Why EHP Can Escape from Detection
By Dr. Wiphada Mitbumrung, Ph.D. Applied Marine Biosciences — Wed Jul 15 2026
Enterocytozoon hepatopenaei, or EHP, is one of the most difficult shrimp pathogens to manage because it does not usually create a dramatic early sign in the pond. Unlike diseases that cause sudden mortality, EHP mainly affects the hepatopancreas, which is the key organ for digestion and nutrient absorption. Infected shrimp may still come to the feeding tray, but digestion efficiency becomes poor. The scary point is that farmers may continue feeding almost the same amount of feed, but the biomass gain is much lower. In other words, the farm pays the same feed cost but receives less shrimp growth.
The main symptom of EHP is growth retardation. Shrimp in the same pond may show uneven size, slow daily growth, poor feed conversion, pale hepatopancreas, weak gut content, and poor overall performance. EHP does not usually kill shrimp directly, but it silently reduces profitability by damaging the organ responsible for nutrient absorption. This is why EHP is often noticed late, when the farmer realizes that the crop is not reaching the expected size despite normal feeding and management. EHP is widely recognized as a major cause of growth retardation and economic loss in shrimp farming.
EHP is transmitted mainly through spores. These spores can be released from infected shrimp through feces, dead shrimp, contaminated pond bottom, water, equipment, live feeds, and infected or contaminated broodstock and post-larvae. In the farm environment, horizontal transmission is very important because healthy shrimp can ingest EHP spores from contaminated organic matter or infected shrimp tissue. Once the spores enter the digestive tract, EHP infects the hepatopancreatic epithelial cells and reduces digestive function. Experimental studies have confirmed that EHP can be transmitted orally and through cohabitation with infected shrimp.
In hatcheries, EHP risk can also start from broodstock. The important point is that this does not always mean true ovarian vertical transmission. EHP is not mainly understood as a classic virus-like infection that must pass inside the egg through the ovary. A more practical risk is non-ovarian vertical transmission, where eggs, nauplii, spawning water, feces, tank surfaces, or hatchery equipment become contaminated with EHP spores from infected broodstock. Broodstock can acquire EHP earlier from contaminated ponds, infected feeds such as fresh polychaetes or other live/fresh maturation feeds, contaminated water, or previous exposure to infected shrimp material.
Hatcheries usually check EHP in broodstock by PCR-based testing, commonly using feces, hepatopancreas-related samples, or other non-lethal samples when the broodstock must be kept alive for spawning. This is where EHP can escape detection. The most accurate tissue for EHP is related to the hepatopancreas, but hatcheries cannot normally kill valuable broodstock just to collect the whole target organ. Therefore, testing may depend on fecal samples, spawned material, water, or limited non-lethal sampling. If the infection level is low, the broodstock is shedding spores intermittently, or the collected sample does not contain enough pathogen DNA, the PCR result may become negative even though the broodstock still carries risk.
Another reason EHP can escape detection is uneven distribution of infection. EHP is an intracellular microsporidian parasite, and the pathogen load may not be uniform in every tissue, every fecal string, or every sampling time. A single negative PCR result only means that EHP DNA was not detected in that particular sample at that particular time. It does not always guarantee that the broodstock, tank, or PL batch is completely free from EHP. This is why sensitive molecular methods, repeated testing, proper sample selection, and biosecurity are important for hatchery control. PCR and qPCR methods are widely used for EHP diagnosis, but sampling quality and pathogen load strongly influence detection reliability.
Therefore, EHP can escape from detection not because PCR is useless, but because the biology of EHP makes sampling difficult. The pathogen may be present at low levels, shed irregularly, hide in the hepatopancreas, or contaminate the hatchery environment without being captured in one sample. For hatcheries, EHP control should combine broodstock screening, repeated fecal or environmental testing, clean maturation feed, disinfection of spawning and larval tanks, PL screening, and strict separation between suspected and clean groups.
