Prevalence and diversity of Salmonella enterica in water, fish and lettuce in Ouagadougou, Burkina Faso

Our investigation on prevalence of Salmonella enterica in water from taps, wells, channels and reservoirs, fish, and lettuce grown in Ouagadougou,
Burkina Faso, indicated that Salmonella contamination is rare in drinking water but common in the samples from the other
sources (15-50 % prevalence). In a previous study in Lagos, Nigeria, Salmonella were isolated from 18,5 % of drinking water samples 19]. Serotyping of the Salmonella isolated in our study revealed the great diversity of serotypes in these sources:
we identified 50 different serotypes among the 98 isolates. Recently, in Burkina Faso,
383 Salmonella strains were isolated from animal faeces (cattle, poultry, swine and hedgehogs),
representing 81 different serotypes 17]. Of those, Agona, Bredeney, Brive, Carmel, Chester, Colindale, Derby, Drac, Eastbourne,
Fresno, Galiema, Kokomlemle, Korlebu, Muenster, Nima, Nottingham, Poona, Rissen, Schwarzenground,
Senftenberg, Typhimurium and Virchow were isolated also in our study indicating a
possible transfer between animals, their faeces, water, fish and irrigated vegetables.
The previously most commonly reported serotypes in Africa (Cameroon, Mali, Marocco,
Senegal and Tunisia), Enteritidis and Anatum 20], were not isolated in our study. Serotypes Muenster (the second most common serotype
isolated from fish), Cubana, Kentucky, Montevideo, Poona, Fresno, Virchow and Typhimurium
detected in our study, were also isolated in stool specimens from children under 5 years
old suffering from acute diarrhea in Ouagadougou and in rural Burkina Faso 14], 21].

Salmonella were isolated from 23 % of the surface water samples (channels and reservoirs). This
frequency is lower than in the survey conducted in Yaoundé, Cameroon, where Salmonella were isolated in 49.4 % of the surface water samples 22]. We isolated 22 different serotypes from surface water, showing more diversity in
Salmonella than reported elsewhere 23]–26]. No same serotypes were found from the two reservoirs studied, which may be due to
their different external factors. The reservoir of Tanghin located in the city is
influenced by people and the reservoir of Yamtenga in the rural area is less frequented
by people but more by animals. In any case, the presence of Salmonella in the surface waters reveals the health risk posed by the use of this water for
irrigation. It has been indicated that the potential hazards may be associated with
poor hygienic standard of waters influenced by human sewage, livestock farming, or
industry 27]. S. Poona, the only Salmonella serotype we isolated from well water, was also isolated from children under 5 years
suffering from acute diarrhoea in Ouagadougou and in rural Burkina Faso 14], 21].

Among the fish samples examined, 57 (24 %) were positive to Salmonella with high diversity of serotypes, 34 different ones were detected. In Kenya, along
the Lake Victoria Beaches, almost 17 % of Tilapia samples were positive for Salmonella with four different serotypes 28]. In Indonesia, about 10 % of fresh fish were reported to be contaminated by Salmonella5]. The high prevalence of Salmonella in fish in Ouagadougou is not surprising since they were caught from water of poor
hygienic quality. S. Schwarzengrund and Colindale were the serotypes found in both the fish and the reservoir
of Tanghin. Although as many as 34 different Salmonella serotypes were detected in the fish from Tanghin, only four serotypes were detected
in its water, presumably because the salmonellas are more sparsely distributed in
the water environment and thus in water samples. S. Typhimurium, an important clinical serotype, was isolated only from one fish sample
(0,4 %) in our study, thus, being much rarer than in the study of fish along the Lake
Victoria Beaches, where it was more isolated (8 %) 28]. In another study, conducted in Winam Gulf of Lake Victoria, Kenya, S. Typhimurium was isolated from 6 % of fish harvested 29]. We did not find any S. Typhi, perhaps because the enrichment technique used is not optimal to this serotype
30].

Our results revealed that 50 % of the analysed lettuce samples were positive for Salmonella with four different serotypes (S. Bredeney, S. Colindale, S. Gerland and S. Korlebu).

S. Colindale was found in lettuce, reservoir water of Tanghin and the channels. This
suggested that the lettuce has been contaminated by irrigating water. On the other
hand, S. Korlebu and S. Gerland found in lettuce samples were not detected in the water sources, but S. Korlebu has instead been identified in cattle faeces in Ouagadougou 17].Bacteria in lettuce might also come from another origin e.g. manure used as a fertilizer.
Salmonella has been demonstrated to contaminate vegetables (carrots, radish, lettuce and parsley)
in field experiments following treatment with contaminated manure compost or irrigation
water 7], 31]. In Finland, iceberg lettuce contaminated with S. Reading and Newport caused a nationwide outbreak, according to a traceback investigation 32]. S. Colindale which was common in our lettuce samples was also described as the most
common serotype in Gambia of non-typhoidal Salmonella in cases of enteric infection under the age of 5 years 10].

Reassuringly, our results showed that the great majority of environmental Salmonella strains are susceptible to the antimicrobials tested. Two strains, S. Wagadugu and S. Adabraka, both isolated from the reservoir water of Yamtenga were resistant to streptomycin.
Domestic animals treated by antibiotics might transfer resistant Salmonella from their faeces to this reservoir leading to antimicrobial resistance. No resistant
Salmonella were detected in fish reflecting the fact that antimicrobial agents are not used
in fish farming in Burkina Faso. On the contrary, 52 (14 %) of the Salmonella isolates from animal faeces in Ouagadougou were resistant to one or more tested antimicrobials
17]. In Nigeria, emerging multidrug-resistant Salmonella serotypes were commonly found from water sources 19].