Sheep can be affected by infectious diseases that affect the productivity of your flock. Some diseases can only be transmitted by close contact between sheep, as the organisms responsible only survive in the environment for a short time. Other diseases can be spread via people, vehicles, birds, insects and wildlife, as the organisms responsible can survive in the environment for extended periods of time.

It is easier to keep diseases that are spread directly from one sheep to another out of your flock:

• Make sure that your animals stay confined to your property
• Keep other sheep fenced out of your property
• Quarantine and biosecurity test any new introductions before introducing them to your flock.

Organisms that can survive outside the sheep for a while can be harder to exclude from your flock:

• Have dedicated farm shoes that never leave your property
• Change your clothes and wash your hands after visiting other farms, shows or auctions, to prevent bringing in disease on your shoes or clothing
• Ask visitors to disinfect their shoes or offer boot covers (just be careful, they can be slippery)
• Make sure that the feed delivery truck does not drive in areas that your sheep have access to; they visit multiple farms and can spread disease from farm to farm
• Keeping birds, insects and wildlife off your farm is a challenge

Small Ruminant Biosecurity Screens test for antibodies to CL, OPP and Johne’s disease. There are other infectious diseases and parasites that you should keep in mind for biosecurity (footrot, brucella ovis, dewormer resistant GI parasites, etc.) This article will discuss CL, OPP and Johne’s disease as well as the different tests that are available for them.

Types of laboratory tests

As part of the approval process, all lab tests have to be used on known infected and uninfected animals and report on how successful their test is at detecting positive animals and giving negative results for uninfected animals. Ideally all tests would give correct answers 100% of the time, but unfortunately in real life there is cross reactivity with related organisms or animals that respond in unexpected ways when infected, resulting in some false positives and false negative test results.

The sensitivity of a test is the ability to correctly identify an animal that is infected as a true “positive”. A 100% sensitive test has no infected animals that test negative, but will probably give you some false positive results.

The specificity of a test is the ability of a test to correctly identify an animal that is not infected as a true “negative”. A 100% specific test will have no uninfected animals that test positive but is more likely to give you some false negative results.

There are two main ways of identifying if an animal has been exposed to a specific organism

• Antibody testing looks for antibodies to the specific organism. Having antibodies shows that an animal has been exposed at some time in their life, or received antibodies from their dam through colostrum. Young animals (under six months old) can have interference from maternal antibodies that cause false positive results but they can also have false negatives as they have not been exposed to the disease for a long time in their home flocks yet. Antibody tests do not detect the actual organism.
• Antigen tests look for the actual organism in blood, tissue, feces etc. Some look for DNA from the organism while others grow the actual organism in a suitable medium.

Antibody tests

Small Ruminant Biosecurity Screens are antibody tests that detect antibodies to CL, OPP and Johne’s disease. ELISA, cELISA, SHI and AGID tests are different kinds of antibody tests. The results are a titer or level with different cut offs for positive, suspect and negative status. These tests are quick to perform with results coming back within days of submitting a test. Blood samples are required for antibody tests.

Different labs use different tests as there are many different companies that make antibody tests for these diseases. Below is a result for a biosecurity panel done through WADDL (Washington Animal Disease Diagnostic Laboratory). Their results show a level and a result as positive or negative. You can see that there are comments below. I encourage you to look at the comments to see what the cut off is for a positive result at the specific lab. If your level/titer is close to the cut off for positive, re-test that animal in a few weeks (and wait to introduce it to the rest of your flock until you can see if the level/titer is going up or down). Some labs only give you the “result” column.

You are very unlikely to get a 0 level, you can see below that it is even possible to have negative levels. Levels change, if you test the same animal again at the same lab you are very unlikely to get exactly the same level, but they should stay in the same ballpark. If I were to retest GBR 1139 and his Lentivirus level suddenly jumps to 25 or 30, I would be concerned. Keep in mind that if you want to compare results over time you need to use the same test at the same lab to do that. Also note that some of the ELISA tests do not have reportable levels/titers, just positive or negative results.

Antigen tests

PCR tests are used to detect the DNA of the infective organism in blood, tissue or fecal matter. Currently, there are commercially available PCR tests for Johne’s disease as well as OPP. Johne’s disease PCR testing uses fecal samples from live animals or tissue samples at necropsy to diagnose Johne’s disease. Johne’s PCR tests take 7-10 days to run. Currently the Johne’s Information Center recommended this test to diagnose Johne’s disease. OPP antigen tests are run from a blood sample.

