Small Ruminant Lentiviruses

Overview

Small ruminant lentiviruses (SRLVs) infect sheep and goats, causing chronic, lifelong diseases that impact various organ systems.

Transmission:

• Vertical transmission (from mother to offspring) occurs mainly through ingestion of infected colostrum or milk.
• Horizontal transmission can happen through direct contact, respiratory secretions, or shared equipment. Close contact in crowded conditions increases the risk.
• In utero transmission is possible but less common.
• No insect vector is involved in SRLV transmission, unlike some other lentiviruses like Equine Infectious Anemia Virus (EIAV).

Pathogenesis:

• SRLVs cause chronic, progressive infections characterized by long incubation periods and persistent viral replication.
• The virus primarily targets macrophages and dendritic cells, resulting in inflammation and damage in various tissues.
• The disease can present in multiple organ systems, particularly the lungs, joints, mammary glands, and central nervous system, leading to different clinical manifestations depending on the host species.

Clinical Signs:

SRLV infection causes multi-system disease - typical symptoms include pneumonia, wasting, paralysis, polyarthritis, and mastitis. Disease manifestations can ultimately lead to organ failure and death. However, infection is usually only apparent following an incubation period of at least 3-4 years, and lifelong subclinical infections often occur. General susceptibility and specific disease symptoms are influenced by genetic factors and vary between small ruminant species and breeds.

Diagnosis:

• Serological tests such as ELISA or AGID (agar gel immunodiffusion) are the primary methods used to detect antibodies against SRLVs in sheep and goats.
• PCR testing is used to directly detect viral nucleic acids, which can be helpful for confirming infection or identifying viral strains.
• Post-mortem examination of tissues may reveal characteristic lesions in the lungs, joints, or brain, which can support a clinical diagnosis.

Control and Prevention:

• There is no vaccine or effective treatment for SRLVs, making prevention strategies critical.
• Test-and-cull programs: Regular testing and removal of infected animals from the flock or herd is an effective method to reduce transmission.
• Separation of offspring: Preventing young lambs and kids from nursing infected mothers by feeding them colostrum and milk from uninfected sources can reduce vertical transmission.
• Management practices: Improving biosecurity measures, minimizing close contact between animals, and preventing sharing of equipment can help limit the spread.

Disease Impact:

• SRLVs cause chronic diseases that can significantly reduce productivity in affected sheep and goat populations due to respiratory issues, arthritis, poor milk production, and early culling.
• The slow progression of disease often leads to economic losses due to decreased animal health and performance over time.
• The disease tends to be more widespread in regions with intensive farming practices, where close animal contact facilitates transmission.

Immune Response and Viral Persistence:

• SRLVs establish a lifelong infection, similar to other lentiviruses, with persistent viral replication in macrophages.
• Infected animals can remain asymptomatic for long periods, during which they still shed the virus and contribute to the spread of the infection.
• The immune response, while detectable, is ineffective at clearing the virus, and infected animals experience chronic inflammation and immune-mediated tissue damage over time.

Management of Infected Animals:

• There is no cure, so managing infected flocks and herds focuses on preventing further spread and reducing clinical symptoms.

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