# 4 Parasite taxis


Fig 1: Flower-like patterns of trypanosoma brucei colonies on agarose. Parasites are confined to a dense quasi two-dimensional monolayer which can be represented by a 2D agent-based in silico model. Each agent i is defined by its position xi and velocity vi.

Collective motion is a phenomenon well known from flocks of bird or fish as well as from colonies of bacteria. In parasitology, not much attention has been put on collective motion so far.
Large groups of Trypanosoma brucei show different movement patterns in vivo inside the tsetse fly gut system. The microswimmers can switch between uncorrelated and highly correlated phases of movement with synchronised flagella beats. So far, it is unclear how this synchronization is modulated and what purpose it serves.

In vitro studies have shown a phenomenon called social motility. Colonies of trypansoma brucei (~ 10^6) on agarose gel confined to a monolayer expand non-homogeneously in characteristic flower-like patterns. Even though measurements of single trypanosomes suggest that they move in an uncorrelated manner. It is hard to explain how such characteristic patterns can emerge when there seems to be no coordination of movement on a colony level.

Early studies suspected similar chemotactic alignment as for bacteria, but this has not been directly measured so far. Therefore, it is hypothesized that the environment plays an essential role both in social motility and the correlated movement phase in vivo.
Theoretical work on hydrodynamic interactions of microswimmers with boundaries has shown that microswimmers show distinct boundary-alignment interactions based on their geometry. Maybe this effect together with the soft boundaries is already sufficient to explain the observed behavior.

As successfully seen for bacteria, agent-based in silico studies can help to increase our understanding of such systems. In simulations, the proposed mechanisms can be switched on and off individually to study their influence on the behavior of the system in great detail. Such an approach is not limited to Trypanosomes. It can also be applied to other parasite systems like Gardia or Heligmosomoides polygyrus, which show hints of collective behavior as well.