OPP, or Maedi (which means "shortness of breath in Icelandic), is a slowly progressive interstitial pneumonia.  Visna (which means "wasting" in Icelandic) is the neurologic form characterized by leukoencephalomyelitis with demyelination.  OPP is the most common clinical manifestation of OvLV infection in the United States.  Other chronic inflammatory conditions associated with OvLV include polyarthritis and mastitis.

  Ingestion of infected colostrum is the primary route of OvLV transmission.  Horizontal transmission by ingestion or inhalation of aerosolized virus from respiratory secretions or ingestion of contaminated food or water also occurs, albeit less efficiently, and is typically associated with close confinement during winter months.  In utero transmission has been reported, but the frequency is unknown.  Initial infection is followed by a latent period of months to years, during which virus replication is limited and seroconversion is delayed.  An important feature of retrovirus replication is that viral genetic material becomes integrated into the host cell DNA, resulting in lifelong infections despite production of specific antibodies.  Approximately 20% of sheep infected with OvLV will eventually develop clinical disease; however, these animals typically do not show clinical signs until approximately 2 years of age.

  Tentative diagnosis of OPP is often based on clinical signs, including chronic afebrile pneumonia with progressive respiratory failure and loss of body condition despite a good appetite.  Expiratory dyspnea, abdominal breathing, and severe tachypnea (up to 80-12 breaths/min) may be observed.  Pregnant ewes often give birth to small or weak lambs.  Other clinical signs include chronic polyarthritis and mastitis ("hard bag").  At necropsy, the lungs have a rubbery consistency, fail to collapse, and may be 3-4 times normal weight.  The basic microscopic lesion in all affected tissues, including lungs, mammary gland, and central nervous system, is lymphocytic interstitial inflammation accompanied by formation lymphoid nodules with germinal centers.  OvLV is tropic for mononuclear phagocytes, and persistent activation of macrophages causes chronic stimulation of the immune system, resulting in the lymphoid hyperplasia and follicle development observed in various tissues.

  Serologic and molecular-based diagnostic tests for OvLV are available.  Serologic tests such as agar gel immunodiffusion (AGID) and enzyme linked immunosorbent assay (ELISA) demonstrate the presence of virus-specific antibodies in serum.  AGID is the most commonly used serologic screening test, but has a lower sensitivity for detection of antibodies than ELISA-based tests.  In experimentally infected animals, ELISA tests were able to detect seroconversion earlier than AGID.  The PCR test detects proviral DNA in whole blood or tissue samples.  The PCR test is able to detect infected animals before they mount an antibody response, but is more costly than the serology-based diagnostic tests.

  OPP is a chronic, progressive disease for which no effective treatments or vaccines are available.  Control and prevention programs are paramount.  Periodic AGID or ELISA screening tests are recommended to identify infected individuals in the flock.  Lambs may be removed from infected mothers at birth and raised in separate flocks.  Preferably, these lambs should be fed colostrum and milk from certified OPP-free ewes.  Colostrum from infected ewes should be heated at 56ºC for 60 minutes and milk should be pasteurized.  Alternatively, seropositive ewes may be culled from the flock.  Total herd replacement or annual purchase of OPP-free replacements should also be considered in lamb-producing flocks.  The most recent National Animal Health Monitoring System (NAHMS) sheep survey in 2001 reported that 24.2% of sheep from the 3,210 operations surveyed nationally were seropositive for OvLV using the ELISA test.  These figures were slightly higher in the central region, in which 24.4% of sheep tested positive and 46.6% of the operations surveyed had one or more seropositive animals.

-by Morgan Hennessey, Class of 2006
-edited by Dr. Kim Maratea, ADDL Graduate Student

References

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