Cultures take blood, pus, tissue, feces, etc., add it to an appropriate growth medium for a period of time, and identify the organisms that were grown. Cultures are available to diagnose CL as well as Johne’s disease. For CL diagnosis, labs will culture samples for up to 72 hours. Fecal cultures for Johne’s grow very slowly and it can take months to culture. Most cultures are done through your veterinarian. The conclusive test to diagnose CL is to culture the organism from a sample of pus.

Genetic tests

Genetic tests are able to identify genes that increase or decrease the susceptibility of an animal to certain diseases. Codon 171 for Scrapie resistance is a good example that most of us are familiar with. RR confers resistance to becoming infected with classical scrapie and QQ animals are susceptible.

TMEM 154 Genetic testing for OPP susceptibility is available. Note that this test gives results as less susceptible or very susceptible status, there is not an OPP resistant category like we have for RR scrapie genotype. TMEM 1,1 or 1,4 or 4,4 animals are less susceptible to OPP while all other combinations are very susceptible. Animals that are less susceptible can still get OPP, so this does not replace antibody/antigen testing before introducing a new animal to your flock. The benefit of buying/breeding animals that are less susceptible to OPP is that they can be exposed to OPP infected animals for a longer time before they get infected.

Below are results from genotyping through NSIP. A tissue sample, collected via ear punch was used for this test.

Diseases included in “Small Ruminant Biosecurity Panels”

There are lots of infectious diseases of sheep that you should keep in mind when bringing new animals onto your farm. Biosecurity panels only test for OPP, Johne’s and CL and those three diseases are discussed below.

OPP (Ovine Progressive Pneumonia)

OPP is caused by a lentivirus that affects the white blood cells and causes chronic inflammation, leading to normal tissue being replaced by infiltrates of immune cells over time. This disease can have a significant impact on flock performance. There are different strains of OPP that vary in severity of the disease that they cause. Different syndromes can be seen depending on the organs being affected.

Hard bag: Normal udder tissue is replaced by infiltrates of immune cells, affecting milk production, leading to:

• increased lamb mortality, due to poor colostrum volume and quality as well as starvation of lambs in larger litters
• more bottle lambs as severely affected ewes lamb with nice looking udders but they produce no milk
• slower growth in lambs as less milk is being produced by the ewe
• culling of ewes at a younger age due to poor performance of their lambs

Thin ewes: Chronic inflammation causing wasting away of otherwise healthy appearing animals

Progressive Pneumonia: Normal lung tissue is replaced by infiltrates of immune cells, leading to:

• Exercise intolerance, animals lagging behind when moving the flock
• Heat intolerance
• Increased susceptibility to other pneumonias
• Weight loss

Arthritis in animals in their prime

There is no vaccine or treatment for OPP. Testing and culling of infected animals is the only “treatment” available to infected flocks. The virus is spread mainly via respiratory secretions when animals have close contact with each other. Thankfully this virus does not survive long outside of sheep so quarantining and repeat testing of new animals before introducing them into your flock can keep this disease off your farm.

A fact sheet on OPP can be found here https://oppsociety.org/wp-content/uploads/2022/03/opp-info-sheet-aasrp-neaton.pdf

Available tests for OPP

These tests are often called “Small Ruminant Lentivirus”, SRLV or OPP tests

• Elitest, available at the University of Minnesota, is the only ELISA test that is validated according to the World Organization for Animal Health standards.
• cELISA tests are available from many laboratories, often as part of a “small ruminant biosecurity panel” in combination with CL and Johne’s testing
• ELISA tests are available from many laboratories, often as part of a “small ruminant biosecurity panel” in combination with CL and Johne’s testing
• OPP PCR antigen testing
• TMEM 154 genetic susceptibility testing

Recent vaccination, especially Chlamydia vaccine, and hormones can affect results, so it is recommended to time your testing so you are not sampling animals shortly after vaccination or around lambing.

