Why McMaster ova counts are largely worthless for detecting changes in helminth population

The Argument Against Egg Counts as an Indicator of Helminth Number

  • Parasitology texts note that per day female hookworm and whipworm ova production varies from 2,000 to 20,000 total. For this reason alone any estimate based on an ova count must have a variance of x10. That is the answer can only be expressed as a range, with the higher end of the range being ten times the low end. So a typical answer would be “from 20-200 hookworm”. Fairly useless, particularly if you are trying to detect the loss of one or two hookworm. On this basis alone the test is too crude for the purposes most with deliberate helminth infections are trying to use it for.
  • Egg counts are a measure of density. So any count is going to be affected by things like the speed of material through the intestines (constipation or diarrhoea at the extremes), amount eaten, amount of fluid drunk, fibre content of food, etc. If you think of the extremes one can see this is going to have an enormous impact on density of ova per gram of faeces. Different foods and drugs affect the speed of material through the intestines.
  • Any count depends on extreme precision and replication of methods and precision from test-to-test. Only an experienced lab technician is capable of accurately counting ova in stool, stains are no aid and identifying each and every ova in a gram of faeces is difficult work. In someone producing 100 ova per gram .1 grams will contain 10 ova, so the slightest variation in weight can have a big effect on the number observed, that effect then being amplified by the multipliers used to derive worm population from egg counts.
  • Ova production varies tremendously with time. Hookworm ova production falls by an estimated 50% about one year after infection. But there is little information on whether this is invariably 50% or exactly when it happens or how fast.
  • Ova production in all helminths is affected by various drugs, some known, some inevitably unstudied. So, antibiotic use so profoundly reduces ova production that parasitology texts recommend not doing ova tests for two weeks after the subject stops taking the antibiotics. Other drugs almost certainly have various effects as well.
  • McMaster egg counts have been abandoned by doctors of veterinary medicine as being useless as a practical tool for estimating worm numbers. They would know.
  • Most people using helminthic therapy, if using N. americanus or T. trichiura, know how many helminths they received. So the only purpose of ova counts is to monitor infection levels. But given all the other factors noted here the test is not sensitive enough to detect even a relatively large change in numbers.

    Egg counts can be used as a broad indicator of worm burden, but only as an indicator. McMaster egg counts were intended for use to determine whether someone has a heavy, moderate or light infection, so appropriate treatment (as in elimination except in light infections) could be prescribed.

    Researchers use egg density measurements in studies to monitor worm burden in their study subjects, but they have specialised equipment and the training to do so reliably and consistently. Even then they cannot indicate much, and beside most studies continue for far less than one year. So any change they are likely to detect would be gross, that is loss of worms. Which is undoubtedly what they are primarily looking for.

    The only method with any real utility to determine helminth population is endoscopy for hookworm and colonoscopy for whipworm.