Clinical Microbiology Newsletter 36:3,2014 | ©2014 Elsevier 17

Introduction

Several years ago, when I coauthored an article inthis newsletter on infections in nail salons (1), oneof my colleagues sent me an email that she wouldnever get a pedicure again. I still get pedicures, soit was certainly not my intention to close downnail salons. However, it turned out that there arecertain precautions that people and salons cantake that will help prevent infection. I do not eatsushi or sashimi, but my sister and many of mybest friends do. I decided to write this articlebecause I wanted to explore the potential conse-quences of eating raw or undercooked seafood. Ihave learned a lot as a result of my research thatmay be comforting or scary depending on yourperspective.

Epidemiology of Food-Borne IllnessesCaused by Seafood

Food-borne diseases cause approximately 76 mil-lion illnesses in the United States each year, andseafood is implicated in 10 to19% of these episodes(2). The actual causative agent can be tracked in44% of seafood-related outbreaks, with virusesaccounting for approximately one-half of theevents. There are about 325,000 hospitalizationsand 5,000 deaths annually.

Sushi and Sashimi

There are certain basic things that should beconsidered at the outset. The first is the difference

between sushi and sashimi. Sushi is actually sweet-ened, vinegared rice combined with other ingre-dients, including sliced raw fish. Sashimi is thesliced raw fish itself. A second key point is that thetype of fish involved, whether marine or fresh-water, will ultimately determine the type(s) ofmicroorganism(s) that may cause infection. Thethird thing is that there is a difference betweenmolluscs (oysters, mussels, cockles, and clams)and crustaceans (crabs, shrimp, and prawns) andthe diseases they cause. Molluscs are more trou-blesome, because they are filter feeders; thismeans that they filter and concentrate environ-mental microorganisms in high numbers in theirtissues. These microorganisms may be viablewhen the molluscs are eaten raw (2).

Diseases from Shellfish

As stated above, viruses (particularly norovirus)cause 50% of seafood-related illnesses.

Norovirus

Norovirus is a calicivirus that was first discoveredin 1972 in feces collected after an outbreak of gas-troenteritis in 1968 in Norwalk, Ohio (3). Thevirus was originally referred to as the Norwalkvirus or Norwalk-like virus (4). An electron pho-tomicrograph of norovirus is shown in Fig. 1; itis a single-stranded RNA virus with an icosahe-dral capsid composed of a single major protein.Norovirus is the leading cause of gastroenteritisassociated with the consumption of raw shell-

Vol. 36, No. 3February 1, 2014www.cmnewsletter.com

I N T H I S I S S U E

17 Infections fromEating Raw orUndercooked Seafood

22 CutaneousLeishmaniasis in aCentral AmericanRefugee

Corresponding Author:Alice Schauer Weissfeld, Ph.D.,D(ABMM), F(AAM), Microbi-ology Specialists Incorporated,8911 Interchange Dr., Houston,TX 77054. Tel.: 713-663-6888.Fax: 713-663-7722. Email:alice@microbiologyspecialists.com

ClinicalMicrobiologyN E W S L E T T E R

CMN

Stay Current...

Stay Informed.

Infections from Eating Raw or UndercookedSeafoodAlice Schauer Weissfeld, Ph.D., D(ABMM), F(AAM), Microbiology Specialists Incorporated, Houston, Texas

Abstract

The bacterial, viral, and parasitic diseases that may be contracted from eating raw or undercooked seafoodare reviewed. Diseases frommarine, as well as, freshwater fish are discussed. The Food and Drug Admin-istration’s sanitation program for shellfish and their guidelines for preparation of raw fish in restaurantsare also described.

CMN

18 Clinical Microbiology Newsletter 36:3,2014 | ©2014 Elsevier

fish. Oysters are frequently implicated, but outbreaks have also beenassociated with consumption of mussels, cockles, and clams.

Norovirus is excreted in the feces of infected individuals, where itpersists in the environment. It is transmitted primarily by thefecal-oral route or via contaminated food and water. In fact, con-tamination of fishing waters by human sewage has played a majorrole in seafood-associated outbreaks (5). The infectious dose is low,with <100 virions capable of causing symptomatic disease. The incu-bation period is 18 to 48 hours, and the duration of illness is 24 to48 hours, depending on the original infectious dose. The attack ratemay be as high as 83% during outbreaks.

