A group of FSU medical students goes to New Orleans for the weekend to celebrate after a big midterm exam. On Saturday evening, they all eat at a seafood restaurant in the French Quarter that might be considered by some to be a bit on the seedy side. The group enjoys a variety of shellfish and other local delicacies, along with the customary ethanol-containing supplements. The next morning, they head back to Tallahassee. Somewhere near Mobile, Alabama, several of them begin to feel a really urgent need for a rest area with working toilet facilities (inconvenient timing to say the least, but microbes wait for no one). Within the next hour, everyone is enjoying an explosive watery diarrhea, vomiting, nausea, abdominal cramps, and headaches. When they finally get to Tallahassee (after numerous intermediate stops), they are still experiencing intense symptoms, so they go to their student clinic. Vital signs are normal, except for one student (out of five) that has a low-grade fever. There are no gross signs of blood or mucus in the stool specimens. Microscopic examination of these specimens fails to detect protozoan cysts or ova.

Question 5.1: What is your preliminary diagnosis?

The patients clearly have an acute form of gastroenteritis. The combination of explosive watery diarrhea and vomiting is somewhat suggestive of cholera. However, even in New Orleans, it is unlikely that they would have been exposed to chlolera. Diarrhea sometimes accompanies vomiting during staphylococcal food poisoning, but the incubation period in this case probably is too long for staphylococcal food poisoning. The symptoms are insufficiently distinctive to permit a specific diagnosis without additional information. Nevertheless, the timing and similarity of the symptoms suggest that the students were exposed to a common source of the disease, very likely the meal they all enjoyed at the seafood restaurant. Whenever food poisoning could be related to contaminated seafood, especially in the Gulf Coast region of the U.S., one should consider infection Vibrio parahaemolyticus.

Question 5.2: What tests should be done to resolve this case?

Stool samples should be tested for blood and leukocytes. Stool cultures should be ordered and, because V. parahaemolyticus infection is being considered, the lab should also be asked to test specifically for this organism. V. parahaemolyticus usually will not grow on the nutrient media used for routine stool cultures, so the lab must run a separate culture for Vibrio (not done unless specifically requested). Species of Vibrio are differentiated with various biochemical tests.

Test Results

None of the stool samples tested positive for blood, and only two of them contained small numbers of leukocytes. The routine cultures for GI pathogens (Salmonella, Shigella, etc.) were all negative, but all of the specific cultures for Vibrio were positive. The lab identified the isolates from all of the patients as Vibrio parahaemolyticus.

Question 5.3: How does the causative agent get into food?

V. parahaemolyticus is a curved, Gram-negative bacillus that is facultatively anaerobic and has a fermentative metabolism. Although it has relatively simple nutrient requirements, it is halophilic (i.e., it requires elevated concentrations of NaCl for growth). It is commonly present in estuarine and marine environments worldwide, where it can easily contaminate shellfish and other types of seafood. Filter-feeding organisms such as oysters are especially vulnerable to contamination because their feeding mechanism may actually concentrate V. parahaemolyticus and other bacteria inside their bodies. In any event, the organism is a problem only if the seafood is eaten raw or is undercooked. (Thorough cooking kills V. parahaemolyticus.)

Question 5.4: How does the causative agent produce disease?

Actually, the precise mechanism by which V. parahaemolyticus causes diarrhea is still unknown. The organism has not yet been shown to produce a toxin, so it may have an invasive mechanism. It produces at least one possible virulence factor, a hemolysin (a 42-kDa heat-stabile protein), but the exact pathophysiologic role of this molecule is uncertain.

Question 5.5: What other diseases does this causative agent produce?

V. parahaemolyticus actually has two distinct gastrointestinal presentations. The more common of the two (and the one described in this case) is a syndrome of watery diarrhea, accompanied in most cases by abdominal cramps, nausea, and vomiting. Fever and chills occur in about 1/4 of the cases. The incubation period ranges from 4 hrs to 4 days, and the symptoms persist for a median of 3 days. The vast majority of North American cases are of this type. The less common gastrointestinal syndrome is one of dysentery and is seen mostly in India and Bangladesh. It is characterized by severe abdominal cramps, nausea, and vomiting, along with bloody or mucoid stools. V. parahaemolyticus is also a rare cause of extraintestinal infections (usually following exposure to seawater), including wound infections, otitis, and (very rarely) sepsis.

Question 5.6: How should the students be treated?

V. parahaemolyticus gastroenteritis seldom requires treatment as it typically self resolves within 72 hours. Fluid replacement can be used to treat dehydration in severe cases. Antibiotics can be used to shorten the duration of severe cases as well. Recommended drugs include tetracycline, doxycycline, gentamicin, and cefotaxime.