Diverse approaches to the study of infectious disease

Abstract

This thesis describes three separate projects. While each is very different from the other, they all represent aspects of the study of infectious disease, and highlight the necessity of utilising diverse approaches to solving complex problems.

Chapter 1:

A novel retrovirus, xenotropic murine leukaemia virus-related virus (XMRV), has been identified in samples from patients with prostate cancer and chronic fatigue syndrome (CFS). Reports of its prevalence vary, but the data suggest that XMRV is circulating in the human population. The virus is closely related to murine leukaemia viruses, which cause lymphoid neoplasias in mice. Samples from human patients with a variety of lymphomas and leukaemias were screened to determine whether there was any evidence of XMRV in these tissues.

DNA from blood or tumour samples from 368 patients with lymphoma/leukaemia and from 139 patients with other diseases was screened for XMRV provirus using three specific quantitative PCR (qPCR) assays. Samples were screened for the presence of the human beta-globin gene to ensure integrity of the DNA. The positive control, consisting of DNA from the XMRV containing cell line 22Rv1, was amplified consistently in each assay. No sample was positive for XMRV in any of the three assays.

The data suggest that XMRV is not directly associated with common forms of lymphoproliferative disease in the UK and does not appear to be a prevalent blood borne infection in this population. It is possible that the prevalence of XMRV infection varies between Europe and North America, as has been suggested by studies of prostate cancer and CFS. The strain of virus present in the UK may be different to that in the US, but previous isolates have been nearly identical, making this unlikely. Further work needs to be carried out to more fully assess the true prevalence of XMRV infection in different geographical areas and in different diseases.

Chapter 2:

The prevalence of scrapie, a fatal neurodegenerative disease of sheep, has been declining in the UK over recent years. This is thought to be a result of nationwide measures put in place to control the disease. Such control measures are expensive, and alternative methods are being explored. This preliminary study aimed to identify flocks that represent a high risk of transmission to others, using UK movement records in a disease transmission model. As breeding ewes are thought to be most important for scrapie transmission, it was decided to restrict the movement dataset to only these animals, where possible.

The movement records were analysed for breeding movements in conjunction with market sales data. Although statistical analysis indicated that movements of small batches in autumn were likely to be those of breeding sheep, no robust criteria to distinguish different types of movements could be found. However, a small subset of movements were examined in the disease transmission model and yielded useful results. High-risk farms, i.e., those becoming infected or spreading infection via sale or purchase of breeding sheep, were significantly more active traders both in buying and selling of all sheep than non-infected farms. This indicates that it may be possible to positively identify such high-risk farms, thus allowing application of targeted control measures.
Though promising, the model, and the movement dataset in particular, require further refinement. Further statistical analysis, potentially of additional factors such as age of sheep or altitude of farm, may be useful. The introduction of individual sheep movement recording from 2011 should improve the accuracy of recording and provide additional data sources, hopefully increasing the predictive power of future studies.

Chapter 3:

Transmission of infectious disease between species, while infrequent, does occur, and can result in high morbidity and mortality in susceptible populations. Feline immunodeficiency virus (FIV) is endemic in many felid species across the world, including large cats and hyenas in the Serengeti region of Tanzania, and has been known to jump across species. The risk of transmission between domestic cats and dogs and wild species in this region is, as yet, unknown. It is also not known whether dogs are capable of infection with FIV at all, particularly as they have no currently recognised exogenous retroviral infections. There is evidence to suggest that this might be possible, including infection of a canine cell line and the loss of TRIM5α, a retroviral restriction factor, in this species.
Sera from domestic cats and dogs in the Serengeti were obtained, and tested against a diverse range of FIV subtypes by Western blot analysis. There was a distinct response to the viral antigen p24 in 51.8% of samples tested, including those from both cats and dogs. The majority of these responses, however, were of weak or intermediate strength, with few reactions to multiple viral antigens, as would be expected with a specific response. Primary canine T-cells were also infected with several strains of FIV. No evidence of productive infection of these cells was noted. It appears that viral entry into canine cells is reduced compared to feline controls. This may be related to the finding that, in dogs, the main FIV cell surface receptor, CD134, contains a mutation at the FIV binding site and is non-functional. There are, however, also signs of post-entry viral restriction.

Whilst the serological evidence suggests that domestic species may have been exposed to FIV, the specificity of such responses has not been established. Further analyses are required to ascertain whether exposure to bona fide FIV has occurred, and to determine the risk of infection to the populations involved. In vitro data suggest that primary canine cells cannot support infection with FIV, and so dogs appear unlikely to be able to transmit infection. Nonetheless, evidence of post-entry restriction in these cells may offer interesting opportunities for future study, particularly if dogs have a unique mechanism to avoid retroviral infection.