Ticks infected via co-feeding transmission can transmit Lyme borreliosis to vertebrate hosts

Abstract

Vector-borne pathogens establish systemic infections in host tissues to maximize transmission to arthropod vectors. Co-feeding transmission occurs when the pathogen is transferred between infected and naive vectors that feed in close spatiotemporal proximity on a host that has not yet developed a systemic infection. Borrelia afzelii is a tick-borne spirochete bacterium that causes Lyme borreliosis (LB) and is capable of co-feeding transmission. Whether ticks that acquire LB pathogens via co-feeding are actually infectious to vertebrate hosts has never been tested. We created nymphs that had been experimentally infected as larvae with B. afzelii via co-feeding or systemic transmission, and compared their performance over one complete LB life cycle. Co-feeding nymphs had a spirochete load that was 26 times lower than systemic nymphs but both nymphs were highly infectious to mice (i.e., probability of nymph-to-host transmission of B. afzelii was ~100%). The mode of transmission had no effect on the other infection phenotypes of the LB life cycle. Ticks that acquire B. afzelii via co-feeding transmission are highly infectious to rodents, and the resulting rodent infection is highly infectious to larval ticks. This is the first study to show that B. afzelii can use co-feeding transmission to complete its life cycle.

Figure 1

The mode of transmission influenced the spirochete load of Borrelia afzelii in the Ixodes ricinus nymphs. Nymphs that acquired B. afzelii as larvae via systemic transmission had a mean spirochete load that was 26 times higher than nymphs that acquired the infection as larvae via co-feeding transmission. Shown are the medians (black line), the 25th and 75th percentiles (edges of the box), and the minimum and maximum values (whiskers).

Figure 2

The spirochete load of B. afzelii per mg of mouse tissue (log10-transformed) is compared between mice that were infected with co-feeding nymphs versus systemic nymphs in four organs: (a) ear, (b) heart, (c) bladder, and (d) ventral skin. There was no difference between the two groups of mice in the mean log10-transformed spirochete load for the ear, heart, bladder, and ventral skin. Shown are the medians (black line), the 25th and 75th percentiles (edges of the box), the minimum and maximum values (whiskers), and the outliers (circles).

Figure 3

Co-feeding nymphs and systemic nymphs induce the same B. afzelii infection phenotype in mice. (a) The two types of nymph (co-feeding versus systemic) induced a similar Borrelia-specific IgG antibody response in the mice. (b) Mice infected with co-feeding nymphs or systemic nymphs had similar levels of host-to-tick (systemic) transmission to xenodiagnostic ticks. Shown are the medians (black line), the 25th and 75th percentiles (edges of the box), and the minimum and maximum values (whiskers).

Figure 4

The production of the co-feeding nymphs and the systemic nymphs is shown. (A) Each pathogen-free mouse was infested with 5 B. afzelii-infected nymphs on day 0 and with 100 larvae on day 2. The larval ticks acquired B. afzelii via co-feeding transmission. The engorged larvae were allowed to moult into what are hereafter referred to as the co-feeding nymphs. The nymphs and larvae were placed in a plastic capsule to enhance co-feeding transmission. (B) At 30 days after the nymphal infestation, the mice had developed a systemic infection with B. afzelii. Each B. afzelii-infected mouse was infested with 100 larvae on day 30. The larval ticks acquired B. afzelii via systemic transmission. The engorged larvae were allowed to moult into what are hereafter referred to as the systemic nymphs.

Figure 5

The design of the infection experiment to compare nymph-to-host transmission of B. afzelii between the co-feeding nymphs and the systemic nymphs is shown. (C) Each of 26 pathogen-free mice was infested with 2 to 9 co-feeding nymphs. (D) Each of 12 pathogen-free mice was infested with 1 to 2 systemic nymphs. Nymphs were placed in a capsule to protect them from mouse grooming and to prevent them from escaping. The engorged nymphs were collected and their B. afzelii infection status was determined using qPCR. At 30 days after the nymphal infestation, all the mice were infested with 50 xenodiagnostic larvae, and the engorged larval ticks were allowed to moult into xenodiagnostic nymphs. These nymphs were used to quantify host-to-tick transmission. At 59 days after the nymphal infestation, the mice were sacrificed and the organs were dissected to determine the mouse infection status.