Investigations of American foulbrood virulence in South Africa and resistance by African honeybees
Date
2021-12
Authors
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Among all bee diseases American foulbrood (AFB) caused by the bacterium
Paenibacillus larvae is considered the most dangerous and economically significant. American
foulbrood is highly contagious and deadly, killing bee larvae, as well as honeybee colonies.
Clinical signss of AFB were identified in Cape honeybee (Apis mellifera capensis) colonies in the
Western Cape, South Africa in 2008. The prevalence and distribution of P. larvae genotypes in South
Africa, as well as the virulence of the identified P. larvae field strain to and tolerance of
honeybee larvae were investigated in an AFB-exposed and unexposed population. Further the
progression and severity of AFB was recorded in full-sized Cape colonies over time and correlated
to colony hygienic behaviour. To date, clinical AFB infections are still limited to the Western
Cape and are caused by P. larvae ERIC I genotype with only two subtypes identified, namely Ab and
ab, which suggest that there is not much genetic diversity of P. larvae strains currently in South
Africa. The local AFB strains were found to be highly virulent, both to Cape honeybee larvae and to
Cape honeybee colonies, while tolerance of individual larvae was low, for both populations, and no
better than the tolerance of European honeybee larvae. Yet the Cape honeybee population is
exhibiting resilience, which clearly is not as a consequence of individual larval tolerance. We do
show however, that Cape honeybees exhibit good hygienic behaviour and this facilitates the fight
against AFB, with those colonies showing good hygienic behaviour exhibiting less severe AFB signs.
Further, the only difference in AFB resistance between the two populations was at the colony level.
Colonies of a non AFB-exposed population, which were sourced from a small-scale managed beekeeping
operation with neglible intervention, suffered less severely from AFB than colonies of a previously
AFB- exposed population, which were sourced from a large-scale managed commercial beekeeping
operation. This indicates that the intensively used, AFB-exposed honeybee population in the Western
Cape had not developed increased AFB resistance or undergone natural selection for enhanced
hygienic behaviour, having been exposed to AFB exposure. Rather, the intensively used large-scale
commercial population has been exposed to a range of stresses that are known to lower overall
resistance of colonies to diseases. Despite the recent reduction of clinical AFB cases in the
Western Cape we are cautious in our recommendations going forward. American foulbrood spores are
extremely environmentally stable and so we advocate for continual monitoring of colonies since the
recent AFB outbreak caused serious damages to the South African honeybee industry and future
outbreaks could endanger the South African honeybee
industry. Thus, to avoid potential losses associated with AFB, the South Africa government
should implement monitoring and eradication programmes.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (PhD)--Stellenbosch University, 2021.