Aphanomyces invadans Life Cycle

Study Notes

Life Cycle of Aphanomyces invadans

The primary zoospore transforms into the secondary zoospore, which is reniform with laterally biflagellate cells and can swim freely in the water.
The secondary zoospore remains motile for a period dependent on environmental conditions and the presence of the fish host or substratum.
The zoospore encysts and germinates to produce new hyphae, although further tertiary generations of zoospores may be released from cysts (polyplanetism).

Disease Information: Host Factors

EUS causes disease and mortality in farmed and wild fish, worldwide, with around 76 species of fish confirmed to be naturally affected by histological diagnosis.
Susceptible host species include yellowfish sea bream, straightfin barb, striped snakehead, and many others.
Some fish, such as common carp, Nile tilapia, and milkfish, are considered to be resistant to EUS.
Juvenile and young adults are usually susceptible stages of the host.
EUS can be readily detected in diseased fish specimens collected from EUS-infected areas using histological techniques.

Disease Information: Disease Pattern

EUS is transmitted horizontally, with Aphanomyces zoospores capable of attaching to the damaged skin of fish and germinating into hyphae.
The pathogen migrates towards the central nervous system, producing proteolytic enzymes that destroy muscle and other tissues.
Hyphae invade widely into the surrounding skin and deeply into underlying muscle tissues, resulting in extensive, bloody ulceration and destruction of tissues.
The prevalence of EUS in the wild and in aquaculture farms is high in endemic areas that share the same water way or system.

Disease Information: Disease Pattern (continued)

Geographical distribution: First reported in Oita Prefecture, Kyushu Island, Japan in 1971, and later reported in estuarine fish in eastern Australia in 1972.
EUS has been detected in many fish species and has a high prevalence in endemic areas.

Disease Information: Sampling

Scoop net, cast net, or seine net are the best choices for catching EUS-infected fish in natural waters or in fish ponds.
Fish specimens should be transported to the laboratory live or in ice-cooled boxes for further diagnosis.
Fixed specimens are transferred to double-layer plastic bags with formalin-moistened tissue paper for semi-dry conditions.

Disease Information: Diagnostic Methods

Diagnosis of EUS is based on clinical signs and confirmed by histopathology.
Diagnosis of EUS in clinically affected fish may be achieved by histopathology or by oomycete isolation.
Positive diagnosis of EUS is made by demonstrating the presence of mycotic granulomas in histological sections or isolation of A. invadans from internal tissues.

Disease Information: Agent Factors

EHNV has large (150–180 nm), icosahedral virions, a double-stranded DNA genome, and replicates in both the nucleus and cytoplasm with cytoplasmic assembly.
EHNV is extremely resistant to drying and can survive for months in water, persisting in frozen fish tissues for more than 2 years and frozen fish carcasses for at least a year.
EHNV is susceptible to 70% ethanol, 200 mg/liter sodium hypochlorite, or heating to 60°C for 15 minutes.

Description

Learn about the life cycle of Aphanomyces invadans, a parasitic fungus, including the transformation of zoospores and their motility, encystment, and germination. Understand the environmental and host-dependent factors influencing its life cycle.

Aphanomyces invadans Life Cycle

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