An article in Emerging Infectious Diseases describes a joint collaboration between the CDC and Mexican health authorities that built a system to monitor the spread of Salmonella through the food chain and into people. One finding shocked me.
The authors examined four Mexican states, and the carriage of Salmonella in asymptomatic children ranged from 1.9% to over 11 percent. That’s a lot of kids with Salmonella who are not exhibiting any symptoms:
Salmonella carriage was strongly correlated with contamination of meat by Salmonella* and by overall poverty. While the healthy kids had fewer of the ‘nasty’ Salmonella types (Enteriditis and Typhimurium) as compared to food animals, they were still found in asymptomatic children. It’s not clear why this is the case. Several explanations present themselves:
- These Salmonella were associated with a previous bout of disease that was not recorded (‘healthy’ was defined as having no diarrhea in the previous week), and these strains persist at a density below the threshold for causing disease.
- Mexican children acquire immunity to Salmonella, allowing for stable coexistence.
- These asymptomatic strains are less able to cause disease, and are essentially commensals.
Now, if there were only a biologist in a position to sequence some of these genomes to get at these questions….
*The table is a summary. Within the poorest communities, food contamination was associated with asymptomatic carriage.
Cited article: Zaidi MB, Calva JJ, Estrada-Garcia MT, Leon V, Vazquez G, Figueroa G, et al. Integrated food chain surveillance system for Salmonella spp. in Mexico. Emerg Infect Dis [serial on the Internet]. 2008 Mar [date cited]. Available from http://www.cdc.gov/EID/content/14/3/429.htm