Spatial epidemiology of amphibian chytridiomycosis in the Orange River system of South Africa

Abstract

South Africa not only hosts a large number of amphibians, but also two different lineages of Batrachochytrium dendrobatidis, the postulated endemic BdCAPE and the Global Panzootic Lineage (BdGPL). The Orange River is one of the largest river systems in South Africa, stretching 2500km and encompassing various climates and areas within South Africa. With more than one lineage being present in South Africa, and similar genotypes isolated from Europe, the possibility exists for new lineages to be introduced to either naïve individuals within a population or into a population where another lineage occurs, due to connectivity and anthropogenic activity within a river system. The Orange River was surveyed, not only to establish which lineages are present in this system, but also to collect isolates for whole genome sequencing. The competition and virulence capabilities of the lineages occurring in South Africa were tested by use of experiments on a South African host — Sclerophrys gutturalis. Because it is important to know the ability of a pathogen to spread through a population, a second experiment was conducted to test the transmission capability to negative individuals and to individuals infected with a different genotype of the same geographical origin. Sampling of the Orange River was done at approximate intervals of 100 km between sites stretching from Vioolsdrif to Aliwal North. Swabbing and toe-clipping were done on all adult frogs and toads encountered and tadpoles of Amietia delalandii were also collected for attempted amphibian chytrid isolation. The first experiment was done to determine the virulence capabilities of South African and European lineages. Competition trials consisted of animals being exposed sequentially to the different genotypes. During this experiment the influence of the different lineages on the rate of mortality and the infection intensities on the animals were compared between the different geographical origins. The transmission experiment was conducted using only local lineages and individuals of different disease status were co-housed after the dosing stage completed. Trials consisted of an individual infected with either lineage being co-housed with a naïve individual and a trial where two individuals infected with different lineages were co-housed. The Orange River survey yielded 27 cultures in total, from both toe-clips (adults) and tadpoles. These cultures were obtained from four different sites. The swab data showed that amphibian chytrid is widespread throughout the Orange River system. Both BdCAPE and BdGPL lineages are present within this system. Although amphibian chytrid was found throughout the entire system, overall prevalence and infection intensities were low. When concerned with the experiments it was seen that the expat lineages showed a significant increase in rate of mortality dependent on dosage, while the same was not seen for the local lineages. In terms of competition between lineages on a single individual, it was seen that both expat lineages managed to establish an infection on a single host, while the local lineages were not as successful in co-infecting the same host. The transmission experiment showed that both local lineages are successful in being transmitted to a naïve individual just by being housed in the same enclosure, with BdCAPE infecting more naïve individuals than BdGPL. The latter being more lethal towards the infected individuals. The competition capabilities of the different lineages, as well as their ability to transmit to other individuals is especially important in a corridor habitat such as the Orange River. Habitat characteristics may change as well as host population composition progressing through the system, and thus the pathogen’s capabilities to spread to the available individuals and establish infections needs to be known, even when just marginally exposed to potentially naïve hosts.