Updated July 21, 2021
What Are Small Ruminant Lentiviruses (SRLV)?
SRLV are a group of viruses that affect small ruminants including sheep and goats. The most commonly known diseases caused by SRLV are Caprine Arthritis Encephalitis (CAE), which primarily affects goats, and Ovine Progressive Pneumonia (OPP), which primarily affects sheep (in many parts of the world, OPP is also called maedi-visna). These viruses are in the same family as HIV, but they cannot cause HIV infections in humans. OPP and CAE are similar to HIV in the sense that once an animal has been infected, the virus can never be cleared from the body. Although there will be periods of latency (when the virus is dormant and may not be detectable), infection is lifelong. Additionally, lentiviruses as a family are slow to replicate, so infected animals usually do not develop signs of disease until they are at least two years of age or older. SRLV infections are common in the United States; approximately 30-70% of sheep and goats in the U.S. are infected with OPP and CAE, respectively. Because so many small ruminants in the U.S. are infected with these lentiviruses, many residents in sanctuary environments are SRLV-positive. SRLV diseases can be transmitted via infected colostrum and milk, such as when a newborn nurses from their SRLV-positive mother or if milk containing the virus is used for bottle feeding. However, it is also possible for one individual to transmit the disease to another via infected blood or prolonged, close contact (this is known as horizontal transmission). While the CAE virus primarily affects goats and the OPP virus primarily affects sheep, transmission between species is possible on rare occasions. Once infected, the individual remains infected for their entire life, even if they never show signs of disease. Currently, there are no known cures or vaccines for SRLV diseases; preventing transmission and treating the symptoms are the best options available.
How Does One Test For A SRLV?
Many sanctuaries submit blood samples from new residents to test for SRLV as part of their incoming protocols. There are various testing methods available including AGID, ELISA, and PCR. AGID and ELISA tests detect antibodies and PCR detects the DNA of the virus. Your veterinarian can recommend the best testing protocol which, due to the limitations of each method, may include a combination of tests. Make sure you understand the limitations of the tests used as well as their sensitivity and specificity. Tests that are highly sensitive will have a reduced risk of a false negative, so these are often used for screening a herd or flock. Tests that are highly specific will have a reduced risk of a false positive and are, therefore, best used for confirmation of disease. Keep in mind that a true positive does not necessarily mean the individual will develop clinical disease; only 25-30% of positive animals will go on to show clinical signs of disease. However, it is still possible for these asymptomatic, positive individuals to transmit disease to others, despite the fact that they may appear healthy. Because the time it takes for the body to develop antibodies that can be detected by AGID or ELISA tests vary, it is possible for an incoming resident who has recently been infected to initially test negative because they are not yet producing antibodies. To account for this possibility, we recommend working with your veterinarian to establish appropriate screening protocols for your sanctuary, which may include annual testing or repeated testing for new residents.
Importantly, maternal antibodies that a lamb or kid receives from their mother in the colostrum (first milk) can affect antibody tests such as an AGID or ELISA test. Therefore, testing should be delayed until kids or lambs are at least 6 months of age or older. Young animals who test positive via AGID or ELISA tests should be retested. Again, the best resource for testing and test interpretation is your veterinarian.
If a resident is showing signs of disease, your veterinarian may recommend additional diagnostic tools to help make a diagnosis.
How Does CAE Manifest?
Although only 25-30% of CAE-positive goats will develop clinical signs of disease, they still have the potential to transmit the virus to other residents if good care practices are not followed. In those goats who do develop clinical signs, CAE can manifest in a variety of ways, but regardless of the manifestation, chronic wasting is a common feature of clinical disease. Individuals progressively lose weight despite a normal appetite, generally seem less healthy than their CAE-negative counterparts, and can have dull or thick and shabby coats. In addition to chronic wasting, clinical disease can manifest as:
- Arthritic CAE: The most common form of CAE in the United States is progressive, chronic polyarthritis/ polysynovitis, which is characterized by viral infection of joints and tendon sheaths. This form of CAE usually affects adults (2 years of age or older) but can rarely be seen in kids who are just six months old. In most cases, lameness starts out mild and progresses over time, though a sudden onset of severe lameness is possible. The rate of progression varies – some individuals will develop a limp but will remain stable for years, while others will deteriorate quickly. Symptoms include arthritis in the joints and sheaths around tendons. The joints most commonly affected include the carpi (often referred to as the “front knees”), tarsi (“hocks”), and fetlocks (“ankles”). Individuals may walk with a limp, or they may walk on their carpi. Advanced arthritis manifests as more definitive swelling and pain in their joints. The individual’s coat often becomes dull, they tend to lose weight, and they generally look less healthy as the condition worsens.
