Six specimens of Melanotaenia oktediensis were examined by our resident
Fishician, Dr. Stephen Pyecroft. All six specimens had what I call
"Blackhead Disease" in varying degrees. This disease presents itself as a
black darkening of one side of the head only. Two of specimens also had
small skin eruptions (pimples) on one side of the body and one also showed
the darkening skin colouration along one side of the posterior portion of
its body. This was the first specimen that I've seen with body
discolouration. In my experience I've only ever seen just the head
discolouration.
The most severely affected fish would swim with their head up and tail down
and showed an increased respiratory rate. This disease (blackhead) seems to
be fairly common among rainbowfishes as I have seen it often and many other
hobbyists have also spoken to me about this disease. It also seems to be
particularly prevalent among Goyder River trifasciata.
Stephen found that most of the fish had enlarged kidneys, which had a
granulated pale colour and protruded beyond their normal position.
Granulomas were also present in the spleens and around abdominal organs.
Acid fast (Ziel-Neilson) stains were preformed on impression smears from
most of the affected organs and the presence of acid fast bacteria was
confirmed.
The Diagnose: Disseminated granulomous inflammation - nephritis, hepatitis
and peritonitis.
Stephen commented "As we have discussed before, the dark areas on the skin
are most likely due to a malfunctioning in either the pigment cells or the
nerves that control the pigment cells in that area of the skin. The findings
of a generalised infection with Mycobacteria sp. would be suggestive that
the localisation of the dark pigmentation is due to the formation of local
abscesses which are then causing the expression of the major clinical sign.
Most of the cases of "blackhead syndrome" in rainbow fish that I have
investigated have had a primary infection with Mycobacteria spp. There may
be other primary causes of this distinct clinical sign but in these fish it
was piscine TB.
Adrian.
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Adrian R. Tappin
"Home of the Rainbowfish"
http://www.ecn.net.au/~atappin/home.htm
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