There is some controversy about the cut off for positive on the cELISA test for OPP. The cELISA test was originally approved for goats with CAE with a cut off for positive at more than 35% inhibition. A study published in 2003 showed that the test works well for sheep for OPP too, but they thought that the cut off should be 20.9%. Testing samples from infected flocks with both Elitest and cELISA did find that Elitest shows positive results for many of the sheep with inhibition above 20.9%. Anecdotally a lot of producers that are working towards eradication find that animals above the 20.9% cut off, often progress to testing positive on cELISA a few months later. https://oppsociety.org/wp-content/uploads/2022/03/vmrd-sheep-vs-goats.pdf

Some considerations about genetic testing for OPP susceptibility:

• TMEM 154 genotype 1,1 or 1,4 or 4,4 are less susceptible to OPP, they are NOT resistant, with enough exposure, they can still get OPP.
• The other TMEM genotypes (1,2 or 1,3 or 2,2 or 2,3 or 2,4 or 3,3 or 3,4) are highly susceptible to OPP
• TMEM 154 genotype 4,4 turns off this gene completely. Research is still ongoing into exactly what this gene does, so 1,1 or 1,4 is more desirable.
• A publication in 2023 shows that about 60% of Katahdins have genotypes that are highly susceptible to OPP. https://academic.oup.com/…/101/Supplement_3/41/7372233

We will be throwing out a lot of great genetics if everyone starts to do single trait selection for OPP genetic susceptibility.

• We can maintain great genetics while “breeding up” to less OPP susceptibility. Breeding highly susceptible individuals to each other if they both have at least one copy of 1 or 4 will result in about 25% of their offspring being less susceptible. Animals with only 2’s and/or 3’s in their genotype will take an extra generation but can be bred to 1,1 or 1,4 or 4,4 animals and about 50% of their offspring should have at least a 1 or a 4 in their genotype and can be strategically bred to produce at least some less susceptible offspring in the next generation.
• If you show sheep, bring in a lot of new sheep or graze your sheep in areas where they can commingle with other sheep, increasing the number of less susceptible sheep in your flock might take a higher priority compared to someone with a closed ewe flock that only brings in rams and is able to quarantine them for 2-3 months and test repeatedly before introducing them.

The OPP Society is a great resource for information about OPP  https://oppsociety.org/

In depth information about OPP testing, including recommended laboratories to use, can be found https://oppsociety.org/opp-testing/  A more in depth blog post about OPP can be found here

Caseous Lymphadenitis (CL)

CL is caused by the bacteria Corynebacterium Pseudotuberculosis. This bacterium causes abscesses in the lymph nodes of sheep and goats. There are two forms:

• External (abscesses in the lymph nodes under the skin) that is visible when you look at the animals
• Internal (abscesses in lymph nodes and organs in the chest and abdominal cavity) where all you see is an animal that loses weight. Occasionally large abscesses in the chest cavity can cause coughing and regurgitation.

Sheep are most commonly infected when they have a wound that comes into contact with pus from infected animals. The bacterium can also enter through intact skin or can be inhaled. Animals with abscesses in their lungs can cough up an aerosol of this bacteria that can infect their flock mates. In wool sheep it is often spread at shearing. Infected goats can be a source of infection for sheep.

The red dots in the photo show the position of lymph nodes that are most commonly affected. In ewes the udder lymph nodes can be affected. Looking from behind, they are at the top of the udder where it attaches to the body.

Most visible abscesses are not caused by CL, they are just caused by soil or skin bacteria that enter a wound and cause an abscess. Some vaccines are very reactive and cause lumps, keep that in mind when you find lumps shortly after vaccination. It is best though to treat all abscesses as if they are CL until proven otherwise. CL is spread by the purulent material and if you contaminate pasture, handling equipment, soil, bedding, etc., the bacteria can live in the environment for months and infect other animals that come into contact with it.

This disease is zoonotic, which means that you can get it from contact with the pus from abscesses. It is rare but farmers, slaughterhouse workers and shearers have been affected. Drinking unpasteurized milk from ewes with affected udder lymph nodes can also be a source of infection. It is a good practice to always wear gloves when working around any abscesses.

