The poor vector competence of fleas probably favoured the evolution of a hypervirulent Yersinia pestis clone through selective pressure, according to researchers at the National Institute of Allergy and Infectious Diseases.

Yersinia pestis, the causative agent of plague, diverged from the closely related Yersinia pseudotuberculosis during the last 1,500–20,000 years. However, Y. pestis differs markedly from other yersiniae — it is highly virulent and is transmitted by fleas. In a recent paper published in the Journal of Infectious Diseases, Lorange and colleagues examined the transmission dynamics of Y. pestis in Xenopsylla cheopis fleas, the most efficient plague vector examined to date.

Using an artificial feeding system, they found an ID50 for X. cheopis of 4.8 × 103 bacteria, whereas that of susceptible mammals is less than 10, indicating that the infectious threshold for fleas is high. In addition, the transmission efficiency of Y. pestis from X. cheopis to mice was found to be low and irregularly distributed. Because Y. pestis grows as a biofilm in the flea digestive tract, this might indicate that bacterial clusters of various sizes break free from the biofilm, causing varying levels of infectiousness. Using epidemiological modelling, Lorange et al. went on to to calculate the flea density required to sustain epizootic plague infection, and their results indicate that a high flea burden per host is required even in a susceptible population, suggesting that flea control may be a more effective disease control strategy than rodent control.

The results of this work not only suggest that the evolutionary change from oral to fleaborne transmission led to the emergence and continued maintenance of a hypervirulent Y. pestis clone, to overcome the high infectious threshold for fleas and the low transmission efficiency from fleas to mammals, they may also have implications for disease-control strategies.