Case Studies: Murder, She Wrote…in the Ocean

With every classic whodunit, there is the crime, the perpetrator, and the victim. While Angela Landsbury always seemed to solve the case, the reality is gathering the evidence, examining the appropriate areas, and fitting all the pieces together is challenging; not to mention that it rarely happens in an hour segment.

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Agatha Christie and other murder-mystery genre novelists have also made the pieces come together seamlessly, however researching disease is normally less clearcut. Today, Carolyn provided 3 case studies which are exemplary marine disease cases.

Case Study #1: Renibacterium salmoninarum is the causative agent of bacterial kidney disease (BKD) in salmonids. Chinook salmon are particularly susceptible. In the 1960’s Chinook salmon were moved to Lake Michigan! The population grew too large, resulting in reduced prey population, and they kept restocking the lake. So the population kept increasing, but eventually fell and continued to be low (regardless of continuing to restock). Eventually the Lake Michigan population did start to re-establish. So what was killing the salmon? Answer: Renibacterium salmoninarum. When tested against the initial stock from WA, high mortality resulted, while the stock in MI was less impacted. Essentially, during this 20+ year period the introduction of the salmon to Lake Michigan was a large selection experiment. Where the natural population is more susceptible to infection, but resistance traits were selected for within the natural genetic distribution, resulting in a more R. salmoninarum-resistant salmon genotypes.

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Case Study #2: Candidatus Xenohaliotis californiensis (bacterium) in Black Abalone (BA) cause Withering Syndrome (WS). The syndrome was magnified by warm water events in 1980’s
BA were densely populated, as sea otters were hunted to almost extinction at the beginning of the 20th century, and the pathogen had a densely susceptible host. During 1985-1990 99% of BA were wiped out. Except on San Nicolas Island (the most distant Channel Island), where 95-98% mortality occurred. Rickettsia (RLO) arrived in 1992 and the pop dropped from 25,000 to 200 in 2001. The population began to increase from 2001. Other locations, e.g. Carmel had naive or non-selected because there are lesions, but not active disease as on San Nicolas. Carolyn & Lisa performed a 490 day exposure trial (CRAZY!) with these two populations. Using a ISH probe they visualized WS RLO and a new (much larger) RLO. It took ~700 days til mortality set in, longer than previous studies. TEM shows phage-like hyperparasites. The pathogen was infected with the phage, causing reduced pathogenicity. Essentially, reduced metaplasia resulted, which decreased the parasite in the digestive gland, then a decrease in pedal catabolism, and ultimately an increase in length of time til mortality. So the phage reduces pathogenicity of WS-RLO.

Interestingly, green abalone was less susceptible to WS, while white abalone had almost 100% mortality. Another interesting trend occurred with temperature and susceptibility in pinto, pink, and red abalone…temperature determined whether the species was susceptible.

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Case Study #3: Selective Breeding of the Eastern Oyster reduces pathogenicity of MSX (Multinucleiated sphere X) or Delaware Bay Disease from Haplosporidium nelsoni. In 1959 MSX was found in Chesapeake Bay. H nelsoni infects cells of the digestive tubule as spores and causes mass mortality. Selection for MSX-resistant C. virginica has helped to reduce mass mortality. Then, DERMO- Perkinsus marinus - thought to be a fungus at first – related to dinoflagellates was introduced in 1992 and spread to warm conditions. This secondary pathogen became the main disease and caused further mass mortality, however climate conditions allow you to predict outbreaks. The question now is whether selection for dual-resistance is possible.

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Selection for these fitness traits were a common theme throughout the day and a clear loophole in natural populations to allow disease to reduce populations (when perhaps they’ve increased beyond sustainable numbers), but also a way for the populations to reseed and be less susceptible in the future. These same types of impacts may also be taking place under ocean acidification (OA) conditions. Carolyn presented data today showing that populations under high OA have lagged and often reduced sporulation. One more player to all of these marine health mysteries!

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