Lentiviral infection causes a progressive, interstitial pneumonia typically observed in mature sheep. Clinical signs include weight loss and increasing respiratory distress. There is no treatment, and control relies on serologic diagnosis and the removal of infected animals.
Ovine progressive pneumonia (OPP), or maedi-visna, is a chronic disease of sheep due to lentiviruses (enveloped, single-stranded RNA viruses in the family Retroviridae). The related disease in goats is referred to as caprine arthritis and encephalitis (CAE). Although the lentiviruses have varying virulence among genetically distinct isolates and were previously thought to be distinct enough to infect either sheep or goats, they are now collectively referred to as the small ruminant lentiviruses (SRLVs). Cross-species transmission is possible, and the same virus isolate can cause a variety of clinical signs and disease syndromes. Maedi (meaning dyspnea) refers to the progressive pneumonia caused by the virus; visna (meaning wasting) refers to the neurologic form of the disease in sheep.
Etiology and Pathogenesis of Lentivirus Pneumonia in Sheep and Goats
Small ruminant lentiviruses persist in lymphocytes, monocytes, and macrophages of infected sheep, evading elimination by the immune system. Infection is transmitted orally, usually by the ingestion of colostrum or milk that contains viral particles, or by the inhalation of infected aerosol droplets. Previously, maternal transmission was thought to account for most infections in lambs; however, research suggests that maternal transmission accounts for only a minority of infections. Intrauterine infection is thought to occur infrequently. All breeds of sheep and goats appear susceptible; however, some resistance to lentivirus infection may exist within breeds.
Epidemiology of Lentivirus Pneumonia in Sheep and Goats
Reported seroprevalence for lentiviral infection in sheep varies widely, ranging in the US from 49% in the West to 9% in the Northeast. This variation has been reported in other countries as well and may result from varied climatic conditions (arid vs humid climates) and management conditions (range conditions vs close confinement).
Management practices can influence morbidity rates.
Clinical Findings of Lentivirus Pneumonia in Sheep and Goats
Clinical signs of progressive pneumonia rarely occur in sheep < 2 years old, and they are most common in sheep >4 years old. The disease progresses slowly; wasting and increasing respiratory distress are the primary signs. Coughing, bronchial exudate, lethargy, and fever are seldom evident unless secondary bacterial infection occurs. A noninflammatory mastitis. sometimes called hard bag, may occur in goats with lentiviral infection. The neurologic form of the disease in adult sheep presents most commonly as an encephalitis; clinical signs may include a head tilt, circling, and altered mentation. The spinal form presents as unilateral or bilateral pelvic limb proprioceptive deficits progressing to paresis and eventually to complete paralysis. Similar neurologic signs may occur in goat kids with lentiviral infection.
Lesions
Macroscopic lesions of progressive pneumonia are confined to the lungs and associated lymph nodes. The lungs do not collapse when the thorax (with obvious rib indentations) is opened on necropsy, and they are abnormally firm and heavy (~2 kg; 2–4 times normal weight). Early lung changes may be difficult to detect; later in the disease, however, lungs are mottled by gray and brown areas of consolidation. The mediastinal and tracheobronchial lymph nodes are greatly enlarged and edematous. Interstitial pneumonia, perivascular and peribronchial lymphoid hyperplasia, and hypertrophy of smooth muscle are evident throughout the lung on histologic examination. CNS lesions, when they occur, are those of meningoleukoencephalitis with secondary demyelination. All lesions are progressive and result from the cellular immune response of the host, rather than directly from viral damage.
Diagnosis of Lentivirus Pneumonia in Sheep and Goats
ELISA or agar gel immunodiffusion (AGID) assay
Differential diagnoses of progressive pneumonia include pulmonary adenocarcinoma, pleural abscesses, and pulmonary caseous lymphadenitis. Ultrasonographic examination is useful to differentiate these various types of pneumonias in the live animal. Listeriosis, scrapie, cerebrospinal nematodiasis, and space-occupying lesions should be considered in cases of the neurologic form (visna) of the disease.
In the live animal, agar gel immunodiffusion (AGID) and ELISA are used. The competitive-inhibition ELISA is highly sensitive and specific but can produce false-negative results in animals very recently infected. Serologic testing is considered a useful tool to detect infected animals, especially if the disease has been confirmed in the flock based on histopathologic findings or virus isolation. PCR assay and virus isolation are sensitive and specific techniques to detect virus. However, both are more expensive and time-consuming than serologic testing, which is more widely available.
Control of Lentivirus Pneumonia in Sheep and Goats
Serologic screening of all new animals
Testing and culling of infected animals
Currently, there is no practical, effective treatment for ovine progressive pneumonia, and no vaccines are available. Therefore, the only means for control and prevention are serologic testing and removal of positive animals. Because of the long incubation period and time to seroconversion, retesting animals once yearly, or even twice yearly, may be indicated. In addition to the approach of testing and culling, it has previously been recommended to raise lambs or kids from seropositive dams separately. Recent research suggests that seroconversion more frequently occurs when young animals join the breeding flock, and separating lambs and kids may not be necessary. Some producers may still feed lambs and kids colostrum from seronegative animals, or heat-treated colostrum, and raise the animals on milk replacer, milk from seronegative animals, or heat-treated milk as the primary method of control.
Key Points
Lentiviral infection is responsible for chronic diseases in sheep and goats, including ovine progressive pneumonia and caprine arthritis and encephalitis.
Control relies on serologic testing of herds and preventing the introduction of infected animals.
