By Jennifer McIntyre
Parasitic worms cause disease and reduce productivity in grazing livestock. Faecal Egg Counts (FECs) are widely used to estimate infection levels of various worm species, which often occur as mixed infections. In sheep, the commonest and most important species of worm parasites belong to the strongyle family, which includes Teladorsagia circumcincta and Haemonchus contortus. When a FEC is performed on a farm, it is a ‘strongyle egg count’ that is usually returned, with other species of interest noted and counted if required. Whilst it is recognised that interpretation of a FEC is complex, guidelines will often still include comments about the mean ‘egg per gram’ and how it relates to infection levels within the sheep such as ‘low infection levels’ being less than 250 eggs per gram.
Faecal Egg Count Reduction Tests
FECs can be used to investigate the efficacy of anthelmintic treatment on a particular farm using the Faecal Egg Count Reduction Test (FECRT). This is increasingly important with the rapid rise in anthelmintic resistance. For the FECRT, FECs are performed before and after anthelmintic treatment (the post-treatment sample time is dependent on which anthelmintic is being used) and the difference between the two is calculated. Ideally the egg count drops to zero, or close to it, so that the anthelmintic can be said to have an efficacy of over 95%. Anything less than this (too high an egg count after treatment) can be interpreted as resistance. Obviously it is important that the FECRT is carried out correctly as otherwise additional factors may confound the outcome – such as under-dosing or the use of an expired product.
What is well known is that the ‘strongyle egg count’ is not providing information about just one species, but rather a wide range of nematode species. Some of these are serious pathogens, like T. circumcincta, whilst others are mild pathogens causing negligible damage. Some of these nematodes have eggs which are shaped differently from the others. For example, Cooperia curticei, a mild pathogen, has eggs which are ‘straighter’ than the other species. For the most part, however, it is very difficult to distinguish one species of worm from another and thus understand the significance of the egg count by looking at the eggs alone.
How to overcome these limitations of the FECRT?
Eggs can be cultured and species identified after a week to ten days by examining larvae. The two larvae below can be differentiated by their tail length, along with other aspects of their anatomy.
Alternatively, DNA-based methods (such as PCR), which are highly specific, can be employed to determine species identity within only a few days following sample collection.
If we rely on the FECRT alone to determine the efficacy of an anthelmintic on farm, we can fall into a trap. Below are two examples of real–life farms where FECRTs have been carried out. The differences are significant!
First we counted the eggs…
Farm 1 carried out a FECRT using a benzimidazole anthelmintic (class 1-BZ). The FEC fell by 80%. This would suggest resistance is present (remember, resistance is present if the efficacy is less than 95%), but that the drug still has a good effect; after all, the count has still dropped to only 20% of the original FEC before treatment.
Farm 2 also carried out a FECRT. They used ivermectin, an anthelmintic in the 3-ML class. Their egg count dropped by 75%. So a similar result to Farm 1. Resistance is present but the ivermectin is still usable.
Next we speciated the strongyles…
We decided to hatch the strongyle eggs from both farms before anthelmintic treatment and culture them over ten days to become larvae. We then picked almost 100 larvae at random and speciated them by PCR to find out what was present. 86% of the population on Farm 1 was identified as T. circumcincta, the nematode we’re most concerned about. But this worm only made up 25% of the sample population collected from Farm 2.
How can we interpret this? Why does it matter?
This means that an 80% reduction in the ‘strongyle egg count’ on Farm 1 will definitely include T. circumcincta, our worm of interest (present at a proportion of 86% in the original population).
But on Farm 2 a reduction of 75% could mean that T. circumcincta, the serious pathogen, has been unaffected by the ivermectin as it only made up 25% of the original sample population.
Can we confirm this?
We decided to look at what was left post treatment. The faeces collected from the lambs after benzimidazole treatment on Farm 1 had mainly mild pathogens, with less than 5% of the sample identified as T. circumcincta. A good outcome.
In contrast Farm 2 had only T. circumcincta in the samples collected after treatment of the lambs with ivermectin. When we looked at the proportion of eggs per gram represented by each of the nematode species we had found after culture, the egg count for T. circumcincta hadn’t changed at all. Ivermectin did not appear to have any effect on this serious pathogen on this farm. So despite each farm having similar results based on the FEC alone, the species identification showed something very different. These results will allow the farmers to make more informed decisions as to their flock management than if only egg counts alone had been used to assess anthelmintic efficacy on the farm.