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Home / Equine Research Bank / Ann Trop Med Parasitol / The transmission and geographical spread of African horse si...
Annals of tropical medicine and parasitology1995; 89(1); 1-15; doi: 10.1080/00034983.1995.11812923

The transmission and geographical spread of African horse sickness and bluetongue viruses.

Abstract: African horse sickness virus (AHSV) and bluetongue virus (BTV) are dsRNA viruses within the genus Orbivirus. Both are able to cause non-contagious, infectious arthropod-borne diseases in their respective vertebrate hosts. AHSV infects equines and occasionally dogs, whereas BTV replicates in ruminants. The disease caused by AHSV is usually at its most severe in horses, whereas certain breeds of sheep are particularly sensitive to BTV infection. AHSV is endemic in sub-Saharan Africa but periodically makes brief excursions beyond this area. BTV occurs much more widely and can be found in a band around the World, stretching from approximately 40 degrees N to 35 degrees S. In the wild, both viruses are transmitted between their vertebrate hosts almost entirely via the bites of arthropod vectors, although dogs can occasionally acquire AHSV by eating virus-contaminated meat and BTV may be infrequently transmitted via infected semen or transplacentally. Because of their reliance upon arthropod vectors, BTV and AHSV have a global distribution which is limited not only by the requirement for susceptible vertebrates but also by the necessity for competent arthropod vectors. The major vectors of AHSV and BTV are certain species of Culicoides biting midge, which are true biological vectors but mosquitoes and/or ticks may also be involved to a greater or lesser extent. Until recently, AHSV has apparently been unable to survive beyond its traditional endemic zones in sub-Saharan Africa for more than 2-3 years at most. This has been interpreted as being due to a number of factors, including the absence of a long-term vertebrate reservoir, a lower prevalence, shorter, seasonal incidence and decreased transmission efficiency of the local vectors and also possibly to the effect of control measures (vector abatement, vaccination). The recent outbreaks of African horse sickness (AHS) in Spain, Portugal and Morocco, which persisted for at least 5 years (1987-1991) therefore seem to have established a new pattern in AHSV survival in an epidemic zone. This extended persistence may be due to the 'all-year-round' presence in the area of adult Culicoides imicola, the major AHSV vector. This is basically an Afro-Asiatic species and its continuous presence in parts of Iberia and may be due to some recent moderation in the climate. Further northerly extensions in the range of Culicoides imicola, in response to 'climatic moderation', cannot be ruled out and could substantially increase the area of Europe 'at risk' to AHS.(ABSTRACT TRUNCATED AT 400 WORDS)
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Publication Date: 1995-02-01 PubMed ID: 7741589DOI: 10.1080/00034983.1995.11812923Google Scholar: Lookup
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Summary

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This research focuses on the transmission and geographical spread of two diseases, African horse sickness and bluetongue viruses. Both diseases are transmitted via arthropod vectors and are studied in the context of their the vectors, the vertebrate hosts, and the environmental conditions that influence their spread.

The Viruses and Their Respective Hosts

  • The African horse sickness virus (AHSV) and bluetongue virus (BTV) are diseases that belong to the genus Orbivirus. Both are non-contagious infectious diseases.
  • AHSV primarily infects horses and, occasionally, dogs, and is especially severe in equines. It is mostly found in areas within sub-Saharan Africa.
  • BTV infects ruminants, with specific breeds of sheep being exceptionally sensitive to the infection. It has a significantly broader geographical distribution, occurring in a band that stretches around the World from approximately 40 degrees N to 35 degrees S.

Transmission Vectors

  • Both these viruses are usually transmitted via arthropod vectors, i.e., through the bites of insects such as mosquitoes or ticks.
  • AHSV can also be contracted by dogs through consumption of virus-contaminated meat. BTV can sometimes spread through infected semen or via intrauterine infection.
  • The spread of these viruses relies heavily on competent arthropod vectors. The primary vectors are certain species of Culicoides biting midge, although mosquitoes and ticks may sometimes be involved.

Geographical Spread and Persistence

  • Historically, AHSV has been unable to persist outside its endemic zones in sub-Saharan Africa for more than 2-3 years due to factors such as the lack of a long-term reservoir, lower prevalence, seasonal incidence, decreased transmission efficiency of the local vectors, and possibly control measures like vector abatement and vaccination.
  • Recent outbreaks in Spain, Portugal, and Morocco that persisted for at least five years (1987-1991) seem to indicate a change in this pattern.
  • Extended persistence may be due to the all-year-round presence of Culicoides imicola, the primary AHSV vector, in these regions – this might be due to a recent change in climate, rendering the area more apt for hosting the vector.
  • If the range of Culicoides imicola continues to extend further north due to climatic moderation, it could put a larger area of Europe at risk of AHSV.

Cite This Article

APA
Mellor PS, Boorman J. (1995). The transmission and geographical spread of African horse sickness and bluetongue viruses. Ann Trop Med Parasitol, 89(1), 1-15. https://doi.org/10.1080/00034983.1995.11812923

Publication

Annals of tropical medicine and parasitology
ISSN: 0003-4983
NlmUniqueID: 2985178R
Country: England
Language: English
Volume: 89
Issue: 1
Pages: 1-15

Researcher Affiliations

Mellor, P S
  • Institute for Animal Health, Pirbright Laboratory, Woking, Surrey, U.K.
Boorman, J

    MeSH Terms

    • Africa South of the Sahara / epidemiology
    • African Horse Sickness / epidemiology
    • African Horse Sickness / transmission
    • Animals
    • Arthropod Vectors
    • Bluetongue / epidemiology
    • Bluetongue / transmission
    • Ceratopogonidae
    • Culicidae
    • Horses
    • Insect Vectors
    • North America / epidemiology
    • Seasons
    • Sheep
    • Spain / epidemiology
    • Ticks

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    This article includes 134 references

    Citations

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