Human listeriosis : sources and routes

Abstract: The bacterium Listeria monocytogenes can cause the disease listeriosis in both humans and animals. For the epidemiological investigation of listeriosis detection and characterisation of the organism are important steps.Paper I. There are few reports on the incidence of L. monocytogenes in clinical samples from humans in India. Therefore, we investigated 144 samples from immunocompromised patients. L. monocytogenes was isolated from two placental bits from women with poor obstetric history, one patient with renal failure and three other patients. Five isolates were positive for the virulence genes hlyA, actA and iap. The sixth isolate was positive for hlyA and actA genes.Paper II. Characterisation of 601 human L. monocytogenes isolates causing invasive listeriosis during the period 1986 to 2007 in Sweden reveals a decrease in serovar 4b strains. Since 1996, serovar 1/2a has become the predominant serovar causing human listeriosis: PFGE analysis revealed two clusters including different serotypes suggesting that we need more studies on genetic relatedness among clinical isolates.Paper III. The incidence of Listeria species in seafood from markets in Goa was studied. One hundred and fifteen raw/fresh seafoods bought at the fish markets were sampled and tested for presence of Listeria spp. L. monocytogenes was detected in 10 samples. L. monocytogenes in raw seafood may pose a health risk in kitchen if contaminating ready-to-eat food.Paper IV. Gravad and cold-smoked salmon are associated with human listeriosis in Sweden. L. monocytogenes was isolated from 11 of 56 products. Serovar 1/2a was predominant, followed by 4b. REA/PFGE typing of the isolates identified five types of L. monocytogenes. One type was identical to a human type, two other were closely related.These findings suggest that gravad and cold-smoked salmon are still possible sources of listeriosis in Sweden.Paper V. Many outbreaks of listeriosis have been related to consumption of dairy-associated products. Therefore, 123 farm bulk milk samples in India and 20 cervico-vaginal samples from dairy cows with reproductive disorders were investigated for L. monocytogenes.L. monocytogenes was isolated from 17.9% of bulk milk samples and from 10% of cervico-vaginal swabs. The virulence gene hlyA was detected in all isolates. These findings represent a public health risk where unpasteurised milk and milk products are largely consumed.Paper VI. Isolates of L. monocytogenes (n=177) from 22 animal species were characterized and compared with human strains isolated between 1986-2006 in Sweden. Although many animals and humans shared pulsovars, they did not appear at the same time or with the same proportion of strains. The pulsovars shared by both animals and humans may indicate that there is an exchange of L. monocytogenes strains between these two groups due to either direct or indirect transmission.