't work, results may be falsely negative) after the animal has been infected for awhile.
   
   
3. TRANSMEMBRANE ELISA
   Antigen for the TM ELISA is produced using the outer "skin" of the virus. Used alone, the TM will pick up the largest number of positives, and sensitivity does not fade with time. Drs. Kwang and Keen are using the TM and p25 together, relying on the p25 to pick up very early positives that the TM might miss. This effective combination has been documented in published research.

       While cross-reaction does occur to some extent between OPP and CAE (the related goat disease), researchers are in agreement that the best antigen for OPP testing *whether AGID or ELISA* is that derived from the OPP virus. As already mentioned, the commonly used AGID test kit for OPP does include the OPP antigen. But, since antigen is expensive and CAE tests are generally more in demand than tests for OPP (due to higher awareness of CAE's depressing impact on production among breeders who milk their animals, as well as the recent interest in Boer goats), CAE antigens are sometimes used for OPP ELISA testing in labs that also test for CAE.

       Through a government laboratory, the ELISA has been available to Canadians at no charge since the mid 1980s. Rob and Shirley Graves and Gerald and Joanne Hunter, of Ontario, have been utilizing this facility since 1986. (After learning their Border Leicester ewes were infected, the Graves and Hunters decided to leave all ewes *negative and positive-testing alike* in one group. Looking back, Shirley says that was the smartest thing they did since they had no way of knowing which ewes were silent carriers. The udders of all pregnant ewes were covered - and all lambs were removed at birth and immediately grafted to a nipple bar. All of the original ewes were then culled as soon as possible.) Even though the tests have always been free of charge, Rob says producer interest in the project has never been high. And he wonders out loud how many breeders will stick with the program as the government looks at across-the-board budget cuts, meaning that producers may have to begin paying for their OPP tests at commercial labs. (As of this posting, January 1997, the Canadian ELISA is not available even on a fee basis to anyone other than Canadian producers.)

       At least two research facilities we are aware of have orphaned 100% negative flocks from 100% positive-testing animals using only the AGID test (one flock of Border Leicesters remains negative after 15 years.) Several individual producers also report success using either test-and-cull or orphaning. Early last year Janet Seavey, from Maine, first learned of OPP infection in her flock of Border Leicesters, Cotswolds and Romneys. After shipping 108 animals to slaughter (positive-testing ewes and their offspring), she began a rigid test- and-cull program using the AGID every 3 months. Now, over a year later, submission of 66 serum samples turned up only 1 positive. Thoroughly convinced of the benefits of eradication (the Seavey flock required only a partial bag of milk replacer this year, 9 bags less than in a recent lambing), Janet says she will continue to test even after her flock reaches 100% negative status.

       Unless a flock has been totally closed, with all animals testing negative for several consecutive years, it is probably risky to stop testing altogether. Consider Minnesota veterinarian, Holly Neaton, who had received several annual negative reports on her accelerated flock of top-producing Polypays before bringing in a new ram. At 16 months of age the ram tested negative on AGID and was put in with the ewes, all of whom had also recently tested negative. When the ram developed symptoms suspicious of OPP at age 3, Dr. Neaton again tested her entire flock. The ram was positive. And, in less than 2 years, her ewe flock had become 75% infected with the virus. Even in flocks believed to be 100% negative, it is probably best to continue monitoring status by testing 10-20% of the adults annually.

       From our very limited comparisons between AGID and ELISA tests, it appears that we are able to at least partially offset the lower sensitivity of the AGID with more frequent testing. Most of our ELISA positives have been detected by the AGID within 3 months (the longest lag time was 9 months against a very early p25 ELISA.) Both tests, when done correctly, are excellent. For the highest degree of accuracy the AGID, though simple and reliable, must be done by a properly trained, experienced technician with good eyesight. For accurate ELISA tests, antigen purity and adequate controls are extremely important. It is best to find a lab you and your veterinarian feel comfortable with, either AGID or ELISA, and stick with it over time you should see consistency in test reports. If you experience wide, unexplainable swings in results from test to test, you may want to consider a different lab. Since the research project involving our flock has ended, we continue to test with the AGID every 3 months.

       Many thanks to Drs. Randall C. Cutlip, Jim Keen, and Jimmy Kwang for taking the time to review this article, and also for their patience in answering my many questions over the years. © 1997 Judy Lewman (This Article Also Appeared in the September 1996 issue of The Shepard magazine)

       First, if you positively don't want to expose your sheep to OPP at shows, don't show! Many of us recognize that, just as youngsters pick up colds and flu more frequently when attending school, showing sheep increases the likelihood of importing not just OPP, but several other "bugs" into the home flock. However, if you are going to show, you might consider taking some simple steps that will help achieve "acceptable risk", or help minimize the chances of horizontal transmission of OPP between sheep. Horizontal, or contact transmission, can occur in closely confined sheep. It can occur when respiratory droplets from infected sheep directly contact uninfected sheep or contaminate feed and water sources.

       Three things a show shepherd can do then, to achieve "acceptable risk" is to:

1. Use your own buckets and tubs to feed and water your stock; don't share or commingle your kit with someone else's.
2. Minimize nose-to-nose contact between your stock and other sheep. This would seem very difficult to do. Perhaps in some situations employing solid panels between pens would be feasible; in other situations, it may not. Avoiding nose-to-nose contact might be easier in an off-the-truck event where a show animal's exposure to other sheep is minimal compared to closely confined sheep occupying a warm, humid barn for several days. Don't permit your sheep to stand around idly at the end of a halter and lead, sniffing noses with other sheep, while you, at the other end of the lead, are visiting breeders, customers, or whomever.
3. Whenever feasible, transport your own animals to and from shows and sales. Transporting your sheep along with someone else's sheep is placing your animals in a close, confined situation with animals whose OPP status may be unknown.

       Show management could take other steps in moving toward a goal of minimizing risk of OPP transmission. To show or transport a horse in the U.S., an owner must be able to prove his animal possesses a "Negative Coggins" that year. The Coggins test identifies the lentivirus which causes Equine Infections Anemia (EIA) in horses, which is related to OPP. Most horses get tested before the start of the show season and carry their Coggins around with them beside their health certificates. Show management could require sheep owners to demonstrate a Negative OPP test along with the sheep health certificate. This policy would not guarantee the absence of OPP-infected sheep, but would most certainly, achieve "acceptable risk".

       While it may be overly optimistic to hope that negative OPP tests will soon be required of show sheep, might it be feasible to pen Negative tested sheep away from untested sheep? Show management can certainly help by providing sheep housing which is airy and ventilated, in pens with solid panels to help discourage nose-to-nose contact. It should be kept in mind, however, that in some instances, solid panels in pens may adversely affect adequate ventilation.

       Another show option would be to allow more events to be off-the-truck, at the owner's discretion. Giving the shepherd the option of keeping his sheep out of pens would help to decrease exposure to OPP, as well as other diseases.

       When show sheep are "mouthed" by the Judge to evaluate bite, a policy can be established to disinfect hands quickly and quietly between sheep.

       Show shepherds need to make their wishes known to management, who in turn, can make their goal the providing of an environment suitable for showing sheep in an atmosphere of "acceptable risk". © by June M. Reed

Groveland Farm Mare & Rusty Jarvis 2862 South Peterson Road Poplar, WI     54864 Email grovelandfarm@supwi.com www.all-animals.com/groveland/sheeph.html