Molecular Epidemiology
Molecular epidemiology is the study of distribution and determinants of health and using molecular biology methods. Not all animals are equal, and neither are all bacteria, parasites or viruses. Some animals are more likely to get disease than others and some micro-organisms are more likely to cause disease than others. Animals’ susceptibility to infection and disease is determined by many factors, such as nutrition, vaccination, pregnancy status, management, etc. Animal species, breeds and genetics play a role too, specifically genes involved in the immune response, such as MHC genes.
For pathogens, we do not talk about species and breeds, but about species and strains. Strains can be identified in many ways, e.g. using PCR, PFGE, DNA sequencing or proteomics. Different methods may suit different purposes and methods are often complementary. For example, investigation of local disease outbreaks requires a different approach than investigation of host-adaptation in a world-wide collection of pathogens. The best method for strain identification of a pathogen may even differ between countries, as we discovered for Mycobacterium avium subsp. paratuberculosis, the causative agent of Johne’s disease.
In a joint effort between EPIC and the Pasteurella group, we characterised a large collection of Pasteurella multocida. These bacteria cause respiratory disease and haemorrhagic septicaemia in cattle, fowl cholera in birds and snuffles in rabbits. They are also found in pigs and sheep. Using RAPD, PFGE and MLST, we explored heterogeneity of the bacteria within animals, farms and countries, and across countries, animal species and disease syndromes. We discovered that the Pasteurella in the respiratory tract of cattle is largely the same in every country, but different from Pasteurella in other organs or animal species. This provides a very specific target for vaccine development. By contrast, bacteria that cause Johne’s disease seem to differ between countries rather than between host species.
Similar approaches are used to study mastitis and streptococcosis in fish. They can also be applied to viruses and parasites, including protozoan pathogens. Studies on Cryptosporidium in dairy calves showed that many strains are potentially zoonotic (transmissible to humans) but some are not. Molecular methods can help to determine whether human cases result from human-to-human transmission or from livestock farming or contaminated environments. Furthermore, transmission mechanisms between farms can be investigated, including the impact of natural boundaries such as rivers and roads. With such information in hand, targeted disease control measures can be implemented.