Enterocytozoon hepatopenaei (EHP) is a microsporidian parasite and a significant issue in the global shrimp aquaculture industry. As an intracellular parasite related to fungi, it lives and reproduces inside the cells of its host. EHP targets the hepatopancreas, a digestive organ in shrimp. First identified in Thailand in 2004, the parasite has since spread to major shrimp-farming regions across Asia.
EHP does not typically cause large-scale death, but its presence leads to hepatopancreatic microsporidiosis (HPM), a disease that undermines shrimp health and development. These subtle but damaging effects pose a considerable economic threat, as farmers may not realize their ponds are infected until growth issues are apparent. The parasite’s resilient spores can be transmitted through contaminated water, feed, and infected shrimp.
The Function of EHP Protein in Parasitic Invasion
The primary mechanism of EHP infection relies on a specialized structure called a polar tube, constructed from unique proteins. This tube is coiled inside the dormant EHP spore. When a spore makes contact with a host cell in the shrimp’s hepatopancreas, environmental triggers cause the spore to rapidly discharge this tube. The process is explosive, functioning like a microscopic harpoon.
The polar tube’s eversion pierces the host shrimp cell’s membrane. Once the cell wall is breached, the parasite’s infectious material, known as the sporoplasm, is injected into the cell’s interior. This invasion depends on the structural proteins that form the polar tube, particularly spore wall proteins (SWPs) that assist in adhering to the host cell. This allows the parasite to bypass the host’s external defenses and begin replicating inside the cell.
Once inside, the parasite multiplies extensively, producing many new spores. These spores are then released from the ruptured host cell to infect neighboring cells within the hepatopancreas or be excreted into the pond environment to infect other shrimp.
Effects of EHP on Shrimp Aquaculture
The most significant consequence of EHP infection in shrimp aquaculture is severe growth retardation. Infected shrimp populations exhibit a wide variation in size, with many individuals failing to reach marketable weight. This leads to a poor feed conversion ratio (FCR), where the amount of feed used does not translate into expected growth, impacting profitability. Farmers often experience an average profit margin loss of 20-30%.
Besides stunting growth, EHP weakens the shrimp’s immune system. This makes them more susceptible to secondary bacterial and viral infections, which can lead to higher mortality rates. For example, EHP is a risk factor for acute hepatopancreatic necrosis disease (AHPND). While EHP itself may not cause high mortality, its presence can allow other, more lethal pathogens to take hold.
Visible signs of an EHP infection can include a pale or white hepatopancreas and reduced feeding activity. EHP is also associated with white feces syndrome, where white fecal strings are seen floating in the pond. These symptoms, combined with poor growth, result in significant economic losses for farmers due to lower yields and less valuable harvests.
Controlling EHP in Shrimp Farming
Detecting EHP is accomplished through molecular methods. Polymerase Chain Reaction (PCR) testing is the most reliable technique for screening shrimp post-larvae (PLs), broodstock, and live feeds for the parasite. This allows hatcheries and farms to identify and exclude infected animals and contaminated feed sources before they enter the production system.
With no direct treatment for EHP in infected shrimp, control strategies center on prevention and biosecurity. The most important measure is to source PLs that have been tested and certified as EHP-free. Thorough pond preparation between culture cycles is necessary. This includes disinfecting pond bottoms with substances like calcium oxide (quicklime), which raises the pH to levels that inactivate EHP spores.
Maintaining high water quality and managing the pond environment are also part of an effective control program. This involves using probiotics for a healthy microbial balance and removing waste from the pond bottom. Because EHP can be introduced through live or frozen feeds, using sterilized or pasteurized feed can prevent this transmission route. A combination of these biosecurity protocols provides the best defense against EHP.