“Spillover” of human disease threatens wild chimps

Ebola, HIV, SARS, bird flu: the most frightening infectious diseases to emerge in recent decades have been zoonoses, meaning illnesses that originated in other species and often can be transmitted from animals to humans. Now, researchers from Emory University in Georgia have found evidence that this “spillover” happens in the opposite direction too, and an intestinal parasite picked up from people may be contributing to population declines among wild chimpanzees.

The researchers collected nearly 700 stool samples from people, livestock, chimpanzees, and baboons living in and near Gombe National Park in Tanzania. They analyzed these samples for the presence of Crypotsporidium, a single-celled intestinal parasite that is one of the most common causes of diarrheal disease in developing countries.

Three species of the parasite circulate in this ecosystem, the researchers report in PLoS Neglected Tropical Diseases. Just under 10 percent of the sheep and goats tested positive for C. xiaoi, a species that commonly infects livestock. About 4 percent of the people sampled were infected with C. hominis, the species most closely associated with humans. And among the non-human primates, 16 percent had Cryptosporidium, with similar rates of infection in chimpanzees and baboons.

Among the study participants were the world’s best-known chimpanzees. Best-known in both a scientific and a popular sense: Jane Goodall began studying the chimpanzees of Gombe more than 50 years ago.

The researchers found 6 chimpanzees from Kasekela, a community of about 65 individuals located near the center of Gombe National Park, infected with C. suis. This species of the parasite is commonly found in pigs, but there are no domestic pigs in the region (the people are predominantly Muslim). The researchers theorize that the chimpanzees may pick the parasite up from wild bush pigs living in the forest.

The remaining primates with Cryptosporidium — 6 chimpanzees from Kasekela; 4 chimpanzees from a smaller community called Mitumba, which is close to the human settlement of Mwamgongo on the northern edge of the park; and all 5 baboons – were infected with C. hominis.

The researchers analyzed DNA from a subset of the human and primate C. hominis cases and found that all belonged to the same strain, IfA12G2C, which is prevalent among humans in Africa.

These results suggest that people may be transmitting the parasite to Gombe’s chimpanzees. This could occur when chimpanzees raid agricultural fields outside the park boundaries and come in contact with exposed sewage, the researchers say.

The Gombe chimpanzee population has declined from about 150 animals 20 years ago to 100 today, and Cryptosporidium could be part of the reason.

An estimated 9 to 16 percent of Gombe’s chimps are infected with Simian Immunodeficiency Virus (SIV), which makes them vulnerable to opportunistic infections. In people with HIV, Cryptosporidium infections can be more severe than in HIV-negative individuals, and even fatal. The parasite could be having a similar effect in SIV-infected chimpanzees.

In the past, conservation planning has focused mainly on limiting hunting and providing adequate habitat for threatened species. But the new study suggests that conservation biologists need to pay more attention to infectious disease, the researchers say. Even when protected areas are well established, their borders are porous – especially to pathogens.

The findings could have human health implications, too, because species that live in close enough proximity to pass one pathogen back and forth may be vulnerable to other spillovers.

Granted, we humans are the only species likely to have nightmares after watching Steven Soderbergh’s Contagion. But we’re all in this together, the Gombe study reminds us: the same forces that terrify us may threaten the survival of other species as well. – Sarah DeWeerdt | 17 March 2015

Source: Parsons M.B. et al. 2015 Epidemiology and molecular characterization of Cryptosporidium spp. in humans, wild primates, and domesticated animals in the Greater Gombe Ecosystem, Tanzania. PLoS Neglected Tropical Diseases DOI: 10.1371/journal.pntd.0003529

Header image: A chimpanzee in Gombe National Park, Tanzania. Credit: Thomas Gillespie.