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Beyond mastitis: molecular epidemiology insights into transmission and control of bacterial, parasitic and viral diseases of cattle
Zadoks R.; Schukken Y.H.
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Introduction: As sequencing of DNA and RNA has become increasingly accessible and affordable, molecular methods have added significantly to our understanding of the transmission and control of bacterial, parasitic and viral disease of cattle. They have provided insight into past movement of pathogens and their current distribution, and inform rational design of future eradication strategies, diagnostic tests, and vaccines. They also help us understand how the pathophysiology of disease and the transmission of organisms and, hence, control strategies may differ, or need to differ, between production systems, countries, or continents. Parasites are comparatively large infectious agents. They are eukaryotic, like cattle and humans, and have large genomes spread across multiple chromosomes, which means that sequencing of entire genomes is still very challenging. Bacteria are prokaryotes, with smaller and simpler genomes than parasites. Less than 15 years ago, sequencing of the first bovine Streptococcus agalactiae genome cost more than the annual salary of the postdoctoral researcher analysing it. Now, it can be done within days or weeks for less than 100 Euro. Viruses are much smaller yet, and sequencing of entire genomes is cheaper and faster than for bacteria, although it can be tricky for segmented genomes such as those of influenza D, which was first described in cattle in 2011.
Parasites: In parasites, sequencing of housekeeping genes is a useful tool for accurate species identification. This approach led to the realization that most rumen fluke in the Republic of Ireland and the United Kingdom did not belong to the species Paraphistomum cervi, as long assumed, but to the species Calicopheron daubneyi. The two species have different life cycles and intermediate host snails. Control of rumen fluke, which has emerged as a common cattle parasite across much of western Europe, requires knowledge of how to manage the environment to limit the snail population and cattle exposure to infectious cysts on pasture. Where the culprit was thought to be a freshwater snail, it is actually a mud snail, completely changing our options for environmental control. Other desirable tools for diagnostics and management of parasitic diseases would be molecular markers of anthelmintic resistance. Although initial results for, e.g., the liver fluke Fasciola hepatica, were promising, they could not be confirmed in other studies, countries, or continents.
Bacteria: Differences in pathogen characteristics between continents can be considerable, and evidence-based control strategies may differ between countries. In our contribution on molecular epidemiology of mastitis, the difference between New Zealand and the UK is used as an example, whereby Streptococcus uberis mastitis is almost exclusively environmental in New Zealand but commonly contagious in the UK, as it is in mainland Europe or the USA. Another striking geographical difference is observed for Coxiella, the causative agent of Q-fever in humans and coxiellosis in ruminants. In the USA, up to 95% of bulk tank milk may test positive for Coxiella but reports of Q-fever are unusual. In Europe, Q-fever is primarily associated with small ruminants. By contrast, cattle are considered the main source of Q-fever in Australia. The difference may be due, at least in part, to differences in Coxiella strains, which may also impact on the efficacy of diagnostic assays coated with antigens from different parts of the world.
Viruses: The most detailed molecular epidemiology information is available for viruses. Strain typing is essential to inform selection of vaccines for a high-impact pathogen like Food and Mouth Disease (FMD) because dominant serotypes may differ between countries or change over time. It may also inform on the role of animal movements and non-bovine host species as reservoirs, both for epidemic viral diseases like FMD and for endemic viral diseases like bovine viral diarrhoea virus disease (BVDV). Viral sequence analysis has shown that sheep, humans, or inanimate fomites may act as source of BVDV. Insight into occurrence and prevention of such exceptions will become increasingly important as BVDV programs progress, which they are likely to do now that it has been add- ed to the OIE list of notifiable diseases.
Although it is impossible to provide a comprehensive overview of the molecular epidemiology of bovine infectious diseases, selected examples will be presented to illustrate their contribution to our ability to understand and manage the health of cattle populations.
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Affiliation of the authors at the time of publication
Sydney University, Australia;
Royal GD, Wageningen University and Utrecht University, The Netherlands
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