Progressive Pneumonia in Sheep and Goats

The causal lentivirus, which persists in lymphocytes, monocytes, and macrophages of infected sheep in the presence of a humoral and cell-mediated immune response, can be detected by several serologic tests. Seropositive sheep and goats must be considered infected and capable of transmitting the virus. Transmission occurs most commonly via the oral route, usually by ingestion of colostrum or milk that contains virus, or by inhalation of infected aerosol droplets. 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. Management practices can influence morbidity rates.

Signs rarely occur in sheep <2 yr old and are most common in sheep >4 yr old. The disease progresses slowly, with wasting and increasing respiratory distress as the main signs. Coughing, bronchial exudate, depression, and fever are seldom evident unless secondary bacterial infection occurs. A noninflammatory, indurative mastitis may occur. The encephalitic form of visna causes head tilt and circling, whereas the spinal form causes unilateral pelvic limb proprioceptive deficits progressing to paresis and eventually to complete paralysis.

Differential diagnoses of progressive pneumonia include pulmonary adenocarcinoma, pleural abscesses, and pulmonary caseous lymphadenitis. Ultrasonographic examination is very useful to differentiate these various types of pneumonias in the live animal. Listeriosis, scrapie, cerebrospinal nematodiasis, and space-occupying lesions should be considered when the neurologic form (visna) of the disease is seen.

In the live animal, agar gel immunodiffusion and ELISA tests are used. The competitive inhibition ELISA is reported to be highly sensitive and specific; it has been reported to produce false-negative results in animals very recently infected. Serologic testing is considered a useful tool to detect infected sheep, especially if the disease has been confirmed in the flock by histopathologic examination or virus isolation. PCR and virus isolation are sensitive and specific techniques to detect virus. However, both are more expensive and time consuming than serologic testing.

Currently, there is no practical, effective treatment, and no vaccines are available. Therefore, the only means for control and prevention is serologic testing and removal of positive animals. Because of the long incubation period and time to seroconversion, retesting animals once a year, or even twice a year, is recommended. In addition to the test and cull approach, consideration should be given to raising neonates in isolation from their dams, especially if the dam is seropositive. Lambs should be fed colostrum from seronegative sheep, or heat-treated sheep colostrum, and raised on milk replacer, milk from seronegative ewes, or heat-treated sheep milk.