Best practice when you notice a lump on any of your animals is to catch them and investigate. Isolate the affected sheep from the rest of the flock and confine them in an area that is easy to clean and disinfect, in case this is CL and the abscess opens up unexpectedly. Sheep do not like to be alone, so ideally keep a buddy with them, just make sure that this is an animal that you are OK culling if this happens to be CL. Abscesses tend to enlarge over time and usually develop a soft spot before breaking open. Ideally you can collect a sample with a needle and syringe before the abscess breaks open. If you are unsure on how to do this, call your veterinarian out the first time, ask them to show you how to aspirate the abscess. Larger needles and bigger syringes are more likely to result in useful samples. If you (or your veterinarian) are able to aspirate a good pus sample, have your veterinarian send it in for a bacterial culture and note that you are looking for CL. You should have the result after the lab has incubated the sample for 48-72 hours.

Ideally you will know if you are working with CL before the abscess breaks open, but if not, isolate the animal ASAP. Make sure you wear gloves and protect yourself and express as much pus from the abscess and catch this material, making sure to collect a sample for testing. After expressing the pus, flush the abscess cavity with a disinfectant solution of betadine, making sure to catch this flushing liquid too. Make sure that you keep flies away from this infected material as they can spread the infected material to other sheep. Infected materials can be burned or buried deep. Keep the affected sheep isolated until you get culture results back.

A small (44 animals, mostly goats) study looked at three treatment regimens for CL and compared their outcomes.

1. Open, drain and flush abscess. Give injectable penicillin under the skin
2. Do not open the abscess, inject saline into the abscess to break up the pus, and thin it out so that it can be removed via needle and syringe instead of opening up the abscess. Then inject Draxxin (tulathromycin) right into the abscess
3. Do the same as #2 but give the Draxxin under the skin

They followed up after 30 days and there was a slightly better outcome in group #1 animals, however the decreased spread of infectious material around the farm in group #2 and #3 animals probably makes up for the lower success rate. You can find the full study https://avmajournals.avma.org/view/journals/javma/234/9/javma.234.9.1162.xml

A vaccine for CL is available. Using this vaccine in flocks that have CL decreases the number of abscesses that infected animals get. Using this vaccine in flocks that do not have CL comes with drawbacks. Yes, it does offer some protection against animals developing CL abscesses if they are exposed but the antibodies produced in response to the vaccine will result in a positive CL antibody test. This will complicate eradication efforts if you do introduce CL into your flock as everybody tests positive for CL whether they are infected or not. If you sell breeding stock, you should mention to customers that your stock is vaccinated against CL, so they will not be surprised by a positive result if they run a biosecurity screen. Some customers might opt to not buy from flocks that vaccinate for CL due to this complication with testing.

Available tests for CL

• Antibody testing, SHI or ELISA are often available as part of a “small ruminant biosecurity panel” in combination with OPP and Johne’s testing.
• Bacterial culture of pus from an abscess, this is the definitive test to diagnose CL

The available antibody tests for CL are not as sensitive and specific as the OPP antibody tests, and therefore harder to interpret for an individual animal. These tests are much better when used on a flock level, as having a whole flock with negative tests makes it much more likely that the disease is not present in a flock when compared to getting a negative result when testing just one animal. If you have an animal with an abscess, culture the pus to see if they have CL, do not do an antibody test.

In the study above they also cultured all the abscesses and drew blood for SHI (antibody) testing on the day of treatment. Seven of the animals were vaccinated for CL between 6 months and a year prior. They found some animals with active CL culture confirmed abscesses that had negative antibody tests, as well as culture negative sheep with positive antibody tests. You can see the details below:

• 43 lesions tested positive for CL on culture
• Five lesions tested positive for another organism and did not grow CL
• Of the 43 culture positive lesions, 35 were in animals that tested positive on SHI testing and eight were in animals that tested negative on SHI.
• Of the five culture negative lesions, three were in animals that tested positive on SHI and two were in animals that tested negative
• Six of the seven vaccinated animals tested positive on SHI, two of these six animals did not grow CL on culture
• The remaining one vaccinated animal tested negative on SHI, but CL was cultured from their abscess.

