Multihost Bartonella parasites display covert host specificity even when transmitted by generalist vectors

Abstract

Many parasites infect multiple sympatric host species, and there is a general assumption that parasite transmission between co-occurring host species is commonplace. Such between-species transmission could be key to parasite persistence within a disease reservoir and is consequently an emerging focus for disease control. However, while a growing body of theory indicates the potential importance of between-species transmission for parasite persistence, conclusive empirical evidence from natural communities is lacking, and the assumption that between-species transmission is inevitable may therefore be wrong. We investigated the occurrence of between-species transmission in a well-studied multihost parasite system. We identified the flea-borne Bartonella parasites infecting sympatric populations of Apodemus sylvaticus (wood mice) and Myodes glareolus (bank voles) in the UK and confirmed that several Bartonella species infect both rodent species. However, counter to previous knowledge, genetic characterization of these parasites revealed covert host specificity, where each host species is associated with a distinct assemblage of genetic variants, indicating that between-species transmission is rare. Limited between-species transmission could result from rare encounters between one host species and the parasites infecting another and/or host-parasite incompatibility. We investigated the occurrence of such encounter and compatibility barriers by identifying the flea species associated with each rodent host, and the Bartonella variants carried by individual fleas. We found that the majority of fleas were host-generalists but the assemblage of Bartonella variants in fleas tended to reflect the assemblage of Bartonella variants in the host species they were collected from, thus providing evidence of encounter barriers mediated by limited between-species flea transfer. However, we also found several fleas that were carrying variants never found in the host species from which they were collected, indicating some degree of host-pathogen incompatibility when barriers to encounter are overcome. Overall, these findings challenge our default perceptions of multihost parasite persistence, as they show that despite considerable overlaps in host species ecology, separate populations of the same parasite species may circulate and persist independently in different sympatric host species. This questions our fundamental understanding of endemic transmission dynamics and the control of infection within natural reservoir communities.

Keywords: Apodemus sylvaticus; Bartonella; Myodes glareolus; fleas; host-generalist; host-specialist; pathogen genotypes; rodents; sequencing; vector-borne diseases.

Figure 1

The proportion of blood samples that tested positive for infection with each Bartonella species in bank voles and wood mice across all sites. Infections were identified to species according to sequencing of the pITS region where possible, and according to the length of the pITS region in all other cases.

Figure 2

The number of each (A) Bartonella grahamii (B) Bartonella taylorii and (C) Bartonella birtlesii variant detected within wood mice and bank voles across all sites. Colour coding represents different variants within each Bartonella species group. Infections that were not sequenced are classed as ‘unknown’ variants (white). Classification of ‘unknown’ variants into their respective Bartonella species groups is based on pITS length.

Figure 3

Mean percentage of individuals correctly identified to host species according to linear discriminant analyses where models were trained on (A) Bartonella infections of the hosts, (B) flea infestations of the hosts (χ² = 0·02, P = 0·88) and (C) Bartonella infections of the fleas infesting the hosts (χ² = 28·7, P < 0·001), using data from all three woodland sites combined. In each case, models were trained on random selections of 75% of host–parasite associations and used to predict the host identity of the remaining 25% of the data. This was done 1000 times in each case. Grey bars represent models trained on true host–parasite associations, while white bars represent models trained on random host–parasite associations. Differences between the predictive capabilities of each model were assessed using chi‐squared analyses. In Fig. (A), models were trained on host‐Bartonella infections identified either to species level (‘Bartonella species’) or to pITS variant level (‘Bartonella variants’), and ten comparisons were made, represented by the letters a–j. a: χ² = 61·8, P < 0·001, b: χ² = 44·8, P < 0·001, c: χ² = 33·5, P < 0·001, d: χ² = 30·9, P < 0·001, e: χ² = 33·8, P < 0·001, f: χ² = 83·7, P < 0·001, g: χ² = 26·8, P < 0·001, h: χ² = 0·99, P = 0·32, i: χ² = 19·5, P < 0·001, j: χ² = 31·2, P < 0·001. Linear discriminant analyses models could not be computed for Bartonella birtlesii variants alone as the distribution of the one variant shared between host species was highly skewed (birtlesii‐1, found only twice in wood mice but 50 times in bank voles; Table S2).

Figure 4

The proportion of flea‐infested wood mice and bank voles that were infested with at least one specimen of each species of flea detected in this study.

Figure 5

The number of (A) Ctenophthalmus nobilis vulgaris (B) Megabothris turbidus (C) Amalareus penicilliger mustelae (D) Hystrichopsylla talpae talpae and (E) Rhadinopsylla pentacantha taken from wood mice and bank voles that tested positive for Bartonella infection. Colour coding represents the host associations (according to this study) of the Bartonella pITS variants found within the fleas: purple = found in wood mice and bank voles, green = found only in bank voles, yellow = found only in wood mice, grey = found only in fleas. White represents Bartonella DNA in fleas that was not sequenced. Horizontal divisions within colour blocks represent multiple pITS variants within a host‐association category. The specific identities of variants identified in each flea species collected from each host species are shown in Table S8.