Aphanomyces invadans Life Cycle

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.