Caprine Arthritis Encephalitis (CAE) Virus
Small ruminant lentiviruses (SRLV) are a group of closely related viruses which can cause chronic disease in multiple organ systems, including pneumonia, arthritis, encephalitis and mastitis. Historically, they were named after the animal species affected which included Caprine Arthritis Encephalitis Virus of goats and Ovine Progressive Pneumonia/Maedi-Visna of sheep. Further research demonstrated that they should all fall under a single name, SRLV.
WSU-WADDL receives numerous inquiries about SRLV, how to test for it, and most importantly, how to diagnose and take steps to control the infection in sheep and goat herds. We have taken some of the most frequently asked questions and presented them along with some short answers.
1. What are the major means of spread of the virus?
SRLV can be transmitted in multiple ways. In goats, it is transmitted via colostrum and milk, but there is evidence that close contact with infected animals will transmit the virus. Blood (e.g., contaminated instruments such as needles, dehorners, etc, and open wounds) is another way of spread. In sheep, close contact appears to be an efficient way to spread the virus in addition to the routes mentioned for goats. There is little evidence to support transmission at breeding, however, in utero infection is possible.
2. May an owner sample goats and send the serum directly to the lab?
The diagnostic laboratory provides services primarily to veterinarians. Although we will test serum samples mailed directly from an owner, we strongly encourage animal owners to work with a veterinarian in developing a SRLV control program. We will send results to the veterinarian, and also to the owner if requested.
3. What type of sample is needed for SRLV testing?
We recommend working with your veterinarian to obtain appropriate samples. Blood should be collected into a 5 or 10 ml "red-top" clot tube or serum separator tube. Leave the blood at room temperature for at least 30-60 minutes to allow clot formation. We recommend centrifugation of the blood sample, separation of the serum into a sterile twist top tube and storage at refrigeration temperature prior to shipping for best results. A minimum of 0.5 ml of sera is required. We will accept uncentrifuged blood tubes, but it is not ideal.
Label the tube with the animal name/number and the owner name or number the tubes consecutively to match a key sheet with the animal names.
Send sample(s) to the lab, preferably by overnight mail. Ideally, the sample should reach the lab within 7 days post collection.
Note: The recommended amount of blood above will be sufficient for multiple tests. It is unnecessary to submit one tube per test requested.
4. How should I ship samples for testing?
Pack the tubes in a plastic sealable bag with absorbent material and put another plastic bag around the first. If submitting blood tubes, the best method is to use padded pouches designed for transporting blood tubes. If you do not have access to these, we recommend using a thick rubber band and grouping your tubes tightly into groups of 7-10 tubes. If you alternate the direction of the tubes, they will stay tightly packed. Pack so the box can be dropped from a four-foot height without breaking any tubes! An ice pack is recommended if the shipment is expected to take several days in warm weather.
5. What documentation should I send with the samples?
Please fill out an accession form found at (https://waddl.vetmed.wsu.edu/forms).
Alternatively, you can use our online ordering system. If you don’t have an online account, please go to: https://w3.vetmed.wsu.edu/newaccount.php
Once approved, you’ll receive an email with a link to submit your order, or you can go directly to the WADDL Client Portal.
6. How long does it take to get results?
SRLV competitive ELISA (cELISA) tests are normally run twice a week, on Tuesday and Thursday morning, with reports going out on Tuesday and Thursday afternoon. During busy times, the test may be set more than one time per week. However, to be tested in the same week of receiving the sample, they must arrive by Wednesday morning. Test results can be mailed or faxed to the veterinarian and/or owner upon request. You can also access laboratory reports electronically. To set up an account to access laboratory reports online, click here. For security reasons WADDL does not e-mail laboratory results.
7. What does a positive or negative mean?
Serology test results must be interpreted considering herd-level prevalence of infection. For example, the predictive value of a negative result is enhanced if all herd mates also test negative. Ideally, a multi-year record for the herd of origin will be available to provide likelihood of risk of infection.
Considering herd prevalence information and/or consistent clinical signs, a positive result means the animal has been infected with the SRLV and has made antibodies reactive with the SRLV antigens used in this test. This animal is regarded as potentially contagious for the virus and can spread virus through contact with milk, blood, and secretions. A young kid or lamb not infected with SRLV may also test antibody positive for several months due to passive transfer of maternal antibodies from the colostrum. We recommend re-testing these animals after six months of age to determine their true infection status. As many as 70% of positive animals may be free of clinical signs of the disease and remain so for years or life.