Another study looking at SHI test for CL in 196 goats and 76 sheep found that 98% of goats and 96% of sheep with abscesses that cultured positive for CL, had positive SHI tests, unfortunately 28% of goats and 10% of sheep without abscesses tested positive for CL

You can find the abstract https://europepmc.org/article/med/3740616

More information on CL can be found here: https://www.lsuagcenter.com/topics/livestock/animal_health/sheep_goats/caseous-lymphadenitis

Johne’s Disease

Johne’s disease is caused by a bacterium, Mycobacterium avium subspecies paratuberculosis. Lambs are most often infected by ingesting the bacteria when feed, water or milk is contaminated by fecal matter from an animal with Johne’s disease. Ewes that are infected with Johne’s can transfer the infection in utero to lambs, as well as via milk and colostrum. The main thing you see in Johne’s infected animals is weight loss and some sheep will progress to having pasty diarrhea very late in the disease process. Typically, animals are infected as lambs but only start expressing the disease as adults. Lambs under 6 months old are the most susceptible. Cattle can be a source of infection for sheep.

This bacterium infects the local white blood cells in the last part of the small intestine and just hangs out there, not causing any problems for two to ten years. During this time an infected animal can shed the bacteria intermittently. At some point the animal starts responding to the bacteria and leading to granulomas forming in the small intestine. They interfere with the normal absorption of nutrients and the sheep starts losing weight. At this point the sheep is shedding ever increasing amounts of bacteria in their manure. Eventually the bacteria are able to spread throughout the body and at this stage ewes can infect their unborn lambs and pass the bacteria in milk and colostrum. These bacteria can stay viable in the environment for up to a year.

No vaccines or treatments are available for Johne’s in the USA

Available tests for Johne’s

• PCR testing on manure or tissue samples
• Blood antibody ELISA testing, often available as part of a “small ruminant biosecurity panel” in combination with OPP and CL testing
• Fecal culture

Fecal PCR testing is the recommended test by the Johne’s Information Center. The animals start shedding Johne’s bacteria before the organism spreads throughout the rest of their bodies. They will only start having antibodies to Johne’s once this systemic spread has already occurred. You can find shedding animals earlier with PCR testing and have less shedding in the environment. You can send in individual samples on all your sheep and some labs will do fecal pooling where they combine samples from 5 animals, which makes this less expensive. If a pool tests positive, then the individual samples that went into that pool will be tested to identify the infected individual.

Antibody testing has low sensitivity so unfortunately there will be many false negatives. It has a 99% specificity though, so a positive test is significant and most likely means that the animal is infected. Once again this shows that antibody testing on individual animals is not very diagnostic (unless you get a positive result) but on a flock level, if you test a lot of animals and they all test negative, then it is likely that the flock is not infected. Farms that keep cattle as well as sheep, should test their cattle as well as they can serve as a reservoir that can infect the sheep.

The Johne’s Information Center https://johnes.org/  has excellent information about Johne’s disease in all species.

Collecting a blood sample

You will need to collect a blood sample for biosecurity screening. You can use a blood collection needle and holder to collect directly into a blood tube; alternatively you can use a needle and syringe to draw blood and then inject the blood into a blood tube.

If possible, do not draw blood for biosecurity testing within a month after vaccinating. Vaccines cause immune reactions and sometimes can lead to false positive results, especially for OPP testing. Vaccinating for CL will cause positive results upon testing. Hormonal changes around lambing can also affect results.

First, decide if you feel comfortable drawing blood and sending in samples. If not, contact your veterinarian; they can draw the blood, process and send in the samples and report the results back to you. Pay attention and ask questions when they draw blood, so that you can learn for the future. It is a handy skill to have.

If you are doing it yourself, first decide which laboratory you are going to send your samples to. There are many labs that offer this service. A quick search shows prices for biosecurity screening (OPP/CL/Johne’s antibody screening) ranging from $20-$35 per sample.

Once you decide where you are going to send your samples, look on their website or call to see how much blood they need and also which type of tube you need to collect the sample in.

Buy the correct size and type of blood tube. Many labs sell tubes too, but supplies can also be obtained from your veterinarian or local or online suppliers. Make sure to buy more tubes than you need; you will have some mishaps, so make sure you have extra.

Buy enough needles and syringes or blood collection needles and a few holders for the number of animals that you are testing, plus a few extra. You need a fresh needle per sheep, otherwise you will get cross contamination between animals and you will not be able to trust your results. If you draw blood into a syringe, you will need a new syringe per animal too. Needle holders can be reused, just changing out the needles between animals, but have at least two in case something goes wrong with one. Also be sure to clean them if you get any blood on them before using them again.

