Abstract
BackgroundThe population dynamics of vector-borne pathogens inside the arthropod vector can have important consequences for vector-to-host transmission. Tick-borne spirochete bacteria of the Borrelia burgdorferi (sensu lato) species complex cause Lyme borreliosis in humans and spend long periods of time (>12 months) in their Ixodes tick vectors. To date, few studies have investigated the dynamics of Borrelia spirochete populations in unfed Ixodes nymphal ticks.MethodsLarval ticks from our laboratory colony of I. ricinus were experimentally infected with B. afzelii, and killed at 1 month and 4 months after the larva-to-nymph moult. The spirochete load was also compared between engorged larval ticks and unfed nymphs (from the same cohort) and between unfed nymphs and unfed adult ticks (from the same cohort). The spirochete load of B. afzelii in each tick was estimated using qPCR.ResultsThe mean spirochete load in the 1-month-old nymphs (~14,000 spirochetes) was seven times higher than the 4-month-old nymphs (~2000 spirochetes). Thus, the nymphal spirochete load declined by 80% over a period of 3 months. An engorged larval tick acquired ~100 spirochetes, and this population was 20 times larger in a young, unfed nymph. The spirochete load also appeared to decline in adult ticks. Comparison between wild and laboratory populations found that lab ticks were more susceptible to acquiring B. afzelii.ConclusionThe spirochete load of B. afzelii declines dramatically over time in domesticated I. ricinus nymphs under laboratory conditions. Future studies should investigate whether temporal declines in spirochete load occur in wild Ixodes ticks under natural conditions and whether these declines influence the tick-to-host transmission of Borrelia.
Highlights
The population dynamics of vector-borne pathogens inside the arthropod vector can have important consequences for vector-to-host transmission
The spirochete load was investigated in engorged larval ticks and unfed adult ticks to obtain a better understanding of how the population size of B. afzelii inside I. ricinus changes over the life-cycle of the tick vector
Effect of nymphal age on the spirochete load of infected nymphs The mean spirochete load of the 1-month-old nymphs (n = 43, mean: 14,094, 95% confidence interval (CI): 13,508–14,705 spirochetes per nymph) was 7.05 times larger than that of the 4-month-old nymphs (n = 42, mean: 1999, 95% CI: 1900–2103 spirochetes per nymph) and this difference was highly significant (t = 12.768, df = 41, P < 0.0001)
Summary
The population dynamics of vector-borne pathogens inside the arthropod vector can have important consequences for vector-to-host transmission. A recent study on the European LB pathogen, B. afzelii, in its tick vector, I. ricinus, observed that the spirochete population in the nymphal tick following the larva-to-nymph moult was high (>10,000 spirochetes) and that it declined dramatically (by almost 90%) over a period of 6 months [33]. This observed decline in spirochete load was a serendipitous discovery, and the study was not designed to answer that particular question [33]. The spirochete load was investigated in engorged larval ticks and unfed adult ticks to obtain a better understanding of how the population size of B. afzelii inside I. ricinus changes over the life-cycle of the tick vector
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