Salt: Friend of the parasite?


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Think you know all about salt? Nathan Hill looks at startling new research that may change your mind.

Salt may not be the cure-all for certain illnesses. After decades touted as the comprehensive remedy, new findings suggests that in some cases sodium chloride (NaCl) actually helps particular parasites.

Researchers, writing in the journal Aquaculture, have tested this treatment not on species unfamiliar to the trade, such as trout, but focussing closer to home on the guppy (Poecilia reticulata) and two known species of Gyrodactylus fluke (G. bullatarudis and G. turnbulli).

This host and parasite combination can be a nuisance. Guppies are natural hosts for both species and in the crowded, stressful conditions in which they are transported and kept the parasite often overcomes the fish’s weakened immune system.

To replicate real life conditions, the researchers selected a variety of known used dosage rates — from long-term low level baths within the regions of 1-7grams per litre to shorter-term dips of higher concentrations as far as 33g/l. They used a range of online forums to determine ‘folk dosage’ of salt, basing studies against actual parameters used by aquarists.

It should come as no surprise to find a direct link between higher dose rates and increased mortality of the parasites, but one result is certain to ruffle feathers. This is the discovery that at a long-term bath at the lower dose rate of 3g/l – a level often touted as good for fluke control – the Gyrodactylus grew faster than on those control fish simply in dechlorinated water. The salt actually helped the parasites establish themselves!

At a higher concentration of 7g/l, a positive effect was seen with both species of fluke hindered. These levels are acceptable to guppies with their higher tolerance of salt, but would be difficult to apply to more sensitive species.

Best results were attained against both species of fluke from a short- term dip. Subjected to a 25g/l bath for 15 minutes (adults) and five minutes (juveniles), all G. turnbulli were eradicated, with 73% of G. bullatarudis going the same way.

Evidently, dipping is more effective in controlling these flukes, although it was noted that, post-treatment, elevated levels of mortality were observed in the juvenile fish.

Speculation regarding why the long-term dose rate increased parasite growth involves the nature of the pathogen itself. The fact that it inhabits the mucous layer of the fish could be to its advantage.

Gyrodactylus flukes feed on mucous, but this layer also protects them from factors in the external environment. Initially, on adding salt to water, fish will increase their mucous production, providing both cover and nutrition to the very pathogen we are trying to control.

Eventually, as mucous cells are depleted, this would both starve and expose the parasites. However, if we need to expose fish to medications that cause them to annihilate their own immune responses, then something has gone very wrong.

This is not to debunk salt as a medication, but it indicates that between pathogens there are marked differences in using salt as a disease control. For example, salt is more effective at lower, ongoing doses in controlling whitespot when the problem is not confined to parasites hiding within the mucous of the fish, but rather free swimming as theronts searching for a host.

When whitespot infection is both below the mucous and embedded in the skin, it’s unlikely that short-term baths will have any effect against this kind of pathogenic behaviour.

Prevention is always preferable in trying to cure established illness and medicines should be used only to treat specific pathogens.

However, guppy keepers should think twice before dosing their aquarium at 3g/l of salt, ‘just in case’ they get Gyrodactylus. In trying to do the right thing they may be promoting the very disease they want to control.

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