| 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.
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