Download The importance of secondary spread in the epidemiology of

The importance of secondary spread
in the epidemiology of Escherichia coli O157
infection in Scotland, 1999-2008
Clifton House, Clifton Place
Glasgow G3 7LN, Scotland
Email: [email protected]
Tel: +44-141 300 1118
Mary Locking1, Kevin Pollock1, Lesley Allison2, Linda Rae1, Mary Hanson2, John Cowden1
1 Health Protection Scotland, Glasgow, Scotland 2 Scottish E. coli O157/VTEC Reference Laboratory, Edinburgh, Scotland
Background and Aims:
Results:
Escherichia coli O157 remains an important public health challenge worldwide, particularly
because of its association with haemolytic uraemic syndrome (HUS)1. The low infectious
dose exacerbates its potential to cause both secondary spread, and large outbreaks2,3,4.
Most information on secondary spread of E. coli O157 worldwide derives from outbreaks,
or from settings or sub-groups not necessarily comparable with the population as a
whole.
From 1999 to 2008, 2228 laboratory-confirmed E. coli O157 cases were reported to HPS
(mean 223 annually). The mean annual incidence rate was 4.4 per 100,000 population
(Figure 1). Children under 10 accounted for 36% of all cases.
Scotland consistently reports higher rates of infection than neighbouring countries (Figure 1).
Historically, most cases are sporadic (Figure 2); and research in Scotland found a strong
association between sporadic infection and exposures to livestock faeces5. In 1999,
in close liaison with the Scottish E. coli O157/VTEC Reference Laboratory (SERL),
Health Protection Scotland (HPS) established enhanced surveillance of all E. coli O157
cases – primary and secondary, outbreak and sporadic – based on the entire population.
We analysed this data to identify the proportion of secondary spread, and to explore
differences between secondary and primary cases, in single households as well as in
outbreaks.
Methods:
A case is defined as one person-infection-episode, with laboratory confirmation as either
“culture positive” (isolates of E. coli O157 cultured from faeces) or “serum positive”
cases (antibodies to E. coli O157 detected in serum). HPS integrates microbiological
information from SERL with epidemiological, clinical and exposure details from local
Health Protection and Environmental Health investigations. Secondary cases are those
symptomatic cases from whose onset date, and an assumed incubation period3, we can
infer that contact3 with a confirmed case was the most likely source of infection. HPS
surveillance systems identify “general” outbreaks ie those affecting members of more
than one household, or residents of an institution. Other cases are therefore sporadic,
or are members of a single household, for ease of reference referred to collectively as
“sporadic cases”. Laboratory confirmed cases also meeting the definitions of HUS6 are
also included in the HPS E. coli O157 enhanced surveillance dataset.
Most cases (2026/2228, 91%) reported gastrointestinal symptoms; 41% were hospitalised
and 9% developed HUS. All 1118 cases reported 2004-2008 provided symptoms details;
59% had bloody diarrhoea.
Of the 2026 symptomatic cases, 12% were secondary infections (11% of all cases) (Figure
3), with similar risk of HUS as primary cases (p = 0.95) (Figure 4); 12% of all HUS occurred
in secondary cases. Secondary cases were younger (median 13 years) than primary
cases (median 20 years, p < 0.0005), with 47% of secondary cases under 10 years old.
Although 20% of all cases occurred in outbreaks, 80% of all cases (and 64% of secondary
cases) were single household cases or clusters (“sporadic cases”) (Figure 2 and Figure
5). Sporadic and outbreak cases had similar risk of HUS (p = 0.89). Bloody diarrhoea was
more common in sporadic (62%) than outbreak cases (46%, p < 0.0005). Sporadic and
outbreak secondaries had the same risk of HUS (p = 0.97); and risk of HUS increased with
bloody diarrhoea, in all cases (p < 0.0005).
Figure 4: Secondary spread, hospital admissions and HUS:
Symptomatic cases, E.coli O157 Enhanced Surveillance in
Scotland 1999-2008 (n=2026)
172
24
58
964
164
644
Primary case
Secondary case
HUS
Figure 1: E.coli O157: Laboratory isolates, 1984-2008
(rate per 100,000 population)
Hospitalised, no HUS
Recovered at home
Figure 5: Secondary spread, outbreak or sporadic cases: Symptomatic
cases, E.coli O157 Enhanced Surveillance in Scotland 1999-2008 (n=2026)
12
Cases per 100,000 population
10
8
6
200
4
160
2
120
0
1984
1986
Scotland
1988
1990
1992
1994
1996
1998
England (2000 onwards)
England & Wales (pre 2000)
2000
2002
Wales (2000 onwards)
2004
2006
N Ireland
2008
Ireland
Data outwith Scotland courtesy of Health Protection Agency Belfast/London, and Health Protection Surveillance Centre Dublin.
