OPA (Ovine Pulmonary Adenocarcinoma or Jaagsiekte), is a contagious lung cancer of sheep caused by jaagsiekte sheep retrovirus (JSRV). OPA occurs in most countries where sheep are farmed and is of significant economic and animal welfare importance.
The typical clinical signs of OPA are that of a progressive respiratory disease that resembles pneumonia. However, a unique feature in many affected sheep is the overproduction of fluid in the lung that can be seen running from the nose when the animal lowers its head. Once these signs appear the disease is invariably fatal. Affected flocks many lose up to 20% of sheep per year to OPA, although 1-2% is more usual. Despite the unique clinical appearance, infection with JSRV is frequently subclinical, and OPA is commonly spread to new flocks and areas through the purchase of infected but apparently healthy animals.
Current control strategies for OPA are confined to managemental interventions, such as culling of clinically-affected animals, and are relatively ineffective in controlling the disease. A major aim of our research is to develop improved methods for controlling OPA, including vaccines and diagnostic tests that can detect infected animals prior to the onset of clinical signs.
Previously we developed a PCR blood test to detect JSRV in the blood of infected animals. Although this test has been useful as a research tool it is not sufficiently sensitive as a diagnostic test for individual animals though it can be applied as a flock test for JSRV. Currently we are trying to develop diagnostic tests with greater sensitivity. One approach is to identify protein or microRNA biomarkers specific to OPA-affected sheep. Another promising approach is using a nasal swab test for OPA as part of a flock health screening programme. We are also investigating trans-thoracic ultrasound scanning for diagnosis or screening for OPA. Ultrasound scanning reliably identifies sheep with OPA tumours of 2 cm3 or more and allows us to identify sheep with OPA at a stage before they become sick and whilst they are still of cull value. We are now investigating the efficacy of ultrasound scanning when trying to identify smaller tumours, and also whether using it as the basis for a test-and-cull programme in OPA-affected flocks will be a useful and economic way to reduce the disease incidence or perhaps even eradicate the disease. However, it is important to note that ultrasound is unlikely to detect very small tumours and therefore cannot ever confirm that a sheep is negative for OPA.
Another area of interest is directed at understanding the interaction of JSRV with its host during the early stages of infection. For example, we are currently studying the cell types that are initially infected and transformed and we are characterizing the innate immune response to infection. Greater understanding of these events may be informative for the design of novel control strategies for OPA. In addition, OPA is regarded as an animal model for some forms of human lung cancers and studies on the early events in JSRV-induced transformation in sheep may be relevant to the human disease.
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