Abstract:
BACKGROUND:
An outbreak of listeriosis was identified in South Africa in 2017. The source was
unknown.
METHODS:
We conducted epidemiologic, trace-back, and environmental investigations and
used whole-genome sequencing to type Listeria monocytogenes isolates. A case was
defined as laboratory-confirmed L. monocytogenes infection during the period from
June 11, 2017, to April 7, 2018.
RESULTS:
A total of 937 cases were identified, of which 465 (50%) were associated with
pregnancy; 406 of the pregnancy-associated cases (87%) occurred in neonates. Of
the 937 cases, 229 (24%) occurred in patients 15 to 49 years of age (excluding
those who were pregnant). Among the patients in whom human immunodeficiency virus (HIV) status was known, 38% of those with pregnancy-associated
cases (77 of 204) and 46% of the remaining patients (97 of 211) were infected with
HIV. Among 728 patients with a known outcome, 193 (27%) died. Clinical isolates
from 609 patients were sequenced, and 567 (93%) were identified as sequence type
6 (ST6). In a case–control analysis, patients with ST6 infections were more likely
to have eaten polony (a ready-to-eat processed meat) than those with non-ST6
infections (odds ratio, 8.55; 95% confidence interval, 1.66 to 43.35). Polony and
environmental samples also yielded ST6 isolates, which, together with the isolates
from the patients, belonged to the same core-genome multilocus sequence typing
cluster with no more than 4 allelic differences; these findings showed that polony
produced at a single facility was the outbreak source. A recall of ready-to-eat processed meat products from this facility was associated with a rapid decline in the
incidence of L. monocytogenes ST6 infections.
CONCLUSIONS:
This investigation showed that in a middle-income country with a high prevalence
of HIV infection, L. monocytogenes caused disproportionate illness among pregnant
girls and women and HIV-infected persons. Whole-genome sequencing facilitated
the detection of the outbreak and guided the trace-back investigations that led to
the identification of the source.