(Figures outwith Scotland are provisional, and include verotoxin positive cases only; 2008 figures outwith Scotland are provisional)
40
Figure 2: E.coli O157: Laboratory confirmed cases,
Scotland 1984-2008
0
Number of cases
Sporadic Secondary
Outbreak Primary
Outbreak Secondary
Discussion and Conclusions:
400
300
200
100
1984
1986
1988
1990
1992
1994
1996
1998
General outbreak
(involves members of more than one household,
or residents of an institution)
2000
2002
2004
2006
Sporadic
(sporadic cases or single household clusters)
Figure 3: Secondary, primary or asymptomatic cases;
E.coli O157 Enhanced Surveillance in Scotland 1999-2008
Number of cases
† Only two outbreak secondary cases were identified each year in 2000 and 2007
Sporadic Primary
500
0
80
2008
†
2007
2006
2005
2004
2003
2002
2001
†
2000
1999
2008
Person-to-person spread accounted for 12% of all symptomatic cases. Most secondary
cases occurred outside outbreaks ie they acquired infection from members of their own
households, as observed in other studies7,8. The extent of secondary spread in Scotland
would not be identifiable without enhanced surveillance. Even so, it may be underestimated
e.g. if contacts submitted no, or late, samples.
Whilst infection control is rightly emphasised in outbreaks, the risk of secondary spread
and HUS within single households may be underestimated. We recommend that all cases
of suspected infectious bloody diarrhoea (particularly if bloody) should be investigated
microbiologically and monitored clinically9. Cases should be given immediate infection
control advice, without waiting for confirmation of an outbreak10.
References:
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365: 1073-86.
300
[2] Scottish Executive Health Department/Food Standards Agency (Scotland). Report of the E. coli O157 Task Force. Edinburgh:
The Stationery Office, July 2001. http://www.food.gov.uk/news/newsarchive/2001/oct/ecolitask.
250
[3] Health Protection Network. Guidance for the Public Health Management of Infection with Verotoxigenic Escherichia
coli (VTEC). Health Protection Network Scottish Guidance 3. Health Protection Scotland; Glasgow (Scotland):2008.
http://www.hps.scot.nhs.uk/giz/guidelinedetail.aspx?id=39336.
[4] Working Group of the Former PHLS Advisory Committee on Gastrointestinal Infection. Preventing person-to-person spread
following gastrointestinal infections: guidelines for public health physicians and environmental health officers. Commun Dis
Public Health. 2004; 7:362-384. http://www.hpa.org.uk/cdph/issues/CDPHvol7/No4/guidelines2_4_04.pdf.
200
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[5] Locking ME, O’Brien SJ, Reilly WJ, Wright EM, Campbell DM, Coia JE et al. Risk factors for sporadic cases of Escherichia coli
O157 infection: the importance of contact with animal excreta. Epidemiol Infect 2001; 127: 215‑220.
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[6] Pollock KGJ, Locking ME, Rae L. Enhanced surveillance of thrombotic microangiopathies in Scotland, 2003‑2008. Health
Protection Scotland Weekly Report 2009; 43 (10): 104‑105. http://www.documents.hps.scot.nhs.uk/ewr/pdf2009/0910.pdf.
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[7] Werber D, Mason BW, Evans MR, Salmon RL. Preventing Household Transmission of Shiga Toxin-Producing Escherichia coli
O157 Infection: Promptly Separating Siblings Might Be The Key. Clin Infect Dis 2008; 46: 1189‑96.
0
1999
2000
2001
2002
Primary symptomatic
2003
2004
Secondary symptomatic
2005
2006
Asymptomatic
2007
2008
[8] Seto EYW, Soller JA, Colford Jnr JM. Strategies to Reduce Person-to-Person Transmission during Widespread Escherichia
coli O157:H7 Outbreak. Emerg Inf Dis 2007; 13(6): 860‑866.
[9] Pollock KGJ, Stewart A, Beattie TJ, Todd WTA, Ahn CK, Tarr PI, Cowden JM. From diarrhoea to haemolytic uraemic syndrome – when
to seek advice. J Med Micro 2009, 58, 397‑398. http://jmm.sgmjournals.org/cgi/reprint/58/4/397.
[10] Locking M, Cowden J. E. coli O157. Early clinical diagnosis and outbreak identification are essential to prevent serious sequelae and further transmission. BMJ 2009; 339:b4076. http://www.bmj.com/cgi/content/extract/339/oct06_3/b4076.
Acknowledgements
HPS and SERL particularly wish to thank participating patients and their relatives; and the following groups across Scotland: Health Protection Teams in NHS Boards; Environmental Health Officers; Consultant Microbiologists and
Diagnostic Laboratory staff; and participating General Practitioners and Clinicians. We also acknowledge other present and previous staff at HPS and SERL.
Poster produced by the Graphics Section, Health Protection Scotland