Why do some fish die from white spot disease when others don’t?

I get this question a lot. This paper attempts to explains that the genetics of the fish play a large role in resistance to the white spot parasite.

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Journal of Fish Diseases
Volume 35, Number 12 (December 2012)
	Susceptibility of three strains of blue catfish, Ictalurus furcatus(Valenciennes), to Ichthyophthirius multifiliis
	Authors:	D-H Xu, P H Klesius, B G Bosworth, N Chatakondi
	Author Affiliations:	no affiliations available
	Source:	Journal of Fish Diseases, Volume 35, Number 12 (December 2012)
	Page Numbers:	887 – 895
	Available Full Text:	Full Text: Subscription Required to view full text Format: PDF Size: unknown Location: Publisher’s Site Authentication: EBSCOhost EJS
	Abstract:	This study compared the susceptibility of three blue catfish strains (D&B,USDA101 and USDA102) to the parasite Ichthyophthirius multifiliis(Ich). In Trial I, a cohabitation study (all strains stocked communally) was conducted and fish were exposed to theronts at 0, 200, 1000, 5000 or 25 000 theronts fish-1, respectively. All fish died when exposed to theronts at 5000 or 25 000 theronts fish-1. When exposed to 1000 theronts fish-1, USDA102 strain of blue catfish showed significantly lower mortality (78.5%) compared to USDA101 and D&B strains (92.7% and 100%). In Trial II, the same three strains of blue fish were evaluated for their susceptibility to Ich with strains challenged in separate tanks by adding Ich theronts at 0, 200 and 1000 theronts fish-1, respectively. All D&Band USDA101 blue catfish died; however, 42.3% of USDA102 strain survived the infection when exposed to 1000 theronts per fish. The results indicate that there are differences among strains of blue catfish for susceptibility to Ich, and these differences will be useful in the development of improved catfish germplasm for commercial aquaculture.
	Citation:	D-H Xu, P H Klesius, B G Bosworth, N Chatakondi . Susceptibility of three strains of blue catfish, Ictalurus furcatus(Valenciennes), to Ichthyophthirius multifiliis. Journal of Fish Diseases, Volume 35, Number 12 (December 2012), pp. 887-895, ;
	URL:	http://ejournals.ebsco.com/direct.asp?ArticleID=4913A625B97643BE36D9