- Encephalitic CAE: The encephalitic form of CAE mostly affects young goats between 2-6 months old, although this form of CAE is occasionally seen in adults. The afflicted individual has poor coordination and difficulty walking, progressing into nerve damage and paralysis from the hind to front legs. It may progress further into depression, seizures, blindness, circling motions, tremors, a tilted head, and facial nerve damage. This form of CAE is unfortunately often fatal, though rarely some individuals can recover with time and supportive care.
- Pneumonic CAE: This form typically affects older goats and manifests as a chronic cough followed by an elevated respiratory rate, labored breathing, weight loss, pneumonia, and enlarged lymph nodes. Infected individuals can transmit the virus in respiratory secretions.
- Mastitic CAE: Individuals with this form of CAE will have a swollen and firm udder. Beyond general discomfort and difficulty or inability for a mother to nurse their young, this form of CAE does not produce many other challenges.
How Does OPP Manifest?
Like goats, only 25-30% of sheep infected with OPP show clinical signs of disease. Those who do develop clinical signs of disease typically do not show signs until they are older than 2 years of age. Chronic wasting is a common feature of clinical disease and individuals often appear to be less healthy than their OPP-negative counterparts. The forms of OPP are similar to CAE, though there are some key differences:
- Pneumonic OPP (or Maedi): In the United States, this is the most common form of OPP, typically affecting adults. Affected sheep develop dull coats, lose weight despite a healthy appetite, and develop labored breathing due to a thickening of their lung tissue. They may experience general exhaustion and may be unable to keep up with the rest of the flock. They may develop a cough that gets progressively worse and may have nasal discharge, but they typically do not have a fever unless they develop a secondary bacterial infection. Infected individuals can transmit the virus in respiratory secretions.
- Mastitic OPP: This form of OPP is also very common in the U.S. Affected sheep have indurative mastitis, meaning the udder is very firm to the touch, and the condition is often called “hard bag”. Swelling and firmness will be in both halves of the udder and will be symmetrical. Some have described the udder as feeling as though there are tennis balls within the mammary tissue. Lambs nursing from sheep with OPP mastitis will be smaller and will gain weight slower than those nursing from unaffected sheep.
- Encephalitic OPP (or Visna): This form is similar to the encephalitic form of CAE, and is similarly rare. Adult sheep as well as lambs as young as 4 months old can be affected. Initial signs are subtle and typically begin with weakness of the hind legs, a head tilt, trembling lips, and weight loss. As the condition progresses, individuals will develop ataxia, incoordination, paresis, and paralysis. Back legs are often more severely affected than front legs. Unfortunately, this form is often fatal.
- Arthritic OPP: Though less common than in goats with CAE, SRLV can manifest as chronic arthritis in OPP- positive sheep and presents similar signs as those described above for goats.
How Can Sanctuaries Prevent The Spread Of SRLV?
As there are no cures or vaccines available, preventing transmission of SRLVs is key. Whereas it was once believed that the majority of SRLV spread occurred via colostrum and milk, more recent evidence shows that spread through close contact is common. Therefore, housing plays a more important role in preventing the spread of SRLV than previously understood. Respiratory droplets that are inhaled by uninfected sheep and goats are the primary mode of transmission of SRLVs, so putting physical distance between infected and uninfected residents is the best way to prevent spread of the disease. Although the exact distance required to prevent infection is not known, most recommendations range from at least 10 feet to more than 100 feet. Ideally, the more space between the two herds or flocks, the better. Because of this, some sanctuaries have created SRLV-positive resident herds that live completely separate from their SRLV-negative residents in order to reduce the risk of transmission as much as possible.
In addition to physical separation, tools and equipment used for infected residents, such as shovels, brushes, pitchforks, etc. should be different from those used for uninfected residents. Alternatively, all equipment should be thoroughly disinfected prior to use elsewhere in the sanctuary. Staff and volunteers working with infected animals should change clothes and footwear and wash hands well before working with other animals.
