zoonoses and communicable diseases common to ... - PAHO/WHO

120 BACTERIOSESSource of Infection and Mode of Transmission: Non-O1 V. cholerae is a naturalinhabitant of the surface waters of estuaries, rivers, streams, lakes, irrigationchannels, and the sea. It has also been isolated from wastewater from an Argentinecity (Corrales et al., 1989). Thus, water constitutes the principal reservoir of this etiologicagent.Non-O1 V. cholerae has been isolated from many animal species in different partsof the world. However, their role as reservoirs is still in dispute (Morris, 1990).Man can be a carrier of the agent and the source of infection for others. In a studyconducted on Iranian pilgrims returning from Mecca, several of their contactsacquired the infection and had diarrhea (Zafarí et al., 1973). The source of infectionis different in each country. In the US, the main source of infection is raw oysters.Of 790 samples of fresh oysters, 14% contained non-O1 V. cholerae. The number ofisolations was higher in the summer months, when there are more vibrios in thewater (Twedt et al., 1981). As expected, the highest number of human cases has alsooccurred in summer and autumn. A variety of contaminated foods have been implicatedin other countries (see the section on occurrence in man). Surface water adjacentto a cistern was possibly the vehicle of infection for an outbreak in Sudan in1968 (Kamal, 1971). In a refugee camp in Thailand, 16% of drinking water sampleswere contaminated by non-O1 V. cholerae (Taylor et al., 1988).A case of cystitis occurred in a woman after she swam in Chesapeake Bay (USA).Ear and wound infections have almost always been caused by exposure to seawater.It is more difficult to establish the source of infection in septicemias. Some are associatedwith diarrhea, which would indicate infection via the oral route. Shellfishhave been suspected in several cases.Only a minority of strains of non-O1 V. cholerae are pathogenic. At present, it canbe stated that strains isolated from patients are more virulent than environmentalstrains. Strains are differentiated based on hemolysins, their ability to colonize theintestine (adherence factor), and the production of a toxin similar to cholera toxin,Shiga-like toxin, and a thermostable enterotoxin similar to that produced by enterotoxigenicEscherichia coli. In India and Bangladesh, non-O1 strains that producecholera toxin were isolated, but this happens less frequently in other countries. InThailand, only 2% of 237 environmental non-O1 strains and none of 44 strains isolatedfrom clinical cases carried gene sequences homologous with the cholera toxingene. In summary, strains of V. cholerae vary greatly in terms of the factors thatcould determine virulence and no single characteristic has been identified that couldbe used to differentiate pathogenic strains from avirulent strains (Morris, 1990).Role of Animals in the Epidemiology of the Disease: Although the agent hasbeen isolated from many animal species and many researchers consider such animalsreservoirs or possible sources of infection for man (Sack, 1973; Sanyal et al.,1974), their actual role is questionable.Diagnosis: Culture, isolation, and characterization of the microorganism is theonly irrefutable method for diagnosing the disease. Alkaline peptone water (APW)and Monsur broth with tellurite and bile salts are useful enrichment media. The recommendedselective medium is thiosulfate citrate bile salts sucrose agar (TCBS)(Corrales et al., 1989).The corresponding antiserum should be used for specific diagnosis of the O139strain (WHO, 1993).