Lack of Evidence for Transovarial Transmission of the Lyme Disease Spirochete Borrelia mayonii by Infected Female Ixodes scapularis (Acari: Ixodidae) Ticks

Abstract

The recently described Lyme disease spirochete Borrelia mayonii is associated with human illness in the Upper Midwest of the United States. Experimental laboratory studies and field observations on natural infection indicate that B. mayonii is maintained by horizontal transmission between tick vectors and vertebrate reservoirs. While maintaining a colony of Ixodes scapularis Say ticks infected with the B. mayonii type strain (MN14-1420), we had an opportunity to examine whether infected females may pass this spirochete transovarially to their offspring. We found no evidence of B. mayonii infection in subsets of larvae originating from 18 infected I. scapularis females (grand total of 810 larvae tested), or in mice exposed to larval feeding.

Borrelia mayonii, a recently described human-pathogenic species within the Borrelia burgdorferi sensu lato complex, is associated with Lyme disease in the Upper Midwest of the United States (Pritt et al. 2016a, b; Kingry et al. 2016). The blacklegged tick, Ixodes scapularis Say, is an experimentally confirmed vector of B. mayonii (Dolan et al. 2016, 2017a), and natural infection was documented from I. scapularis nymphs and adults in Minnesota and Wisconsin (Pritt et al. 2016a, b). Moreover, the house mouse (Mus musculus L.) is an experimental reservoir for B. mayonii (Dolan et al. 2016, 2017b), and natural infection was documented in Minnesota from two rodent species, the white-footed mouse (Peromyscus leucopus Rafinesque) and the American red squirrel (Tamiasciurus hudsonicus Erxleben) (Johnson et al. 2017). These findings collectively indicate that B. mayonii is maintained in part by horizontal transmission between tick vectors and vertebrate hosts, similar to Borrelia burgdorferi sensu stricto (Piesman and Gern 2004). However, the potential for transovarial (vertical) transmission of B. mayonii from infected females to their offspring, which could place humans at risk for bites by infected larval ticks, is unknown. Experimental evidence of transovarial transmission is lacking for I. scapularis females infected with well characterized B. burgdorferi sensu stricto strains. Wild spirochete strains detected by microscopy or immunofluorescence assays from unfed Ixodes larvae in older studies can no longer be assumed to belong to B. burgdorferi sensu lato (Richter et al. 2012, Rollend et al. 2013) because these detection methods may not differentiate B. burgdorferi sensu lato spirochetes from the relapsing fever spirochete Borrelia miyamotoi, which is passed transovarially from infected I. scapularis females to the resulting larvae (Scoles et al. 2001, Breuner et al. 2017).

In order to conduct experiments on transmission of B. mayonii by feeding ticks, we established a colony of I. scapularis ticks infected with the MN14-1420 type strain that was originally isolated from a human patient (Pritt et al. 2016a, b). The maintenance of this colony of B. mayonii-infected ticks also allowed us to opportunistically examine whether infected females may pass this recently discovered Lyme disease spirochete transovarially to their offspring.

Materials and Methods

Infected female ticks were produced by feeding nymphs from our in-house CT (Connecticut) and MN (Minnesota) colonies infected with the B. mayonii type strain (MN14-1420) (Eisen et al. 2017) ad libitum on naïve 1–3 mo old female CD-1 white mice (Charles River Laboratories, Wilmington, MA) as described previously (Dolan et al. 2016). The nymphs had acquired B. mayonii by feeding as larvae on 4 individual infected mice from which other resulting nymphs had infection rates ranging from 5.4–20.0% (Dolan et al. 2016, Eisen et al. 2017). Mixed potentially infected females from the CT and MN colonies were then fed together with male ticks on a single female New Zealand white rabbit (Charles River Laboratories). The ticks were allowed to attach to the ears of the rabbit, contained within cloth bags covering each ear and secured with athletic tape at the base of the ears (Piesman et al. 1991, 1999), with each ear receiving approximately 25 female and 25 male ticks. The rabbit was fitted with an Elizabethan collar to prevent disruption and damage to the ear bags and the ticks within. Recovered fully fed females (n=39) were placed singly into glass vials with a plaster of Paris and activated charcoal mixture at the bottom and a mesh top, and then transferred to desiccators (90–95% relative humidity) in a growth chamber maintained at 24°C with a 16:8 hour light:dark cycle.

Whole bodies of fed females that had finished ovipositing (n=23) were examined for presence of B. mayonii DNA by multiplex polymerase chain reaction (PCR) as described previously (Dolan et al. 2017, Johnson et al. 2017). Larvae originating from females found to be infected (n=18) were similarly examined for presence of B. mayonii DNA by multiplex PCR. This included 45 larvae per infected female, tested in pools of 15 larvae. Moreover, larvae originating from 3 B. mayonii-infected females were allowed to feed ad libitum on 10 CD-1 mice (>100 larvae per mouse). These mice were assayed, as described previously (Dolan et al. 2017a, Eisen et al. 2017), for B. mayonii infection by culture of ear biopsies (taken 4 wk after the larval feed) in modified Barbour-Stoenner-Kelly medium with antibiotics (in-house BSK-R medium), and for seroreactivity to B. mayonii (8 wk after the larval feed).

Animal use and experimental procedures were in accordance with approved protocols on file with the Centers for Disease Control and Prevention Division of Vector-Borne Diseases Animal Care and Use Committee.

Results and Discussion

Of the 23 I. scapularis females that fed and oviposited, 18 (78%) contained B. mayonii DNA. This infection rate was much higher than expected from previous examination of unfed nymphs of the same cohort (5–20%) that originated from larval feeds on the same 4 individual infectious source animals as the females examined here. Indeed, the observed infection rate of 78% (18/23) for the fed females is significantly higher than an upper end prediction of 20% infected unfed females based on the data for the preceding nymphal stage (Likelihood Ratio Test; P<0.001). Co-feeding transmission where spirochetes were passed from feeding infected to non-infected females is one possible explanation for the observed rise in B. mayonii infection rate of the fully fed females. Co-feeding transmission from infected to non-infected I. scapularis ticks was previously observed for uncharacterized B. burgdorferi sensu lato spirochetes in females fed together within ear bags on naïve rabbits, with half of previously uninfected females acquiring spirochetes via co-feeding transmission (Piesman et al. 1998). Moreover, co-feeding transmission from infected to non-infected I. scapularis ticks was documented for B. burgdorferi sensu stricto (B31) and uncharacterized B. burgdorferi sensu lato spirochetes in immatures fed in close proximity to each other on rodents (Patrican 1997, Piesman and Happ 2001). However, as we did not test the blood of the rabbit the females were feeding on for presence of B. mayonii in the present study, we cannot rule out the possibility that the females acquired spirochetes via infected blood rather than by co-feeding transmission.

Regardless of whether they were infected prior to feeding or became infected while feeding, we found no evidence that any of the 18 B. mayonii-infected I. scapularis females passed spirochetes to the resulting larvae (based on testing of a grand total of 810 larvae). As we cannot be sure of how many females were infected prior to feeding, and we tested only subsets of the resulting larvae, the possibility of ineffective transovarial transmission resulting in occasional larval infection cannot be ruled out. Moreover, none of 10 mice exposed to larval feeding were found to be either infected with or seroreactive to B. mayonii. We conclude that while horizontal transmission between vector ticks and vertebrate reservoirs appear to be important for the natural maintenance of B. mayonii evidence for transovarial transmission of this Lyme disease spirochete is lacking.