Transovarial Transmission

Another feature distinguishing LB group species from RF group species is the frequency of transovarial transmission of the agent from one generation of ticks to the next (Burgdorfer & Varma, 1967). Transovarial transmission allows the persistence of the micro-organism in a vector population in the absence of sufficient numbers of suitable vertebrate hosts for maintenance. Although some studies suggested that a small percentage of larvae of field-collected Ixodes spp. are infected with B. burgdorferi or a related LB group species, at least some of these organisms are probably B. miyamotoi s. l. spirochaetes and not transovarially transmitted B. burgdorferi (Scoles et al., 2001). Experimental studies of vertical transmission of B. burgdorferi in I. scapularis and Ixodes pacificus indicate that it would be insufficient in frequency to maintain Borrelia populations for long in the absence of a vertebrate reservoir (Magnarelli et al., 1987; Piesman, 1991; Schoeler & Lane, 1993). When transovarial transmission of the LB group species was demonstrated in the laboratory it was in Ixodes hexagonus, a species that is much less frequently infected than the principal disease vector, I. ricinus, under natural conditions in western Europe (Toutoungi & Gern, 1993).

In contrast, among the RF group of species transovarial transmission is the rule rather than the exception. Fig. 2 summarizes the results of several studies from five to eight decades ago of this phenomenon in the agents of RF and fowl spirochaetosis. Of the nine species in the reports, only two, Borrelia parkeri and Borrelia mazzottii, neither of which are common causes of RF (Barbour, 2004), did not demonstrate a high frequency of transovarial transmission. There is also evidence of transovarial transmission of B. lonestari in the metastriate tick Amblyomma americanum (Stromdahl et al, 2003).

Fig. 3. Borrelia sp.-Ixodes sp. tick relationships. The tree on the left is based on partial mitochondrial 16S rDNA sequences from Ixodes spp., while the unrooted tree on the right is based on 16S rDNA sequences from selected Borrelia spp. of the B. burgdorferis. l. group. Both trees were constructed using maximum-likelihood analysis; values above the nodes are bootstrap values based on 100 replicates; values below the nodes are bootstrap values expressed as a percentage of 1000 replicates, generated from a neighbour-joining distance analysis performed using the same dataset. For the analysis of the taxa connected by solid grey lines, the sequences from two regions of the 16S rDNA genes and from the datasets of Black & Piesman (1994) and Xu et al. (2003) were used; Ixodes uriae, a member with I. holocyclus of a sister group to the other Ixodes species in the figure, was the outgroup (Klompen et al., 2000). Taxa terminating branches shown in dashed lines occurred in only one of the two datasets, and their placement is more tentative and is shown here without bootstrap support.

Fig. 3. Borrelia sp.-Ixodes sp. tick relationships. The tree on the left is based on partial mitochondrial 16S rDNA sequences from Ixodes spp., while the unrooted tree on the right is based on 16S rDNA sequences from selected Borrelia spp. of the B. burgdorferis. l. group. Both trees were constructed using maximum-likelihood analysis; values above the nodes are bootstrap values based on 100 replicates; values below the nodes are bootstrap values expressed as a percentage of 1000 replicates, generated from a neighbour-joining distance analysis performed using the same dataset. For the analysis of the taxa connected by solid grey lines, the sequences from two regions of the 16S rDNA genes and from the datasets of Black & Piesman (1994) and Xu et al. (2003) were used; Ixodes uriae, a member with I. holocyclus of a sister group to the other Ixodes species in the figure, was the outgroup (Klompen et al., 2000). Taxa terminating branches shown in dashed lines occurred in only one of the two datasets, and their placement is more tentative and is shown here without bootstrap support.

In I. scapularis there was transovarial transmission of B. miyamotoi s. l. but not B. burgdorferi (Scoles et al., 2001).

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