Diagnosis of norovirus infection was originally performed byimmune electron microscopy, since norovirus does not grow in tis-sue culture. Today, diagnosis is by detection of viral antigen eitherby enzyme-linked immunosorbent assay (ELISA) or by reverse tran-scription-PCR. Treatment is usually just supportive therapy, asnorovirus disease is usually self-limiting.

Hepatitis A Virus

Hepatitis A virus is also a single-stranded RNA virus surroundedby a protein capsid (Fig. 2) and is a picornavirus of the enterovirusgroup. The first seafood-related infection with hepatitis A virusoccurred in Sweden in 1956, when an outbreak of 629 cases asso-ciated with the consumption of raw oysters was reported. Thefirst seafood-associated outbreak in the U.S. occurred in 1961 andwas linked to the consumption of raw oysters and clams. Thevirus is transmitted by the fecal-oral route. Interestingly, hepatitisA is more resistant to chlorination than some other enteric viruses.The infectious dose is also low, between 10 and 100 virions. Shell-fish concentrate the virus severalfold in their tissues through activefiltration.

In seafood-associated outbreaks, disease is usually mild and is pri-marily characterized by a prodrome of fatigue, anorexia, nausea, andupper abdominal pain. Many patients develop icterus (jaundice) anddark urine. The incubation period is relatively long (3 to 6 weeks),and the disease may be prolonged in the elderly (4 to 6 weeks).However, mortality is low, and there is no chronic carrier state, asthere is, for example, with hepatitis B virus. Diagnosis is made sero-logically using an IgM anti-hepatitis A virus test. Treatment issupportive. In rare cases of fulminant liver failure, patients mayrequire a liver transplant.

There is currently a vaccine available for hepatitis A virus. It is rec-ommended for children, international travelers to areas of endemic-ity, individuals with chronic liver disease, and persons with clottingfactor disorders. Immunization is given starting with an initialinjection, which is followed by a booster injection 6 months later(6).

Vibrio spp.

Vibrio spp. are gram-negative comma-shaped or straight rods thatare oxidase positive and motile and ferment glucose without the pro-duction of gas. Vibrio parahaemolyticus is the commonest cause ofseafood-borne disease in the U.S., followed by Vibrio vulnificusand non-epidemic Vibrio cholerae. Vibrio spp. are widespread in

marine and brackish-water (estuarine) environments and growbest between 10°C and 30°C at a salinity between 5 and 30%. Con-tamination of oysters is greatest from May through October,although folk wisdom only tells us not to eat raw oysters in amonth without an “r” in it. Shrimp, clams, oysters, mussels, cock-les, crab, lobster, and scallops are all implicated in food-bornedisease transmission. Oysters can concentrate Vibrio spp.; and,subsequently, Vibrio are present at a much higher level in the oys-ters’ tissue than viruses. Twenty percent of bacterial illnessesrelated to shellfish consumption are caused by Vibrio spp.; oystersare implicated in 49% of the cases caused by Vibrio spp. in the U.S.,and clams are implicated in an additional 38%. Gastroenteritisaccounts overall for 80% of all cases of Vibrio sp. infection (2).

A large outbreak of cholera was traced to food served on an inter-national flight (7), and multistate outbreaks of acute gastroenteri-tis have been traced to oysters harvested in Louisiana (8). In 1989,at the Interscience Conference for Antimicrobial Agents andChemotherapy (ICAAC) held in New Orleans, Louisiana, severalindividuals conducted a study looking at the incidence of seafood-borne illness of attendees (9). Twelve percent of respondents

Figure 1. Transmission electron micrograph of norovirus virions.(Public Health Image Library no. 10708, CDC, Atlanta, GA.)

Figure 2. Electron micrograph of hepatitis A virus. (Public HealthImage Library no. 2739, CDC, Atlanta, GA.)

reported a diarrheal illness, and the risk of diarrhea was significantlyhigher in individuals who ate raw or cooked oysters. In 1993, theNew York Times published an article regarding findings first reportedat another ICAACmeeting. In this study, researchers reported thatLouisiana hot sauce killed some Vibrio spp. found in raw shellfishwhen tested in the laboratory (10). The lead investigator reportedthat he “eats raw oysters, but only with plenty of hot sauce.”

