Continuing Source

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Continuing source outbreaks are characterized by the sustained presence of an infectious agent, often leading to a continuous or intermittent series of cases over an extended period. [1]

Definition and Characteristics

A continuing source outbreak occurs when a common source of exposure to a pathogen persists over time, resulting in an ongoing transmission of the disease. The outbreak may be localized or widespread and may involve a single or multiple transmission routes. Examples of continuing source outbreaks include waterborne diseases caused by contaminated water supply or foodborne diseases stemming from the persistent contamination of food products.

The following characteristics are often associated with continuing source outbreaks:

  • Cases occur over an extended period without a distinct pattern.
  • Transmission may be direct or indirect and involve multiple exposure pathways.
  • Identification and elimination of the source can be challenging due to its persistence.
  • Affected individuals may have varying exposure levels, leading to a wide range of disease severity.

Challenges in Investigating Continuing Source Outbreaks

Identifying the source
Continuing source outbreaks often involve complex and persistent exposure pathways, making it challenging to pinpoint the exact source of infection.
Heterogeneous exposure
Varying exposure levels among affected individuals can lead to a wide range of disease severity, further complicating the investigation.
Prolonged time frame
The extended duration of continuing source outbreaks can make it difficult to establish the temporal relationship between exposure and disease onset.
Multiple transmission routes
The presence of multiple transmission routes can complicate the identification of the primary source and mode of transmission.

Examples of Continuing Source Outbreaks

Here are five examples of continuing source outbreaks, along with literature references for further reading:

Legionnaires' Disease Outbreak in Flint, Michigan (2014-2015)
During the Flint water crisis, there was a notable increase in cases of Legionnaires' disease, a severe form of pneumonia caused by the bacterium Legionella pneumophila. The contamination of the water supply with Legionella was attributed to the city's switch to the Flint River as its primary water source, which led to a significant decline in water quality. The outbreak resulted in at least 87 cases and 12 deaths. [2]
Salmonella Typhimurium Outbreak Linked to Peanut Butter (2008-2009)
A widespread outbreak of Salmonella Typhimurium infections occurred in the United States, affecting over 700 individuals across 46 states. The source of the outbreak was traced to contaminated peanut butter products from a single processing facility. The contamination was likely due to persistent issues with sanitation and the presence of rodents in the facility. [3]
Hepatitis A Outbreak Linked to Frozen Strawberries (2016)
A multistate outbreak of hepatitis A infections in the United States was traced back to frozen strawberries imported from Egypt. The contaminated strawberries were used in smoothies served at a popular restaurant chain. The outbreak affected at least 143 people across nine states and resulted in 56 hospitalizations. [4]
Cryptosporidiosis Outbreak in Milwaukee, Wisconsin (1993)
The largest waterborne disease outbreak in U.S. history occurred in Milwaukee, Wisconsin, when over 400,000 people became ill due to Cryptosporidium parvum, a protozoan parasite. The outbreak was attributed to the contamination of the city's water supply, resulting from inadequate water treatment and heavy rainfall that led to the overflow of contaminated water into Lake Michigan. [5]
Salmonella virchow ST16, EU (2017-2023)
Continuing source outbreaks can cover very long periods. One example is the Salmonella Virchow ST16 outbreak that was first detected in 2017, and that continued for several years. At the moment of this article update in April 2023, the outbreak is still ongoing, and ECDC has published a new risk assessment. Since June 2017, a persistent cross-border outbreak of Salmonella Virchow ST16 has been ongoing in five European Union/European Economic Area (EU/EEA) countries, the United Kingdom (UK), and the United States (US). A total of 210 cases have been reported from the following countries: Denmark (2), France (111), Germany (26), Ireland (4), the Netherlands (34), the UK (32), and the US (1). Among the interviewed cases (55), hospitalisation rates ranged from 16.7% (2/12) in the UK, to 29.4% (5/17) and 38.5% (10/26) in France and Germany, respectively. No deaths have been reported. A majority of cases have been linked to local restaurants serving kebab meat. The number of confirmed cases represents only a small proportion of all infections in the EU/EEA, partly due to the varying sequencing capacities of countries. ECDC concludes the report with "New infections are likely to occur in the EU/EEA affecting any age group, until further investigations are performed to identify the source(s) and point(s) of contamination along the chicken meat production chain, including the primary production upstream lines. This will allow appropriate control measures to be implemented." [6]

References

  1. ↑ This text was originally written by ChatGPT4.0 on April 6, 2023, and edited by Arnold Bosman
  2. ↑ Zahran, S., McElmurry, S. P., Kilgore, P. E., Mushinski, D., Press, J., Love, N. G., ... & Swanson, M. S. (2018). Assessment of the Legionnaires' disease outbreak in Flint, Michigan. Proceedings of the National Academy of Sciences, 115(8), E1730-E1739.
  3. ↑ Cavallaro, E., Date, K., Medus, C., Meyer, S., Miller, B., Kim, C., ... & Braden, C. (2011). Salmonella Typhimurium Infections Associated with Peanut Products. New England Journal of Medicine, 365(7), 601-610.
  4. ↑ Foster, M., Ramachandran, S., Myatt, K., Donovan, D., Bohm, S., Fiedler, J., ... & Miller, K. (2018). Hepatitis A Virus Outbreaks Associated with Drug Use and Homelessness — California, Kentucky, Michigan, and Utah, 2017. MMWR. Morbidity and Mortality Weekly Report, 67(43), 1208-1210.
  5. ↑ Mac Kenzie, W. R., Hoxie, N. J., Proctor, M. E., Gradus, M. S., Blair, K. A., Peterson, D. E., ... & Davis, J. P. (1994). A massive outbreak in Milwaukee of cryptosporidium infection transmitted through the public water supply. New England Journal of Medicine, 331(3), 161-167.
  6. ↑ European Centre for Disease Prevention and Control, European Food Safety Authority, 2023. Multi-country outbreak of Salmonella Virchow ST16 infections linked to consuming meat products containing chicken meat – 30 March 2023.
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