NOVEL LINEAGES OF GIARDIA-INTESTINALIS IDENTIFIED BY GENETIC-ANALYSISOF ORGANISMS ISOLATED FROM DOGS IN AUSTRALIA

Infection of suckling mice with Giardia trophozoites recovered from th e intestines of 11 dogs autopsied in Central and Southern Australia in each case produced an established isolate. In contrast, only 1 isolat e was obtained by inoculation of faecal cysts. The organisms grew poor ly in comparison with isolates from humans or non-canine animal hosts. Light microscopy revealed that the trophozoites had median bodies wit h the 'claw hammer' appearance typical of G. intestinalis (syn. G. duo denalis, G. lamblia) but that they differed in shape and nuclear morph ology from axenic isolates of human or canine origin. Allozymic analys is of electrophoretic data representing 26 loci and phylogenetic analy sis of nucleotide sequences obtained from DNA amplified from the gluta mate dehydrogenase locus showed that the 11 isolates examined from Aus tralian dogs were genetically distinct from all isolates of G. intesti nalis that have been established previously from humans and animals, a nd also from G. muris. Both analytical methods placed 10 of the Austra lian canine isolates into a unique genetic lineage (designated Assembl age C) and the eleventh into a deep-rooted second branch (designated A ssemblage D), each well separated from the 2 lineages (Assemblages A a nd B) of G. intestinalis that encompass all the genotypes known to inf ect humans. In contrast, 4 axenic isolates derived from dogs in Canada and Europe (the only other isolates to have been established from dog s) have genotypes characteristic of genetic Assemblages A or B. The fi ndings indicate that the novel Giardia identified in these rural Austr alian dogs hare a restricted host range, possibly confined to canine s pecies. The poor success rate in establishing Giardia from dogs in vit ro suggests, further, that similar genotypes may predominate as canine parasites world-wide. The absence of such organisms among isolates of Giardia that have been established from humans by propagation in suck ling mice indicates that they are unlikely to infect humans. However, infection of humans by those dog-derived genotypes that grow in vitro cannot be excluded.