How a Tiny Parasite is Disrupting India's Seafood Treasure
Imagine a shrimp farmer in West Bengal, patiently tending to his ponds for months, anticipating a healthy harvest of Pacific White shrimp. As the harvest day approaches, he notices something puzzling—his shrimp show extreme size variations, with many remaining disappointingly small despite months of feeding.
The mystery culprit behind this economic tragedy isn't a virus or bacterium, but a microscopic parasite called Enterocytozoon hepatopenaei (EHP), silently wreaking havoc in aquaculture ponds across Eastern India 1 2 .
This article explores the fascinating detective story of how scientists identified and characterized this emerging threat to India's vital shrimp industry, focusing on groundbreaking research from West Bengal that revealed one of the highest infection rates ever documented in the country 1 .
Extreme differences in shrimp size within the same pond
Shrimp remain small despite adequate feeding
Undersized shrimp command lower market prices
EHP (Ecytonucleospora hepatopenaei, previously known as Enterocytozoon hepatopenaei) is a microsporidian parasite—a type of fungal-related, spore-forming organism that can only survive and reproduce inside the cells of its host 5 8 .
The parasite primarily infects the hepatopancreas of shrimp, a crucial digestive organ that functions similarly to the human liver and pancreas combined 6 . This organ is responsible for producing digestive enzymes, absorbing nutrients, and storing energy reserves—all essential processes for growth and development 6 .
EHP was first reported in black tiger shrimp in Thailand in 2004 6 8 but has since spread to numerous shrimp-farming countries across Asia. The parasite made its way to India with the introduction of the non-native Pacific White shrimp (Litopenaeus vannamei) in 2009, which revolutionized Indian aquaculture but inadvertently introduced new disease challenges 4 .
Fungal-related, spore-forming intracellular parasite
Primary infection site - the shrimp's digestive organ
In 2019, scientists conducted a comprehensive investigation into the growing reports of stunted shrimp across West Bengal's coastal districts 1 . What they discovered was alarming—EHP had firmly established itself throughout the region with devastating prevalence.
| District | Samples Collected | Infection Rate |
|---|---|---|
| East Midnapur | 119 samples | 84.9% overall prevalence |
| North 24 Parganas | 50 samples | Part of overall 84.9% prevalence |
| South 24 Parganas | 50 samples | Part of overall 84.9% prevalence |
Shrimp farmers first noticed concerning signs in their ponds:
Farmers report unusual patterns in shrimp growth and behavior that signal potential EHP infection.
When these symptomatic shrimp arrived at laboratories, scientists employed multiple techniques to confirm EHP infection:
Researchers examined thin sections of hepatopancreas tissue under the microscope, revealing telltale signs of EHP infection—eosinophilic to basophilic inclusions within the epithelial cells of the hepatopancreatic tubules, along with moderate necrotic tubular detachment from the basal membrane 1 7 .
The most definitive diagnosis came from polymerase chain reaction (PCR) testing. Scientists extracted DNA from hepatopancreatic tissues and used specialized primers targeting two different EHP genes:
| Detection Method | Target Gene | Amplicon Size |
|---|---|---|
| Conventional PCR | ssu rRNA | 951 bp and 510 bp |
| Second-generation PCR | Spore wall protein (SWP) | Varies |
| Nested PCR | ssu rRNA and SWP | 514 bp and 148 bp |
Modern EHP research relies on sophisticated laboratory tools and reagents:
| Research Tool | Specific Example | Function in EHP Research |
|---|---|---|
| DNA Extraction Kits | DNeasy Blood & Tissue kits | Isolate high-quality DNA from shrimp tissues |
| PCR Primers | SWP_1F/SWP_1R and SWP_2F/SWP_2R | Amplify specific EHP gene sequences for detection |
| Histology Reagents | Davidson's AFA solution | Fix and preserve tissue for microscopic examination |
| Staining Agents | Hematoxylin and Eosin (H&E) | Visualize tissue structure and pathological changes |
| Molecular Biology Reagents | Taq DNA polymerase, dNTPs, MgCl2 | Enable DNA amplification through PCR |
Genetic sequencing of PCR products revealed 100% homology with EHP sequences from other affected countries including Vietnam, Thailand, China, and other parts of India 1 .
Histopathological analysis provides visual confirmation of EHP infection through characteristic cellular changes in hepatopancreatic tissues.
The hepatopancreas, as the primary digestive organ, becomes increasingly compromised as EHP multiplies within its cells. The parasite causes:
Impaired digestive enzyme synthesis
Impaired nutrient uptake
Wasting away of hepatopancreatic tubules
With their digestive system compromised, infected shrimp cannot effectively convert feed into growth, leading to the characteristic stunting despite adequate feeding.
Researchers also noted an association between EHP infection and White Feces Syndrome (WFS), a condition where shrimp produce white, stringy fecal matter that floats on the pond surface 1 7 . Studies confirmed that these floating white feces contained EHP spores, creating an efficient transmission route within ponds 7 .
Floating white fecal strings containing EHP spores serve as transmission vectors.
A 2022 study revealed that EHP infection follows a degenerative cyclic pattern in the hepatopancreas microbiome 9 . In early stages, high diversity of beneficial microorganisms related to nutrient assimilation persists. As the infection progresses, these beneficial organisms decrease while opportunistic pathogenic fungi increase. In late stages, opportunistic bacteria and fungi associated with disease processes displace the beneficial microorganisms 9 .
Transcription analysis revealed significant changes in immune gene expression:
Gut samples from symptomatic shrimp contained pathogenic V. parahaemolyticus, V. harveyi, and V. campbellii—all known to cause serious diseases in shrimp 6 . This helps explain why EHP-infected shrimp often succumb to secondary bacterial infections.
Currently, no approved therapeutics exist to treat EHP infections in shrimp 5 . Control strategies therefore focus on prevention and management.
Preventing introduction of EHP through PCR-screened, EHP-free postlarvae 5
Proper pond preparation between cycles to eliminate spores from previous crops 5
Avoiding practices that promote EHP transmission, such as collecting floating white feces 7
Exploring microbiome manipulation to enhance shrimp health and resistance to EHP 9
The story of EHP in West Bengal's shrimp farms represents a classic case of an emerging pathogen finding opportunity in a rapidly expanding industry. The 84.9% prevalence rate discovered in 2019 served as a wake-up call to the region's aquaculture sector 1 .
While EHP doesn't typically cause mass mortality like some viral diseases, its impact on growth and size uniformity creates substantial economic losses for farmers who must harvest undersized shrimp that command lower market prices 4 5 .
As the global demand for seafood continues to grow and aquaculture expands, understanding and managing microscopic threats like EHP will become increasingly important—not just for West Bengal, but for seafood production worldwide.
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