Research veterinarian Charles McGurk works at ARC headquarters in Stavanger, Norway. He explains, using the gut or intestine, as an example. “As well as being the focus of digestive and absorptive processes, the intestine is also a massive surface area for potential infection to take hold. Pathogens—whether viral, bacterial or parasitic—can pass through the gut wall and find their way to other target organs. Therefore the immune mechanisms in the intestine are a first line defence, alongside those of the skin and gills. We know non-specific immunity is fundamental in protecting fish, and these mechanisms are particularly suited to modification by immune-modulators delivered in the feed, which explains our interest.”
Conventionally, a wide range of health assessments are made by the examination of specially stained thin slices of tissue under a microscope. These histological methods are essential for diagnosis of disease but they also can reveal subtle changes in tissue structure that might be beneficial. However it is notoriously demanding and potentially inconsistent for human operators to make subjective assessments. Various semi-quantitative methods have been documented but they are still highly laborious and generally limited to relatively small-scale specialist research studies.
“At ARC we invested in the latest advance in quantitative histology; a high resolution histology slide scanner that can digitise microscope slides,” says McGurk.
“The system is used in human medicine for specialised pathologists to examine samples from patients anywhere in the world. ARC is now pioneering its use in fish health and nutrition research and it is enabling us to generate masses of data with relatively little user input. In addition, we are using customised image analysis software to extract quantitative data from the tissue sections. This refinement has revealed previously undetectable differences in gut structures. For example, a newly developed diet was shown to reduce subclinical changes in gut morphology in fish reared in water temperatures above the optimum. The potential of this approach is not limited to studying intestines,” adds McGurk.
As the many probiotic products marketed for human consumption suggest, the microbes resident within the intestinal tract, the microbiota, are highly relevant to gut health. Skretting ARC is cooperating with other Nutreco and external research organisations working with pigs, poultry and ruminants on advanced methods for assessing the range and quantity of microbiota, especially bacteria, in the guts of fish. A technique called DNA pyrosequencing is proving especially effective according to McGurk. “We are using DNA-pyrosequencing to assess the impact on the gut micro environment of Skretting diets utilising the MicroBalance™ concept. The results show that replacement of dietary fish protein sources with vegetable proteins has no demonstrable impact on the delicate balance of microbes in the gut.”