Maedi-Visna / OPP

Maedi-visna, also known as ovine progressive pneumonia in North America, is a chronic, progressive, and ultimately fatal viral disease of sheep caused by a lentivirus. The disease is characterized by slowly developing respiratory disease, encephalomyelitis, mastitis, and arthritis, with affected animals typically not showing clinical signs until they are several years old. The name maedi-visna comes from Icelandic, with maedi meaning dyspnea or labored breathing and visna meaning wasting or shrinking, describing the two main clinical presentations of respiratory and neurological disease respectively.

The disease occurs worldwide wherever sheep are raised, though prevalence varies considerably between regions and flocks. Infection is lifelong once acquired, with the virus persisting in infected animals despite an immune response. The insidious nature of the disease, with its long incubation period and slow progression, means that infection can become widespread within a flock before any clinical cases are recognized. This silent spread makes maedi-visna one of the most challenging sheep diseases to control and has significant implications for flock health and productivity.

The economic impact of maedi-visna extends beyond direct mortality from clinical disease. Infected animals have reduced productive lifespan, decreased wool production, impaired milk production, and increased susceptibility to other diseases. Subclinically infected ewes have poorer reproductive performance and raise lighter lambs than uninfected contemporaries. The presence of infection also limits trade opportunities, as many buyers and breeding programs require documented freedom from the virus. These cumulative effects make maedi-visna a significant economic burden even when clinical disease is relatively uncommon.

There is no vaccine or treatment for maedi-visna, making control dependent on testing and removal of infected animals or maintenance of closed, tested-negative flocks. Understanding the transmission routes and epidemiology of the virus is essential for implementing effective control programs. While eradication from individual flocks is achievable through systematic testing and culling, the widespread distribution of the virus in many sheep populations makes regional or national eradication challenging. Producers must make informed decisions about the level of control appropriate for their circumstances and market requirements.

Maedi-visna is caused by a virus belonging to the genus Lentivirus within the family Retroviridae. The maedi-visna virus is closely related to caprine arthritis encephalitis virus of goats, and these viruses are now often collectively referred to as small ruminant lentiviruses. Like other lentiviruses, including human immunodeficiency virus, maedi-visna virus integrates its genetic material into the host cell genome, establishing lifelong persistent infection. The virus is non-oncogenic but causes progressive degenerative disease through chronic inflammation and immune-mediated tissue damage.

Transmission of maedi-visna virus occurs primarily through two routes: respiratory transmission between adult animals and transmission from infected ewes to their lambs. Respiratory transmission occurs when susceptible sheep inhale virus-containing respiratory secretions from infected animals. This route is favored by close confinement, housing, and any situation that increases respiratory contact between animals. Virus is shed in respiratory secretions throughout the prolonged incubation period, meaning that subclinically infected animals serve as important sources of infection for their flockmates.

Transmission from ewes to lambs occurs through ingestion of infected colostrum and milk, which contain both free virus and infected cells. This vertical transmission route is highly efficient and establishes infection in lambs during the first hours and days of life. Lambs infected through this route may subsequently transmit infection to other lambs and to adult animals through respiratory contact. In utero transmission can occur but is considered relatively uncommon compared to postnatal transmission through milk.

Environmental and management factors significantly influence the rate of transmission within and between flocks. Intensive housing during winter or for lambing concentrates animals and increases respiratory transmission. Dairy sheep operations, with their intensive management and close confinement, typically have higher infection rates than extensively managed meat sheep flocks. Introduction of infected animals is the primary means by which infection enters naive flocks, making biosecurity and purchase policies critical for maintaining disease-free status.

Host factors influence susceptibility to infection and the rate of disease progression. All sheep breeds appear susceptible to infection, though there may be breed differences in disease expression. Some genetic variation in susceptibility within breeds has been identified, raising the possibility of future genetic selection for resistance. Once infected, individual animals show considerable variation in the rate of disease progression, with some developing clinical disease relatively quickly while others remain subclinical for many years.

