Babesia bovis merozoites invade human, ovine, equine, porcine and caprine erythrocytes by a sialic acid-dependent mechanism followed by developmental arrest after a single round of cell fission.

This research investigates the specificity of Babesia bovis parasites in invading erythrocytes (red blood cells) of humans and various animals, and the role of sialic acid in this process. The study finds that all but goat erythrocytes support parasite growth and development, yet repeated invasion only takes place in bovine cells, suggesting host specificity happens late in the parasite’s lifecycle.

Study Rationale and Method

• While Babesia bovis infections are typically limited to bovines, unlike the more indiscriminate Babesia divergens, the aim of this research was to establish whether this discrimination happens during the initial invasion of host cells.
• An in vitro assay was used to measure erythrocyte invasion by free merozoites, which are a stage in the parasite’s lifecycle, within an hour.

Findings on Erythrocyte Invasion

• Human erythrocytes were found to be the most susceptible to invasion, ahead of bovine erythrocytes. Erythrocytes from sheep, pigs, and horses were slightly less affected, while goat erythrocytes were largely refractory to invasion.
• The level of invasion varied substantially amongst individuals from the same species.
• All species apart from goats supported intracellular development of the parasites, seen as a high number of duplicated parasites.
• Only in bovine erythrocytes did the parasites continue to invade in further rounds, leading to a larger parasite population, suggesting that host specificity emerges later in the parasite’s lifecycle.

The Role of Sialic Acid

• Erythrocyte treatment with neuraminidase, an enzyme that removes sialic acid from cells, resulted in an 80% reduction in invasion efficiency across most species. Removing either type of sialic acid molecule yielded similar results, and simultaneous removal did not compound the effect.
• Pre-treatment of merozoites with N-acetylneuraminyl-lactose, a sialic acid compound, resulted in a 45% reduction in invasion efficiency. Applying it just before invasion does not significantly alter the result.
• This evidence suggests that the invasion of erythrocytes by Babesia bovis merozoites is largely dependent on sialic acid-containing receptors on the erythrocyte membrane that interact with merozoite ligands—proteins that bind with specific cellular agents.