Case Number: 17 - 33

Comments

This was a nationwide outbreak attributed to chocolate bars (16 g each) contaminated with S. Napoli. Although the overall attack rate in the population exposed cannot be determined, details were given on three individuals: a mother and two sons. All three ate two bars on the first day, and one son ate two more bars on the second day. The son that ate chocolate bars on both days became ill. He may have received a larger dose, or, alternatively, not all the bars were contaminated and the ill child ingested a single contaminated bar. We can only state that the attack rate for the one child that ate four chocolate bars was 100%.

A box of chocolates, which consisted of 8 packets with 6 bars in each packet, was obtained from a retailer from whom two patients had purchased chocolate. This box of chocolates was analysed and 42 of the 48 bars examined were positive, with the average concentration for the positive bars reported to be 16 organisms per 10 g. The highest concentration for one bar was 240 organisms per 10 g, and the lowest was 3 organisms per 10 g. It was also observed that the level of contamination per packet was not consistent. Packets consisting of 6 bars that were all positive also tended to have a higher contamination level. Of the 8 packets examined, the packet with the highest average concentration was 58.5 organisms per 10 g.

Since information is only known about one case, these data were not considered for further analysis.

Comments

In order to incorporate uncertainty in the dose, the concentration and amount consumed were assumed to have a potential range of 25% of the one reported. The lower and upper bounds for the dose were estimated to be 135 000 (1500 CFU/g × 90 g) to 375 000 (2500 CFU/g × 150 g). Since the size of the exposed population was reasonably small, it can be assumed that the uncertainty associated with the exposed and positive populations is minimal.

Outbreak parameter uncertainty

Comments

In order to incorporate uncertainty in the dose, the concentration and amount consumed were assumed to have a potential range of 25% of the one reported. The lower and upper bounds for the dose were estimated to be 1 080 000 (24 000 CFU/g × 45 g) to 3 000 000 (40 000 CFU/g × 15 g). The exposed and positive populations in this case were potentially uncertain. Since the degree of uncertainty is unknown, it was assumed that the reported exposed population could not have been exceeded; however, there could have been 10% fewer people actually exposed. The number of positives reported was assumed to represent the most likely number, but a 5% under- and over-reporting were allowed for.

Outbreak parameter uncertainty

Comments

The number of potentially exposed elementary school students (6 to12 years old) was very large, since a central cooking facility served 15 schools. Patients were found from almost all the schools, but there was an indication that most of the exposures occurred at 5 schools. It is highly unlikely that all 10 775 people were exposed to contaminated food. As a result, it was assumed that only a proportion, ranging from 1/2 to 1/4 of the total potentially exposed population, were actually exposed. There could also be uncertainty in the number of positives, but given the size of the denominator (exposed population) and the size of the numerator (positives), incorporating a 5% allowance for under- and over-reporting has minimal effect on the attack rate uncertainty range.

In order to incorporate uncertainty in the dose, the concentration and amount consumed were assumed to have a potential range of 25% of the one reported. The lower and upper bounds for the dose were estimated to be 495 (30 CFU/g × 16.5 g) and 1375 (50 CFU/g × 27.5 g), respectively.

Outbreak parameter uncertainty

Comments

In order to incorporate uncertainty in the dose, the concentration and amount consumed were assumed to have a potential range of 25% of the values reported. The lower and upper bounds for the dose were estimated to be 1 057 500 (35 250 CFU/g × 30 g) and 2 937 500 (58 750 CFU/g × 50 g). The exposed and positive populations in this case were also potentially uncertain. Since the degree of uncertainty is unknown, it was assumed that the reported exposed population could not have been exceeded, and also that there could have been 10% fewer people actually exposed. The number of positives reported was assumed to represent the most likely number, but 5% under- and over-reporting was allowed for.

Outbreak parameter uncertainty

Comments

In order to incorporate uncertainty in the dose, the concentration and amount consumed were assumed to have a potential range of 25% of the one reported. The lower and upper bounds for the dose were estimated to be 337 500 (4500 CFU/g × 75 g) and 937 500 (7500 CFU/g × 125 g), respectively. Since the size of the exposed population was reasonably small, it can be assumed that the uncertainty associated with the exposed and positive populations is minimal.

Outbreak parameter uncertainty

Comments

The attack rate that was reported for this outbreak was based on exposure of the entire school population that received lunch from the central kitchen. With such a large exposed population, which can be highly uncertain, the estimated attack rate can vary widely. It is highly unlikely that the entire reportedly exposed population was actually exposed to the contaminated food. Unlike the prior school outbreak (Case Number 20), there was no indication in this case of some schools being more likely to have been exposed than others. As a result, it was assumed that only a proportion, ranging down to 1/2 of the total potentially exposed population, were actually exposed. There could also be uncertainty in the number of positives, but given the size of the denominator (exposed population) and the size of the numerator (positives), incorporating a 5% allowance for under- and over-reporting has minimal effect on the attack rate uncertainty range.