The clinical presentation of maedi-visna is characterized by its insidious onset and slowly progressive nature. Infected animals typically remain clinically normal for years after initial infection, with clinical disease usually not appearing until animals are three to six years of age or older. The long incubation period means that infection is often well-established in a flock before any clinical cases draw attention to its presence. Four main clinical syndromes are recognized: respiratory disease, neurological disease, mastitis, and arthritis, though not all syndromes occur with equal frequency in all regions.

The respiratory form, maedi, is the most commonly recognized clinical presentation in many countries. Early signs include subtle exercise intolerance, with affected ewes lagging behind the flock when moved. As the disease progresses, increased respiratory rate and effort become apparent, initially only after exertion but eventually present at rest. Affected animals develop open-mouth breathing with nostril flaring and may adopt an extended head and neck posture to ease breathing. Coughing is notably absent or rare, distinguishing maedi from bacterial pneumonia.

Progressive weight loss accompanies the respiratory disease, giving rise to the term thin ewe syndrome. Affected animals lose condition despite adequate nutrition, and their fleece may become poor quality with increased fiber shedding. The combination of labored breathing and poor condition is highly suggestive of maedi-visna in adult sheep, though similar presentations can occur with other chronic diseases. Terminal animals are severely emaciated and die from respiratory failure or are humanely euthanized.

The neurological form, visna, is characterized by slowly progressive hindlimb weakness and ataxia. Early signs may include subtle stumbling or difficulty rising from recumbency. Over weeks to months, the neurological deficits worsen, with affected animals developing progressive hindlimb paresis that eventually involves the forelimbs. Affected sheep remain bright and alert with good appetite until late in the disease course. Eventually, affected animals become recumbent and unable to rise, at which point euthanasia is indicated on welfare grounds.

Maedi-visna virus infection causes indurative mastitis, sometimes called hard bag or hard udder. Affected ewes develop firm, enlarged mammary glands with markedly reduced milk production. The udder becomes progressively harder and less functional over successive lactations. Lambs from affected ewes may show poor growth rates due to inadequate milk supply and may be at increased risk of infection through the limited milk they do consume. The mastitis form is particularly significant in dairy sheep, where milk production is the primary economic output.

Arthritis associated with maedi-visna infection causes progressive lameness and joint swelling. The carpal joints are most commonly affected, becoming enlarged and firm. Affected animals may show shifting lameness as multiple joints become involved. The arthritis is non-responsive to antimicrobial treatment, helping differentiate it from infectious arthritis. Joint involvement adds to the debilitation of affected animals and contributes to their progressive decline.

It is important to recognize that clinical disease represents only a small proportion of infected animals at any given time. Most infected sheep remain subclinically infected for extended periods, serving as sources of infection while appearing healthy. These subclinically infected animals may show subtle decreases in productivity that go unnoticed without careful monitoring. The iceberg concept applies to maedi-visna: clinical cases represent just the visible tip, while subclinical infection is far more widespread.

Diagnosis of maedi-visna involves clinical assessment of affected animals combined with laboratory testing to detect infection. Clinical suspicion should be raised in adult sheep presenting with chronic progressive respiratory disease, wasting, hindlimb weakness, indurative mastitis, or arthritis, particularly when these syndromes occur in the absence of response to treatment and in the absence of other identifiable causes. However, because most infected animals are subclinical, laboratory testing is essential for identifying infection in flocks.

Serological testing for antibodies against maedi-visna virus is the primary method for diagnosing infection in live animals and for flock screening programs. The agar gel immunodiffusion test has been widely used historically and remains a reliable method for detecting infected animals. Enzyme-linked immunosorbent assays offer higher throughput and sensitivity and are increasingly used for large-scale screening programs. It is important to note that antibodies may not be detectable in recently infected animals, so negative tests in individual animals cannot completely rule out infection, particularly if exposure was recent.

