Typhoid fever in the park: epidemiology of an outbreak at a cultural interface

Within-host mathematical modelling of the incubation period of Salmonella Typhi

Mechanistic mathematical models are often employed to understand the dynamics of infectious diseases within a population or within a host. They provide estimates that may not be otherwise available. We have developed a within-host mathematical model in order to understand how the pathophysiology of Salmonella Typhi contributes to its incubation period. The model describes the process of infection from ingestion to the onset of clinical illness using a set of ordinary differential equations. The model was parametrized using estimated values from human and mouse experimental studies and the incubation period was estimated as 9.6 days. A sensitivity analysis was also conducted to identify the parameters that most affect the derived incubation period. The migration of bacteria to the caecal lymph node was observed as a major bottle neck for infection. The sensitivity analysis indicated the growth rate of bacteria in late phase systemic infection and the net population of bacteria in the colon as parameters that most influence the incubation period. We have shown in this study how mathematical models aid in the understanding of biological processes and can be used in estimating parameters of infectious diseases.

Incubation period of typhoidal salmonellosis: a systematic review and meta-analysis of outbreaks and experimental studies occurring over the last century

BACKGROUND

Salmonella Typhi is a human pathogen that causes typhoid fever. It is a major cause of morbidity and mortality in developing countries and is responsible for several outbreaks in developed countries. Studying certain parameters of the pathogen, such as the incubation period, provides a better understanding of its pathophysiology and its characteristics within a population. Outbreak investigations and human experimental studies provide an avenue to study these relevant parameters.

METHODS

In this study, the authors have undertaken a systematic review of outbreak investigation reports and experimental studies, extracted reported data, tested for heterogeneity, identified subgroups of studies with limited evidence of heterogeneity between them and identified factors that may contribute to the distribution of incubation period. Following identification of relevant studies, we extracted both raw and summary incubation data. We tested for heterogeneity by deriving the value of I2 and conducting a KS-test to compare the distribution between studies. We performed a linear regression analysis to identify the factors associated with incubation period and using the resulting p-values from the KS-test, we conducted a hierarchical cluster analysis to classify studies with limited evidence of heterogeneity into subgroups.

RESULTS

We identified thirteen studies to be included in the review and extracted raw incubation period data from eleven. The value of I2 was 84% and the proportion of KS test p-values that were less than 0.05 was 63.6% indicating high heterogeneity not due to chance. We identified vaccine history and attack rates as factors that may be associated with incubation period, although these were not significant in the multivariable analysis (p-value: 0.1). From the hierarchical clustering analysis, we classified the studies into five subgroups. The mean incubation period of the subgroups ranged from 9.7 days to 21.2 days. Outbreaks reporting cases with previous vaccination history were clustered in a single subgroup and reported the longest incubation period.

CONCLUSIONS

We identified attack rate and previous vaccination as possible associating factors, however further work involving analyses of individual patient data and developing mathematical models is needed to confirm these as well as examine additional factors that have not been included in our study.

Survival of Salmonella Typhi and Shigella dysenteriae in dehydrated infant formula

Powdered infant formula has previously been linked to the transmission of various bacterial pathogens in infants resulting in life-threatening disease and death. Survival studies of 2 common foodborne pathogens, Salmonella enterica serovar Typhi and Shigella dysenteriae, in powdered infant formula have not been previously studied despite the potentially devastating consequences from ingestion of these organisms, particularly by newborns, in case of a natural or deliberate contamination event. Therefore, to better predict the risk of S. Typhi and S. dysenteriae infection from consumption of infant formula, the present study was undertaken to determine survival of these microorganisms in dry infant formula under varying atmospheric conditions. A 2-strain cocktail of S. Typhi and a 3-strain cocktail of S. dysenteriae were stored for up to 12 wk in dehydrated infant formula in an ambient air or nitrogen atmosphere. Viable counts of S. Typhi at 12 wk in infant formula revealed a 2.9- and 1.69-log decrease in ambient air and nitrogen atmosphere, respectively. Viable counts of S. dysenteriae at 12 wk in infant formula revealed a 0.81- and 0.42-log decrease in ambient air and nitrogen atmosphere, respectively. These results show that S. Typhi and S. dysenteriae can remain viable for prolonged periods of time in powdered infant formula, and the presence of nitrogen enhances survival.

PRACTICAL APPLICATION

Our goal in this work was to study the survival of S. Typhi and S. dysenteriae in dehydrated storage conditions in infant formula. This interest is partially generated by the possibility of using these 2 microorganisms to deliberately contaminate the food supply. The outcome of this study will help us to have a better idea how to respond and react to the risk of deliberate food contamination.