In order to incorporate uncertainty in the dose, the concentration and amount consumed were assumed to have a potential range of 25% of the one reported. The lower and upper bounds for the dose were estimated to be 28 (1.05 CFU/g × 26.25 g) and 77 (1.75 CFU/g × 43.75 g), respectively.

Outbreak parameter uncertainty

Comments

The food was a rice dish covered with cooked chicken and eggs. Of 133 exposed people, 16 were adults (3 became ill) and 117 were children (50 became ill). Day care-aged children were assumed to be of increased potential susceptibility to foodborne pathogens. Because of the outbreak setting (day care), the exposed and positive populations were assumed to be well characterized in this case.

In order to incorporate uncertainty in the dose, the concentration and amount consumed were assumed to have a potential range of 25% of the one reported. The lower and upper bounds for the dose were estimated to be 2278 (20.25 CFU/g × 112.5 g) and 6328 (33.75 CFU/g × 187.5 g), respectively.

Outbreak parameter uncertainty

Comments

The attack rate that was reported for this outbreak was based on exposure of the entire school population that received lunch from the central kitchen. With such a large exposed population, which can be highly uncertain, the estimated attack rate can vary widely. It is highly unlikely that the entire reportedly exposed population was actually exposed to the contaminated food. In addition, the reported concentration per gram of food was less than 100 CFUs, which introduces a second significant uncertain parameter.

Comments

Eggs were pooled in the preparation of this food. The number exposed was the number of people who were served with this dish. Of the 191 served, 128 answered the food-intake questionnaire. Some of the hospital patients could not talk. Among 128 responses, 36 did not actually consume this dish. Among the 45 cases, 2 were tuberculosis (TB) patients and apparently had taken antibiotics. The number of TB patients in the actual exposed population is unknown. This outbreak is highly unusual because the dose is very high but the attack rate is very low. In addition, the outbreak is reported to have occurred in a hospital, an environment in which one might expect, depending on the circumstances, the exposed population to be more susceptible than the overall population. Because of the uncertainties in these data and the potential confounding factors, this outbreak was not included for further analysis.

Comments

This outbreak is unusual, like the previous hospital-associated outbreak (Number 27). Eggs were pooled and mixed well in preparing this dish. The actual number of individuals exposed is suspected to be lower than originally reported. The reported attack rate is lower than would be expected at this high dose level. It should be noted that some of the patients had antibiotic treatment, which may be a confounding factor in interpretation of these data.

Comment

This outbreak is also unusual, similar to the previous two hospital outbreaks, although in this case the dose is not as high as reported in Numbers 27 and 28. Information about confounding factors in these hospital outbreaks, such as diagnoses and treatments that patients were undergoing, was not available. Therefore, the three Japanese hospital outbreaks were not included in further analysis.

Comment

In order to incorporate uncertainty in the dose, the concentration and amount consumed were assumed to have a potential range of 25% of the values reported. The lower and upper bounds for the dose were estimated to be 3375 (150 CFU/g × 22.5 g) and 9375 (250 CFU/g × 37.5 g), respectively. The exposed and positive populations in this case were also potentially uncertain. Since the degree of uncertainty is unknown, it was assumed that the reported exposed population could not have been exceeded, and it was assumed that there could have been 10% fewer people actually exposed. The number of positives reported was assumed to represent the most likely number, but 5% under- and over-reporting was allowed for.

Outbreak parameter uncertainty

Comment

In order to incorporate uncertainty in the dose, the concentration and amount consumed were assumed to have a potential range of 25% of the values reported. The lower and upper bounds for the dose were estimated to be 236 (11 CFU/g × 22.5 g) and 656 (18 CFU/g × 37.5 g), respectively. The exposed and positive populations in this case were also potentially uncertain. Since the degree of uncertainty is unknown, it was assumed that the reported exposed population could not have been exceeded, and it was assumed that there could have been 10% fewer people actually exposed. The number of positives reported was assumed to represent the most likely number, but 5% under- and over-reporting was allowed for.

Outbreak parameter uncertainty

Comment

This outbreak was assumed to represent a susceptible population since the outbreak occurred in a day care facility. In order to incorporate uncertainty in the dose, the concentration and amount consumed were assumed to have a potential range of 25% of the values reported. The lower and upper bounds for the dose were estimated to be 13 (0.59 CFU/g × 22.5 g) and 37 (0.98 CFU/g × 37.5 g), respectively. The exposed and positive populations were assumed to be well characterized in this case because of the outbreak setting (day care).

Outbreak parameter uncertainty

Comment

In order to incorporate uncertainty in the dose, the concentration and amount consumed were assumed to have a potential range of 25% of the one reported. The lower and upper bounds for the dose were estimated to be 4.22E+9 (3.75E+7 CFU/g × 112.5 g) and 1.17E+10 (6.25E+7 CFU/g × 187.5 g), respectively. Since the size of the exposed population was reasonably small, it can be assumed that the uncertainty associated with the exposed and positive populations is minimal.

Outbreak parameter uncertainty