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Does resistance to filarial reinfections become leaky over time?
4 Aug 2008
Citation: Duerr HP, Hoffmann WH, Eichner M (2008). Does resistance to filarial reinfections become leaky over time? Trends Parasitol. 2008 Jun 19.
When it comes to infecting a host, how many parasites are too many? One is lonesome, two may be perfect, and three ...three might be a crowd.
Duerr and colleagues have reviewed the recent literature on onchocerciasis and lymphatic filariasis to design theoretical models that explain why only a small number of infective larvae are able to successfully infect the host, and why this number tends to be limited, even when the host lives in areas where larvae are abundant.
It has been long known among parasitologists that well-adapted parasites have been selected through evolution to survive within the host, without killing it. The question about filariasis infection, which the work of Duerr and colleagues tries to answer, is why and how a stable self-limiting colony of parasites is established and maintained inside the host.
Filarial nematodes are long-lived parasites; some can live for as long as 11 years inside the host. Such an extended, and most likely unwelcome, stay within the host’s lymphatic system and tissues is the result of an orchestration played by the host’s immune surveillance mechanisms and the parasite’s ability to morph into less antigenic forms.
The host’s immune system must know how to co-exist with the parasitic antigenic stimulation, even though the host still tries to eliminate the parasite. The goal though is to eliminate the parasite but avoid all the collateral damages caused by an intense inflammatory response. The models proposed by Duerr and colleagues consider that the immune system is very dynamic and thus immunity against the parasite must co-exist along with tolerance, to limit tissue damaging. The authors present several theoretical models of infection that would be worth testing experimentally.
One point that comes to mind, when pondering this delicate equilibrium between long-lived parasites and their hosts, is the population control that is imposed by the parasite itself. How is this control achieved? What are the mechanisms that are controlled by the parasites that somehow limit their population size? Are the parasites like a small community that decides to limit its annual growth so that resources continue to be abundant to all, equally? And if so, how do parasites communicate among themselves how large their community is? And how do they avoid the establishment of those parasites that try to relocate into a community that do not welcome them?
It is possible that the parasite uses the host immune system as a way to control its own population density and thus ensure its survival and long stay in the host. In other words, the host immune system must be able to eliminate some parasites, but for some reason not yet fully known, the host immune system cannot eliminate all parasites. From the parasite’s perspective this is a favourable situation because it avoids an ‘over infection’ of the host, which could lead to the host’s death.
These are all very interesting aspects of the relationship between vertebrate hosts and filariases that are worth investigating and the work by Duerr and colleagues provides a well-thought and organized picture of the matter.
Over one billion people, mainly in African and Latin American countries, are at risk of being infected by filariases. As with other infectious diseases, prevention is the best course of action, but understanding the dynamic interactions that take place between host and parasites may offer alternative and unexpected new approaches for treatment.
Note: This article is published in a journal which is not open access. To see the full article a subscription to Trends in Parasitology is therefore required. In some developing countries, readers who are based in institutions may be able to access it through the HINARI programme.
Copyright © 2008 Elsevier Ltd All rights reserved.
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