Diagnosing Internal Parasites in Horses: A Veterinarian’s Guide to Fecal Testing

Effective parasite control is a key component of equine health management. While routine deworming with anthelmintic medications was once the primary method of parasite prevention, growing evidence of anthelmintic resistance has shifted the focus toward targeted treatment.

Diagnostic tests provide valuable insight into which parasites are present in a horse or herd and the extent of infection, allowing veterinarians to design evidence-based control programs that preserve drug efficacy and protect equine well-being.

The most common diagnostic approach involves examining fecal samples for parasite eggs, larvae, or oocysts using fecal egg counts (FECs). These tests identify individuals shedding the highest numbers of eggs and help monitor the effectiveness of deworming treatments over time.

Regular testing, combined with management practices like pasture hygiene and selective treatment, supports a sustainable approach to parasite control. Rather than relying on calendar-based deworming, diagnostic testing allows veterinarians and owners to make informed decisions tailored to each horse’s risk level and environment.

By integrating these tools into herd health programs, owners can work with their veterinarian to optimize parasite control, maintain drug effectiveness, and improve long-term outcomes for equine populations.

Diagnosing Equine Internal Parasites

All grazing animals, like horses, are infected by internal parasites at some point in their lives. ^[1] The vast majority of these infections do not cause problems for the horse, as horses and their internal parasites can maintain a balanced relationship.

However, in some cases, an overwhelming number of parasites within the body can cause clinical disease. ^[1] Heavy parasite burdens can damage the intestinal wall and interfere with nutrient absorption.

Signs of significant parasitic infection in horses include: ^[1]

• Diarrhea
• Colic
• Lethargy
• Pot-bellied appearance
• Weight loss
• Poor growth
• Tail rubbing

The most common intestinal parasites that affect horses are: ^[1]

• Parascaris (roundworms)
• Large and small strongyles
• Anoplocephala tapeworms
• Pinworms

Veterinarians use diagnostic testing to identify pathogenic (disease-causing) parasitic infections, and to assess the efficacy of a deworming program.

Regular testing helps distinguish horses who require treatment from those who don’t, supporting targeted parasite control and reducing the risk of anthelmintic (dewormer) resistance in equine populations.

Sample Collection

Collecting fecal samples for parasitic testing is straightforward and is typically performed by owners. In some cases, veterinarians may collect a sample themselves as part of a brief rectal examination.

To collect an equine fecal sample for testing: ^[2]

1. Label a sandwich-sized zip lock bag with your horse’s name and the date of collection
2. Turn the bag inside out and place it over your hand
3. Use your covered hand to pick up 1-2 fresh fecal balls
4. Use your other hand to invert the zip lock bag and turn it right side out, with the fecal balls inside
5. Remove as much air as possible and zip up the bag. It is important to remove air, as oxygen can stimulate egg hatching
6. Store in a cool place, such as the fridge
7. Deliver the sample to your veterinarian within 48 hours of collection

Available Tests

Your veterinarian may perform a combination of tests depending on what type of parasites they are looking for.

The most common diagnostic tests used for identifying intestinal parasites in horses are: ^[3]

• Fecal flotation
• Fecal culture
• Fecal sedimentation
• Cellophane tape test
• Baermann procedure
• McMaster’s procedure
• Modified Wisconsin procedure

Fecal Flotation

Fecal flotation is one of the most commonly used laboratory methods for identifying intestinal parasite eggs in horses.

The procedure works by using a solution with a specific gravity greater than that of most parasite eggs, allowing the eggs to “float” to the surface while heavier fecal debris sinks to the bottom. ^[3] This separation makes it possible to collect and examine the eggs under a microscope.

The technique typically uses a dense solution such as sodium nitrate, zinc sulfate, or saturated sugar solution. When a diagnostician combines the fecal sample with flotation solution, the lighter parasite eggs float to the top of the solution where they can be sampled for analysis. ^[3]

The diagnostician observes the eggs under the microscope for speciation, identifying the species of parasite eggs present. They look at the eggs’ size, shape, color, and other characteristics to distinguish between common internal parasites of horses.

Fecal flotation samples can also be used for estimating fecal egg count using the McMaster method, described below.

Fecal Culture

Fecal culture involves growing the sampled parasite eggs in a laboratory to identify their larvae. ^[3] Eggs within a parasite group, such as strongyles, have a similar appearance under the microscope.

This means diagnosticians cannot reliably identify the specific species present based on the appearance of the eggs alone. This makes it challenging to prescribe the most effective deworming schedule for the individual horse. ^[3]

Parasitic larvae have more distinct appearances than eggs, which allow experienced parasitologists to identify their species. Fecal culture can take 10 to 12 days to grow larvae, so this diagnostic test is typically slower than other types of tests.

Fecal Sedimentation

Tapeworm eggs are too heavy for normal fecal flotation methods, meaning they can be missed on routine parasitic examination. To avoid this, parasitologists use fecal sedimentation to identify these eggs. ^[3]

This method involves combining the fecal sample with sedimentation solution and allowing the heavier particles to settle to the bottom of the fluid. They then filter the sample to produce a smaller amount of fluid heavily concentrated with eggs. ^[3] This sample is examined under the microscope to detect tapeworm eggs.

