Maedi

Maedi, also known as ovine progressive pneumonia (OPP) or sheep progressive pneumonia, is a chronic and progressive viral disease affecting sheep. It is caused by the Maedi-Visna virus (MVV), a member of the Lentivirus genus within the Retroviridae family. The disease is characterized by progressive weight loss, respiratory distress, and an increased susceptibility to secondary infections due to immunosuppression. Maedi is of significant concern in the sheep industry due to its impact on animal welfare and productivity.

Etiology

Maedi is caused by the Maedi-Visna virus, which infects monocytes and macrophages, leading to a prolonged inflammatory response in affected tissues. The virus is closely related to the Visna virus, which primarily causes neurological symptoms in sheep. Both viruses are strains of the same virus, hence the combined name Maedi-Visna for the disease complex that can cause both respiratory and neurological symptoms.

Transmission

Transmission of MVV is primarily through inhalation of aerosolized secretions from infected animals. The virus can also be spread through ingestion of contaminated colostrum and milk, making lambs particularly susceptible. Direct contact and fomites (inanimate objects that can carry infection) are less common modes of transmission. The virus has a long incubation period, often several years, which allows it to spread silently within flocks.

Clinical Signs

The clinical signs of Maedi are insidious and progressive. Affected sheep may show:

• Progressive weight loss despite normal appetite
• Difficulty breathing, increased respiratory rate, and abnormal lung sounds
• Weakness and lethargy
• Enlarged lymph nodes

In advanced cases, severe respiratory distress can lead to death. Due to the slow progression of the disease, many infected sheep may appear asymptomatic for extended periods.

Diagnosis

Diagnosis of Maedi is challenging due to its slow progression and the nonspecific nature of its clinical signs. Diagnostic methods include:

• Serological tests to detect antibodies against MVV
• Polymerase chain reaction (PCR) tests to detect viral RNA
• Post-mortem examination revealing characteristic histopathological changes in the lungs, lymph nodes, and other affected tissues

Prevention and Control

There is no cure for Maedi, making prevention and control critical. Strategies include:

• Testing and culling infected animals to prevent spread
• Segregating flocks based on serological status
• Implementing strict biosecurity measures to prevent introduction of the virus
• Avoiding the use of contaminated colostrum and milk to feed lambs

Economic Impact

Maedi significantly affects the sheep industry by reducing productivity through decreased weight gain, lower wool quality, and increased mortality. The costs associated with testing, culling, and implementing biosecurity measures also contribute to the economic burden of the disease.

Public Health

While Maedi-Visna virus is specific to sheep and closely related species, it poses no known risk to human health. However, the economic impact on the sheep industry can indirectly affect communities dependent on sheep farming.

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