What does a fecal egg count tell you?


The adult worms inside sheep/goats lay eggs. The eggs pass out of the sheep/goat in their manure. The eggs can be detected with a microscope (100x magnification). A fecal egg count is a procedure performed on a sample of manure to detect the presense of worm eggs (and to quantify them). There are two types of fecal testing: qualitative and quantitative.

A qualitative test is performed by mixing a small amount of feces with a flotation solution in a small vial. A simple slide and cover slip are used to examine the solution. This is the type of fecal testing many veterinarians perform. Results of a qualitative test are usually "positive" or "negative." Some qualitative tests use plus signs (+) to give a subjective opinion of the number of eggs present. More pluses means more eggs.

Quantitative fecal egg counts are reported in eggs per gram of feces (EPG). The common method of determining a quantitative fecal egg count is the modified McMaster procedure. McMaster is the name of the specialized slide that is used to count the eggs. In order to do a fecal egg count, a known amount of feces is combined with a known amount of flotation solution. The number of eggs counted in the chambers of the slide by a dilution factor to get EPG (a ratio). There are slight variations to the McMaster technique.

A single fecal egg count tells us the number of worm eggs in a particular fecal sample. Infections with one parasite are rare. Mixed infections are common. In sheep/goats the species of worms we are interested in are Trichostrongylus which includes Haemonchus spp., Trichostrongylus spp., and Teladorsagia spp. and to a lesser extent Oesophagostonum spp. and Cooperia spp. It is not possible to visually differentiate the aforementioned species due to the similarity in the size and shape of the eggs. Thus, a fecal egg count for sheep/goats is only for tricholstrongylus. Other egg types (tapeworm, whipworm, and coccidia) can be noted or counted. Coccidia oocyst (egg) counts are less useful because not all species of Eimeria are pathogenic.

Just because no (zero) eggs are detected in a fecal sample does not mean that the animal is free from parasites. Failure to see eggs can be the result of many things including a fecal egg counting technique that is not sensitive enough or from the random chance that no eggs were deposited in the particular portion of the manure sample that was collected or examined. Nor does a low egg count mean the animal is not clinically parasitized. A sheep/goat could be anemic (FAMACHA© score 4-5) and have a low fecal count because immature worms cause damage but do not lay eggs. With coccidiosis, animals often become clinical before they shed many oocysts.

A fecal egg count is a "snapshot in time" and tells us very little about the actual worm burden in the animal. Logic suggests that a higher egg means a high worm count (burden). But things aren't that simple. Many factors determine the egg laying rate of female worms. Some worms, such as Haemonchus, are more prolific. The female barber pole worm may lay 5000-15000 eggs per day, whereas the Nematodirus female only lays 50-100 eggs per day. Trichostronylus spp. only lay a few hundred eggs per day. There are many other factors affecting the number of eggs that are layed, including the age and production status of the animal and the host animal's level of immunity. What the animal eats or how much moisture is in its feces can also affect fecal egg count. Scours (diarreha) supresses egg count. Fecal egg counts are less accurate if the number of eggs is low.

Regardless, fecal egg counts do correlate with worm burdens; thus, have many applications. A low fecal egg count following treatment with an anthelmintic (dewormer) signifies that the treatment was effective. Dead worms don't lay eggs. If a group of animals has a high fecal egg count following deworming, there is resistance to that drug or combination of drugs. Drug resistance (or treatment efficacy) is determined by comparing before and after fecal egg counts on a group of animals. When fecal egg counts are very high, you can assume the level of pasture contamination is also very high. Animals grazing highly contaminated pastures need to be monitored for signs of clinical parasitism and the need for deworming. Susceptible animals should be removed from highly contaminated pastures.

Sheep/goats with consistly low fecal egg counts are more resistance to gastrointestinal parasites. They have greater natural immunity. They are depositing fewer eggs onto the pasture. Their resistance has a genetic component and is passed onto their offspring. A sheep/goat that doesn't require deworming may be resilient to parasites, but it isn't necessarily resistant to parasites. It might be, but you would need to verify it with a fecal egg count. Using FAMACHA© scores or deworming need will help to identify resistant animals, but it won't be as effective as using fecal egg counts.

An animal with a "high" fecal egg count may or may not need dewormed. Fecal egg counts can help make deworming decisions, but they should not be used as the sole critiera. Other criteria should be considered including FAMACHA© and the Five Point Check©. In areas when the barber pole worm is not prevalent, fecal egg counts are relied upon more for making deworming decisions, though targeted selective treatment is being gradually adopted.


Additional reading
What are fecal egg counts used for?
Can you learn to you own fecal egg counts?
Fecal egg count uses and limitations
Are fecal egg counts approaching their "sell by" date?