Although sedimentation can improve the likelihood of detecting tapeworm infections, it is still less sensitive than other diagnostic options because eggs are shed inconsistently and in low numbers.

Cellophane Tape Test

Pinworms rarely shed larvae or eggs into the horse’s feces, preventing diagnosis through traditional methods. Instead, the eggs of these parasites adhere to the skin around the horse’s anus. ^[3]

A tape test involves pressing a piece of tape around the anus to pick up eggs, then observing the tape surface under the microscope for egg identification. ^[3]

Baermann Procedure

The Baermann procedure detects motile (moving) larvae in the feces. This method is particularly valuable for identifying parasites that pass larvae rather than eggs in the feces.

Because these larvae are alive and capable of movement, the technique relies on their natural behavior to migrate out of the fecal material and into surrounding water.

In the procedure, the fecal sample is wrapped in gauze and placed into a funnel apparatus filled with warm water. The sample sits for 2 hours or more, allowing the larvae to escape into the fluid. The parasitologist drains the fluid from the funnel into a small Petri dish and observes the contents under a microscope. ^[3]

From there, they can identify the larval species based on features such as their length, shape, esophageal structure, and movement patterns.

McMaster’s Procedure

The McMaster’s procedure helps estimate the level of parasite contamination within the feces. In this procedure, the parasitologist performs a standard fecal flotation and the resulting fluid is poured into a McMaster chamber. This chamber has several “lanes” that standardize the amount of fluid within each microscope view.

The parasitologist counts the number of eggs within each microscope view and uses that information to calculate the number of eggs found in each gram of feces. ^[3]

The McMaster’s procedure is most commonly used to determine whether antiparasitic treatment strategies are effective at managing parasite burden. ^[3] Comparing the fecal egg count (eggs per gram of feces) allows veterinarians to assess whether their deworming protocol successfully reduced parasite load.

In general, “high shedder” horses are those with more than 500 eggs per gram of feces. ^[4] These horses typically require more intensive deworming protocols to reduce egg contamination in the environment. Horses under 200 eggs per gram of feces are “low shedders” and may only require deworming once per year. ^[4]

Modified Wisconsin

The Modified Wisconsin procedure is another method that allows parasitologists to estimate fecal egg count. This method does not require specialized equipment such the McMaster chamber. ^[5]

In this method, the diagnostician weighs out 3 grams of feces and combines it with a flotation solution. They strain the sample through gauze or cheesecloth and collect the fluid.

They centrifuge the solution to collect the eggs at the bottom of a test tube. This ensures all of the eggs in the sample are collected. ^[5]

Finally, they place the fluid onto a microscope slide and count the eggs. Dividing the total egg count by 3 gives an egg count per gram of feces. ^[5]

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Interpreting Results

There are several factors to consider when interpreting the results of parasitic testing. Parasitic testing is particularly prone to false negatives, where the test returns negative results despite an active, significant infection in the horse.

Veterinarians must have a thorough understanding of the parasite’s life cycle to draw conclusions from testing results.

There are multiple potential reasons for a false negative result on parasitic testing, including: ^[4][6]

• Only larval stages are present: Traditional diagnostic methods cannot identify larval parasites. If only larval stages are present due to recent infection or deworming, the test returns a false negative. For many species of parasites, such as cyathostomins, the larval stages are the most problematic for the horse.
• Eggs or larvae do not pass in the feces: Some parasites, such as pinworms, do not pass eggs or larvae in the feces. These parasites require special testing methods to identify.
• Timing: The number of eggs or larvae found in a fecal sample may vary depending on the activity of the adult parasite population around the time of defecation. Some samples may show lower parasite burdens than are actually present.
• Feces storage: How the fecal sample is stored can affect the results of testing. Fecal samples should be kept refrigerated, never frozen, until processing. Exposure to high temperatures can promote egg hatching, resulting in lower egg counts.

Finally, it’s important to note that complete elimination of parasite eggs after deworming is not feasible. Horses are continuously exposed to parasite eggs in their environment, so low levels of parasites are always expected. Most infections do not cause significant disease, but appropriately scheduled deworming is a key component of preventive medicine for your horse.

Work with your veterinarian to determine an appropriate deworming protocol for your horse based on fecal egg count results. This helps avoid excessive deworming, which is contributing to anthelmintic resistance where parasite species no longer respond to traditional deworming medications. ^[4]

Frequently Asked Questions

Here are some frequently asked questions about diagnostic testing for equine parasites:

Summary

Diagnostic testing for parasites in horses helps guide targeted deworming programs, monitor treatment success, and slow the development of anthelmintic resistance.

• Most parasitic infections in horses do not cause significant disease
• The most common diagnostic tests are fecal flotations and variants of that procedure that allow estimation of fecal egg count
• Fecal egg counts identify horses shedding high numbers of parasite eggs
• Regular testing supports evidence-based parasite control strategies and reduces antiparasitic resistance