Salmonella Serovars from Humans and Other Sources in Thailand, 1993–2002

We serotyped 44,087 Salmonella isolates from humans and 26,148 from other sources from 1993 through 2002. The most common serovar causing human salmonellosis in Thailand was Salmonella enterica Weltevreden. Serovars causing human infections in Thailand differ from those in other countries and seem to be related to Salmonella serovars in different food products and reservoirs.

We serotyped 44,087 Salmonella isolates from humans and 26,148 from other sources from 1993 through 2002. The most common serovar causing human salmonellosis in Thailand was Salmonella enterica Weltevreden. Serovars causing human infections in Thailand differ from those in other countries and seem to be related to Salmonella serovars in different food products and reservoirs.
S almonella enterica is one of the most common causes of human gastroenteritis. The infection is caused primarily by improper handling and digestion of uncooked food; a large number of food animal sources have been identified as reservoirs of the bacteria (1). More than 2,500 serovars of S. enterica have been identified; most have been described as the cause of human infections, but only a limited number of serovars are of public health importance. S. enterica serovars Typhimurium and Enteritidis have been reported to be the most common causes of human salmonellosis (1,2). However, in some regions other serovars are of greater importance (3,4).
Different serovars in one country can be of global importance because of travel and animal and food product trade. Knowledge about the occurrence and epidemiology of different serovars in different countries and geographic regions may assist in the recognition and tracing of new emerging pathogens. We review the trends in serovars of Salmonella causing infections in humans and potential reservoirs in Thailand during 1993 to 2002.

The Study
The On arrival, all isolates were purified and confirmed to be Salmonella on the basis of reactions on triple sugar iron agar and lysine indol motility agar. All strains identified as S. enterica were serotyped according to the Kauffman-White serotyping scheme (5). Salmonella antisera (S & A Reagent Laboratory LMT, Bangkok, Thailand) were used for serotyping. From 1993 through 2002, a total of 70,235 isolates received were confirmed as S. enterica and serotyped.
Samples have not been systematically taken from the different reservoirs for Salmonella infections in humans. However, data from samples were available from chicken, seafood, other food products, and water for all 10 years. Data from ducks were only available from 1998 to 2002.
The 10 most common serovars from all sources are given in Table 2. All serovars that were represented with >6% of the isolates are given in Figure 2.
Similar trends were detected for some of the serovars among isolates causing infections in humans and contami-

Conclusions
S. enterica continues to be one of the most important causes of foodborne gastrointestinal infections in humans. During the last few decades S. Enteritidis and S. Typhimurium have emerged as the two predominant serovars in most Western countries. The epidemiology of these serovars has been studied, and different programs have been established to limit the spread of these serovars. However, other serovars may have a different epidemiology, and conditions are optimal for spread between reservoirs in some countries.
Our report indicates that S. Weltevreden was the most common serovar isolated from humans in Thailand. A similar finding has been reported from Malaysia (3), and S. Weltevreden was the most common serovar to cause human infections in India during the early 1970s (6).  raw vegetables, suggesting that this is a potential reservoir of this serovar in Malaysia. Raw vegetables may, however, be contaminated by both feces and water. In a recent study from the United States, S. Weltevreden was the most common serovar found in seafood mainly imported from Thailand and Malaysia (8). These observations could point to a water-related source for S. Weltevreden. S. Weltevreden was the most common serovar in isolates from seafood, water, and duck, which suggests a waterrelated source for this serovar in Thailand. S. Enteritidis infections in humans in Thailand increased from 1.3% in 1990 to 14% in 1993 to 1994 (4). Thus, Thailand has also been part of the global pandemic of S. Enteritidis observed in the late 1980s (9). The 1995 global survey conducted by WHO showed that the global pandemic has continued and expanded (2). The S. Enteritidis pandemic appears to have ended in 1997; this finding is similar to the decrease observed in Thailand, where S. Enteritidis has decreased during the last decade. However, this serovar is still an important cause of human infections, reflected in the increase in 2002. The frequent occurrence of this serovar in chickens suggests that poultry may be an important reservoir, a finding that is consistent with almost all other studies in other countries (10). Eggs have also been found to be important reservoirs in other countries (10) but were not examined in this study.
S. Anatum has consistently been one of the most important causes of salmonellosis in Thailand. The main reservoirs seem to be other food products and water. This serovar has previously been isolated from a large number of different animal sources.
The importance of S. Derby has decreased in Thailand. S. Derby has been associated with pigs (11,12). Pork and other swine products were not sampled in this report, but a frequent occurrence of the serovar was observed among unspecified other food products, which could include pork.
Salmonella isolates of serovar (1,4,5,12:I) were frequently found in isolates from humans, but infrequently in isolates from the different food reservoirs. Thirty isolates from Thailand were examined by phage typing and susceptibility testing (data not shown). A variable resistance pattern was observed, and five (17%) had a resistance pattern and phage reaction in agreement with S. Typhimurium U302. Twenty (67%) did not react with any phages. Some of these isolates from Thailand might be S. Typhimurium, but a large number might belong to other serovars.
S. Typhimurium is among the most prevalent serovars in Europe and America and of growing importance in Southeast Asia, Africa, and the Western Pacific (2). In Thailand, the importance of this serovar has not increased and continues to account for 5% of all human infections. S. Typhimurium can be found among a large number of different animal reservoirs; no specific source has been found.
S. Rissen has been isolated infrequently as a cause of human infections, and limited information about the potential reservoirs are available. The importance of this serovar seems to be increasing in Thailand. A specific reservoir for