Marine Pathobiology

Dr. Overstreet Honored

July 22, 2014 - Dr. Robin Overstreet, professor emeritus in the Department of Coastal Sciences, received the Eminent Parasitologist Lectureship Award at the American Society of Parasitologists (ASP) Conference, July 24-27 in New Orleans.
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Marine pathology investigates the infectious diseases of wild and farmed marine organisms.

  • Crustacean Disease Research. Investigations of diseases of wild and aquaculture shrimp are conducted through the Crustacean Disease Laboratory at the Gulf Coast Research Laboratory. Jeffrey M. Lotz, Ph.D.
  • Finfish Disease Research. Amyloodinium ocellatum

Crustacean Disease Research

The Crustacean Disease Laboratory at the Gulf Coast Research Laboratory investigates diseases of wild and aquacultured shrimp and crabs.

Shrimp Diseases

Marine shrimp aquaculture is the most valuable marine aquaculture industry in the world at about $15 billion annually. The United States imports more than 90 percent of the shrimp it consumes, creating a trade deficit of $3.5 billion dollars.

A primary impediment to growth of a shrimp aquaculture industry in the United States is control of diseases. The shrimp diseases of most importance to the U.S. are: Taura syndrome virus (TSV), white spot syndrome virus (WSSV), and the necrotizing hepatopancreatitis bacterium (NHP-B).

Taura Syndrome Virus (TSV)

Taura Syndrome Virus (TSV) has had a devastating economic impact on the shrimp aquaculture industry of the Americas since its initial recognition in the early 1990s. TSV is a 30 to 32 nm, icosahedral virus particle containing positive-sense, single-stranded RNA of about 10.2 kb in length. Initially the virus was allied with the family Picornaviridae. However, TSV is more closely related to the cricket paralysis-like viruses (CrPV-like viruses) and is a member of the “picornavirus superfamily,” the family Dicistroviridae and the genus Cripavirus.

At the Gulf Coast Research Lab we are dedicated to the eventual eradication of TSV by means of exploring further the ecological and epidemiological characteristics of this disease. Recent work at our lab includes the modeling of the pathogen’s epidemiology, estimating viral vital rates, TSV-resistant selective breeding, the effect of host size on TSV virulence, and disease control via an SPF-based shrimp aquaculture industry.

Tissue damage due to TSV infection in L. vannamei, solid arrow. Normal tissue is designated by the broken arrow.

Tissue damage due to TSV infection in L. vannamei, solid arrow. Normal tissue is designated by the broken arrow.

White Spot Syndrome Virus (WSSV)

White spot syndrome virus is the most virulent pathogen of penaeid shrimp. WSSV was recognized in the early 1990s in eastern and southeastern Asia. WSSV is an ovoidal, enveloped particle about 80- 20 nm wide by 250-380 nm long with a small tail-like appendage at one end. The virion’s genome consists of a single, circular, double-stranded DNA molecule of about 300 kb in length The virus belongs to the viral family Nimaviridae and the Genus Whispovirus.

The progression of a WSSV infection in affected ponds begins when shrimp become anorexic; after 1 -2 days, mass mortalities ensue. By 3 -10 days following onset of mortalities, deaths typically reach 80 percent and frequently all of the population succumbs

At the Gulf Coast Research Lab we are investigating the ecological and epidemiological characteristics of this disease. Recent work at our lab include the modeling of the pathogen’s epidemiology, estimating viral vital rates, pathogenesis, the geographic variation, and disease control via an SPF-based shrimp aquaculture industry.

WSSV infected cell nuclei in hypodermis of L. vannamei (Solid arrow). Dashed arrow indicates normal cell nucleus.

WSSV infected cell nuclei in hypodermis of L. vannamei (Solid arrow). Dashed arrow indicates normal cell nucleus.

White spots on shell are gross signs of white spot syndrome viral disease.

White spots on shell are gross signs of white spot syndrome viral disease.

Necrotizing Hepatopancreatitis (NHP)

Necrotizing Hepatopancreatitis, NHP, is a severe bacterial disease affecting penaeid shrimp aquaculture. NHP was first reported in 1985 from shrimp ponds in Texas, USA, and resulted in significant mortalities and devastating losses to shrimp crops. NHP has since been observed in penaeid shrimp aquaculture in Central and South American countries and is possibly in the Eastern Hemisphere as well. Elevated salinity and temperature appear to be factors associated with NHP outbreaks. Reported hosts are Litopenaeus vannamei, L. setiferus, L. stylirostris, Farfantepenaeus aztecus and F. californiensis.

The NHP-bacterium (NHPB) is a gram-negative, pleomorphic, obligate intracellular bacterium that is non-culturable through established cell lines or traditional bacteriological methods. Therefore, laboratory research of NHP is dependent on maintaining the disease agent in live animals. Gross signs of NHP include reduced feed intake, empty gut, lethargy, anorexia, and a pale and atrophied hepatopancreas.

Shrimp affected by NHP. Note pale hepatopancreas and empty gut.

Shrimp affected by NHP. Note pale hepatopancreas and empty gut.

Our research on NHP involves experimental infection in Kona stock Specific Pathogen Free L. vannamei to estimate epidemiological parameters of NHP spread in a shrimp population. We will use what we learn to generate predictive mathematical epidemic models of NHP infection in shrimp ponds. In addition, bioassays examining salinity and temperature will be attempted to elucidate the role of these environmental factors in NHP outbreaks.

Crab Diseases

Hematodinium

Crustacean health is becoming increasingly important as mounting economic pressure is placed on ocean resources.  Outbreaks of the disease caused by Hematodinium spp. have been identified in several commercially important crustacean hosts.  Of the health concerns associated with the blue crab, Callinectes sapidus, the parasitic dinoflagellate, Hematodinium sp. is quickly gaining attention as a cause of crab population decline in several geographic areas. Hematodinium is a parasitic dinoflagellate, believed to reside and proliferate in crustacean hemolymph. This disease induces pathological alterations in host organs and hemolymph resulting in lethargy, anorexia, and eventually death. 

Much of our work is focused on laboratory challenge model and surveys of blue crabs in Mississippi Sound.  The parasite displays seasonal prevalence with greatest abundance between the months of November and March.

Seasonal variation in mean prevalence of Hematodinium sp. in blue crabs from the Northern Gulf of Mexico

Seasonal variation in mean prevalence of Hematodinium sp. in blue crabs from the Northern Gulf of Mexico

Laboratory infections in blue crabs are highly lethal. The time to death varies between 10  and 60 days. 

Hematodinium organisms (trophonts) in the hemolymph of an experimentally infected blue crab

Hematodinium organisms (trophonts) in the hemolymph of an experimentally infected blue crab

Finfish Diseases

Amyloodinium

Amyloodinium ocellatum is a parasitic dinoflagellate that infects warm water marine and estuarine fishes and causes mortalities in aquaculture. Its life cycle consists of 3 stages: a feeding trophont that parasitizes the gills and skin where it interferes with gas exchange, osmoregulation and tissue integrity, a detached reproductive tomont, and a free-swimming infective dinospore.

Life cycle of Amyloodinium ocellatum

Life cycle of Amyloodinium ocellatum

Our work revolves around the population biology of Amyloodinium ocellatum. In particular we have developed an epidemiological model and are estimating the vital rates for infection dynamics in the laboratory. The important rates are, the mean number of dinospore produced per tomont,  dinospore infection rate (β), the tomont sporulation rate, and the dinospore mortality rate.  The population model helps understand the relative importance of the different vital rates on the parasite’s population growth rate.