Variation among strains of Borrelia burgdorferi in host tissue abundance and lifetime transmission determine the population strain structure in nature

Pathogen life history theory assumes a positive relationship between
pathogen load in host tissues and pathogen transmission. Empirical
evidence for this relationship is surprisingly rare due to the difficulty
of measuring transmission for many pathogens. The comparative method,
where a common host is experimentally infected with a set of pathogen
strains, is a powerful approach for investigating the relationships
between pathogen load and transmission. The validity of such experimental
estimates of strain-specific transmission is greatly enhanced if they can
predict the pathogen population strain structure in nature. Borrelia
burgdorferi is a multi-strain, tick-borne spirochete that causes Lyme
disease in North America. This study used 11 field-collected strains of B.
burgdorferi, a rodent host (Mus musculus, C3H/HeJ) and its tick vector
(Ixodes scapularis) to determine the relationship between pathogen load in
host tissues and lifetime host-to-tick transmission (HTT). Mice were
experimentally infected via tick bite with 1 of 11 strains. Lifetime HTT
was measured by infesting mice with I. scapularis larval ticks on 3
separate occasions. The prevalence and abundance of the strains in the
mouse tissues and the ticks were determined by qPCR. We used published
databases to obtain estimates of the frequencies of these strains in wild
I. scapularis populations. Spirochete loads in ticks and lifetime HTT
varied significantly among the 11 strains of B. burgdorferi. Strains with
higher spirochete loads in the host tissues were more likely to infect
feeding larvae, which molted into nymphs with a higher probability of B.
burgdorferi infection (i.e., higher HTT). Our laboratory-based estimates
of lifetime HTT were predictive of the frequencies of these strains in
wild I. scapularis populations. For B. burgdorferi, the strains that
establish high abundance in host tissues and that have high lifetime
transmission are the strains that are most common in nature.