Cracking the Pathogen Code: A Scientific Approach to Tackling EHP, AHPND, and WFD in Shrimp Farms

By: Rico Wibisono — Geneticist, Bioinformatician, and CEO of FisTx

I spend my time in two vastly different settings: in front of a screen analyzing genomic data, and out on the pond banks in muddy boots. It is from these dual perspectives that I view today’s shrimp disease crisis.

EHP, AHPND, and White Feces Disease (WFD) are not merely "seasonal disease outbreaks" that come and go. They are systems operating via precise biological mechanisms; if we understand how they function at the molecular level, we stand a far better chance of controlling them.

Three Diseases, One Chain of Attack

EHP: The Invisible Energy Thief

*Enterocytozoon hepatopenaei* (EHP) is a microsporidian parasite with a remarkably simple genome—simpler even than that of most bacteria. It has intentionally shed its own energy-producing genes because it has a singular goal: to steal energy directly from the shrimp's cells.

Using specialized transporter proteins, EHP siphons ATP—the cell's fuel—directly from the shrimp's hepatopancreas (HP) cells. Infected shrimp do not die immediately; they simply stop growing. Feed consumption continues, yet conversion into body mass stalls. This is why losses caused by EHP often go unnoticed until harvest time, when yields fall far short of targets.

AHPND: It’s Not About the *Vibrio*, It’s About the Plasmid

AHPND (Acute Hepatopancreatic Necrosis Disease) is often referred to as a *Vibrio* disease. While not incorrect, that description is not entirely precise. The true danger lies not in the bacterium itself, but in a circular DNA fragment known as the pVA1 plasmid, carried by certain *Vibrio* strains.

This plasmid encodes two toxins: PirA and PirB. They function like the two components of a bomb—one seeks out the target, while the other triggers the explosion. PirA attaches to receptors on the surface of the shrimp host cell, while PirB punctures the cell membrane from within. Damage occurs very rapidly.

What complicates matters further is that the pVA1 plasmid transfers easily between *Vibrio* species through a process known as horizontal gene transfer. This means the killing capability can "spread" from one bacterium to another—even across different species.

WFD: A Symptom, Not the Root Cause

This is where many farmers miss the mark. White Feces Disease (WFD)—manifested as white feces floating on the pond surface—is often treated as the primary problem. In reality, it is the final outcome of a chain of damage that has already occurred.

Here is the sequence: EHP weakens hepatopancreas (HP) cells by depleting their energy. In this weakened state, AHPND toxins act far more effectively than usual. The microvilli (fine, hair-like structures) on the intestinal wall become damaged, slough off, and clump together, forming a mass known as Aggregated Transformed Microvilli (ATM). It is this mass that appears as white feces.

By the time WFD becomes visible, the damage within the HP is already severe. Treatment focused solely on visual symptoms will not suffice.

A Defensive Approach: From the Outside In

Understanding how these pathogens operate paves the way for more targeted strategies. The most effective approach works on two fronts simultaneously: clearing pathogens from the aquatic environment (external defense) and protecting the shrimp's digestive organs from within (internal defense).