Yellow Head Disease (YHD)

Name

Yellow Head Disease

Clinical signs

The hepatopancreas turns yellowish in color. As the disease's name suggests, the cephalothorax or the shrimp's head to thorax also turns yellowish and becomes swollen. Other parts of the body may turn white, while the gills can change color to white, yellow, or brown. The shrimp will appear to be dying near the surface or edges of the pond and will suddenly stop eating. Mass mortality of up to 100% can occur 2-4 days after the shrimp stop feeding.

Method of diagnosis

As shrimp with YHD do not always show the mentioned clinical signs, it is important to confirm diagnosis by coloring of gills and hemolyph test. Further diagnosis can be done with an RT-PCR test.

Name

Yellow Head Virus genotype 1 (YHV1)

Pathogen type

Virus

Synonyms

Yellow-head baculovirus (YBV), Yellow-head disease baculovirus (YHDBV).

Characteristics

YHV1 are bacilliform virus or rod-shaped virus with their cytoplasm wrapped in ssRNA and have a size of 44 ± 6 x 173 ±13 nm. This virus is under the C-1 pathogen category, in which they can cause mass mortality and spread in one location, making it hard to treat.

The YHV is found to have 10 discovered genotypes. Genotype 2 is known as Gill Associated Virus (GAV). Genotypes 3, 4, 5, 6, 9, and 10 have not been found to cause disease, while genotypes 7 and 8 cause diseases in Penaeus monodon and Fenneropenaeus chinensis shrimp, although they have not been found in vannamei shrimp.

Toxicity

If this disease infects shrimp, the mortality rate increases to 100% within 3 to 5 days after infection. This disease has been recorded to appear at DoC 50-70 when the shrimp are between 5-15 grams in size.

Predispositional factors

Sudden change in pH or DO.

Transmission

Transmission of YHV1 generally occurs horizontally through cannibalism or pond water, where it infects the eggs. Transmission can also occur vertically from male or female broodstock through surface infections or contamination of tissues surrounding fertilized eggs. The virus can still infect if it remains in water for up to 72 hours.

Epidemiology

YHV1 was first recognized in the 1990s when it caused mass mortality to Penaeus monodon in Thailand. It has since spread to Sri Lanka, Indonesia, the Philippines, China, Taiwan and Malaysia.

Host or vector

YHV1 can spread from host, broodstock, or fry that are not certified. Some species of farmed shrimp (Penaeus monodon, P. japonicus, Litopenaeus vannamei, and P. stylirostris) are potential hosts, and some among them can be carriers for the virus.

Infectious dose

No data.

Incubation period

Clinical signs show up 2-4 days after infection, and require 3-5 days to reach mortality.

Susceptibility to drugs

No data.

Susceptibility to disinfectants/probiotics

No data.

Physical inactivation

YHV1 can be inactivated by heating at 60°C for 15 minutes or by chlorine treatment at 30 ppm.

Early warning

YHD might occur if shrimp appetite suddenly increases, and then decreases drastically until the final phase of the disease. Physical detection can be done by observing the change in color of shrimp’s head to yellow.

Prevention

The prevention measures that can be done include keeping shrimp stress-free by monitoring water conditions and ensuring they are at ideal parameters. After harvest, remove sediments and organic matter build up thoroughly. Choose shrimp fry that is certified to be virus-free.

Treatment

If YHD occurs, prevent sudden changes in water physical and chemical conditions. There is no vaccination currently developed for YHD.

Eradication

The pond should be disinfected using 30 ppm of chlorine for at least 4 days to kill the infected shrimp and any potential carriers. The dead shrimp are buried or burned, and the pond is then dried. The water used for the next cycle should also be disinfected to eliminate potential carrier organisms by adding 20-30 ppm of chlorine.

Bower, S.M. 1996. Synopsis of Infectious Disease and Parasites of Commercially Exploited Shellfish: Yellow-head Virus Disease (YHD) of Penaeid Shrimp. Fisheries and Oceans Canada.

FAO. FAO Fisheries Technical Paper 402/2.

Flegel, T.W., D.F. Fegan, S. Kongsom, S. Vuthikomudomkit, S. Sriurairatana, S. Boonyaratpalin, C. Chantanachookhin, J.E. Vickers and O.D. Macdonald. 1992. Occurrence, diagnosis and treatment of shrimp diseases in Thailand. In: W. Fulks and K.L. Main (eds.). Diseases of Cultured Penaeid Shrimp in Asia and the United States. The Oceanic Institute, Honolulu, p. 57-112.

Ganjoor, M. 2015. A Short Review on Infectious Viruses in Cultural Shrimps (Penaeidae Family). Journal of Fisheries Science. 9 (3): 9-33.

Genics Pty Ltd. 2023. Genics Education Series: Yellow Head Virus (YHV) variants 1 and 7.

Lotz, J.M. 1997. Special Topic Review: Viruses, Biosecurity and Specific Pathogen-free Stocks in Shrimp Aquaculture. World Journal of Microbiology & Biotechnology. 13: 406-413.

OIE. 2009. Manual of Diagnostic Tests for Aquatic Animals: Yellow Head Disease.