This document summarizes the classification, characteristics, life cycle, and development of myxozoan parasites. Myxozoans are metazoan parasites that infect aquatic hosts and cause serious disease outbreaks. They have a complex life cycle involving both fish and annelid hosts. The document describes the multicellular spore structure, actinospore and myxospore stages, and the proliferative, gametogenic, and sporogenic phases within hosts. It also compares the life cycles of myxosporeans and the malacosporean Tetracapsuloides bryosalmonae.

1. Presented by
Avijit Pramanik
AAH-MA-09-02
M.F.Sc, CIFE, Mumbai

2. INTRODUCTION
ØMyxozoan are highly specialized metazoan parasite of aquatic hosts with a very
wide host range.
ØBoth marine water and freshwater species affected.
ØCause serious disease outbreaks in farmed fish species and wild stock.
ØCause severe economic loss.
ØThey are spore forming , endoparasite.
ØMulticellular spores with polar capsule.

3. CLASSIFICATION:
Phylum:
Myxozoa
Class:
Myxosporea
Order:
Bivalvulida
e.g.myxobolus,
ceratomyxa,
parvicapsula &
sphaerospora
Order:
Multivalvulida
e.g kudoa
Class:
Malacosporea
e.g.
T. bryosalmonae

5. Thalohanellus Myxobolus
HenneguyaCeratomyxa Ellipsomyxa
Myxidium
Chloromyxum

6. Myxospore of Bivalvulida
Spore
1)Myxospore:
ØSize in fish host is 10-20 µm.
Ø 2-12 shell valves
Ø1-13 polar capsules & sporoplasm
ØSurrounded by mucus envelope
ØSporoplasm(1 binucleate/ 2 uninucleate).
ØSpores may be rough/ smooth.

7. PCL: polar capsule length, PCW: polar capsule width,
(C & E, top view, D: side view)
Myxospore of multivalvulida

8. CHARACTERISTICS
vMulticellular spores with extrudable polar filaments coiled tightly within polar
capsule.
vInfection by amoeboid sporoplasm that are released from multicellular
spores.
vThe polar filament is twisted longitudinally.
vInfects tissues, organs and nervous system.
vEither histozoic or coelozoic.
vcause extensive myoliquefaction making fish unmarketable.

9. ØActinospore stage released from oligochaete host
is always triradiate with either 3 or 6 spore valve.
ØMade of a single style -150 µm long and three
processes or “ tails” -200 µm long.
ØA sporoplasm packet at the end - germ cells
surrounded by cellular envelope.
Ø3 polar capsules with polar filament of 170 -
180 µm long.
Triactinomyxon stage:

10. v The spore valve act as floats in maintaining actinospore position in water column.
Myxospore stage Actinospore stage

11. 1.Guyenotia
2.Aurantiactinomyxon 'A'
3.Aurantiactinomyxon 'B1'
4.Aurantiactinomyxon 'B2
5.Neoactinomyxum 'A1'
6.Neoactinomyxum 'A2'
7.Neoactinomyxum 'B1
8`Neoactinomyxum 'B2

12. Fish host
Annelid host
Life cycle of myxosporea

13. v Undergoes three phases:
INTRA - OLIGOCHAETE DEVELOPMENT
Schizogony or proliferative stage
Gametogony
Sporogony

14. Schizogony or proliferative stage
Binucleate/ 2 uninucleate sporoplasm
Nuclear division
Multinucleate cell
Plasmotomy
Uninucleate cells & invade intercellular spaces
Schizogony
More multinucleate cells
& subsequently uninucleate cells
Plasmogamy
Binucleate stage

15. Gametogony
Binucleate stage
Karyogamy
4 nuclei stage
Plasmotomy
4 uninucleate cells
(2 - somatic cells & 2 - α & β cells)
mitosis twice - 8 somatic cells mitosis thrice- diploid gametocytes of 8 α & 8 β cells
Meiotic division
16 haploid gametocytes & 16 polar bodies
•Each α cell fuses with each β cell to form a pansporocyst - 8 zygotes surrounded by
8 somatic cells.

16. Around 60–90 days post infection, sexual cell stages of the parasite undergo
sporogenesis
2 mitotic division of the zygotes to a 4 cell stage
3 of these cells surround the 4th & divide by mitosis resulting in 6 cells- 3 will
become capsulogenic & 3 will become valvogenic cells.
4th cell become sporoplasm- divide by internal cleavage
Generative cell undergoes mitosis to produce infective germ cellls.
At maturity, each pansporocyst contains 8 actinospores.
Sporogony

17. Actinospore stage is infective to fish host.
Polar filament is extruded to anchor it to the host
Within five minutes, sporoplasm enters the fish epidermis, and within a
few hours, splits into individual cells
Cell wall of the sporoplasm disintegrates
Endogenous budding-enveloping primary cell & an internal secondary cell
INTRA PISCINE DEVELOPMENT

18. Extrasporogonic proliferative stages
Secondary cell
Mitotic divisions
Produce numerous secondary cells
Endogenous budding
Tertiary cell within secondary cell
Rupture of primary cells
Release of secondary cells into host cytoplasm or into the blood system.

21. Myxospore are released into environment via a number of
different routes:
1) Rupture of cysts on body surface.
2) Excretory products.
3) After death decay materials.
Myxospore of M. cerebralis retain infectivity after 4 months in mud
and freezing at -20℃ for 2 months.

22. Polar filament discharge

23. ØThe malacosporean,T. bryosalmonae causative agent of PKD of salmonids does
not follow the patterns observed in the myxospore- actinospore life cycle.
ØThe malacospore entry and developed inside bryozoan host .
ØIn T. bryosalmonae the wall of the sac in which spores develop consist of a
single layer of flattened or mural cells.
ØProliferation leads to formation of sac wall and produce sporogonic cell within
the sac.
ØThese sporogonic cells take two forms:
Malacospore Development
Sporoplasmogenic
cells (denser)
Stellate cells

24. 2 uninucleate sporoplasm
(contain secondary cell derived by endogenous budding)
Sporoplasms are enveloped by stellate cells which differentates to
form atleast 4 valve cells and 4 capsulogenic cells.
Synchronous maturation of the spores occures within the sac, which
contains up to several hundred spores, valve cells remain soft.
ØThe exit pore of polar filament is not covered by cap or valve walls.
ØMalacospore - short lived, surviving for less than 12h
Sporoplasmogenic cells (denser)
Mitosis

26. References
ØRoberts, R.J., 2012. Fish pathology. John Wiley & Sons.
ØWoo P.T.K , FISH DISEASES AND DISORDERS vol- 1
ØGoogle search
ØRamudu, K.R. and Dash, G., 2016. Prevalence,
morphology and scanning electron microscopy study of
myxozoan parasites. Journal of parasitic diseases, 40(2),
pp.339-347.
ØClass notes
Ø You Tube