Costiosis, also known as blue slime disease, is caused by the protozoan parasite Ichthyobodo necator. The parasite infects fish through the skin and gills, attaching and feeding on epithelial cells. This causes skin irritation and hyperplasia, increased mucus production, and respiratory distress in the fish. Heavily infected fish appear lethargic with damaged fins and are often unable to swim normally. The parasite can be diagnosed by examining skin or gill biopsies microscopically. Treatment involves salt baths, formalin baths, or raising water temperature above 30 degrees Celsius to kill the parasites. Proper tank management can also help prevent outbreaks.
3. Introduction
– A protozoal disease of fishes of wide species
– Often frequently occurs in winter tanks for young trout
and where fish are grouped together.
– Lack of food during hibernation or end of growth period
of carp increases susceptibility to costiasis
Kingdom; Animalia - animals
Subkingdom; Protozoa
Class; Flagellata
Order ; Protomonadina
Family; Eumonadidae
Genus ; Costia
Genus ; Ichthyobodo
4. Etiology
– Caused by a skin flagellate called Icthyobodo necator ( previously
known as : Costia necatrix)
– Has bean shaped form (i.e. reniform) and is constant in shape
– Body measures: 10-12 micron in length and 6-8 micron in width
– Two locomotory flagella arises from the convex side of the body
– Outside fish the parasite become rounded off and dies in majority
of cases after 30-60 minutes.
– Parasite dies at 30 ֯ C
5. Mode of transmission &
pathogenesis
– Transmitted from infected fish to other
through water
– Parasite exists in two forms:
a. motile / free swimming having 2 or 4 flagella
b. Attached or non motile form attached to
epithelium
6. Motile form swim freely and while feeding on fish, it is curled
into pyriform shape and is attached to and penetrate
epithelium (Attached form)
Transition between forms is within a few minutes.
Epithelial sloughing and irritation occurs which may lead to
epithelial hyperplasia and increased mucus production
The parasites feed on the gill and skin cells and can quickly
kill fish by depriving oxygen .
Also may allow secondary infection of fungus and bacteria in
to the wounds.
7. Signs
– Fish with light infections may roll in the water and rub against
immersed object & sides of tank due to skin irritations.
– Tissue irritation also leads to epithelial hyperplasia and increased
mucus production giving fish a bluish cast (slime)
–Fish with heavy infestations are often listless, anorexic and debilitated
with respiratory distress
–Damage of fins may occur & fish become unable to maintain an upright
position or to swim to the water surface.
–High mortality (40-75%) is associated with heavy infections.
9. Diagnosis
Along with signs, symptoms and lesions, from histopathology of skin or gills
with parasite, the diagnosis can be made:
– Can be easily made from skin or gill biopsies
a. Free form exhibit characteristic flickering motion when it
turns its crescent-shaped body
b. Attached parasite more difficult to detect
– in heavy infestations can be located by adjusting the magnification on
the edge of the gill epithelium, where they form palisade
10.
11. Prevention and Treatment
– Infected young trout and carp are bathed for 20 minutes in 1% NaCl
and then placed in fresh tank with abundant food
– Formalin bath may be applied
– Simple increase in temperature to above 30 C also kills Costia
– Fish should never be placed too closely together
– In aquarium, longer bathe in trypaflavine ( 1gm/100 litres) kills the
parasites within 2 days
– PH fluctuation should be corrected since they damage the skin and
favor infection by Costia
12. References:
– Noga E., Eds (2014). - Fish Disease Diagnosis And Treatment, second edition
– Amlacher E., Eds (2009). – Textbook of fish diseases,
– J Savage - Transactions of the American Fisheries Society, 1935 - Taylor & Francis
– Study materials provided by prof. Dilip Kumar Jha
