Atypical cases of cholera due to serogroup O1 in the native population of Oman and its relation to non-O1/O139.

C is a major public health problem especially in developing countries and the seventh pandemic is still ongoing. In 2004, 56 countries officially reported 101,383 cases and 2,345 deaths to WHO and Africa accounted for 94% of these reported cases. Vibrio cholerae (V. cholerae) is an autochthonous inhabitant of marine and freshwater environments and also a facultative pathogen for humans. It is an acute bacterial enteric disease with high infection rate but has low disease rate. Important distinctions within the species are made on the basis of serogroups and production of cholera toxin (CT), which is responsible for severe diarrhea and potential epidemic spread. Only 2 sero-groups namely V. cholerae O1 and O139, have been considered as causative agents of cholera. All other strains namely, non-O1/O139 do not produce CT and are not usually involved in epidemics. These strains are frequently isolated from environmental sources and are associated with sporadic cases of gastroenteritis or extra-intestinal infections. Although occasionally non-O1/O139 strains can produce CT or other virulence factors; none of them have caused large scale epidemics. Diarrhea caused by V. cholerae is attributed to cholera enterotoxin (CT) codified by the ctx operon and regulated by a number of environmental factors and virulence protein genes such as toxT, toxR and toxS.1 Vibrio cholerae O1 may produce mild illness without causing outbreak and V. cholerae non-O1 produces enterotoxin similar to O1 causing cholera-like outbreak. A single factor cannot explain the entero-pathogenicity. However the ctx genetic element is increasingly associated with toxigenic V. cholerae O1 and O139 strains.1 In Oman, cholera is a notifiable disease since the beginning of communicable disease surveillance in March 1991. As early as 1999, 7 cholera cases were reported from Sohar, a coastal town in the North Batinah region. However, these cases were imported and amongst the non-nationals with no secondary transmission locally. Vibrio cholerae O1, Ogawa, biotype El Tor was isolated. In June 2000 the first indigenous cases of cholera were reported in Oman from Dhahira region. A descriptive study was conducted to investigate the atypical characteristic of the disease due to V. cholerae O1 and its probable relationship with non-O1/O139 strains from June to December 2002. A formal case-based epidemiological investigation for all the diagnosed cases was carried out upon notification. The stool samples for close family contacts and water samples were collected for examination. Stool samples were processed at the regional hospital laboratory using standard culture method. The stool samples were inoculated into Alkaline Peptone water and incubated for 6-8 hours, then further inoculated into TCBS selective media and incubated for 18-24 hrs at 37OC. The suspected colonies were sub-cultured in nutrient/blood agar and incubated overnight at 37OC. Presumptive V. cholerae identification was based on the positive oxidase test and reading from API 20 E system as V. cholerae. Grouping was carried out by using polyvalent antiserum (Mast group, UK) by slide agglutination method and later on reconfirmed by the Central Public Health Laboratory, Muscat. Total 26 cases of cholera were reported in Oman during the study period. Vibrio cholerae O1, El Tor was isolated from 12 cases while non-O1 strains from 14 cases. The median age was 17.5 years with a range of 2-48 years among the O1 positive patients. The female to male ratio was 1:1.4. The point prevalence of cholera O1 for the study period was 0.51/100,000 population. All the cases presented with mild symptoms of diarrhea with mild to moderate dehydration except in one imported case. The distribution of cases was focal namely restricted to only one Wilayat (District) of Dhahira region. The salient and atypical features observed were1 no explosive epidemic despite water contamination. The total numbers of cases were small, no clustering in time or place, mild clinical presentation, no fast spread and a wide range of incubation periods.2 The relation between the organism isolated from cholera patients and contacts could not be directly established. On the contrary some of the contacts were positive for nonO1.3 The organism isolated from the patients (O1) was different from the organism isolated from the environment (non-O1) except in one case where O1 was isolated from falaj water (a water stream).4 No significant exposure history could be elicited except in one imported case which had no relation to other cases.5 The degree of dehydration was minimal with no associated mortality.6 Multiple serotypes were isolated during the study period namely, V. cholerae O1, Ogawa from case no. 1-8, Inaba from case no. 9, 10 and 12, and Hikojima from case no. 11. The reasons for this atypical presentation of V. cholerae O1 in Oman probably are: 1) Persistence of the organism in a free, perhaps in an altered state in Brief Communication