How best to use povidone iodine (Betadine) topically to treat fish ulcers?

Thank you to everyone who’ve contributed to this discussion. I thought I’ll summarise my thoughts.

Some of our colleagues do not use topical disinfectants, some dilute it prior to use, and some use it neat. So, the questions were:
1. Whether or not to use topical disinfectants?
2. If yes, is dilution necessary?
3. If yes, will diluting betadine negate its antiseptic properties?

In my notes (below), it states that excess free iodine is indeed irritant to skin and mucus membranes (but dose rate unrecorded). To err on the side of caution, some veterinarians choose to not use betadine at all. Additionally, there is argument that immunoprotective qualities of mucus would be destroyed by application of chemicals.


Some of our colleagues choose to apply betadine topically for the treatment of ulcers and post-surgical wound-closure. This likely reflects extrapolation from our veterinary training in the treatment of terrestrial animals. It seems logical that we need to prevent bacterial colonisation while allowing the skin to heal. Is this justified?

I could not find literature specifically on the toxic effects of topical use on fish. When used on fish eggs, it is applied at 0.002-0.01% for 1-15 minutes. For spawning fish, they are immersed in solutions at a concentration of 0.006-0.007% for up to 30 minutes (see link to article). This significantly lower dose may be because ionophores are particularly toxic to gills. I was sent an article stating the efficacy, and at the same time, no signs of toxicity were shown in opthalmic surgeries in canines when used at 0.2%. Can fish be likened to eyeballs with fins and scales? Perhaps the fish is more different to the eye of a dog.

One of our colleagues suggests that the antiseptic properties are lost once betadine is diluted by more than 50%. Consequently they routinely use it neat, and in their experience, have found no evidence of delayed healing. Could it be that much of the betadine washes off once fish are replaced in the water as evidenced by the brown tint disappearing within minutes?

But what if we use betadine, combined with a fish "bandage" (see link). Here, betadine is used at 1:10 dilution, and the brown tint lasts for at least 2 hours after application. This may be evidence that the contact-time is extended. Should the betadine continue to be used diluted in this situation?

I suspect the debate continues because:
– The solutions may come different concentrations: as 0.5% liquid spray, 1% betadine solution and even up to 7% (tincture of iodine).
– The iodophore may come in different forms (Wescodyne reported more toxic than Betadine) – see link.
– Toxicity depends on pH (more toxic at pH <6, and becomes ineffective at pH >8).

Though empirical data is useful, I believe we need controlled studies to settle the debate.

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Yours sincerely,

Dr Richmond Loh
DipProjMgt, BSc, BVMS, MPhil (Pathology), MANZCVS (Aquatics & Pathobiology), CertAqV, CMAVA, NATA Signatory.
THE FISH VET, Perth, Western Australia, AUSTRALIA.
Mobile Aquatic Veterinary Medical & Diagnostic Services.
Ph: +61 (0)421 822 383
Skype: thefishvet

President WAVMA 2014

Adjunct Lecturer Murdoch University | Secretary Aquatic Animal Health Chapter – ANZCVS.

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