As already noted, Malagasy lemurs may be especially susceptible to pathogens introduced through recent human occupation. Of importance are several pathogens that could play an important role in lemur disease ecology when associated with increased human presence. The first, Toxoplasmosis gondii, is a protozoanl infection that is carried by domestic cats. When the parasite enters an aberrant host, it often causes significant illness and death. T. gondii was first documented in captive L. catta in 1962 and is believed to have been brought to Madagascar by domesticated cats (Sureau et al., 1962). Of these domesticated cats, many are now feral (locally known as Ampaha), and some reside in lemur habitats. T.gondii infection is highly pathogenic in some lemur species (e.g., it is usually fatal for captive ring-tailed lemurs, Dubey et al., 1985), but may be less so for other lemur species (e.g., ruffed lemurs, Junge, 1999). Seropositive titers have been detected for both wild black lemurs (Eulemur macaco) in Lokobe reserve and golden-crowned sifaka (Propithecus tattersalli) near Daraina (Junge, unpublished data). Screenings of 19 ring-tailed lemurs using antibody titers at the Tsimanampetsotsa reserve (Dutton et al., 2003) and 20 ring-tailed lemurs at Beza Mahafaly using PCR analysis (Miller et al., in press) have been negative. At the first ring-tailed lemur site, which is an isolated reserve with reduced human contact, it is unlikely that domestic or feral cats are common. At the second site, the reserve is intact but the surrounding areas have had considerable human impact. Nearby villages do contain domestic cats in low numbers, and a preliminary screening of several cats found that 50% of the sample (3 of 6) exhibited positive titers to toxoplasmosis (Mills, Conrad, and Lappin, personal communication). A potentially greater threat comes from the more numerous Ampaha (feral cats), which have been seen stalking ring-tailed lemurs in areas outside of the reserve (Sauther, personal observation). It remains to be seen if the Ampaha exhibit positive titers, but if so they could provide an avenue for transmission to the lemurs. Similarly, old and new world primate species are susceptible to herpesviruses, which are generally chronic, and relatively apathogenic for species-adapted strains (King, 2001). However, herpes simplex virus (Herpesvirus hominis), which uses humans as its natural reservoir, can be pathogenic to other primates. In captive ring-tailed lemurs the duration of the disease can run from 1 day to 7 months but in all cases it is fatal (Flügger and Pfeiffer, 1992). It has also been associated with encephalitis in captive ruffed lemurs (Kornegay et al., 1993). At Beza Mahafaly a screening of 50 individuals was negative (Sondgeroth et al., in review). This may indicate that the lemurs have not been exposed to H. hominis, but it may also indicate that immunity to this introduced pathogen has not been developed.
In both cases it is possible that the apparent high sensitivity of some lemurs to toxoplasmosis and H. hominis is a result of geographical isolation. As there are no native felid species in Madagascar, lemurs have not evolved with exposure to tox-oplasmosis. Similarly, H. hominis presumably arrived with the first humans, only 1500 years ago. With no evolutionary exposure, no resistance has evolved making these types of pathogens particularly problematic as human and lemur habitat converge.
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