Correlation between Buruli Ulcer Incidence and Vectorborne Diseases, Southeastern Australia, 2000–2020

Researchers have hypothesized that mosquitoes are vectors involved in Mycobacterium ulcerans transmission. Previous findings of a correlation between incidence of M. ulcerans, which causes Buruli ulcer, and locally acquired vectorborne diseases in southeastern Australia further strengthened this argument. However, our updated data indicate that this correlation has not continued beyond 2008.

mosquitoes are involved in M. ulcerans transmission because covering limbs with clothing would also help to protect against other potential environmental sources of M. ulcerans, such as possum excreta or soil contamination of wounds. Furthermore, mosquitoes rarely act as vectors for bacteria, and no other species of Mycobacteria are known to have arthropod vectors.
The apparent role of mosquitoes in the transmission of M. ulcerans may be explained by mechanical vectoring. Within M. ulcerans-endemic areas of southeastern Australia, the geographic locations of BU cases are highly focal. Cases are often clustered together, and adjoining communities only a few kilometers away may be spared (7). However, Aedes camptorhynchus mosquitoes, one of the main species thought to be involved in M. ulcerans transmission, are widespread within M. ulcerans-endemic and -nonendemic areas, and many species of mosquito may have the capacity to fly distances that would take them outside of BUaffected regions. Therefore, the transmission model for M. ulcerans may not be explained by mosquitoes alone.
In coastal regions of southeastern Australia, Ross River virus (RRV) and Barmah Forest virus (BFV) cause locally acquired vectorborne diseases. RRV and BFV are transmitted by Ae. camptorhynchus mosquitoes, which were the main species captured during an M. ulcerans outbreak in Point Lonsdale, southeastern Australia (4). The incidence of RRV and BFV peaks sporadically, especially during years of above average rainfall or La Niña events, as occurred during the 2020-21 summer in Australia (8). These environmental changes often favor increased mosquito population sizes, thus giving rise to RRV and BFV outbreaks (9). It is also thought that BU incidence is associated with environmental factors; increased incidence has lagged 12 months behind periods of greater rainfall (10). However, no association has been found between rainfall and BU cases on the Mornington Peninsula, the main M. ulceransendemic site driving the increased incidence of BU in southeastern Australia.
During 2002-2008, BU incidence correlated with combined RRV/BFV incidence in Victoria, southeastern Australia (r 2 = 0.52) (3). It was argued that this correlation strengthened the link between mosquitoes and M. ulcerans transmission in southeastern Australia. However, this observation was made over a short time.
Using the square of the Pearson product-moment correlation coefficient (coefficient of determination) analysis over a 21-year period (2000-2020), we found little to no correlation between BU and combined RRV/BFV incidence in southeastern Australia (r 2 = 0.05; p = 0.69) (Figure)  or Legionella (water-associated bacteria). A lack of correlation between BU incidence and locally acquired vectorborne diseases does not disprove that mosquitoes are involved in M. ulcerans transmission. Nevertheless, this lack of correlation may suggest that the worsening BU epidemic in southeastern Australia is not caused by increased mosquito populations or other environmental changes that favor RRV and BFV outbreaks. We believe that other independent factors may be driving the increased BU incidence, although the effects of recent La Niña events on BU incidence in 2021 are not yet known. Planning and implementing successful public health interventions to control M. ulcerans are substantially hindered by lack of knowledge of the mechanism of disease transmission.

About the Author
Mr. Linke is a final-year doctor of medicine student at the Geelong Clinical School, Deakin University. His research interests include mosquitoborne flaviviruses and cellulitis.