What is it?
For the purpose of this guide, infection with Perkinsus marinus is considered to be infection with P. marinus, the causative agent of dermo disease in oysters.
Infection with P. marinus is usually fatal depending on host and environmental conditions. Persistent
infection with lifelong carriers can occur.
Where and when might it occur?
The disease is found on the East coast of North America from Maine, US to Campeche, Mexico. It has also recently been introduced to the Pacific coast of Mexico.
Prevalence and intensity of P. marinus infections are greatest at salinities greater than 12 practical salinity units (psu). Transmission can occur between 9 and 12 psu, but infections remain low in intensity.
Perkinsus marinus can persist for long periods in hosts at salinities less than 9 psu, but replication is low and no host mortality occurs.
Temperature controls the annual cycle of P. marinus, with maximum prevalence and intensity lagging one to two months behind maximum summer water temperatures and minimum prevalence and intensity lagging one to two months behind minimum winter temperatures.
Thus, P. marinus infections are most intense in autumn and least intense in early spring.
Transmission is direct from host to host. All life stages are infective.
Perkinsus marinus causes a chronic wasting disease. Clinical signs may be dead or gaping bivalves with thin, watery tissue but these clinical signs are not specific to infection with P. marinus.
Infected hosts may be slow to close their valves when disturbed but these behavioural changes are not specific to infection with P. marinus.
Infection is often lethal for C. virginica. Death usually occurs one or two years after infection, during or shortly after the warmest annual water temperatures. Infection intensity in clams is very low and there is no indication of mortality.
Control and treatment
No vaccine is currently available.
Farming in areas where salinity is less than 12 psu and use of fast-growing, disease-tolerant strains has shown some benefit to preventing the disease.