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The sequences of the nuclear ribosomal DNA region spanning the first internal transcribed spacer, the 5.8S rRNA gene and the second internal transcribed spacer were determined for Ascaris samples from pigs and humans from different geographical regions. The sequences of the 5.8S gene and the second internal transcribed spacer were the same for all samples examined, whereas all Ascaris samples from humans had six (1.3%) nucleotide differences in the first internal transcribed spacer compared with those from pigs. These differences provided some support for the existence of separate species of Ascaris or population variation within this genus. Using a nucleotide difference within a site for the restriction enzyme HaeIII, a PCR-linked restriction fragment length polymorphism method was established which allowed the delineation of the Ascaris samples from pigs and humans used herein. Exploiting the sequence differences in the first internal transcribed spacer, a PCR-based single-strand conformation polymorphism method was established for future analysis of the genetic structure of pig and human Ascaris populations in sympatric and allopatric zones.

Genomic DNA was isolated from adult Strongylus asini collected from zebra. The second ribosomal transcribed spacer (ITS-2) was amplified and sequenced using polymerase chain reaction (PCR) based techniques. The DNA sequence was compared with previously published data for 3 related Strongylus species. A PCR-linked restriction fragment length polymorphism method allowed the 4 species to be differentiated unequivocally. The ITS-2 sequence of S. asini was found to be more similar to those of S. edentatus (87.1%) and S. equinus (95.3%) than to that of S vulgaris (73.9%). This result confirms that S. Asini and S vulgaris represent separate species and supports the retention of the 4 species within 1 genus.