Virology
Malignant Catarrhal Fever Research
Dr. David Haig and Dr. George Russell
Figure 1 OvHV-2 infected animal with the head and eye form of MCF showing characteristic nasal discharge |
Malignant Catarrhal Fever (MCF) is a fatal disease of cattle and deer caused by Ovine herpesvirus-2 (OvHV-2), a widespread, persistent virus of sheep. MCF is only clinically recognised in the acute stages, when the animal shows signs similar to other mucosal diseases such as FMD, BVD and rinderpest (see Figure 1). These factors make correct diagnosis important and make MCF a difficult disease to study. The disease is sporadic but has serious consequences, since most animals showing clinical MCF either die or are culled. However, surveillance of this disease is difficult with traditional diagnostic tools due to the ability of the virus to persist for the lifetime of the reservoir host, sheep, with little detectable virus or viral antigen. The virus is assumed to be passed from sheep to cattle, however there are reports of MCF in cattle that had not been in contact with sheep for several months, raising the possibility of sub-clinical infection in cattle with acute disease induced by some unknown external factor.
MCF research at Moredun has three main goals: to understand how the virus infects cattle; to predict the range of consequences for the animal and the herd; and to improve the diagnostic tools available.
Our approach has been to understand the disease by understanding the virus. OvHV-2 belongs to the gamma-herpesvirus family. Most of the OvHV-2 genes, especially those involved in replicating the virus and making new virus particles, are common to all of the gamma-herpesviruses, but each virus has a small number of genes (about 10) that are unique to that species. Some of these genes encode factors that can influence the host immune system, helping the virus to evade the immune response. The number and nature of these ‘immuno-modulatory’ genes is different in each virus, and it is thought that the unique combination of genes in each virus is responsible for the unique pathology of the resulting disease.
Infected cell lines can be established from MCF-affected animals and these cell lines have indiscriminate cytotoxic behaviour. We think these cells are an important feature of the disease and may cause much of the pathology found in diseased animals.
Figure 2 Diagramatic representation of the OvHV-2 genome sequence. Conserved and unique genes are boxed. |
At Moredun we are exploiting our ability to study infected animals and cell lines, focusing on the small number of unique OvHV-2 genes that may be responsible for the unique pathology of MCF. To this end we have been involved in a collaboration which has recently resolved the 132kb DNA sequence of OvHV-2 and has identified all of the virus genes (see Figure 2). Within this sequence, 12 genes unique to MCF viruses were found and work has begun to understand their function. The unique genes encode: two proteins involved in the regulation of cell death; two glycoproteins that may be involved in determining the types of cells that can be infected; two regulatory proteins that may alter the expression of host cell genes; a cell-surface regulatory protein and a viral copy of the host interleukin-10 molecule. Interleukin-10 normally functions to regulate immune responses and prevent them getting out of control. The viral IL-10 molecule may help the virus by damping down the host anti-viral immune response. We have shown that the gene is expressed as expected and appears to have IL-10-like functions (see Figure 3). To investigate the function of the other unique genes we are using two general approaches. First, switching off each gene individually, by using RNA interference, may help us identify the function of the gene in infected cells. Second, by analysing the proteins that associate with each of the unique gene products we may be able to understand how they work.
Figure 3 3-D structure of the 02.5 gene product modelled on the human 1L-10 structure (click for larger image) |
We have also begun work to improve diagnosis of MCF by developing an ELISA test that may identify infected animals. Using the OvHV-2 genome sequence, we can identify potential diagnostic antigens and produce them as recombinant proteins for testing with MCF case sera available in the institute. Understanding the function of the unique genes of OvHV-2 will help us select good candidate molecules for this test.
This programme will yield information on the route of MCF infection and factors that influence its severity. This may be used to improve current viral surveillance and to advise farmers and their vets on best practice to reduce the occurrence of disease.
This research is funded by SEERAD & BBSRC.