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The State of the Bay Galveston Bay Area Project

Pathogens in Seafood

The potential presence of human pathogens in bay waters is an important health concern related to human use of Galveston Bay. Exposure to these pathogens could occur through contact and non contact recreation or through the consumption of seafood. Molluscan shellfish, primarily oysters, are especially associated with the risk of infection because they are often consumed raw. However, all types of seafood can serve as vectors for some form of disease agent. History provides valid reasons for concern about such diseases as infectious hepatitis, dysentery, and cholera. Thanks to scientific understanding and effective regulation by public health agencies, outbreaks related to environmental conditions are now rare. However, occasional instances still occur, reminding us that the sources for contamination still exist and that some pathogens occur naturally in the bay ecosystem.

Diseases Transmitted by Seafood

Two groups of bacteria and two groups of virus commonly cause illness after ingestion with seafood: Vibrio and Salmonella bacteria; and Norovirus and Hepatitus A viruses.

Common Bacterial Pathogens

Species of the bacterial genus Vibrio have been identified as pathogenic for humans with the potential to cause extreme illness and sometimes death. Several Vibrio species pose a concern in coastal waters, the most common being Vibrio vulnificus, which can cause rapid and devastating disease symptoms in humans. Vibrio cholerae and Vibrio parahaemolyticus also occur in estuarine waters and do occur in Galveston Bay. Vibrio infection is usually associated with eating raw or undercooked shellfish, particularly oysters.

Vibrio bacteria occur naturally in coastal waters and are most common in warm waters with a temperature range of 10 to 30 degrees Celsius (C) and a salinity range of 5 to 30 practical salinity units (psu) (FDA 2004; WHO and FAO 2005); conditions representative of Galveston Bay. Vibrio species are fast growing food-borne pathogens of increasing concern for public health. In 2007, in the U.S., there were 549 Vibrio illnesses reported to the Centers for Disease Control (CDC) and 36 deaths (CDC 2008). Between 1996 and 2004, 50 percent of the reported 549 Vibrio infections were caused by V. parahaemolyticus and 12 percent by V. vulnificus (Qadri et al. 2005).

V. parahaemolyticus was first identified in Japan in 1950 and was first confirmed in the U.S. in 1971 when it produced an outbreak associated with consumption of steamed crabs. This bacterium causes gastroenteritis and occasionally sepsis (a bacterial infection in the blood stream or body tissues) with a mortality rate of approximately one percent (FDA 2004). V. vulnificus was first identified by the CDC in 1976. In the US it is the leading cause of death associated with consuming seafood, and has a fatality rate of nearly 40 percent (WHO and FAO 2005).

Salmonella bacteria are found in over seven percent of the market oysters in this country. Salmonella enterica serotype typhi is associated with outbreaks of seafood borne illness and is responsible for the first documented large shellfish outbreak in the U.S. Salmonella enterica typhi has been identified in illnesses tied to consumption of oysters from Galveston Bay in 2003 (Gaul 2005). A more common species of Salmonella in oysters is S. newport, which accounts for 78 percent of the Salmonella isolated from market oysters (Brands et al. 2005).

Current fecal coliform monitoring does not detect these estuarine bacteria and their occurrence cannot be correlated to concentration of sewage or abundance of coliform bacteria. Therefore, they present a problem for current methods of certifying seafood and regulating harvest areas.

Common Viral Pathogens

Bacteria are not the only pathogens that should be included in a discussion of human health. Hepatitis A is caused by a virus that comes from the same family of viruses as the polioviruses and rhinoviruses (causal agent for the common cold). The hepatitis virus is excreted in the waste of infected persons and can contaminate water or food products, including shellfish. There is great concern that hepatitis may be contracted by consumption of raw or improperly cooked seafood.

Norwalk viruses or norovirus is the most common cause of outbreaks of food-borne illness. The only known reservoir for norovirus is the human intestinal tract, so outbreaks are linked to poor sanitation (CDC 2009). These viruses can be ingested in a wide variety of raw or under-cooked food, not just seafood. They can be accumulated in the gut of oysters and outbreaks of acute gastroenteritis due to norovirus have been traced to the consumption of raw oysters from Texas. As recently as 2007, oysters from San Antonio Bay were implicated in a norovirus outbreak (TMN 2007).

Other viruses that can be traced to raw or partially cooked shellfish include rotaviruses and parvo-like viruses. These can cause viral gastroenteritis resulting in symptoms such as nausea, vomiting, diarrhea, malaise, abdominal pain, headache, and fever (USFDA 2000b).

Monitoring for Pathogens

Monitoring of water pollution usually includes determination of the concentration of bacteria in water samples. The testing procedure used to determine whether water is safe for contact recreation has changed from fecal coliform bacteria (general bacterial types associated with the mammalian intestine) to Escherichia coli in freshwater and Enterococci in brackish or salt water. This regulatory system does not currently incorporate the importance of Vibrio bacteria in determining the safety of consuming raw oysters.

Fecal coliform bacteria are used in the Texas Department of State Health Services (TDSHS) Shellfish Sanitation Program as an indicator of human health risk. Currently the bacterial monitoring results from the Galveston Bay watershed document a serious problem that does not appear to respond to current management practices. Most of the monitored water bodies in Harris County and an increasing number in surrounding counties exhibit bacterial concentrations that exceed the regulatory standards for contact recreation. There are regulatory processes in place (e.g., Total maximum Daily Load (TMDL) procedures) to address the high concentrations of fecal bacteria in bay tributaries (e.g., Buffalo Bayou and Dickinson Bayou), which should be suitable for contact recreation. Similarly, a TMDL project for the bay is underway to address high levels of fecal bacteria in waters that should be suitable for shellfish harvest.

