What is it?
Infection with Marteilia refringens, also known as Aber disease, digestive gland disease or marteiliosis, is a protozoan parasite (phylum Cercozoa, order Paramyxida)
that affects the digestive system of multiple bivalve species, including oysters, mussels, cockles and clams.
Where and when might it occur?
M. refringens infections result in high cumulative mortality (50 to 90 percent), associated with sporulation of the parasite in the epithelial cells of the digestive tubules. Highest cumulative mortalities usually occur during summer and autumn. Earlier stages of sporulation occur in epithelia of the digestive ducts and possibly the gills.
Several intermediate hosts or a free-living stage are thought to be required during the lifecycle of M. refringens. The copepod Paracartia grani is one intermediate host and may be involved in transmission of M. refringens between bivalves. M. refringens can exist in a carrier state in some oysters, which can be potential reservoirs of infection.
Factors triggering a pathogenic host response are not clearly established, but may include environmental stresses and differences in susceptibility to disease between stock.
The temperature threshold for parasite sporulation and transmission is 17˚C; however, this is thought to vary with other environmental factors.
The disease is often identifiable through high mortality, reduced growth rate and gaping.
Gross pathological signs are:
- poor condition and emaciation
- pale digestive gland
- inhibited gonad development
Microscopic pathological signs are:
- tissue necrosis
Source: Australian Government, Department of Agriculture, Fisheries and Forestry