Viral detection methods including polymerase chain reaction can identify viral genetic material in blood or tissues, potentially detecting infection before antibodies develop. However, the intermittent nature of viremia and low levels of circulating virus in many infected animals can result in false-negative results. Polymerase chain reaction testing is more commonly used as a confirmatory test or research tool rather than for routine screening. Combination of serological and molecular testing may improve detection sensitivity in some situations.

Post-mortem examination provides valuable diagnostic information in clinical cases and should be performed on animals suspected of having died from maedi-visna. Characteristic findings in the respiratory form include enlarged, heavy lungs that fail to collapse, with gray discoloration and a rubbery texture. The mediastinal lymph nodes are typically markedly enlarged. Histopathological examination reveals diffuse lymphoid hyperplasia and interstitial pneumonia. Similar lymphoproliferative changes are seen in affected nervous tissue, mammary glands, and joints.

Differential diagnosis for maedi-visna includes other causes of chronic respiratory disease, wasting, and neurological disease in adult sheep. Ovine pulmonary adenocarcinoma causes progressive respiratory disease but produces characteristic tumors visible at necropsy. Chronic bacterial pneumonia, parasitic pneumonia, and caseous lymphadenitis may cause respiratory signs and wasting. Scrapie causes progressive neurological disease but typically includes pruritus. Careful clinical assessment and appropriate laboratory testing allow differentiation between these conditions.

There is no effective treatment for maedi-visna virus infection, and once an animal is infected, it remains infected for life. The lentiviral nature of the causative agent, with integration of viral genetic material into the host genome, means that the virus cannot be eliminated by the immune system or by any available therapeutic intervention. This incurable nature of infection is a fundamental characteristic of the disease that profoundly influences management decisions and control strategies.

Supportive care may provide temporary improvement in clinical cases but does not alter the progressive course of the disease. Animals with respiratory disease may benefit from stress reduction, good nutrition, and avoidance of dusty environments that could exacerbate respiratory distress. Antimicrobials may be useful if secondary bacterial infection is suspected but will not address the underlying viral disease. Anti-inflammatory drugs may provide temporary symptomatic relief but again do not affect disease progression.

Management of clinical cases must balance animal welfare considerations against economic factors and biosecurity concerns. Clinically affected animals should generally be culled from the flock for both welfare and disease control reasons. Retaining clinical cases prolongs suffering and allows continued virus shedding that can infect other susceptible animals. Cull animals should be sent directly to slaughter rather than through markets to prevent spread to other flocks. Veterinary guidance should be sought regarding appropriate timing of slaughter relative to any treatments administered and associated withdrawal times.

The absence of effective treatment means that flock-level management must focus on prevention and control rather than treatment of individual animals. Resources that might otherwise be directed toward treatment are better invested in testing programs to identify and remove infected animals, biosecurity measures to prevent introduction of infection, and management changes to reduce transmission within infected flocks. These approaches offer the only realistic path to reducing the impact of maedi-visna on flock productivity and health.

Decision-making regarding culling of seropositive animals must consider the overall flock status and control program goals. In heavily infected flocks, immediate culling of all positive animals may not be economically feasible. Instead, a phased approach may be adopted, with priority given to culling clinically affected animals, seropositive ewes before they lamb, and high-risk animals. Retention of seropositive animals should be viewed as a temporary compromise during control program implementation rather than a long-term strategy.

Veterinary involvement is essential for developing appropriate management plans for affected flocks. Veterinarians can assist with interpreting test results, designing testing and culling strategies, and implementing management changes to reduce transmission. They can also help producers understand the economic implications of different control approaches and make informed decisions appropriate to their circumstances. Regular flock testing and review of control program progress should be incorporated into routine veterinary visits.

Recovery from maedi-visna virus infection is not possible because the virus establishes permanent infection in the host. Once an animal is infected, regardless of whether it develops clinical disease, it remains infected and potentially infectious for life. The lifelong nature of infection is a fundamental characteristic of lentiviral diseases and distinguishes maedi-visna from most other sheep infections where recovery and clearance of the pathogen is possible.

