Infectious Hypodermal and Haematopoietic Necrosis (IHHN) in Shrimp

IHHN virus is the smallest known shrimp virus. It causes mass mortality in Penaeus stylirostris and runt deformity syndrome in Penaeus vannamei. There is no treatment.

Overview

Infectious hypodermal and haematopoietic necrosis (IHHN) is a viral disease of penaeid shrimp caused by infectious hypodermal and haematopoietic necrosis virus (IHHNV), also known as Penaeus stylirostris penstyldensovirus 1 (genus Penstyldensovirus, subfamily Densovirinae, family Parvoviridae). It is a non-enveloped, single-stranded DNA virus and the smallest of the known penaeid shrimp viruses, about 22 nm in size with a genome of roughly 4.1 kilobases. IHHN has been a notifiable crustacean disease of the World Organisation for Animal Health (WOAH) since 1995.

Affected species and disease forms

The clinical outcome depends strongly on the shrimp species. In the Pacific blue shrimp, Penaeus stylirostris, IHHNV can cause acute epizootics with mass mortality of more than 90%, with juveniles and subadults most affected. In the Pacific white shrimp, Penaeus vannamei, the virus instead causes a chronic condition called runt deformity syndrome (RDS), characterized by reduced and irregular growth and cuticular and rostral deformities rather than mass mortality.

Clinical signs

• In Penaeus stylirostris: rapid onset of high mortality in juveniles and subadults
• In Penaeus vannamei: stunted, irregular growth and pronounced size variation
• Deformed, bent, or shortened rostrum and other cuticular deformities (RDS)
• Reduced market value and uneven harvests from runting and deformity
• Often subclinical carriage in tolerant or resistant stocks

Transmission

IHHNV spreads both horizontally, through water, cannibalism, and contact with infected shrimp, and vertically from infected broodstock to offspring. The virus is highly prevalent in farmed and wild Pacific shrimp populations across the Americas, Asia, and Australia, and the movement of live infected shrimp is a major route of spread between regions.

Diagnosis

Diagnosis is confirmed by PCR, supported by histopathology, where the virus produces characteristic Cowdry type A intranuclear inclusion bodies in cells of tissues of ectodermal and mesodermal origin. Molecular testing is important because tolerant carriers can harbor and spread the virus without obvious disease.

Treatment, control and prevention

There is no treatment for IHHN. Control depends on stocking clean animals and on biosecurity, and the industry has made major progress by developing resistant and pathogen-free broodstock lines.

• Stock specific-pathogen-free (SPF) and PCR-screened postlarvae
• Use specific-pathogen-resistant (SPR) broodstock lines developed for IHHNV
• Screen broodstock and incoming animals and avoid IHHNV-positive sources
• Disinfect ponds and equipment and control crustacean carriers
• Follow WOAH surveillance and movement-control requirements