Concurrent Conditions and Human Listeriosis, England, 1999–2009

Piers Mook, Sarah J. O'Brien, and Iain A. Gillespie

Disclosures

Emerging Infectious Diseases. 2011;17(1):38-43. 

In This Article

Discussion

We analyzed surveillance data that included detailed denominator data by using an internationally recognized diagnostic classification system and found that a wide variety of conditions seem to increase the risk for serious infection with L. monocytogenes. Malignancies accounted for more than one third of conditions, and cancer patients had a 5-fold increased risk for development of listeriosis. Cancers of the blood seemed to have the greatest effect. Other high-risk conditions included diabetes mellitus; alcoholism; certain diseases of the circulatory system and the musculoskeletal system and connective tissue; noninfective enteritis and colitis; and diseases of the liver and kidney. For most high-risk conditions, the risk for infection was higher among older patients.

Case identified by the national surveillance program in England are laboratory confirmed, and most cases result in serious illness requiring hospitalization or death. Given this finding, a hospitalized population better represents the population at risk than a community population, which was used in previous studies.[10,11]

The response rate to the clinical questionnaire that captured information on concurrent conditions was high and not influenced by age or sex of the case-patient, which minimized differential ascertainment of clinical data. However, we could not assess concurrent conditions for which completed clinical questionnaires were not returned. This issue indicates that the role of some conditions might be underestimated if clinicians were unwilling to return questionnaires and disclose information for certain case-patients (e.g., those with AIDS). Similarly, but less likely, reporting bias might exist if the propensity to report certain concurrent conditions were affected by the presence or absence of others conditions, or if only concurrent conditions considered relevant to L. monocytogenes infection were reported. Concurrent conditions were reported by the clinical microbiologist rather than by the consultants responsible for the care of the patients with concurrent conditions. These consultants might be better informed of existing concurrent conditions. However, hospital microbiologists need to be aware of such conditions to provide treatment accordingly, and questioning several consultants for each case-patient may have a negative effect on questionnaire response because questionnaires might be lost if passed between multiple consultants.

Misclassification was minimized by grouping conditions only to 3-character ICD-10 code levels. Although we acknowledge that such grouping might mask high-risk conditions apparent at the 4-character ICD-10 code level, routine surveillance data were not specific enough to further discriminate among conditions. In some instances, in which treatments were reported in the absence of relevant conditions (e.g., chemotherapy, dialysis, splenectomy), we made assumptions about the conditions requiring such treatment and coded accordingly (online Technical Appendix). Although these assumptions could inflate the incidence rates for certain conditions, they occurred relatively infrequently and were not used for treatments that could be prescribed for a range of conditions (e.g., broad-spectrum antimicrobial drugs).

Because only single-variable analysis could be performed, we could not assess the extent to which concurrent conditions were correlated, which led to the potential for uncontrolled confounding. Such method limitations might explain the high incidence associated with both diabetes and kidney disease and reinforce the need to consider these findings as highly refined hypotheses to be tested by other methods.[12]

To our knowledge, few studies have attempted to quantify the risk for listeriosis by patient concurrent conditions. As part of a risk assessment of L. monocytogenes in ready-to-eat foods, researchers from the World Health Organization (WHO) and the Food and Agriculture Organization (FAO) calculated the relative susceptibility to listeriosis for certain conditions.[10] Furthermore, risk levels for listeriosis by predisposing condition in Denmark have also been estimated.[11] Despite differences in methods between those studies and our study, several high-risk conditions were also identified in those studies: malignancies (most notably those of the blood), kidney disease (recorded as dialysis[10] and renal transplant,[11] diabetes, alcoholism, and increased age in all 3 studies; liver disease and pulmonary cancer in the WHO/FAO study and our study; and systemic lupus erythematosus in the study in Denmark and our study (as systemic connective tissue disorders). Such commonality would seemingly validate our estimates.

The absence of AIDS as a high-risk condition in our study and its presence in both previous studies,[10,11] might reflect improved treatment for HIV infection that prevents AIDS and, consequently, L. monocytogenes infection[13] or highlight a reporting bias by the consultant microbiologist. A general transplantation status, identified as a condition leading to the highest relative susceptibility in the WHO/FAO study, was not coded in our study because it is a treatment. Noninfective enteritis and colitis and certain diseases of the circulatory system were identified as additional high-risk conditions in our study but not in the previous studies. These additional conditions might be the result of improved accuracy, use of ICD-10 coding and a hospitalized reference population instead of the general population, different susceptibility calculations, or changes in the prevalence of certain conditions in the interim period (the previous studies used data from 1992[10] and 1989–1990.[11] However, we acknowledge that links between these conditions and listeriosis have been reported.[14–18]

With these caveats in mind, our findings have implications for clinical practice and food safety policy makers. The number and diversity of conditions that appear to increase the risk for listeriosis imply that physicians working in all specialties should consider listeriosis when treating patients with concurrent conditions and provide appropriate food safety advice. Similarly, current UK government food safety advice on avoidance of listeriosis, which is delivered passively and is specific mainly for pregnant women,[19,20] should be communicated actively to all high-risk groups. In prioritizing advice, policy makers should consider not only the associated risk but also the prevalence of the concurrent condition. Cancer patients accounted for more than one third of listeriosis cases, and high risks were observed for most cancer subgroups. Because we are not aware of any appropriate food safety advice that is tailored specifically for cancer patients in the UK, emphasis on this group might help to prevent further cases.

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