Clinical disease, once it develops, follows a progressive course without recovery. Animals showing clinical signs of respiratory disease, neurological disease, mastitis, or arthritis will deteriorate over time, though the rate of deterioration varies between individuals. Some animals decline rapidly over weeks, while others may remain stable or progress slowly over months. Regardless of the rate, the direction of change is consistently toward worsening disease, and there is no possibility of recovery or reversal of clinical signs.

The lack of recovery possibility means that prognosis for clinically affected animals is uniformly poor. Humane euthanasia should be considered when clinical signs begin to significantly affect quality of life or when animals are no longer able to maintain adequate body condition or mobility. Delaying euthanasia does not benefit the animal and allows continued virus shedding that puts other susceptible animals at risk. Early culling of clinical cases is both a welfare decision and a disease control measure.

Subclinically infected animals may remain apparently healthy for years and maintain reasonable productivity levels. However, they remain at risk of eventually developing clinical disease and continue to pose a transmission risk to susceptible flockmates. From a production standpoint, subclinically infected animals have shorter productive lives on average than uninfected animals, even if they do not develop overt clinical disease. This reduced longevity is a hidden cost of infection that contributes to the overall economic impact of maedi-visna in infected flocks.

Prevention of maedi-visna relies on maintaining closed flocks free of infection or implementing systematic control programs in infected flocks, as no vaccine is available. Maintaining a closed flock with verified negative status is the most effective prevention strategy for flocks that are currently free of the disease. This approach requires strict biosecurity including no introduction of sheep or goats from untested or infected sources, secure perimeter fencing to prevent contact with neighboring animals, and regular serological testing to confirm continued freedom from infection.

For flocks that wish to purchase animals, sourcing only from certified maedi-visna free flocks or flocks participating in accredited health schemes significantly reduces introduction risk. Purchased animals should ideally have two negative serological tests at least six months apart before joining the flock, with quarantine maintained until testing is complete. Even with these precautions, some risk remains, as recently infected animals may test negative before seroconversion. Using artificial insemination with semen from tested-negative rams eliminates live animal introduction risk while still allowing genetic improvement.

Control of maedi-visna in infected flocks typically involves a test-and-cull program combined with management changes to reduce transmission. All adult sheep should be tested serologically, with seropositive animals culled or segregated from seronegative animals. Management changes to reduce transmission include separating lambs from seropositive dams immediately at birth and feeding heat-treated colostrum and milk replacer, improving ventilation in housing to reduce respiratory transmission, and reducing stocking density to minimize respiratory contact.

Lamb removal programs, sometimes called snatch-rearing protocols, are highly effective at preventing maternal transmission. Lambs are removed from their dams immediately at birth before they nurse, dried, and fed heat-treated colostrum from seronegative ewes or bovine colostrum substitute, followed by artificial rearing on milk replacer. This labor-intensive approach prevents infection of lambs by seropositive dams and is particularly useful for saving valuable genetics during flock eradication programs.

Eradication from infected flocks is achievable but requires sustained commitment over multiple years. Typically, all animals are tested annually, with seropositive animals culled. As prevalence decreases, testing frequency may increase to twice-yearly. The flock must remain closed to outside introductions throughout the program. Complete eradication may take three to five years or longer depending on initial prevalence and program compliance. Ongoing monitoring through periodic testing confirms continued freedom from infection after eradication is achieved.

Ongoing management for maedi-visna prevention in free flocks centers on maintaining strict biosecurity and conducting regular surveillance testing. Biosecurity policies should be clearly documented and communicated to all farm personnel. Physical barriers including adequate perimeter fencing should prevent contact with neighboring sheep. Policies regarding shared equipment, personnel movement between farms, and handling of visitors should minimize any potential routes of virus introduction. Regular review and reinforcement of biosecurity practices helps maintain vigilance over time.

Surveillance testing programs provide assurance that free flocks remain uninfected. Annual testing of all adult sheep is recommended, with testing conducted according to the requirements of any health scheme the flock participates in. New additions from closed biosecurity breaches should prompt immediate testing and potential expansion of testing frequency. Maintaining detailed records of all test results, animal movements, and biosecurity events supports flock health certification and facilitates investigation if problems occur.

