Diagnostic morphology of four larval ascaridoid nematodes that may cause visceral larva migrans: Toxascaris leonina, Baylisascaris procyonis, Lagochilascaris sprenti, and Hexametra leidyi.

The gross and histological morphology of the larvae of 4 ascaridoid nematodes, Toxascaris leonina, Baylisascaris procyonis, Lagochilascaris sprenti, and Hexametra leidyi, are described. The larvae of T. leonina, B. procyonis, and L. sprenti were recovered from experimentally infected mice at 32, 14, and 75 days of infection, respectively. Hexametra leidyi larvae used for morphological study were collected on day 159 postinfection from a rhesus monkey, Macaca mulatta, while H. leidyi larvae used for histological study were collected from mice when they had reached the same size as those found in the monkey, i.e., at 23 days postinfection. Larvae for morphological study were collected by pepsin digestion, fixed in glacial acetic acid, and cleared in glycerin. Tissues for histological study were fixed in 10% formalin or Bouin's fixative. All sections were stained with hematoxylin and eosin. The differences in the morphology of the larvae of these 4 species were found to be length of the body, shape of the anterior end, and shape of the tail. The major differences in the histological anatomy of these larvae were found to be the body diameter, form of lateral alae when present, presence or absence of internal cuticular bars, shape and internal patterns of the excretory columns, and size and number of intestinal cells. These 4 larvae are differentiated from other described species of ascaridoid larvae that may cause visceral larva migrans, and keys have been devised to aid in the making of a diagnosis.