Trypanosomes are a group of protozoan parasites, some of which cause human diseases, one of which is Trypanosoma brucei, and causes African sleeping sickness. Infections start with the relatively minor symptoms of fevers, joint pain and itching, but once the parasite penetrates the blood-brain barrier (a remarkable feat that stumps most pathogens and parasites) then the symptoms get really nasty. The trypanosome invades the central nervous system leading to confusion, reduced coordination, and even worse — it is a textbook example of parasites controlling host behavior. Sleeping sickness gets its name because of the sleep disturbances it causes. Infected people tend to be lethargic during the day and can be awake and active at night, thereby increasing the chances that they will be bitten and the parasite transmitted to its next host. Without treatment, African sleeping sickness is invariably fatal.
A recent paper has now shown that this parasite is — at least under some circumstances — social. A colony will actively seek out and recruit new members to increase its ranks, and then explore its environment. We usually think of social organisms as being, well, bigger and more charismatic than microbes, but some of the best examples of social behavior actually come from these tiny creatures. Microbes can, in fact, hunt together, communicate, divide labor among themselves, form structures, protect each other and attack intruders. (see a great review of this by B. Crespi)
So, while it’s not that unusual to hear of microbes cooperating, this is the first time that trypanosomes have been found to, and this finding could lead to new insight to how the disease works.
Figure from Oberholzer et al. 2010 showing the growth of the trypanosome on a solid culture medium (NB the last image is of a similar pattern in bacterial culture)
To understand how this might be important we must first look at this parasite’s life cycle. T. brucei infects two hosts, the tsetse fly, and humans. A fly bites a person carrying the parasite and ingests it with its blood meal. The parasite then has to move from the gut all the way up to the salivary glands in order to be transmitted to its next human host. And that’s a long freaking way for a single cellular organism to go by itself!
The lifecycle of T. brucei in both insect and human hosts
This is where the sociality comes in. As a group, the parasites may be better able to navigate their host and penetrate tissue boundaries between organs and eventually make it to their next host. So, the parasite arrives in the fly’s stomach after a blood meal and it then has to move through the body of the fly to get to the salivary glands. By searching around within the gut and recruiting others to their cause, these phalanxes of parasites may stand a better chance of reaching their goal.
The insect environment for T. brucei to navigate
While this all sounds very scary, it does also offer some possibilities for control of this incredibly nasty disease. Drugs that target aspects of motility (like flagellar development) or the ability to communicate properly may help treat people with this disease. A simple shot might be enough to stop these bugs in their tracks.
Relevant references
Oberholzer M, Lopez MA, McLelland BT, Hill KL (2010) Social Motility in African Trypanosomes. PLoS Pathog 6(1):e1000739. doi:10.1371/journal.ppat.1000739
Legros D, Ollivier G, Gastellu-Etchegorry M, Paquet C, Burri C, et al.(2002) Treatment of human African trypanosomiasis–present situation and needs for research and development. Lancet Infect Dis 2: 437–440.
Crespi B, (2001) The evolution of social behavior in microorganisms. Trends Ecol Evol 16 (4), 178-183.