Why do parasites seem to attack the gills first?

[HumbleFish]

I always thought it was because no mucous layer existed inside the gills, or what was there was much thinner than on the skin. However, I can find no evidence to back this up. In fact, what I have found (thanks to Paul B) suggests the composition of the mucous layer inside the gills is equivalent to what's on the skin. See my next post for more details.

So why does it seem like ich, velvet, brook usually starts off inside the gills? What makes the gills the path of least resistance? We see people practice ich management, and claim they never see evidence of trophonts on the skin/fins for months or even years. So we always just assumed the trophonts were coming & going from inside the fish's gills, out of sight. What exactly makes the gills so much more vulnerable?

 

[HumbleFish]

Originally posted by Paul B:

Actually, there is protection from parasites in the gills:
Quote:

3. The Cutaneous Mucus Layer
The mucosal surfaces of fishes (gill, skin, and gastrointestinal tract) form a thin physical barrier between the external environment and the internal milieu, and they are important sites of microbial exposure. Host defence mechanisms and their epithelia (with living cells) are covered by a protective mucus overlay [65, 78]. Cutaneous mucus is considered the first line of defence against infection through skin epidermis [16, 65].

The fish skin mucus acts as a natural, physical, biochemical, dynamic, and semipermeable barrier that enables the exchange of nutrients, water, gases, odorants, hormones, and gametes. Concomitantly, mucus plays a critical role in the defence mechanism of the fishes by also acting as a biological barrier [79–81]. Skin mucus has evolved to have robust mechanisms that can trap and immobilize pathogens before they can contact epithelial surfaces, because it is impermeant to most bacteria and many pathogens [82]. This occurs because in this mucus layer, particles, bacteria, or viruses are entrapped and removed from the mucosa by the water current [83]. Furthermore, mucus in most fishes is continuously secreted and replaced, which prevents the stable colonization of potential infectious microorganisms as well as invasion of metazoan parasites

Reference:
ISRN Immunology
Volume 2012 (2012), Article ID 853470, 29 pages
http://dx.doi.org/10.5402/2012/853470
Review Article
An Overview of the Immunological Defenses in Fish Skin
María Ángeles Esteban
Fish Innate Immune System Group, Department of Cell Biology and Histology, Faculty of Biology, University of Murcia, Regional Campus of International Excellence “Campus Mare Nostrum”, 30100 Murcia, Spain

Received 26 August 2012; Accepted 12 September 2012

Academic Editors: A. Bensussan, E. Flaño, J. D. Hayball, and P. Puccetti

Copyright © 2012 María Ángeles Esteban. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

 

[snorvich]

If you want an educated guess:

The infectious stage is water born and water passes through the gills. As such, the contact probability is greater.

 

[ThRoewer]

If you want an educated guess:

The infectious stage is water born and water passes through the gills. As such, the contact probability is greater.

^^ This.
Plus the gills are also kind of a filter so the likelihood to catch a parasite there is quite high. Also, despite the protective mucus layer, in order to function as a gas exchanger the skin on the gills is very thin and vulnerable.

 

[HumbleFish]

Thanks guys. What you both said makes a lot of sense.

 

[snorvich]

^^ This.
Plus the gills are also kind of a filter so the likelihood to catch a parasite there is quite high. Also, despite the protective mucus layer, in order to function as a gas exchanger the skin on the gills is very thin and vulnerable.

I agree.