Duncan Griffiths

Hexamita or Octomita as it was formerly known until 1960 is actually more common as an endo-parasite.
{Internal} as opposed to the more common ecto-parasites {external}of the usual types found on fish by hobbyists. Although Hexamita can be an ecto-parasite. The two most common species are H Salmonis or H Truttae and H Intestinalis. {H salmonis and H truttae being classed by some as the same sub-species}

Hexamita is pyriform in shape and has two nuclei at the anterior end, it was renamed Hexamita because it possesses 6 anterior flagella, it also has 2 posterior flagella hence the former name of Octomita, the flagella are usually about 1.5 times the body length. With the posterior flagella just trailing where as the anterior flagella are very motile.

Most Hexamita in koi are found internally in the alimentary track as commensal symbionts, but
Hexamita can be found in the heart, liver, gall bladder, blood and the peritoneal cavity. Hexamita reproduce in/on a healthy fish at a normal rate by longitudinal binary fission and although are pyriform
in shape measuring about 6 micrometers across to 12 micrometers long,they can change to a more spherical shape just prior to spitting.

There are conditions however, when the host fish is either weak, ill or stressed when Hexamita can take advantage of this situation and reproduce really fast as the opportunity is presented, with a type of Schizogony (multiple divisions of the nucleus allowing multiple splitting's) resulting in a population explosion.

Schizogony is actually achieved by Hexamita invading the epithelial cells of the intestine where the nucleus combines to form a single nucleus this single nucleus can then divide half a dozen times within one single organism. This division is really quite rapid when you consider the normal binary fission
method of reproduction can take place in less than 24 hours at which time many newly formed Merozoites are ready to re-infect new cells and the whole process starts over.

Transmission is by mouth although there is evidence Hexamita can infect by the anal route. Scientific evidence has also confirmed the presence of Hexamita internally in frogs and other amphibians. The encysted stage can survive for weeks away from the host lying in faeces, so we can deduce that transmission and infection can be very varied indeed.

Hexamita forms into a cyst to infect the host internally by the oral route, as it cannot get past the part
of the intestine where typical stomach acids are made for digestion in fish that do develop stomach acid {typical ph 1 to 2.5}, this would destroy them so they are protected from this by the formation of a

Hexamita make their way to the Lumen in the upper intestine or ceca and the free-swimming stage will live in the stomach fluids and as previously mentioned will reproduce normally and live along side the host, provided thefish remains healthy.

Flashing may occur, the fish may look emaciated with sunken abdomen, you may be excused for thinking the fish was suffering from nutritional deficiency. Other signs are anaemia (light coloured gills lamellae), and a yellow cast to the faeces. In severe cases fish suffering from hexamitiasis will also go listless and point into a corner of the pond/tank and not feed at all, the fish will go literally off colour, {skin will darken}
If I fish dies and there are no outward signs other than the descriptions given above squash tissue samples and blood samples should be taken ASAP and viewed under a microscope, in a suspected live fish a fresh sample of faeces should be prepared on a slide and examined under a microscope, this is
not possible in advanced stages of the disease as the fish will have long stopped eating and the stomach will be empty.

Hexamita is easily seen under the scope at any magnification between x100 and x400, provided a thin sample is presented under the objective, easy enough with a blood sample, not so easy with tissue or faeces samples, in this case a squashed tissue or faeces sample should be spread over the slide and another slide pressed down onto the sample and squeezed together between the thumb and for-finger, after separating the two slides this process should be repeated with another clean slide and if necessary repeated until a very thin sample is obtained, then finish with a cover slip.
A good light source is a positive advantage turning the light well up and shutting down the iris for good contrast, focusing the condenser so the light is concentrated onto the parasite for good viewing.


Treatment is very difficult with advanced stages of the disease The most effective treatment is with Metronidazole Flagyl tm, in the form of 250 mg tablets available from some vets in the UK and the US, it's used for intestinal bacterial infections also intestinal protozoa. It is however an antibiotic, so I strongly recommend you do not use Metronidazole in the pond/fish tank water, as this will result in the immediate and total demise of your biological filter, its an indiscriminate antibiotic.

As a bath, one 250mg tablet crushed and dissolved into a little warm water then poured into 50 litres of water then submerse the fish for three days with a raise in temperature and air stones, either water changes or filtering water over activated carbon to rid the system of the medication

Metronidazole will also destroy plants.}

Best by far, if the disease is not to advanced and the fish is still feeding, is Metronidazole as a food additive at 1% by weight = 10 mg per 1 gm or one 250mg tablet to 25 gm of pellets of food daily for 5 days mixed with warm water and agar or knox gelatine, divide into daily portions and freeze and thaw daily as needed.