Genotypic diversity of merozoite surface antigen 1 of Babesia bovis within an endemic population

Abstract

Multiple genetically distinct strains of a pathogen circulate and compete for dominance within populations of animal reservoir hosts. Understanding the basis for genotypic strain structure is critical for predicting how pathogens respond to selective pressures and how shifts in pathogen population structure can lead to disease outbreaks. Evidence from related Apicomplexans such as Plasmodium, Toxoplasma, Cryptosporidium and Theileria suggests that various patterns of population dynamics exist, including but not limited to clonal, oligoclonal, panmictic and epidemic genotypic strain structures. In Babesia bovis, genetic diversity of variable merozoite surface antigen (VMSA) genes has been associated with disease outbreaks, including in previously vaccinated animals. However, the extent of VMSA diversity within a defined population in an endemic area has not been examined. We analyzed genotypic diversity and temporal change of MSA-1, a member of the VMSA family, in individual infected animals within a reservoir host population. Twenty-eight distinct MSA-1 genotypes were identified within the herd. All genotypically distinct MSA-1 sequences clustered into three groups based on sequence similarity. Two thirds of the animals tested changed their dominant MSA-1 genotypes during a 6-month period. Five animals within the population contained multiple genotypes. Interestingly, the predominant genotypes within those five animals also changed over the 6-month sampling period, suggesting ongoing transmission or emergence of variant MSA-1 genotypes within the herd. This study demonstrated an unexpected level of diversity for a single copy gene in a haploid genome, and illustrates the dynamic genotype structure of B. bovis within an individual animal in an endemic region. Co-infection with multiple diverse MSA-1 genotypes provides a basis for more extensive genotypic shifts that characterizes outbreak strains.

Fig. 1

Frequency distribution of 28 distinct B. bovis MSA-1 genotypes isolated from two separate samplings of a B. bovis infected herd of cattle in Nayarit, Mexico. A distinct genotype is defined by greater than or equal to a single amino acid change from the rest of the genotypes.

Fig. 2

Neighboring-joining tree depicting the relationships between known B. bovis MSA-1 genotypes in Mexico and the 28 B. bovis MSA-1 genotypes isolated within an endemic cattle herd in this study. The phylogeny analysis was carried out with 1000 replicates. Tie of tree was randomly selected and not dependent on the order of the aligned sequences; treatment of gaps was distributed proportionally. Number shown at the node represents the percentage of sequence identity. Based on sequence homology, these MSA-1 genotypes as well as those newly identified genotypes can be grouped into three clusters. Group 1 consists of genotypes 1–14, 21–28 (dark grey) while groups 2 and 3 consist of genotypes 15–18 (light grey) and 19–20 (black), respectively.

Fig. 3

Distribution of B. bovis MSA-1 genotype groups in samplings 1 and 2. Prevalence of genotype groups in the first sampling is shown in black and that in second sampling is shown in white. Clusters of genotype based on sequence homology were grouped into three groups. Genotypes 1–14, 21–28 represent group 1 genotype while groups 2 and 3 include genotypes 15–18 and genotypes 19–20, respectively.

Fig. 4

Distribution of 28 distinct B. bovis MSA-1 genotypes in both samplings of the population. Genotype prevalence in first sampling is shown in black while genotype prevalence in subsequent sampling is shown in white. Samples were collected 6 months apart.