A study into the genomic makeup of American foulbrood (AFB) in New Zealand has not only unveiled some findings that are at odds with previous research, but put in place world-leading data that could help limit the spread of the honey bee disease.
Until the Ministry for Primary Industries (MPI) research was published recently, only about 300 genomes of the AFB causing bacteria Paenibacillus larvae were known worldwide. However, the New Zealand study, which relied upon AFB samples collected by inspectors from the national Management Agency between November 2019 and January 2022, has now added a further 164 genomes to the global database.
“New Zealand has added greatly to the international known sequence data,” points out Richard Hall, an MPI scientist and one of the study’s authors.
“The genomes we’ve identified have similarities to international types of AFB already in the database, but they are different. We have really blown apart the international sequence database. There are now nearly 500 AFB genomes in the world and we have just deposited 164 of those. That’s more than what is known in all of Asia, just coming from New Zealand.”
What does it mean for the knowledge of AFB in New Zealand though? While two previous studies have concluded that both major genogroups of AFB known globally, ERIC I and ERIC II are present in New Zealand, MPI’s study revealed three sequence types only, all of which belonged to the ERIC I group.
With samples taken from 22 ‘sub-regions’ on New Zealand, spanning much of the North and South islands, the presence of one ERIC type is a departure from conventional wisdom.
“As far as a beekeeper is concerned, ERIC is a genomic thing, because ERIC I and II both produce a ropey mass,” Hall explains.
“AFB is AFB, but there are a lot of technical reasons why ERIC types are interesting. ERIC I is more virulent at the colony level, but ERIC II is more virulent at the larval level. The ERIC II strain sees larvae killed just days after infection. Ninety percent of larvae killed by ERIC II AFB occur before cell-capping, and about 10 percent go on to be capped first and thus form a ropey mass if inspected. So, both ERIC types can be lethal to pupae and degrade pupae, but there are differences.”
Drilling down further, the three sequence types mapped in the study were ST18, which accounted for more than 90% of cases and spanned large areas of the country; ST5 which was interestingly located in the geographically diverse regions of the West Coast, Canterbury and Auckland; then ST23 which was found only in Otago.
The study concluded that hive movements by beekeepers are likely to be the reason for large distances between ST5 cases, and that there could be cases in other regions which were not sampled for the study. On the other hand, the research also showed that AFB cases in close proximity to one another often share related genotypes, likely due to the natural behaviours of honey bees to rob out colonies weakened by AFB.
Building a baseline of data on the makeup of AFB in New Zealand could prove valuable to national elimination efforts and localised outbreaks. As for efforts so far, the paper reasons why only three sequence types of P. larvae were observed and only ERIC I.
“AFB elimination strategy over the past 24 years has impacted the population structure of P. larvae. It is likely that the natural evolution and spread of P. larvae strains has been disrupted. While some strains have survived, others could have become extinct,” it states.
Outside of the study’s own findings, Hall sees wider value to MPI’s project.
“Importantly, this provides a collection of AFB isolates that can be shared with other researchers,” he says, adding “when combined with the Honey Bee Collection – gathered during previous MPI research and which we have permission to make available – there are great resources available for those researchers who wish to apply to access them.”