Enrichment-ELISA for detection of Salmonella typhi from food and water samples

OBJECTIVE

Development of monoclonal antibody based sandwich enzyme linked immunosorbant assay (sELISA) for rapid detection of Salmonella enterica serovar typhi (S. typhi) from food and water samples and optimization of enrichment procedures for use with the developed sELISA to increase the detection sensitivity of the assay.

METHODS

Spleen cells from BALB/c mice immunized with flagellin (H=d) antigen of S. typhi were fused with Sp2/0 myeloma cells. The hybridoma cell line specific to H=d antigen was established, characterized and ascites raised against one of these clones. The hyperimmune serum to flagellin antigen was raised in New Zealand White rabbits. An sELISA was developed using polyclonal antibody as capture and monoclonal antibody as detection antibody. To design the efficient culture strategies for use with the sELISA, different pre-enrichment and enrichment broths were evaluated. The media included buffered peptone water (BPW) and brain heart infusion broth for pre-enrichment and selenite F broth and Rappaport-Vassiliadis broth as enrichment broths. The developed sELISA with preceding enrichment step in BPW (Enrichment-ELISA) was evaluated in various food samples artificially inoculated with S. typhi bacteria. Various food (30) and water (35) samples collected from field were also tested by Enrichment-ELISA and culture method.

RESULTS

Out of four specific clones to H=d antigen, one clone (# 2/56, IgG2a isotype) was used in sELISA. The sELISA had the detection limit of 10(4)-10(5) cfu of S. typhi. Of the various broths used with sELISA, BPW was found to yield maximum ELISA values. Enrichment-ELISA, when tested in artificially inoculated food samples, generally, could detect 10(2) S. typhi cfu/mL within 10 h from various food rinses (meat, vegetable) and milk samples. After overnight enrichment in BPW, as less as 2 bacteria per 10 mL of milk, meat rinse, and chicken rinse could be detected. Only one of the field samples (water) gave false positive result by Enrichment-ELISA.

CONCLUSION

In comparison to culture, the Enrichment-ELISA is a rapid, sensitive, and specific method for detection of S. typhi from food or water samples. This method may be used as rapid screening procedure for environmental monitoring during outbreak situation.

Application of BAX system, Tecra Unique Salmonella test, and a conventional culture method for the detection of Salmonella in ready-to-eat and raw foods

AIMS

To compare the BAX system, the Tecra Unique Salmonella test, and a conventional culture method for the detection of Salmonella in various foods.

METHODS AND RESULTS

Ready-to-eat and raw foods were inoculated with Salmonella serotype Typhimurium, Salmonella serotype Enteritidis, Salmonella serotype Typhi, or Salmonella serotype Derby. Incubated pre-enrichment cultures were examined using the BAX system, the Tecra Unique Salmonella test, and a conventional culture method. Salmonella could be detected in all ready-to-eat food samples inoculated with S. Typhimurium, S. Enteritidis, or S. Derby, with any of the three test methods. However, false negatives were obtained with the Tecra test and the culture method when samples with higher background flora were inoculated with S. Typhi. Sensitivity test results suggested the two rapid tests performed as well as the culture method in the detection of 10(1) CFU of S. Typhimurium in 25-g cooked or raw food.

CONCLUSIONS

The BAX system and the Tecra Unique Salmonella test demonstrated results comparable with those of the culture method in the detection of Salmonella serotypes used except S. Typhi.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first evaluation of the BAX system, the Tecra Unique Salmonella test, and a culture method in the detection of Salmonella in a variety of western and oriental foods.

Detection of Salmonella enterica serovar Typhi (S. Typhi) by selective amplification of invA, viaB, fliC-d and prt genes by polymerase chain reaction in mutiplex format

AIMS

Development of a PCR assay that can target multiple genes for rapid detection of Salmonella enterica serovar Typhi (S. Typhi) from water and food samples.

METHODS AND RESULTS

PCR primers for invasion, O, H and Vi antigen genes, invA, prt, fliC-d and viaB were designed and used for the rapid detection of S. Typhi by multiplex PCR. Internal amplification control, which co-amplified with prt primers, was also included in the assay. The results showed that all cultures of Salmonella were accurately identified by the assay with no nonspecific amplification in other cultures. The assay had 100% detection probability when a cell suspension of 10(4) CFU ml(-1) (500 CFU per reaction) was used. Salmonella Typhi bacteria were artificially inoculated in the water and food (milk and meat rinse) samples and detected by mPCR after overnight pre-enrichment in buffered peptone water. No Salmonella bacteria could be detected from water samples collected from the field by mPCR or standard culture method.

CONCLUSIONS

The developed mPCR assay provides specific detection of S. Typhi.

