I don't know about the overall conditions inside/outside the forest area where you are, but here is a general observation from a 10-year long parasite inventory in the Area de Conservacion Guanacaste in Costa Rica. There we were able to compare parasite diversity and ecology in tropical dry and tropical wet forests, as well as cloud forests. I know you are looking at lemurs, and I am going to talk about amphibians, but perhaps this will help. We found substantially higher loads of nematode species with an obligate soil-dwelling stage in the amphibians in the dry forest than in the wet forest. Funding was withdrawn before we had a chance to follow up on this, so now I conjecture. First, in some cases it was the same parasite species, in other cases close relatives living in the different forest types. So, in some cases this seems to be a case of plasticity and in others it is perhaps a case of wet-adapted and dry-adapted relatives. In the dry forest, it is very wet for 6 months each year and bone-dry for 6 months. I assumed that what was happening was that in the dry forest, the greatest amount of moist habitat available half of the year was inside the vertebrate host, whereas in the wet forest, the soil was more or less as wet as an amphibian gut year-round. So, through stochastic processes associated with plasticity or through responses to selection, you end up with the primary reservoir of the parasites being adults living in the amphibians in the dry forest, whereas in the wet forest, there is a good reservoir of adults in the amphibian and larvae in the soil. Half the year the dry forest is "wet" so that's when the larvae can develop in the soil. Of course, there are also other potential things happening - the larvae may be capable of a kind of diapause (dauer), so in the dry forest they "sit" in the soil waiting for the rains to come, just like a lot of the insects and plants (and the amphibians themselves. In any event, don't assume the phenomenon is simply a matter of anthopogenic changes producing forest fragments. That could be the answer but it might be more interesting than that. 

not easy answer as many factors involved. probably you can start with the life cycle of each parasite and observe if differences between both habitats you are studying that can influence the life cycle.

In case it's not on your radar, I'd recommend having a look at C. Nunn's work. Although it's generally comparative, you'll find plenty of themes and references relevant to your question:

http://nunn-lab.evolutionaryanthropology.duke.edu/recent-publications

e.g. http://rsbl.royalsocietypublishing.org/content/2/3/351

Dear Bortali

It is good question, but at first, I would like to know something about the human inhabiting these adages?

Edge effects can be very idiosyncratic. The type and quality of the original forest and the type and quality of the introduced matrix work together to determine the severity of fragmentation effect (along with the amount of habitat lost during the act of fragmentation). One common scenario, where a contiguous forest is cut into smaller pieces, I think you can expect to see a few general responses. After fragmentation, you should expect there to be more 'blowdown' on edges as you expose more trees to the strong wind, drying on the edges due to increase exposure to sun and wind, increased prevalence of weedy and invasive species responding to the disturbance, and the increased density of individuals that need the remnant forest to survive.

    So, my best guess is that its a combination of (1) drying and heating on the edges making it more habitable for the parasite, (2) the primate population increasing in density as individuals withdraw to remnant patches, and (3) increased susceptibility to invasive species moving into the area.

   The idiosyncratic nature of forest fragmentation research likely means that your future peer reviewers will have a high burden of proof for linking edge effects to parasite loads, so make sure to collect at least some data on microclimate, population density, community composition change due to habitat fragmentation, and the amount of lost vs. fragmented habitat.

Hope this helps,

Ty

Great Answers. Mr Abdelrahim Sayied the people that inhabit the surrounding areas of the forest fragment are just comprised of a number of small traditional Malagasy villages. Approximately 40 people per village. They do farm the areas surrounding the fragment so are frequently coming in contact with the lemurs living at the fragment edge. Hope this is what you were looking for. 

It is possible that the presence of pets (dogs etc.) accompanying those human populations in the areas surrounding the fragment edges are the source of parasites?

I would like to support Alexis Ribas' suggestion to have a look on life cycle of each parasite species, possible vectors vectors and reservoirs.

Two things, you might have a higher input of, more reservoirs for parasites and more favourable habitat for certain stages or the primate's response to parasites is weaker because of factors that might be caused by habitat edge effects and other stress factors (people, dogs, primate population crowding etc.).

a further reason might be, that it is a consequence o reduced and concentrated habitat ranging (that might favour self-reinfection), in the case that oportunistic lemurs find enough ressources in that reduced ranges.

In one study we experienced and assumed different and interdependend effects on primate (lion tailed macaques ) parasitic load and diversity in a fragmented landscape in South West India. I would suggest that In relative small forest patches and edges you might have a higher diversity and/or abundance of potential vectors, especially more generalist species that cross between different habitats. The species might be different between edges and interior forest. People, dogs, pigs and primates share more parasites in edges and small patches, usually as a consequence of sanitary use of these areas and dogs and pigs spreading parasite stages further on.

Anyway in edges that are frequented by human you have local accumulation of parasite input at "preferred sites" . In the interior forest or continuous forest you might have a wider distribution of infection sites and infected vectors. It will help to check dogs, people and most generalistic species for parasites to exclude at least these immidiate anthropogenic effects on commonly shared parasites. Afterwards have a look on "typical interior forest life cacle parasites that will help to think about further ecological factors.

If you haven't done so already, check out Chapman et al. 2006 (Do food availability, parasitism, and stress have synergistic effects on red colobus populations living in forest fragments?) and Gillespie and Chapman 2006 (Prediction of parasite infection dynamics in primate metapopulations based on attributes of forest fragmentation).