Diagnostics – Step 1

Today was a full day; in the field examining transects of eelgrass in False Bay and then a very informative lecture on Diagnostic Methods. Carolyn reminded us of an important, but often forgotten, first step when examining an impacted site or organism: assess the environment. For example if you were to test what caused a massive die-off at an oyster aquaculture facility, you would take the follow steps for gross examination:

1st thing: look at the environmental correlates
(e.g. aquaculture facility – what’s new? any animals added? any new pumps or change in water quality?)
2nd thing: look for lesions or external deformities
3. examine scraping the exterior of organism
4. Gill squash
5. Examine internal organs or viscera: tissue squashes, stain tissue sections, fat content, whether they’ve been feeding

In addition, we were informed of two resources from the World Organisation for Animal Health: International Aquatic Animal Health Code & Diagnostic Manual for Aquatic Animal Diseases



We also discussed about the pros and cons of various approaches to diagnostic methods, which was informative. Such as Ossiander & Wedemeyer’s method, 1973, where they proposed that to observe pathogen prevalence in 50 samples you would need to look at all 50 at 2% prevalence, at 5% you would need to sample 35 animals, and 10% you would need to sample 20. In 1000 samples, you would need 140 samples at 2%, 55 samples at 5%, 27 samples at 55%. However, the criticism to this method is that as you increase population size you run the risk of false negatives.  Rather, as you increase size, you may not pick up the presence of a pathogen, however that doesn’t mean it’s not present.

Many other approaches beyond histology (e.g. TEM, ELISA, PCR, qPCR, in situ hybridization) were also discussed and critiqued for their application and limitations.

Finally, we rounded out the day with fine talks by Allison, Amanda, (myself), and Morgan summarizing the work we are doing when we’re not participating in EIMD.


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