For flocks undertaking control programs in the presence of infection, management involves ongoing testing, culling, and implementation of transmission-reducing measures. Testing schedules should be designed in consultation with a veterinarian, typically involving at least annual whole-flock testing initially, with frequency adjusted as prevalence decreases. Record keeping should track the status of individual animals, enabling management decisions about culling priorities and segregation groups.

Management of lambing in infected flocks requires particular attention to preventing transmission from seropositive ewes to lambs. Options range from complete snatch-rearing of all lambs to selective removal of lambs from high-risk dams. Heat treatment of colostrum at fifty-six degrees Celsius for sixty minutes effectively inactivates the virus while preserving antibody function. Feeding bovine colostrum or colostrum replacer followed by milk replacer eliminates transmission through milk entirely. The approach chosen depends on flock size, labor availability, and control program intensity.

Economic monitoring helps assess the success and cost-effectiveness of control programs. Tracking productivity parameters including lamb weights, wool production, ewe longevity, and replacement rates allows comparison before and after control program implementation. Economic analysis should consider both the direct costs of testing and culling and the productivity benefits of reducing infection prevalence. For many flocks, the improved productivity of seronegative animals compared to infected animals justifies the investment in control programs.

Maedi-visna virus can infect sheep of any breed, and no breed is known to be completely resistant to infection. However, there are apparent differences between breeds in disease susceptibility and expression that influence the practical impact of infection in different production systems. These differences may reflect true genetic variation in susceptibility, differences in management intensity between breeds, or combinations of these factors.

Dairy sheep breeds may experience higher infection rates than meat breeds due to management factors rather than inherent susceptibility. The intensive housing, close confinement, and prolonged lactation periods typical of dairy sheep production favor respiratory transmission and colostral transmission respectively. Breeds commonly used in dairy production, including East Friesian, Lacaune, and their crosses, frequently show high seroprevalence in intensive dairy systems. The mastitis form of disease is particularly important in dairy sheep, where udder health directly affects economic output.

Within any breed, some genetic variation in susceptibility and disease expression likely exists. Research has identified genetic markers associated with susceptibility to infection and with rate of disease progression. Selection for resistance may become increasingly feasible as these genetic tools develop. However, any selection program must balance resistance traits against other economically important characteristics. Currently, focusing on maintaining negative status or implementing test-and-cull programs remains more practical than attempting genetic selection for resistance in most flocks.

Maedi-visna is closely related to caprine arthritis encephalitis virus infection in goats, and these viruses are now often grouped together as small ruminant lentiviruses. Cross-species transmission between sheep and goats can occur, making mixed sheep-goat operations particularly challenging to manage for lentivirus control. Joint housing of sheep and goats should be avoided in flocks attempting to maintain freedom from these viruses, and biosecurity planning should consider both species when either is present.

Ovine pulmonary adenocarcinoma, also known as jaagsiekte, is another chronic progressive respiratory disease of sheep that may be confused with the respiratory form of maedi-visna. Ovine pulmonary adenocarcinoma is caused by a retrovirus that induces lung tumors, resulting in progressive respiratory disease and weight loss. Unlike maedi-visna, pulmonary adenocarcinoma typically produces copious watery fluid from the lungs when affected animals are lifted by the hindquarters. Definitive differentiation requires post-mortem examination showing characteristic tumor nodules in pulmonary adenocarcinoma.

Chronic wasting and respiratory disease in adult sheep can result from numerous other conditions that should be differentiated from maedi-visna. Caseous lymphadenitis causes internal abscesses affecting the lungs and other organs, leading to chronic wasting. Chronic parasitism, particularly haemonchosis, causes wasting and ill-thrift. Johne's disease, while more common in cattle, can cause chronic wasting in sheep. Careful clinical assessment, appropriate diagnostic testing, and post-mortem examination when possible allow accurate diagnosis and appropriate management responses for these various conditions.