RVC/FAO guide to Veterinary Parasitology
McMaster egg counting technique: Interpretation of results

When interpreting McMaster results, it must be remembered that a number of factors can influence the occurrence, recognition or numbers of helminth eggs found in a faecal sample. In particular, the number of eggs is not necessarily indicative of the number of worms present. Reasons for this include:

  • Eggs are produced only by fertile adult female (or hermaphrodite) worms and will, therefore, be absent in immature or single sex infections
  • The daily output of eggs by fertile females is influenced by host-physiological factors such as stress or lactation ( increased ) or immunity ( decreased )
  • Chemotherapy can also affect egg-production e.g. corticosteroids ( increased ) or sub-lethal anthelmintic doses ( decreased )
  • Some food-stuffs may have a similar effect e.g. tannin-rich forages ( decreased )
  • The concentration of eggs (per gram of faeces) is influenced by the daily volume of faeces being produced by the host, the rate of passage by the ingesta through the intestine, and the distribution of eggs throughout the faecal mass
  • Some types of eggs are heavier than others and may not float well in solutions of lower specific gravity (e.g. Fasciola)
  • Some eggs from different species are indistinguishable (particularly trichostrongylids and strongylids). This complicates clinical interpretation as some species (e.g. Haemonchus) produce many more eggs per day than others (e.g. Ostertagia).

To learn how to recognise the different types of helminth egg click on Egg identification.