Molecular and pathological studies on Angiostrongylus species in southeast Queensland

Abstract

Despite an apparent increase in cases of angiostrongyliasis in humans and animals in Australia, the epidemiology of infection with the two species of Angiostrongylus that co-exist in this country, namely A. cantonensis and A. mackerrasae, is poorly understood. This knowledge gap is particularly important with respect to Angiostrongylus mackerrasae, a species evidently native to Australia, as its ability to cause disease in humans is unknown. Likewise, there is little information on the roles of native and introduced species of rodents and molluscs as hosts of Angiostrongylus species in Australia. This thesis focuses on the gaps in the knowledge about the epidemiology of two species in southeast Queensland, highlighting the hotspots for transmission of the parasite in urban Brisbane. A comprehensive survey of rats belonging to the species Rattus rattus, Rattus norvegicus and Rattus fuscipes was conducted. The prevalence of infection with Angiostrongylus spp. was 16.5% in Rattus spp. trapped in urban Brisbane and surrounds which is much higher than estimates of earlier studies. This highlights the possible risk of zoonotic infection in children, dogs and wildlife in this region and indicates the necessity for public awareness as well as more detailed epidemiological studies on this parasite in eastern Australia. Of the two Australian species, Angiostrongylus mackerrasae appears to be adapted to life in the native bush rat, Rattus fuscipes. This species evidently has an identical life cycle to that of Angiostrongylus cantonensis. A. mackerrasae may have greater potential to infect non-rodent hosts as it has been found as adult form in the lungs, as has been recorded for an individual flying fox. This raises concerns as to the potential of this native angiostrongylid to infect domestic animals and humans. To date, data on the taxonomy, epidemiology and population genetics of A. mackerrasae are poor or unknown. Here, we describe the mitochondrial (mt) genome of A. mackerrasae with the aim of addressing some of these knowledge gaps. The complete mitochondrial (mt) genome of A. mackerrasae was amplified from a single morphologically identified adult worm, by long-PCR in two overlapping amplicons (8 kb and 10 kb). The amplicons were sequenced using the MiSeq Illumina platform and annotated using an in-house pipeline. Amino acid sequences inferred from individual protein coding genes of the mt genomes were concatenated and then subjected to phylogenetic analysis using Bayesian inference. The mt genome of A. mackerrasae is 13,640 bp in size and contains 12 protein coding genes (cox1-3, nad1-6, nad4L, atp6 and cob), and two ribosomal RNA (rRNA) and 22 transfer RNA (tRNA) genes. The mt genome of A. mackerrasae has similar characteristics to those of other Angiostrongylus species which is explained in detail in chapter 3. Phylogenic comparisons reveal that A. mackerrasae is closely related to A. cantonensis and the two sibling species may have recently diverged compared with all other species in the genus with a highly specific host selection. This mt genome will provide a source of genetic markers for explorations of the epidemiology, biology and population genetics of A. mackerrasae. The potential pathogenicity of A. mackerrasae for human and animals was also investigated in this PhD and the results reveal that A. mackerrasae causes severe eosinophilic meningitis in mice and guinea pigs and can potentially cause similar neurological disease in human and animals. It was also revealed that mice immune response is less responsive to A. cantonensis comparing to the similar species A. mackerrasae, as A. cantonensis live for a longer time in meninges of the accidental host and this can help the parasite causing a more patent infection in human and animals. The data of this project raises the possibility that cerebral meningitis remains a potential problem in South Eastern Queensland, particularly as people encroach onto forested areas. Angiostrongylus mackerassae appears to be a species that diverged from A. cantonensis after invasion of the Australian mainland. This species may have greater pathogenesis both in definitive and accidental hosts.