The most common manifestation of V. parahaemolyticus infectionis gastroenteritis (60 to 80%), followed by wound infections (34%)and septicemia (5%). The incubation period is 15 to 19 hours, andgastrointestinal symptoms (abdominal cramps, nausea, vomiting,fever, and diarrhea) last 2 to 3 days before complete recovery inpatients who have no underlying liver disease or any other immuno-suppressive condition. V. parahaemolyticus gastroenteritis is treatedwith hydration and supportive measures.

Salmonella spp.

Approximately 7% of all food-borne outbreaks of Salmonella infec-tion are linked to seafood. In fact, between 3% and 54% of seafoodtested is contaminated with Salmonella. Salmonella gastroenteritishas an incubation period of 8 to 72 hours. Symptoms includeacute non-bloody diarrhea, abdominal pain, muscle aches, andfever. As with any limited Salmonella gastrointestinal illness, non-typhoidal Salmonella is usually not treated with antibiotics, whichcan promote a carrier state.

Other Organisms

Aeromonas spp., Plesiomonas spp., Listeria monocytogenes, Clostridiumbotulinum, Campylobacter spp., and Streptococcus pyogenes have all beenimplicated in seafood-borne illness but do not occur as frequentlyas the organisms mentioned above and will not be discussed here.

Parasitic Diseases from Marine and Freshwater Seafood

Parasites, discussed below, cause the majority of freshwater andmarine seafood-borne illness. An estimated 50 million people areinfected worldwide (11).

Anisakiasis

Anisakiasis is primarily an acute gastrointestinal disease caused byinfection with either the herring worm (Anisakis simplex) or the codworm (Pseudoterrannova decipiens) (12). The highest prevalence ofanisakiasis is in Southeast and East Asia. One thousand cases arereported annually from Japan, but only about 50 cases have beenreported from the U.S. Nearly all marine animals are infected withparasites. Most worms do not cause disease in their natural host orin humans. However, some of these parasites may survive thefood preparation process and cause human infection. The lifecycle of the anisakid nematode is shown in Fig. 3.

A. simplex accumulates within the host during the fish’s lifetime.Anisakids are commonly found in marine mammals, such as dol-phins and sea lions, but are difficult to see within the fish muscu-lature. They excrete eggs, which turn into larvae. After ingestionby the human host, a viable juvenile anisakid may penetrate intoor through the wall of the gastrointestinal tract, causing invasiveanisakiasis.

These larvae are ingested by crustaceans. Fish then ingest thecrustaceans and become intermediate hosts, with the larvae embed-ded in their flesh. When humans ingest the infected fish, the lar-vae can attach to the gastric mucosa or penetrate the stomach orintestinal wall, leading to acute abdominal pain, nausea, and vom-iting within a few minutes to several hours; abscess formation oreosinophilia granulomatosis (13) may also be present. Severe aller-gic reaction, including angioedema, uticaria, or even systemicanaphylaxis, is also possible (14).

An antigen capture ELISA with a reported sensitivity and speci-ficity near 100% can be used for serodiagnosis of anisakiasis (15).Diagnosis can also be made by extirpation (total destruction) of theparasite during an upper gastrointestinal tract endoscopic exami-nation. Although many patients expel the worm spontaneously,albendazole has been shown to be an effective treatment.

Diphyllobothriasis

The fish tapeworms Diphyllobothrium latum and Diphyllobothriumdendriticum are the common cestodes implicated in seafood-bornedisease episodes.D. latum competes for vitamin B12 in the humanhost, causing pernicious anemia, in which the body cannot makeenough red blood cells. Fifty-two cases were reported in 1980fromWest Coast states in the U.S., and salmon was implicated in82% of them. Infective larvae, also known as plerocercoids, alsoreside in the muscles of trout, pike, and sea bass. The plerocercoidsattach to the mucosa of the small intestine, where they mature intoadult worms. Proglottids can protrude out of the patient’s anus tohelp make the diagnosis when observed. Niclosamide is used fortreatment.