SIGNIFICANCE AND IMPACT OF THE STUDY

Rapid methods for detection of S. Typhi from complex environmental matrices are almost nonexistent. The mPCR assay reported in this study can be useful to identify S. Typhi bacteria in field environmental samples.

Risk factors for typhoid fever among adult patients in Diyarbakir, Turkey

We conducted a case-control study to assess risk factors for typhoid fever in Diyarbakir, Turkey, a region where transmission of Salmonella typhi is endemic. We prospectively identified febrile patients from Diyarbakir and the surrounding area who were admitted to hospital. Cases were defined as patients who had S. typhi isolated from at least one blood culture. Sixty-four cases with blood culture-confirmed S. typhi were identified between May 2001 and May 2003. In total, 128 age- and sex-matched controls selected from neighbourhoods as cases were enrolled. We hypothesized that consumption of raw vegetables contaminated with sewage would be associated with an increased risk of typhoid fever. Conditional logistic regression modelling revealed that living in a crowded household (OR 3.31, 95% CI 1.58-6.92, P=0.002), eating cig kofte (a traditional raw food) (OR 5.29, 95% CI 2.20-12.69, P=0.000) and lettuce salad (OR 3.55, 95% CI 1.52-8.28, P=0.003) in the 15 days prior to symptoms onset was independently associated with typhoid fever. We conclude that living in a crowded household and consumption of raw vegetables outside the home increase the risk of typhoid fever in this region.

Current trends in typhoid Fever

Typhoid fever, a systemic infection caused by Salmonella enterica serotype typhi, remains an important worldwide cause of morbidity and mortality. Endemic cases in the United States are unusual, with most following foreign travel to the Indian subcontinent, Africa, Asia, or Latin America. The classic findings of typhoid fever include rose spots, relative bradycardia, and stepwise fevers, but unfortunately these signs are frequently absent. Gastrointestinal manifestations may include diffuse abdominal pain, bleeding, perforation, cholecystitis, and cholangitis. The diagnosis should be suspected after collection of the appropriate clinical and travel history with confirmation by blood or bone marrow culture. Novel methods are in development to establish the diagnosis when cultures are negative or unavailable. Multidrug resistance has increased worldwide, and decisions on antimicrobial therapy must take such resistance into account. The empiric treatment of choice is a fluoroquinolone drug; ceftriaxone and azithromycin are alternatives. Preventive strategies include good sanitation and food handling practices along with vaccination of selected groups.

Poor food hygiene and housing as risk factors for typhoid fever in Semarang, Indonesia

To identify risk factors for typhoid fever in Semarang city and its surroundings, 75 culture-proven typhoid fever patients discharged 2 weeks earlier from hospital and 75 controls were studied. Control subjects were neighbours of cases with no history of typhoid fever, not family members, randomly selected and matched for gender and age. Both cases and controls were interviewed at home by the same trained interviewer using a standardized questionnaire. A structured observation of their living environment inside and outside the house was performed during the visit and home drinking water samples were tested bacteriologically. Univariate analysis showed the following risk factors for typhoid fever: never or rarely washing hands before eating (OR = 3.28; 95% CI = 1.41-7.65); eating outdoors at least once a week (OR = 3.00; 95% CI = 1.09-8.25); eating outdoors at a street food stall or mobile food vendor (OR = 3.86; 95% CI = 1.30-11.48); consuming ice cubes in beverage in the 2-week period before getting ill (OR = 3.00, 95% CI = 1.09-8.25) and buying ice cubes from a street vendor (OR = 5.82; 95% CI = 1.69-20.12). Water quality and living environment of cases were worse than that of controls, e.g. cases less often used clean water for taking a bath (OR = 6.50; 95% CI = 1.47-28.80), for brushing teeth (OR = 4.33; 95% CI = 1.25-15.20) and for drinking (OR = 3.67; 95% CI = 1.02-13.14). Cases tended to live in houses without water supply from the municipal network (OR=11.00; 95% CI = 1.42-85.2), with open sewers (OR = 2.80; 95% CI = 1.0-7.77) and without tiles in the kitchen (OR = 2.67; 95% CI = 1.04-6.81). Multivariate analysis showed that living in a house without water supply from the municipal network (OR = 29.18; 95% CI = 2.12-400.8) and with open sewers (OR = 7.19; 95% CI = 1.33-38.82) was associated with typhoid fever. Never or rarely washing hands before eating (OR = 3.97; 95% CI = 1.22-12.93) and being unemployed or having a part-time job (OR = 31.3; 95% CI = 3.08-317.4) also were risk factors. In this population typhoid fever was associated with poor housing and inadequate food and personal hygiene.