A negative result means that an animal is either not infected or has been recently infected and is producing amounts of antibody too low to be detected. While the latter case does not appear to be common, it is a good reason to retest all negative animal when not in a closed herd. Animals that are negative should be periodically tested (twice a year for the 1st year, and annually thereafter).
Lastly, although the SRLV cELISA test is a USDA licensed test showing excellent ability to detect virus antibody, it is not a perfect test. Therefore, if the test result is unexpected further testing and investigation is recommended.
8. Can an animal testing positive ever test negative on future tests?
Animals infected with SRLV are infected for life and an animal that tested true positive by the cELISA test would not later clear the infection. However, antibody levels can wax and wane and the animal could test negative at some point during infection, but this is rare. Occasionally a very young animal, fed heat-treated colostrum containing SRLV antibodies may test positive and later negative from the decline of passively acquired antibodies in the colostrum. In some sheep and goats, seroconversion may be delayed for months to years after exposure. These "silently" infected animals test negative for antibody until the viral infection is activated by stress or other factors. It is unclear whether these animals are infectious to others during the time they harbored the virus but remained seronegative. Lastly, although the SRLV cELISA test is a USDA licensed test showing excellent ability to detect virus antibody, it is not a perfect test. Therefore, if the test result is unexpected further testing and investigation is recommended.
9. Is there a difference in the types of serology tests available for making a diagnosis of SRLV infection?
Yes, WADDL uses a validated and USDA licensed test (cELISA) for SRLV antibodies. This test is more sensitive (ability to detect true positive animal) than the agar gel immunodiffusion (AGID) test.
10. Is it okay to drink raw milk containing the infectious SRLV?
There is no evidence that the SRLV virus is transmissible to humans. However, there are other serious human pathogens which have been transmitted through raw milk. Consult your physician regarding the public health hazards of consuming raw milk.
11. Biosecurity Screen
We recommend this screen as part of a herd health program developed with your veterinarian. This screen includes Small Ruminant Lentivirus, Johne's Disease, and Caseous Lymphadenitis. There is a discount for ordering the screen rather than individual tests.
12. In heat treating colostrum, what times and temperature should I use?
Heat treating colostrum will inactivate the SRLV and prevent spread from the dam to her offspring. Colostrum from any dam may be heated to between 133 degrees and 138 degrees F (56 to 59 degrees C) and held at that temperature for one hour to inactivate the virus. An accurate thermometer is important. It is recommended to use a water bath or double boiler to regulate the temperature more closely. A large batch may be heat-treated and frozen in small feeding size portions for later use. If heated higher than 140 degrees F, the usefulness of the colostrum will be greatly reduced due to denaturing of beneficial proteins, including antibodies to other infectious microorganisms.
13. How often should I test my animals?
Twice a year initially followed by annual testing is suggested for herds which are primarily negative, with testing before parturition recommended. Any new animals brought into the herd should be quarantined and tested twice (at least 60 days apart) before introduction with other negative animals. For herds with both positive and negative animals, negative animals should be tested more often to adjust the milking order so that negative animals are milked first.
14. What should I do if my animal tests positive for SRLV?
We recommend working with a veterinarian to develop an on-farm disease control and testing plan. Generally, test results must be interpreted in conjunction with estimated prevalence of infection on the premise and other risk factors (introduction of animals, management procedures). Confidence of the positive status of an animal is increased if other animals test positive in the herd or the herd of origin has known positives. Retesting is recommended if the likelihood of infection is low based on risk analysis. Animals under 6 months of age should not generally be tested, but if a young animal is test-positive, it should be re-tested after 6 months of age because of the potential for influence of maternal antibodies passed from the dam to offspring.
15. Can sheep and goats transmit SRLV to each other?
16. Is there a vaccine for SRLV?
No. It is very difficult to create effective vaccines against retroviral diseases. Several different vaccine types have been experimentally tested, but none have provided adequate protection against SRLV infection.
Additional information on SRLV virus and other infections of livestock can be obtained by contacting your local veterinarian or the diagnostic laboratory at 509-335-9696, FAX 509-335-7424.
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Page revised May 2022