Flukes

Heterophyes andMetagonimus are trematodes that are acquired byeating raw, marinated, or improperly cooked fish. Most cases arereported from theMiddle East and Asia. However, one case ofHet-erophyes heterophyes infection was reported in the U.S. in 1986; thepatient had never traveled outside the U.S. but had eaten sushi ina restaurant that served fish flown in from Asia (11).

Liver Flukes

Clonorchis sinensis and Opisthorchis viverrini cause human infectionfollowing the ingestion of raw freshwater or brackish-water fish car-rying infective larvae (metacercariae). The larvae migrate to the bileduct, where they develop into adult worms. A heavy infectionmay lead to obstructive jaundice and eventually liver cirrhosisand cholangiocellular carcinoma. Diagnosis is made by seeingcharacteristic eggs in stool. Treatment is with either mebendazole,albendazole, or praziquantel.

Lung Flukes

Paragonimiasis is caused by the lung fluke Paragonimus westermanior Paragonimus skrjabini. The disease is endemic in Thailand,Japan, Korea, andChina. Infective larvae (metacercariae) are encystedin freshwater crabs. These larvae penetrate the peritoneal cavity andmigrate across the diaphragm into the lung parenchyma. Theyreach maturity in the lung parenchyma and form solid wormcysts. Typical manifestations include remittent fever, chest pain, and

Clinical Microbiology Newsletter 36:3,2014 | ©2014 Elsevier 19

20 Clinical Microbiology Newsletter 36:3,2014 | ©2014 Elsevier

a chronic cough with hemoptysis. Chest X rays show characteris-tic infiltrative, nodular, and cavitating lesions. Confirmation isby detection of the eggs in sputum, stool, or gastric aspirates,although serological tests are also available. Pleural effusions (if pre-sent) must be drained before praziquantel is given.

FDA National Shellfish Sanitation Program

The Food and Drug Administration (FDA) is responsible for reg-ulating seafood, as they are with other foods (16). The main pointsin this program are (i) harvest fish from approved areas only and

process them immediately after capture, (ii) monitor the numberof fecal coliforms in the area, and (iii) candle fish to look for wormsprior to food preparation. Candling involves shining a bright lightthrough the fillet to observe worms not removed by gutting the fish.

The FDA also sets guidelines for restaurants. These include the fol-lowing: (i) defrost fish fillets at room temperature within 14 hours;(ii) do not store fillets for more than 3 days; (iii) use trays of fishremoved from the refrigerator within 3 hours; (iv) shell shrimpimmediately and preferably before they are completely defrosted;

Figure 3. Life cycle of A. simplex and P. decipiens, the causal agents of anisakiasis. (Public Health Image no. 3378,CDC, Atlanta, GA.)

(v) remove defrosted shrimp from the refrigerator as needed, anddo not keep them at 4°C for >48 hours; (vi) use clams within 3 days,including the time required for defrosting; (vii) flash-freeze all fishand shellfish intended for consumption raw, semi-raw (marinated),or partly cooked to -35°C or below for 15 hours, or freeze themslowly over time to -30°C or below for 7 days.

Conclusions

It seems presumptuous to give advice to consumers on whether theyshould or should not eat raw or undercooked seafood. However,a few salient facts are worth summarizing here.

People with liver disorders or weakened immune systems shouldavoid raw or undercooked seafood. Individuals consuming a sin-gle serving of raw shellfish from an approved harvesting site in theU.S. have an estimated probability of 1 in 100 of becoming infectedwith an enteric virus (2). The risk is even smaller for anisakiasis, withfewer than 10 cases of reported infections each year (17). The attackrate for Vibrio is small, as well, and the infectious dose of V. para-haemolyticus is 105 CFU.While the FDA allows the sale of oysterswith ≤104 CFU per gram of oyster meat to be sold in the U.S.,transmission has occurred from oyster beds where colony countshave been <200 CFU (2). Of note, pasteurization at 50°C for 10minutes significantly reduces the number of V. vulnificus and V. para-haemolyticus bacteria in oysters. Finally, in one study in the UnitedKingdom (2), 2% of Salmonella spp. from molluscs were found inbeds with “acceptable” fecal coliform counts.