The size of a needle is expressed as a gauge (G) and a length in inches, so when you purchase needles, you will see for example a 1” 20G needle. The length is the length that is available to stick into the animal while the gauge is how big the circumference of the needle is. Bigger needles will make bigger holes and animals might react more to being poked with a bigger needle, however the blood will come out quicker and the red blood cells are less likely to break (hemolysis) and turn your serum pink or red. The higher the number of the gauge the smaller the needle diameter, so a 16G needle is much thicker than a 24G needle. You need a long enough needle to get through the skin and solidly into the jugular vein of the sheep. Personally, I like a 1 ½’ needle, you will need at least a 1” needle and once you get to 2 ½ or 3”, those might become a little awkward. Most people use 18G or 20G needles for drawing blood.

Make sure you have shipping supplies: cooler, ice packs, Ziploc bags. Some labs offer mailing supplies and shipping labels. The shipping is often much less expensive this way.

Plan the best day to draw blood. Find out what days they do testing and make sure that your blood will not get stuck in transit over the weekend or a holiday. Make sure you have room in a refrigerator to keep the blood samples if you are not mailing them right away.

If you are collecting from a large flock, work out a system beforehand of how to label the blood tubes. When I collected from my flock of 160 ewes, I pre-labeled the tubes in the house and arranged them in ascending number order in the Styrofoam tube holder that the tubes shipped in. That way I did not have to label the tubes chute side, I could just find the right tube and draw blood. An added benefit was that if I accidentally used the wrong tube for a sheep, I did not have a tube available for that sheep when she came through, so I could sort out a problem like that easier.

With hair sheep, drawing blood is much easier during the summer when they are shed out vs the winter when they have a thicker coat. If you are still learning, leave the rams with thick manes for the end when you have practiced on a few animals. On hot days veins are more prominent.

Having a helper (or two) makes it much easier.

A comprehensive video about drawing blood for OPP testing, but the principles apply to biosecurity testing, can be found https://www.youtube.com/watch?v=jgiPEkCijwo

Do I really need to test my flock?

Well, that depends. If you are seeing signs of any of the three diseases, finding out if your flock is infected can make a big difference in your flock performance. It is expensive, no doubt about that. Test your oldest animals, they have been around the longest and are the most likely to test positive if you have disease on your farm. Animals that are struggling to maintain or regain weight after lactation are also good candidates. It is ideal to initially test your whole flock, however you can test just a proportion of your flock to get a good idea of where you stand. Below is the recommended number of animals to test according to your flock size. Be sure to focus on older animals that have been in your flock for years. This chart is from the OPP Society website https://oppsociety.org/opp-testing/ .Hopefully you will get all negative results, but if you find any positives, you should test your whole flock.

At least test and quarantine incoming animals. Fecal PCR testing is a better test than Johne’s ELISA if you are only going to be testing one or a few animals.

You are less likely to introduce CL, OPP or Johne’s disease into your flock if you buy from flocks that do biosecurity testing. Expect to pay more for tested animals; there is a lot of money invested in testing whole flocks. You can ask for testing to be done if you buy somewhere, but you need to be willing to pay for it. Ask to test a sample of older animals for better results.

There are alternatives to small ruminant biosecurity screens to do disease surveillance in your flock. OPP requires antibody testing. For CL you can culture all abscesses in your sheep; if CL is present, you will find it this way. If your flock is large and you do not get many abscesses this might be less expensive. Pooled fecal PCR testing for Johne’s is a superior test to antibody testing, so you can monitor for Johne’s in your flock this way.

When you buy directly from a producer ask about their biosecurity practices:

• Do animals leave and come back to the farm – fairs, exhibitions, breeding? If so, are they quarantined and tested before integrating back into the flock?
• Are new additions quarantined and tested? How often are new animals brought in?
• Look at the other animals on the farm. Are there signs of abscesses or scars? Are there very thin ewes that are not lactating or have not just weaned lambs?
• Do they ask you to disinfect your shoes/boots or offer boot covers?

All antibody testing has better accuracy in animals over six months old.

A few words about terminology:

Test negative flock. Having your whole flock test negative does not guarantee a disease-free flock. You have a Johne’s/OPP/CL test negative flock. Unfortunately, even with repeat testing you cannot guarantee that your animals are disease free. They could have had false negative tests or have been infected/started shedding Johne’s since you last tested them.

