Amoebic gill disease

Amoebic Gill Disease (AGD) is a significant parasitic infection affecting marine and freshwater fish, particularly salmonids. It is caused by the amoeba Neoparamoeba perurans, which infects the gills of fish, leading to severe respiratory distress and, if untreated, can result in high mortality rates. This disease has emerged as a critical concern in aquaculture, especially in regions with intensive salmon farming such as Tasmania, Australia, and parts of Chile.

Etiology

The causative agent of AGD, Neoparamoeba perurans, is a single-celled, free-living amoeba that transitions into a parasitic phase upon encountering a suitable host. The amoeba attaches to the gill tissues, causing hyperplasia, lamellar fusion, and necrosis. This attachment and subsequent tissue damage impair gas exchange and can lead to systemic hypoxia.

Symptoms and Diagnosis

Fish affected by AGD exhibit a range of symptoms including increased mucus production on the gills, labored breathing, reduced feeding, and lethargy. In severe cases, the gill tissue may appear pale and swollen. Diagnosis of AGD is typically confirmed through microscopic examination of gill swabs, where the characteristic amoebae can be identified. Molecular techniques, such as PCR, are also employed for definitive diagnosis.

Transmission

Transmission of Neoparamoeba perurans occurs through direct contact with infected individuals or through water containing the amoeba. The exact mechanisms facilitating the spread and persistence of N. perurans in marine environments are not fully understood, but factors such as water temperature, salinity, and host density are believed to influence disease prevalence.

Treatment and Control

Management of AGD involves a combination of strategies aimed at reducing amoeba load and improving fish health. Freshwater baths are a common treatment, exploiting the intolerance of N. perurans to low salinity. Other methods include the use of chemical treatments, such as hydrogen peroxide, and adjustments to farming practices, such as reducing stocking densities. Vaccination and selective breeding for resistance are areas of ongoing research.

Impact

AGD poses a significant threat to aquaculture, leading to economic losses through increased mortality, reduced growth rates, and higher costs associated with treatment and prevention. The disease also raises concerns regarding animal welfare and the sustainability of fish farming practices.

Prevention

Preventative measures focus on biosecurity protocols to minimize the introduction and spread of N. perurans. These include quarantine procedures for new stock, regular monitoring of water quality and fish health, and the implementation of effective treatment strategies at the first sign of disease.