Parasitic nematode infective larva (L3) Identification organograms - small ruminants and cattle

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TIPS FOR ROUTINE LARVA IDENTIFICATION (see Van Wyk & Mayhew, 2013. Onderstepoort J. Vet. Res., 2013.   http:// v80i1.539)

  1. Larva preservation:  Advisable to have at least Trichostrongylus colubriformis L3 for comparison. For life-like L3 appearance, add formalin to suspension to concentration 1-2%, heat to 55-57oC, maintain for a minute, then add formalin to about 10% (4% formaldehyde).
  2. Stain L3 with Lugol’s Iodine diluted sufficiently for dark staining to be delayed few minutes.
  3. Microscope ocular lens graticule for measuring: Advisable to have, especially for training.
  4. Start by measuring one or two L3, then proceed by visual comparison with the rest in the sample (differences as small as 0.5 ‘X’ can usually be estimated without having to measure).
  5. Transition from L3 tail sheath filament to rest of loose sheath tail:  There is no clear-cut point of transition, and depth of microscope focus is of crucial importance.
  6. Points of comparison: For routine differential larval counts, select few points of comparison for routine counts, then refer to other points for  identification if not clear initially.
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