Less susceptible to OPP. 1,1 or 1,4 or 4,4 animals are not OPP resistant. They can still get OPP; they are genetically less susceptible. They should still be tested for OPP before introducing them to your flock.

What do you do with the results?

Everyone tested negative for everything. Congratulations!  You have a test-negative flock. You can test the whole flock annually, or a subset as recommended above (make sure to focus on the older animals and thin animals). Make sure that you keep up with good biosecurity practices.

Oh no, some animals tested positive. Bummer, this is going to take some time, effort and money to figure out.

• Multiple animals test positive for the same disease – you most likely have this disease in your flock. You should isolate the positive animals from the rest, contact your veterinarian and see what your options are for eradication. The OPP society and the Johne’s Information Center have good information. Eradication programs are expensive and take a lot of dedication (are you willing to cull your favorite animals, do you have the facilities to keep infected animals and test negative animals separate?), time and money. You do have the option of deciding to just live with the disease in your flock, but be sure to not sell breeding stock (ideally), or if you do, that you disclose the presence of disease in your flock.
• One or two animals tested positive – this could be a false positive or this disease could be present in your flock. Bummer, this is also going to take some effort and money to figure out. Treat animals that test positive as if they are infected and segregate them from the rest of the flock while you investigate. If you only tested part of your flock and had animals that tested positive, I highly recommend testing the other animals for this disease too. If the animals tested positive for Johne’s, do fecal PCR testing on the other animals rather than Johne’s antibody testing.

You could just cull the animals BUT, what if they were really infected? If you do this you should assume that your flock is infected and act accordingly. You do not have a test negative flock if some tested positive and you culled them. It is best to investigate further. KEEP THESE ANIMALS SEPARATE AND TREAT THEM AS IF THEY ARE INFECTED WHILE YOU INVESTIGATE

1. Did the test that you used come with a titer/level? If not, send a sample to a lab that will report a level ASAP and see where you stand (marginal vs off-the-charts high). Now you can keep sending samples to this particular lab and track this animal’s change in level/titer
2. Did this particular animal have vaccines close to testing, was it ill with another disease, did she recently lamb, was she stressed by something (lamb died, being bullied by the other sheep)? This is more likely to be a false positive, but you still need to retest to be sure.
3. For positive CL tests: Has this sheep been vaccinated for CL? CL vaccination can cause false positive CL antibody tests and can be hard to sort out. Carefully evaluate this animal, focusing on the areas where CL abscesses occur. Are there any swellings or scars in these areas? If this is your only positive and you are going to cull this animal anyhow, consider having a necropsy done to check for internal CL abscesses, as well as culture any lymph nodes that look suspicious. Tell them that you have an animal with a positive CL antibody test and you want to rule in/out that it is infected. It is hard to 100% rule out CL but an animal that has never had a draining external abscess and no internal abscesses would not be able to spread CL to the rest of your flock.
4. If Johne’s positive, collect a sample for fecal PCR testing ASAP. If fecal PCR testing is negative, repeat this test in 2-4 weeks.
5. Does this sheep show signs of disease? If so, it is probably a real positive test, but can still be a false positive.
6. Retest the animal two weeks later. Are they negative now, or have a markedly lower titer/level, or are they still positive with the same or higher titer?
7. Retest animals that are negative on retest again two weeks later (four weeks after the initial test). If they are still negative, the original test was most likely a false positive. If you are very risk averse you can cull this animal now, you have proof that levels came down on re-testing, or if you keep them in your flock, re-test them in six months instead of waiting for the annual re-testing. If you are only testing part of your flock annually, make sure to include these animals in your set of animals that you test.
8. Retest animals that are positive on retest again two weeks later (four weeks after the initial test. Are titers/levels decreasing?  If this animal is still positive and titers are not decreasing, it already had three positive tests in the past month, and is less likely to be false positive. Consider culling and necropsy with testing. hopefully you can get a more definitive answer as to whether the disease is present in your flock.

If you get bad news and find infection in your flock, it can be shocking and can feel overwhelming. The OPP Society and Johne’s Information Center has resources and veterinarians that you can reach out to for help in eradication. The OPP Society has testimonials from producers that eradicated the disease in their flocks. Find other shepherds that have been in the same boat and pick their brains for eradication tips as well as support.

By: Isabel Richards, EAPK Communications Committee