Virology
Understanding Ovine Pulmonary Adenocarcinoma
Dr David Griffiths
|
|
Fig 1. The JSRV virus particle observed by electron microscopy. The diameter of the virus is approximately 1000 times smaller than the thickness of a human hair. |
Fig 2. Collecting lung fluid from a sheep with OPA. |
Ovine pulmonary adenocarcinoma (OPA) is a contagious lung cancer of sheep also known as sheep pulmonary adenomatosis (SPA), or by its Afrikaans name, jaagsiekte (‘driving sickness’). OPA is caused by a virus (jaagsiekte sheep retrovirus, JSRV), which infects fluid-producing cells in the lung and causes them to become cancerous (Figure 1). The excess fluid produced in the lungs can be collected from some affected animals (Figure 2). The disease occurs in most sheep-rearing countries and once symptoms appear it is always fatal. At Moredun, we are seeking to develop effective control strategies for JSRV and OPA and specific areas of work include the development of diagnostic tests and vaccines for JSRV and an analysis of the basic processes of JSRV replication and tumour induction.
Diagnostics and Epidemiology
Fig 3. Histological appearance of OPA tumour. The dense mass of tumour cells at the top of the figure can be distinguished from the normal air filled alveoli at the bottom of the figure. |
At present, there is no approved ante-mortem diagnostic test for OPA and a positive diagnosis requires histological examination of the lungs at post-mortem, in addition to the clinical symptoms (Figure 3). As a consequence the true prevalence of JSRV is uncertain. Despite this, it is clear that OPA is a significant problem to individual farmers and losses due to this disease can be as high as 10% per year within some affected flocks.
A major advance in recent years has been the development at Moredun of a blood test that can detect JSRV in sheep many months before the clinical signs of OPA appear. This test will allow us to investigate the prevalence of JSRV infection, both in individual flocks and on a national scale, and to compare infection rates with disease prevalence and incidence. Within the last year we have
obtained the first data from a longitudinal survey of one affected flock. In this study, ewes and their lambs were tested for JSRV over several years. The results reveal that over 70% of adult ewes in this flock are infected with JSRV. Testing of lambs has shown that they are infected at a young age and within a year the prevalence of infection is the same as that seen in the adult sheep. Over this period however, there have been no clinical cases of OPA in the group of animals under study.
These results are important since they show that flocks may contain many infected sheep with no clinical signs, although they may still harbour the cancer in their lungs. It has been shown that asymptomatic carrier animals such as these present a risk for infection spreading to previously uninfected sheep and flocks.
Quantification of JSRV
Research on JSRV and OPA has been complicated by the lack of a quantitative assay for JSRV. We have now developed a new technique, known as quantitative PCR, which allows the amount of JSRV in a sample such as blood or lung fluid to be measured. Quantitative PCR is a valuable tool that has several applications, for example, this assay will be used to determine how long JSRV can survive in the environment, such as on pasture and bedding. It will also be used to standardise the amount of virus contained in challenge doses in vaccine trials.
Fig 4. Detection of antibodies to JSRV capsid protein in immunised sheep. Before the treatment (pre) neither infected nor uninfected sheep have anti-capsid antibodies but these can be detected following immunisation with recombinant capsid protein (post). |
Vaccines
Vaccination has the potential to be an important method for controlling OPA and the development of vaccines against JSRV is a major focus of our research at Moredun. An unusual feature of the disease is that immune responses to JSRV are not detectable in infected sheep. Despite this, antibodies and cell mediated immune responses to JSRV proteins can be produced experimentally in sheep (Figure 4). We are currently optimising the vaccination procedure to promote stronger immune responses before determining whether this treatment can protect sheep against OPA. In addition, this year we have demonstrated that OPA can be induced experimentally in lambs up to 6 months of age thereby providing a model system for determining the effectiveness of any candidate vaccine.
National Survey
The tools for detecting and quantifying JSRV have
opened up new possibilities for studying the biology of the virus and its role in OPA. Together with colleagues from SAC and BioSS we have recently undertaken a large study across Scotland to determine the importance of the various environmental, virological and immunological factors likely to be involved in the spread of JSRV and OPA. In addition to laboratory analysis, the success of this study will rely on information about management practices obtained from farmers participating in the study. The results will provide a more accurate picture of the spread of JSRV infection in Scottish flocks and aim to identify the most important risk factors for spread of the disease. Together with advances made in parallel studies on the molecular mechanisms involved in the development of the disease, the information obtained will further add to our understanding of OPA and will lead to new strategies for controlling this disease.
This research is funded by SEERAD and EU.