The importance of wild gecko as a source of human Salmonella infection.

A total of 201 wild geckos from the region of Mekong Delta, Vietnam were collected to determine the viable number and survival period of Salmonella in their feces. Of the 101 samples examined, 24 (23.8%) were Salmonella positive. These 24 geckos excreted Salmonella in their feces in a range of 1 to 8.6 log CFU/g with a mean of 4.5 ± 3.2 log CFU/g. Among the Salmonella serovars, Salmonella Weltevreden was the most predominant serovar (37.5%). Moreover, Salmonella could survive for 6 weeks in gecko feces at room temperature in Vietnam. These results indicate that the wild gecko seems to play an important role as a reservoir for Salmonella and a source of Salmonella infection in humans in Southeast Asian countries.

Salmonella is known to be one of the important human bacterial pathogens in both developed and developing countries. Human salmonellosis was found in 0.6 to 7% of the total human diarrhea cases in Southeast Asian countries such as Laos, Myanmar, and Vietnam [3]. Wild geckos were commonly found in the residential areas of these countries [12] and are widely distributed and seen in close contact with humans. Recently, some researchers indicated that geckos could harbor Salmonella at a relatively high rate in Southeast Asian countries [1, 5]. Humans could be infected with Salmonella via contact with reptiles, and reptiles were considered to be a reservoir for Salmonella and a source of human salmonellosis [4, 7, 9,10,11, 14, 15]. However, no report has been published on quantification and survival analysis of Salmonella in gecko feces. Therefore, this study was carried out to determine the number and persistence of Salmonella in gecko feces.

In this study, a total of 201 wild geckos (138 Hemidactylus frenatus and 63 Hemidactylus platyurus) were captured in the Mekong Delta region, located in the South of Vietnam. Of 201 gecko fecal samples, 101 were examined for the number of Salmonella in their feces. About 0.1 g of feces (rectum contents) in each sample was collected and suspended in 9 times volume of phosphate buffer saline (PBS, pH 7.2). Following this, 0.1 ml of the suspension diluted 10-fold with PBS was plated on mannitol lysine crystal violet brilliant green agar (MLCB, Nissui, Tokyo, Japan) and deoxycholate hydrogen sulfide lactose agar (DHL, Nissui). The number of Salmonella colonies was counted on these selective media, after incubation at 37°C for 24 hr. If suspected Salmonella colonies did not appear on the selective media, samples were enriched with Hajna tetrathionate broth (Eiken, Tokyo, Japan) at 37°C for 24 hr. A loopful of enrichment broth was then streaked on the selective agars. The suspected Salmonella isolates were examined for biochemical characteristics, and serovars were identified by following the methods described by Tran et al. [13].

Of 101 gecko samples, 24 (23.8%) were Salmonella positive. Among these positive samples, 14 geckos excreted Salmonella ≥4 log CFU/g in their feces. The highest number of Salmonella in gecko feces was 8.6 log CFU/g, and the mean was 4.5 ± 3.2 log CFU/g (Fig. 1). These results indicated that wild geckos excreted a high number of Salmonella in their feces. Moreover, of 24 Salmonella isolates, S. Weltevreden was the most predominant serovar (37.5%), followed by S. Worthington (12.5%), S. Lexington (8.3%), S. Albany (4.2%), and S. Bellevue (4.2%) (Table 1). S. Weltevreden is known to be the predominant serovar in human salmonellosis in Southeast Asian countries such as Thailand, Malaysia, and Philippines [2, 6]. Ly et al. [8] also reported that S. Weltevreden was isolated from human patients with diarrhea in the region of Mekong Delta, Vietnam. Therefore, wild geckos seem to be the potential source of Salmonella infection, especially S. Weltevreden serovar, and spread Salmonella in the environment of these countries.

Fig. 1.

The number of Salmonella excreted in gecko feces (log CFU/g). The number of Salmonella in gecko feces <2 (log CFU/g) was calculated as 1 (log CFU/g).

Table 1.

Prevalence of Salmonella serovars isolated from gecko feces (n=24)

Serovar	No. of isolates (%)
S. Weltevreden	9 (37.5)
S. Worthington	3 (12.5)
S. Lexington	2 (8.3)
S. Albany	1 (4.2)
S. Bellevue	1 (4.2)
Biovar IIIb	1 (4.2)
Biovar IV	3 (12.5)
Untyped	4 (16.7)
Total	24 (100.0)

Of 201 gecko fecal samples, 101 were used for quantification analysis of Salmonella in gecko feces and 100 were divided into 2 groups. Fecal samples in each group were mixed and put into sterilized Erlenmeyer flasks (300 ml). These fecal mixtures were kept at the room temperature (25–30°C) of Vietnam for 10 weeks. About 1 g of fecal sample was taken from each mixture once a week for 10 weeks after storage. Isolation and identification of Salmonella from fecal samples were also done following the same methods as described above. In this experiment, Salmonella was isolated from fecal mixtures of both groups for 6 weeks after storage. However, no Salmonella was detected from fecal samples after 7 weeks. These results indicate that Salmonella can survive for a long time in gecko feces in normal environmental conditions in Vietnam. However, the mechanism involved in the survival of Salmonella for a long time in gecko feces in the environment is still unclear.

The present study indicated that the wild gecko seems to play an important role as a reservoir for Salmonella and a source of Salmonella infection in humans in Southeast Asian countries. Further research needs to be performed to evaluate the epidemiology of Salmonella in wild geckos of this region.

CONFLICTS OF INTEREST

No conflict of interest was declared.