September 5, 2024 – The first hints of trouble in the gorillas were subtle. Weight loss. A scruffy haircoat.
However, the changes were noticed by the many researchers and caretakers who monitor their habitat. They realized that gorillas were literally wasting away before their eyes, something the team had not witnessed before, and they needed to find out why – fast.
The Bwindi Impenetrable National Park in Uganda is a biodiversity hotspot, home to thousands of species of animals and plants, including the endangered mountain gorilla. These majestic animals are some of the park’s best-known residents. The new disease, chronic wasting syndrome (CWS), posed a major threat to this already fragile population.
An important clue to a possible cause noted by the park’s caretakers and managers was the presence of large numbers of strongylid nematodes (a parasitic worm) found on post-mortem examination. Although parasites are not new to the population, their presence raised an interesting question – could they be the driving force behind the decline? And if so, why now? And where?
Morris Animal Foundation-funded researchers from the Institute of Vertebrate Biology CAS in the Czech Republic, and staff from the Gorilla Doctors, Inc. team wanted to find out.
They systematically studied past and newly occurring cases of CWS to confirm if strongylid nematodes were responsible for the cases of CWS and what factors related to
its occurrence. The team also attempted to evaluate the effect of antiparasitic medication used to treat CWS cases.
The researchers analyzed the Gorilla Doctors’ veterinary records database between 2017 and 2022. They collected observational data and studied the worm and microbial communities in fecal samples from healthy and ill gorillas in the park. The team looked at samples from retrospective, ongoing and new cases.
The project was massive, employing a wide variety of techniques to analyze the data from
traditional microscopy to state-of-the-art metagenomics and advanced statistical analyses to reach their conclusions.
The team noted that cases of CWS steadily increased during the study period and that most cases were detected during the dry season. The majority of the CWS cases occurred in groups ranging near the edge of the forest compared to those ranging inside the forest. Lastly, the team found that the disease was slightly more common in adult females than adult males or infants, although the numbers were similar (26, 21 and 21 respectively).
The team also looked at tapeworms as part of their study. Other results were based on analysis of fecal samples directly.
Internal parasites shed their eggs in the feces, an important way the disease is transmitted, and egg number can be used to indicate the severity of infection. However, the team did not find any significant relationship between the egg counts and the CWS occurrence. The team did not note any association between the gut bacteria communities in CWS-affected and normal individuals, although there were associations between specific microbiota and parasitic worms.
Oesophagostomum (a nodular worm) were associated with gorilla groups that had higher numbers of CWS cases.
Analyses of samples before and after anthelmintic treatment in cases where the treatment was used showed that the treatment was not always effective and did not eliminate all parasites from the intestine. The team concluded this finding warranted further attention as anthelmintic resistance could develop in these nodular worms. The team also reported that the anthelminthic praziquantel was effective against tapeworms, although no association was detected between tapeworm infection and CWS.
One challenge encountered by the team was collecting feces from specific individuals after they received treatment since this requires observing the individual defecating and then locating and collecting the feces in a timely fashion.
The research confirmed that nodular worms are associated with CWS in Bwindi gorillas. Because they couldn’t collect enough feces from sick individuals to provide statistical analysis, they weren’t able to determine whether fecal analysis was useful for diagnosis. However, the team determined that current treatment methods might not be effective and could lead to anthelminthic resistance, a serious concern.
The team was surprised to find that treating affected gorillas with medications directed against the parasites didn’t always work, and they reported concerns about resistance.
The significant finding from the study was that the worms are likely the driving force behind chronic wasting syndrome, and that simply deworming affected animals might not be as effective as thought. The team is now looking at predictive markers for disease and leveraging their findings to find the reservoir of disease – hoping to protect this iconic species.
The study provided a diverse group of students and early career scientists with a remarkable training experience. Multiple publications are in progress, and the results were presented at numerous conferences in Europe, South America, Asia, Africa and the United States.
The epidemiology of parasitic infections is being studied as part of a follow-up project funded by the U.S. Fish and Wildlife Service. The team will also focus on unhabituated gorillas and other individuals, including chimpanzees, baboons, livestock, dogs and humans, to identify potential reservoirs of infection for gorillas. Future studies will also continue searching for markers that could be used to predict if a gorilla will develop CWS or not.
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