Sources of Pathogens

Studies suggest that the principal source of fecal coliform bacteria to Galveston Bay is wet weather runoff from upland areas, with urbanized areas being one of the major components (Jensen 1992). Part of the reason fecal coliform levels are high in urbanized areas is the contribution from sewer leaks and overflows. However, even when the collection systems are not leaking, urban area runoff generally has high fecal coliform levels, and runoff occurs in much greater volumes than sewage leaks or overflows.

Neither septic systems along the bay's shoreline, nor permitted point source discharges are major contributors of fecal coliform bacteria to the bay as a whole (Jensen 1992). Locally, however, septic systems and permitted discharges can both be important contributors of bacteria. This is especially true for enclosed tributaries. Runoff from totally undeveloped land also tends to be high in fecal coliform bacteria (TDSHS 1990), with low incidence of pathogenic organisms.

Studies of the bacterial and viral contamination of the canals in bayside communities from Galveston Bay show that pathogens survive longer in the sediment than in the water column (Gerba and McLeod 1976; Smith et al. 1978). This suggests that the sediments of Galveston Bay hold a reservoir of an assortment of human pathogens.


Literature Cited

Brands, D. A., A. E. Inman, C. P. Gerba, C. J. Mare, S. J. Billington, L. A. Saif, J. F. Levine and L. A. Joens. 2005. "Prevalence of Salmonella spp. in Oysters in the United States." Applied and Environmental Microbiology 71(2): 893-897.

CDC. 2008. Summary of human Vibrio cases reported to CDC, 2007. CDC.

CDC. 2009. Norovirus Technical fact Sheet., U.S. Centers for Disease Control. Chin, E. and D. M. Allen. 1957. "Toxicity of an insecticide to two species of shrimp, Penaeus aztecus and Penaeus setiferus." Texas Journal of Science 9(3): 270–278.

FDA. 2004. Pathogenic Vibrio parahaemolyticus in Raw Oysters. Quantitative Risk Assessment on the Public Health Impact of Pathogenic Vibrio parahaemolyticus in Raw Oysters. USFDA, Department of Health and Human Services. Washington, D.C.

Gaul, L. 2005. "Shifting Paradigms in Epidemiology: The Expanded Role of the Public Health Laboratory (ppt)." Web Page of the TDSHS, 2009, from

Gerba, C. P. and J. S. McLeod. 1976. "Effect of sediments on the survival of Escherichia coli in marine waters." Applied and Environmental Microbiology 32: 114-120.

Jensen, P. 1992. Characterization of selected public health issues in Galveston Bay. Galveston Bay National Estuary Program Publication GBNEP-21. Webster, Texas.

Qadri, F., N. R. Chowdhury, Y. Takeda and G. B. Nair. 2005. Vibrio parahaemolyticus -- seafood safety and associations with higher organisms. In Oceans and health: pathogens in the marine environment. S. S. Belkin and R. R. Colwell (Eds). New York, New York, Springer: 277-293.

Smith, E. M., C. P. Gerba and J. L. Melnick. 1978. "Role of sediment in the persistence of enteroviruses in the estuarine environment." Applied and Environmental Microbiology 35(4): 685-689.

TDSHS. 1990. Cow Trap Lake a study of pathogens, indicators, and classification of a Texas shellfish. Austin, Texas, Texas Department of Health, United States Food and Drug Administration, and United States Fish and Wildlife Service.

TMN. 2007. FDA investigating Norovirus linked to San Antonio Bay oysters. The Medical News.

USFDA. 2000a. "Foodborne pathogenic microorganisms and natural toxins handbook: hepatitis A virus by U.S. Food and Drug Administration." Web Page of the U.S. Food and Drug Administration Retrieved 2000 Nov 16, from

USFDA. 2000b. "Foodborne pathogenic microorganisms and natural toxins handbook: other gastroenteritis viruses by U.S. Food and Drug Administration." Web Page of the U.S. Food and Drug Administration Retrieved 2000 Nov 16, from

WHO and FAO. 2005. Risk Assessment of Vibrio Vulnificus in Raw Oysters: Interpretative Summary and Technical Report. Rome; Geneva, World Health Organization; Food And Agriculture Organization Of The United Nations.





Raw oyster. Image courtesy of Earl Nottingham, Texas Parks and Wildlife Department.




Click for more information on Vibrio vulnificus.
Vibrio vulnificus under microscope, courtesy of CDC Public Health Image Library
Vibrio vulnificus under microscope. Image courtesy of CDC Public Health Image Library.





Water flowing out of a storm drain. Image courtesy Texas Commission on Environmental Quality.





Impaired Waters Mapping Application
Impaired Waters Mapping Application, Houston Advanced Research Center
Use the impaired waters mapping application to view Galveston Bay tributaries on the Texas 303(d) List in 2002 and 2004.




TCEQ logo Bacteria: TMDL Projects for Buffalo Bayou and Dickinson Bayou






Development along the shores of Clear Lake and Clear Creek entering Galveston Bay. Image courtesy Houston-Galveston Area Council, Google Earth.

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