Since the number of worms increases with the age and size of thefish, many people feel that harvesting younger, smaller fish willreduce the number of parasitic infections. Freezing can be effec-tive in killing the metacercariae, but salting, smoking, and mari-nating are unreliable for the purpose. Most sushi chefs are trainedto detect infected fish and usually prepare sushi from more expen-sive fish, such as tuna, yellow tail, red snapper, salmon, and flatfish/flounder. This is in contrast to some cheaper fish, such as cod,herring, mackerel, and squid, which tend to be more heavilyinfected with Anisakis.

References

1. Trevino, E.A. and A.S. Weissfeld. 2008. Infections in nail salons.Clin. Microbiol. Newsl. 30:9-11.

2. Butt, A.A., K.E. Aldridge, and C.V. Sanders. 2004. Infectionsrelated to the ingestion of seafood. Part 1. Viral and bacterial

infections. Lancet Infect. Dis. 4:201-212.

3. Centers for Disease Control and Prevention. Norovirus technicalfact sheet. http://www.cdc.gov/norovirus/hcp/clinical-overview.html. Accessed 22 November 2013.

4. Shieh, Y. et al. 2000. Detection of Norwalk-like virus in shellfishimplicated in illness. J. Infect. Dis. 181:S360-S366.

5. Widdowson, M.A. et al. 2005. Norovirus and foodborne disease,United States, 1991-2000. Emerg. Infect. Dis. 11:95-102.

6. Centers for Disease Control and Prevention. Hepatitis A. InThepink book: epidemiology and prevention of vaccine-preventablediseases http://www.cdc.gov/vaccines/pubs/pinkbook/downloads/hepa.pdf. Accessed 22 November 2013.

7. Eberhart-Phillips, J. et al. 1996. An outbreak of cholera from foodserved on an international aircraft. Epidemiol. Infect. 116:9-13.

8. Berg, D.E. et al. 2000. Multistate outbreaks of acute gastroenteritistraced to fecal-contaminated oysters harvested in Louisiana. J.Infect. Dis. 181(Suppl. 2):S381-S386.

9. Lowry, P.W. et al. 1989. Vibrio gastro-enteritis in Louisiana: a pro-spective study among attendees of a scientific congress in NewOrleans. J. Infect. Dis. 160:978-984.

10. Altman, L.K. 1993. What’s sauce for the oyster may also keep thedoctor away. The New York Times. http://www.nytimes.com/1993/10/19/health/what-s-sauce-for-the-oyster-may-also-keep-the-doctor-away.html. Accessed 22 November 2013.

11. Butt, A.A., K.E. Aldridge, and C.V. Sanders. 2004. Infectionsrelated to the ingestion of seafood. Part II. Parasitic infections andfood safety. Lancet Infect. Dis. 4:294-300.

12. Nawa, Y., C. Hatz, and J. Blum. 2005. Sushi delights and parasites:the risk of fishborne and foodborne parasitic zoonoses in Asia. Clin.Infect. Dis. 41:1297-1303.

13. Yoshimura, H. 1990. Clinical patho-parasitology of extra-gastro-intestinal anisakiasis, p. 145-154. In H. Ishikura and K. Kikuchi(ed.), Intestinal anisakiasis in Japan. Springer-Verlag, Tokyo, Japan.

14. Lopez-Serrano, M.S. et al. 2000. Gastroallergic anisakiasis: findingsin 22 patients. J. Gastroenterol. Hepatol. 15:503-506.

15. Matsuoka, H. et al. 1994. A case report of serologically diagnosedpulmonary anisakiasis with pleural effusion and multiple lesions.Am. J. Trop. Med. Hyg. 51:819-822.

16. Huss, H.H. 1992. Development and use of the HACCP conceptin fish processing. Int. J. Food Microbiol. 15:33-44.

17. Oshima, T. 1987. Anisakiasis: is the sushi bar guilty? Parasitol.Today 3:44-48.

Clinical Microbiology Newsletter 36:3,2014 | ©2014 Elsevier 21