In epidemiology, a sporadic disease is an infectious disease which occurs only infrequently, haphazardly, irregularly, or occasionally, from time to time in a few isolated places, with no discernible temporal or spatial pattern, as opposed to a recognizable epidemic outbreak or endemic pattern.[1][2][3][4] The cases are so few (single or in a cluster) and separated so widely in time and place that there exists little or no discernable connection within them. They also do not show a recognizable common source of infection.[note 1]

Examples

Examples depend on the time and place, because a disease that is common in one area may be rare in another.

In the United States, tetanus, rabies, and plague are considered examples of sporadic diseases. Although the tetanus-causing bacteria Clostridium tetani is present in the soil everywhere in the United States, tetanus infections are very rare and occur in scattered locations because most individuals have either received vaccinations or clean wounds appropriately. Similarly the country records a few scattered cases of plague each year, generally contracted from rodent animals in rural areas in the western part of the country.[5]

In another example, World Health Organization defines malaria to be sporadic when autochthonous cases (i.e. between two individuals in the same place) are too few and scattered to have any appreciable effect on the community.[6]

Potential source for an epidemic

However, if the conditions are favorable for its spread (pathogenicity, susceptibility of hosts, contact rate of individuals, population density, number of vaccinated or naturally immune individuals, etc.), a sporadic disease may become the starting point of an epidemic.

For example, in developed countries, shigellosis (bacillary dysentery) is normally considered a sporadic disease, but in overcrowded places with poor sanitation and poor personal hygiene, it may become epidemic. Shigellosis was a sporadic disease in South Korea for many years, until 1998. Beginning in 1998 South Korea experienced a sudden epidemic of shigellosis among school children. Contaminated school meals were identified as the major source of infection, and after several years, the infection rate declined significantly.[7]

In another example, the South Asian country of Bangladesh experienced sporadic cases of dengue fever, a mosquito-borne disease, from its first outbreak in 1964 until 1999.[8] However, in 2000, the arrival of a Thai/Myanmar strain of the highly pathogenic dengue type 3 virus into the overpopulated and poorly urbanized country (which increases human-mosquito contact), with highly favorable breeding grounds (such as open water reservoirs used by poor people and accumulation of rainwater) for the vector, and very little public awareness gave rise to a sudden epidemic of dengue, with 5,551 reported cases that year.[8] The type 3 Dengue virus subsided after 2002 and re-emerged in 2017, once again causing an outbreak in 2019.[9]

Difficulty of measuring

Molecular epidemiologist Lee Riley claims that most sporadic infections are actually part of unrecognized outbreaks, and that what appears to be endemic disease (from a traditional population-based epidemiology approach) actually consists of multiple small outbreaks (from a molecular epidemiology approach) in which seemingly unrelated (i.e., sporadic cases) are in reality epidemiologically related, because they belong to the same genotype of an infectious agent. Riley considers the differentiation of a disease occurrence as either endemic or epidemic to be not really meaningful. According to Riley, since most so-called sporadic occurrences of an endemic disease are actually small epidemics, rapid public health interventions against such occurrences can be made in the same way as they are done for recognized acute epidemics (i.e. epidemic in the traditional sense).[10]

Notes and references

Notes

  1. ↑ According to Fullerton et al. 2012, pp. 281–292: "...sporadic cases do not necessarily share a single specific common contaminated source..."

References

  1. ↑ Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health (7th ed.), Saunders, 2003
  2. ↑ Principles of Epidemiology in Public Health Practice (3rd ed.), U.S. Department of Health and Human Services and Centers for Disease Control and Prevention (CDC), 2006, p. 72
  3. ↑ Miquel Porta; John M. Last, eds. (2018), A Dictionary of Public Health (2nd ed.), Oxford University Press
  4. ↑ Miquel Porta, ed. (2016), A Dictionary of Epidemiology (6th ed.), Oxford University Press, pp. 46–47
  5. ↑ "Disease and Epidemiology", Microbiology by OpenStax, XanEdu Publishing Inc, 2016, p. 699
  6. ↑ WHO Malaria Terminology, World Health Organization, 2019, p. 30
  7. ↑ Hyunjoo Pai (March 2020), "History and Epidemiology of Bacillary Dysentery in Korea: from Korean War to 2017", Infection and Chemotherapy, 52 (1): 123–131, doi:10.3947/ic.2020.52.1.123, PMC 7113447, PMID 32239814, S2CID 214772371
  8. 1 2 Sifat Sharmin, Elvina Viennet, Kathryn Glass and David Harley (September 2015), "The emergence of dengue in Bangladesh: Epidemiology, challenges and future disease risk", Transactions of the Royal Society of Tropical Medicine and Hygiene{{citation}}: CS1 maint: multiple names: authors list (link)
  9. ↑ Mohammed A. Mamun; Jannatul Mawa Misti; Mark D. Griffiths; David Gozal (December 14, 2019), "The dengue epidemic in Bangladesh: risk factors and actionable items", The Lancet, 394 (10215): 2149–2150, doi:10.1016/S0140-6736(19)32524-3, PMID 31839186, S2CID 209333186
  10. ↑ Lee W Riley (July 2019), "Differentiating Epidemic from Endemic or Sporadic Infectious Disease Occurrence", Microbiology Specrum, 7 (4)

Works cited

  • Fullerton, Kathleen E.; Scallan, Elaine; Kirk, Martyn D.; Mahon, Barbara E.; Angulo, Frederick J.; de Valk, Henriette; van Pelt, Wilfrid; Gauci, Charmaine; Hauri, Anja M.; Majowicz, Shannon; O'Brien, Sarah J. (2012), "Case-Control Studies of Sporadic Enteric Infections: A Review and Discussion of Studies Conducted Internationally from 1990 to 2009", Foodborne Pathogens and Disease, 9 (4): 281–292, doi:10.1089/fpd.2011.1065, PMC 4568830, PMID 22443481
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