Date Published: June 20, 2018
Publisher: Public Library of Science
Author(s): Courtney A. Miller, Geraud Canis Tasse Taboue, Mary M. P. Ekane, Matthew Robak, Paul R. Sesink Clee, Corinne Richards-Zawacki, Eric B. Fokam, Nkwatoh Athanasius Fuashi, Nicola M. Anthony, Stefan Lötters.
The amphibian disease chytridiomycosis in amphibians is caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd) and has resulted in dramatic declines and extinctions of amphibian populations worldwide. A hypervirulent, globally-dispersed pandemic lineage (Bd-GPL) is thought to be largely responsible for population declines and extinctions, although numerous endemic lineages have also been found. Recent reports of amphibian declines have been linked to the emergence of Bd in Cameroon, a major hotspot of African amphibian diversity. However, it is not known whether Bd-GPL or other lineages have been found in this region. This study therefore aims to examine Bd lineage diversity in the region and predict the distribution of this pathogen under current and future climate conditions using data from this study and from historical records. Almost 15% (52/360) of individuals tested positive for Bd using a standard quantitative PCR diagnostic. Infected amphibians were found at all eight sites sampled in this study. Species distribution models generated in BIOMOD2 indicate that areas with highest predicted environmental suitability occur in the Cameroon highlands and several protected areas throughout the country. These areas of high environmental suitability for Bd are projected to shift or decrease in size under future climate change. However, montane regions with high amphibian diversity are predicted to remain highly suitable. Phylogenetic analysis of the ITS sequences obtained from a set of positive Bd samples indicate that most fall within the Bd-GPL lineage while the remainder group with isolates from either Brazil or South Korea. Although more in depth phylogenetic analyses are needed, identification of Bd-GPL lineages in areas of high amphibian diversity emphasizes the need to continue to monitor for Bd and develop appropriate conservation strategies to prevent its further spread.
The infectious disease chytridiomycosis is a leading cause of global amphibian population declines. Clinical symptoms of the disease most commonly include excessive shedding of the skin, hyperkeratosis, and skin redness or discoloration. In general, the disease is diagnosed by the presence of maturing zoosporangia of the amphibian chytrid fungus, Batrachochytrium dendrobatidis (Bd) which infects the keratin-containing layers of amphibian skin . According to a recent global assessment, Bd has been detected in over 500 amphibian species  and is established on every continent where amphibians are found. In Africa, Bd has been documented in South Africa , the Albertine Rift in the Democratic Republic of Congo  and Uganda , Ethiopia , Kenya , Tanzania , Malawi , Morocco , and recently Mozambique . Across the rainforest belt of central equatorial Africa, Bd presence has been reported in Gabon , Nigeria [13,14], the island of Sao Tomé , and Cameroon [8,16,17]. Most of these reports from Africa consist of presence/absence assessments and estimations of Bd prevalence, thus there is little known about lineage diversity within Bd in this region.
While there is still much to be discovered regarding Bd diversity and host-pathogen-environment interactions, the present study provides further insight into genetic diversity and environmental suitability of Bd in an important amphibian biodiversity hotspot in Equatorial Africa. The Cameroon highlands, and specifically Mt. Cameroon, are designated as high conservation priority areas . Presence of the hypervirulent Bd-GPL in this region, while not detected at the montane site, could be potentially linked to the community declines observed in the highlands. With ongoing habitat destruction and degradation, as well as reductions of species climate envelope, amphibians could be pushed into smaller pockets of habitat that could potentially exacerbate Bd transmission due to over-crowding. Much progress has been made monitoring amphibian populations in the light of Bd emergence, but more work is needed to fully understand the environmental drivers of habitat suitability and map more precisely the global distribution of Bd lineages. To further understand the level of threat that Bd poses to Central African amphibians it is important for future research to further characterize Bd lineages in this region and determine how susceptible local amphibians are to different strains. Infectious diseases, primarily chytridiomycosis, are considered a major threat to amphibian populations so understanding which factors contribute to or limit infection across the globe is essential for amphibian conservation.