Update On Caprine Arthritis Encephalitis (CAE) Virus
Caprine arthritis encephalitis (CAE) is a retroviral infection of goats which may lead to chronic disease of the joints and on rare occasions encephalitis in goat kids less than six months of age. The CAE virus is intimately associated with white blood cells; therefore, any body secretions which contain white blood cells are potential sources of virus to other goats in the herd. Since not all goats that become infected with CAE virus progress to disease, it is important to test goats routinely for infection by means of a serology test which detects viral antibodies in the serum.
Over the years, we have had numerous inquiries about CAE virus, how to test for it, and most importantly, how to take steps to control the infection in goat herds. It is important to remember that ‘goat infection status, not clinical disease, is the element of interest in assessing risk factors and designing control programs for CAE virus’ (Rowe & East, 1997). 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? The CAE virus is primarily transmitted to kids via colostrum in the first few feedings after birth. Blood (e.g., contaminated instruments, open wounds, etc.) is regarded as the second most common way of spread. Contact transmission between adult goats is considered to be rare except during lactation.
2. May an owner sample goats and send the serum directly to the lab? The diagnostic laboratory provides services to veterinarians. Although we will test goat serum samples mailed directly from an owner, we strongly encourage goat owners to work with a veterinarian in developing a CAE control program. We will send results to the veterinarian, and also to the owner if requested.
3. How should I ship samples for CAE viral antibody testing? We recommend working with your veterinarian to obtain appropriate samples. In general, blood should be collected into a five or ten ml. "red-top" clot tube or serum separator tube, and immediately sent to the lab by overnight mail. There is no need for an ice pack if shipped by an overnight mail service. We do not recommend separating the serum from the clot prior to shipment. Please number the tubes consecutively to match a key sheet with the animal names. (See key sheet for example) Overnight package delivery should be sent to: WSU-WADDL, 155N Bustad Hall, Pullman, WA 99164-7034. WADDL offers discounted shipping through Federal Express (1-3 lbs. $12, 4-5 lbs. $14.50, outside WA state, excl. AK & HI). Use the WADDL account number on the air bill - #2112-4551-4 and specify "Standard Overnight Service." Shipping fees will be added to the testing charges. The address for post office mail is Washington Animal Disease Diagnostic Laboratory (WADDL), P.O. Box 2037, Pullman, WA 99165-2037. An ice pack is recommended if shipment is expected to take several days in warm weather.
4. How long does it take to get CAE virus serology results? CAE cELISA tests are generally run once a week, on Thursday morning, with reports going out on Friday. During busy times, the test may be set more than one time per week. However, to be tested on Thursday, samples must arrive by Wednesday afternoon. Depending on the day the samples arrive, it may take up to a week to get results. Results can be phoned, FAXed, or e-mailed to the veterinarian and/or owner.
5. What does it cost for testing at WADDL? In-state (WA) costs are a $10 accession fee per case, and $3.20 for each. Costs for out-of-state residents are a $10 accession fee and $4.80 for each. Please do not send payment with the samples. We will bill the veterinarian or owner directly.
6. What does a positive or negative mean? A positive result means the goat has been infected with the CAE virus and has made antibodies reactive with the CAE antigens used in this test. This goat is regarded as potentially contagious for the virus, especially if lactating. The antibody against CAE is not a protective antibody, and although strong antibody reactions may be detected in this test, infectious virus can still be spread in milk and blood of this goat. As many as 90% of positive goats may be free of clinical signs of the disease, and remain so for years or life. A young goat which has received heat-treated colostrum containing CAE antibodies may also test positive for several months because of passive transfer of maternal antibodies. We recommend retesting these kids between six and nine months of age to determine their true status. A negative result means that this goat 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 goats when not in a closed herd. Goats that are negative should be periodically tested (twice a year for the 1st year, and annually thereafter). Predictability (or reliability) of a test result is often used to assess the overall accuracy. For the CAE cELISA, the predictability of positive and negative test results is very high.
7. Can an animal testing positive ever test negative on future tests? It is unlikely that a CAE virus infected adult goat which has tested positive would ever test negative in the sensitive cELISA test. Occasionally a very young animal, fed heat-treated colostrum containing CAE antibodies may test positive and later negative from the decline of passively acquired antibodies in the colostrum. In some goats, seroconversion may be delayed for months after exposure. These "silently" infected animals test negative for antibody until the viral infection is activated by stress or other factors. It has not been determined whether these goats were infectious to other goats during the time they harbored the virus but remained seronegative.
8. Is there a difference in the types of serology tests available for making a diagnosis of CAE virus infection? Yes, WADDL has a competitive Enzyme Linked Immunosorbent Assay (cELISA) for CAE virus antibodies. This test is more sensitive (ability to detect true positive animal) than the agar gel immunodiffusion (AGID) test. Values for the CAE cELISA have been set by double testing goat sera by ELISA and a very sensitive research assay, called immunoprecipitation. The positive cutoff score for the cELISA had a sensitivity of 94%, with no false positive results. This is a great improvement over the CAE/OPP AGID test, which had a sensitivity of 56.1% in one study. It is also an improvement over the 90.7% sensitivity of the CAE virus AGID test.
9. Is it okay to drink raw milk containing the infectious CAE virus? There is NO evidence that the CAE virus is transmissible to humans. However, there are other serious human pathogens which have been transmitted through raw milk. Consult your veterinarian regarding the public health hazards of consuming raw milk.
10. In heat treating colostrum, what times and temperature should I use? Colostrum from any doe may be heated to between 133o and 138o F (56 to 59o 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 (about one pint per kid). If heated higher than 140 ºF, the usefulness of the colostrum will be greatly reduced due to denaturing of proteins, including beneficial antibodies to other infectious microorganisms.
11. 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 kidding recommended. Any new animals brought into the herd should be quarantined and tested twice (at least 30 days apart) before introduction with other negative animals. In addition to CAE infection, new goats should be tested for Johne’s disease, and Brucellosis as a biosecurity check. 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.
12. Are there other test methods on the horizon? Yes. WADDL is working with USDA scientists in the development of a polymerase chain reaction (PCR), which tests for CAE virus specific nucleic acid. Current PCR assays have lower specificity (more false positives) than the antibody assays. The PCR assay may become practical and financially affordable enough for routine testing, especially in goats that have delayed seroconversion.
Additional information on CAE virus and other infections of livestock can be obtained by contacting your local veterinarian or the diagnostic laboratory at 509-335-9696, FAX 335-7424.
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