Because it was once believed that horizontal spread in a sanctuary setting was rare, some sanctuaries have housed SRLV-positive and negative residents together for many years. Others may not have the capacity to create separate herds for positive and negative residents, but it’s important to stress that if SRLV- negative residents are housed with those who are positive, they are at risk of infection. While all infected individuals can transmit the virus in their respiratory droplets regardless of whether they have signs or not, the risk of transmission increases when infected individuals are coughing or have nasal discharge. Because of this, sanctuaries that have not typically created separate herds for positive and negative residents may choose to create an alternative living arrangement if a resident develops signs of pneumonic CAE or OPP. When considering living arrangements, keep in mind that no resident should ever live without at least one companion unless it’s absolutely necessary.
If you’ve been housing SRLV- positive and negative residents together because of the previous understanding of transmission and are wondering if you should separate them now, the answer is probably no. If the residents have been living together for a long time, separation at this point likely isn’t the best option. Instead, consider creating separate herds or flocks going forward and implementing annual, whole-herd testing to identify individuals who are infected. Knowing your residents’ infection status can help you make housing decisions, and even if separate herds are not an option, this information is helpful. Residents who test positive for SRLV can be watched more closely for clinical signs, which may allow for earlier implementation of interventions such as those listed below.
Regardless of whether you separate SRLV-positive residents from the rest of your herd, you should always use separate needles for all residents and dispose of them after a single use. This practice will help prevent transmission of numerous diseases, including CAE and OPP.
If you rescue a pregnant sheep or goat who is SRLV-positive, you’ll need to consider whether or not you will separate mother and baby in order to protect the baby from infection. Transmission of OPP or CAE in utero (from mother to fetus) is rare and accounts for less than 5% of lentivirus infections. Whether or not to separate mother and baby is a difficult decision and what you decide will ultimately depend on your Philosophy of Care. Because the virus can be transmitted via colostrum (first milk) and milk, some sanctuaries choose to use a sheep or goat bra to prevent the baby from nursing in an attempt to reduce their risk of infection. This arrangement will not eliminate all risk of transmission via respiratory droplets, and may still cause distress for mother or baby, but will likely result in less distress than full separation. You can then milk the mother and pasteurize the milk to kill the virus or feed colostrum and milk from a non-infected source. Be sure to talk to your veterinarian to ensure you know exactly what you will need to do to make the milk and/ or colostrum safe and to protect the mother from developing mastitis. Because not all SRLV-positive individuals develop clinical disease, some sanctuaries choose not to intervene and instead allow the baby to nurse freely.
Are There Any Treatments That Can Help Residents With Clinical Signs Of SRLV Infection?
Generally, the best you can do for SRLV-positive residents is treat the symptoms as they arise. Consider the following:
- Individuals with the arthritic form should be treated with analgesics to keep them comfortable. They may also benefit from other arthritis treatments such as chondroprotective agents or supplements recommended for joint health. More frequent hoof trimming may be necessary due to their limited mobility. Make adjustments to their environment to make it as arthritis-friendly as possible. Ensure they have ample soft bedding to prevent pressure sores if they are spending more time laying down and avoid living spaces that have steep grades.
- Individuals who have trouble maintaining a healthy weight should be offered nutritional support and may be more sensitive to the cold so may require coats during the winter.
- Individuals with the pneumonic form may develop secondary bacterial infections and require antibiotic treatment as prescribed by your veterinarian.
- SRLV- positive residents may be more vulnerable to other health issues such as parasitic infections.
- Because OPP and CAE are chronic, degenerative diseases, it will be important to monitor your positive resident’s quality of life and work closely with your veterinarian to provide them with the supportive care necessary.
Keep a positive outlook! Just because your resident is SRLV-positive, doesn’t mean they can’t continue to lead a happy, comfortable life. Remember that most SRLV-positive individuals never show signs of disease. While every situation is different, many SRLV-positive sanctuary residents have continued to maintain a good quality of life with the proper care.
Overview of Caprine Arthritis and Encephalitis | Merck Veterinary Manual (Non-Compassionate Source)
Progressive Pneumonia in Sheep and Goats | Merck Veterinary Manual (Non-Compassionate Source)
Caprine Arthritis Encephalitis Virus (CAE) | Extension (Non-Compassionate Source)
CAE | Washington State Extension (Non-Compassionate Source)
Large Animal Internal Medicine 5th Edition | Bradford P. Smith (Non-Compassionate Source)
Sheep And Goat Medicine, Second Edition (Non-Compassionate Source)
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