Lecture 8- Gastrointestinal Nematodes of Horses: Strongyles & Cyathostomes

What are the two main categories of Strongyle parasites in horses?

Large Strongyles and Small Strongyles (Cyathostomes).

Where do adult Strongyles reside in horses?

colon and large intestine

How do horses become infected with Strongyle parasites?

By ingesting L3 larvae while grazing on contaminated pasture.

Why are Strongyles a primary target for control by equine owners?

They cause significant gastrointestinal disease and can lead to severe complications.

Describe the size and sexual dimorphism of Large Strongyles.

Males: 14-35 mm; Females: 20-47 mm; Males have a copulatory bursa.

What anatomical structure is prominent in Large Strongyles?

Buccal capsule.

Where are adult Large Strongyles parasitic?

In the large intestine of equines.

How long does it take for Strongyle eggs in feces to develop into L3 on pasture?

About 2 weeks.

What is the migration pattern of Large Strongyle larvae before they mature?

They migrate extra-intestinally before arriving at the large intestine to reproduce.

What are the prepatent periods for Large Strongyles?

6 months, 8 months, or 12 months, depending on the species.

Why do Large Strongyles have long prepatent periods or egg reappearance periods after treatment?

Due to their extended larval migration and development cycles.

What is the migration pattern of Strongylus vulgaris larvae?

They migrate in the walls of small arteries and the anterior mesenteric artery before arriving in the cecum/colon.

What is the prepatent period of Strongylus vulgaris?

~6 months

What is the migration pattern of Strongylus equinus larvae?

They migrate through the peritoneal cavity and liver before arriving in the cecum/colon.

What is the prepatent period of Strongylus equinus?

~8 months

What is the migration pattern of Strongylus edentatus larvae?

They migrate through the peritoneal cavity and liver before arriving in the cecum/colon.

What is the prepatent period of Strongylus edentatus?

~12 months

When does disease from Large Strongyles typically occur in older foals?

During the prepatent period.

What stage of Large Strongyles causes the most pathology?

The larval stages during long extra-intestinal migration.

Which species of Large Strongyles is the most pathogenic?

Strongylus vulgaris, due to larval migration through arteries.

What are the main host inflammatory responses caused by Large Strongyles?

Aneurysm, infarction, and colic due to interruption of blood supply.

How can Large Strongyles contribute to colic?

By interrupting the blood supply to the intestines.

What clinical sign is associated with adult Strongyle worms in the large intestine?

diarrhea

How do adult Large Strongyles obtain nutrients?

By sucking blood.

Are adult worms the most significant cause of disease in Large Strongyle infections?

No, the primary pathology is caused by the migrating larvae.

Can fecal exams be negative in horses infected with Large Strongyles?

Yes, especially during the prepatent period.

How is a diagnosis of Large Strongyles typically made?

Based on clinical signs, age of the horse, and risk factors.

What are the main risk factors for Large Strongyle infection?

Wormy pastures, overcrowding, and exposure to older animals with high FEC (Fecal Egg Counts).

Why are older animals with high FEC a risk factor for younger horses?

They serve as a source of environmental contamination.

How does an inconsistent deworming program contribute to Large Strongyle infections?

It allows larvae to persist in the environment and continue their lifecycle.

How do Small Strongyles compare in size to Large Strongyles?

They are comparatively smaller in size.

What are the size ranges for Cylicocyclus and Cyathostomum species?

Cylicocyclus: 10-25 mm; Cyathostomum: 5-12 mm

Are Small Strongyles sexually dimorphic?

Yes, males and females have distinct morphological differences.

What structure is present in male Small Strongyles for reproduction?

Copulatory bursa.

What is the defining feature of the anterior end of Small Strongyles?

A prominent leaf crown called corona radiata.

Where are adult Small Strongyles parasitic in horses?

in the large intestine

How long does it take for Small Strongyle eggs in feces to develop into L3 on pasture?

Approximately 2 weeks.

How do horses become infected with Small Strongyles?

By ingesting L3 larvae while grazing.

Where do Small Strongyle larvae go after ingestion?

They penetrate and encyst in the intestinal wall.

What happens after Small Strongyle larvae encyst in the intestinal wall?

Late L3 emerge, develop into L4, and then mature into reproducing adults.

When are eggs first detected in feces post-infection if there is no seasonal arrested development?

At 9 weeks post-infection.

What is the normal prepatent period for Small Strongyles?

9 weeks

When does disease from Small Strongyles occur in foals?

Before the adult worms are sexually mature.

What is the primary cause of pathology in Small Strongyle infections?

The mass emergence of encysted larvae from the wall of the large intestine.

What common condition in foals is associated with Small Strongyles?

"Foal heat diarrhea."

Can fecal exams be negative in horses infected with Small Strongyles?

Yes, especially during larval encystment.

How is a diagnosis of Small Strongyles typically made?

Based on clinical signs, age of the horse, and risk factors.

What are the main risk factors for Small Strongyle infection?

- Wormy pastures

- Overcrowding

- Exposure to older animals with high fecal egg counts (FEC)

- Lack of a consistent deworming program

What stage of Small Strongyles is often passed in feces following emergence?

L4 (Red Worms).

When palpating an infected horse, where might Small Strongyle larvae be observed?

On gloves after rectal palpation.

What parasite should Small Strongyle larvae not be confused with?

Pinworms (Oxyuris equi).

How many species of Large Strongyles are there?

three

What is the key pathogenic feature of Large Strongyle larvae?

Long extra-intestinal migration.

What type of disease is associated with Large Strongyle infections?

Disease caused by host inflammatory response to larval migration.

What is another name for Small Strongyles?

Cyathostomes.

Where do adult Small Strongyles reside in horses?

Colon and large intestine

Approximately how many species of Small Strongyles exist?

~40 species.

What is the key pathogenic feature of Small Strongyle larvae?

Larval stages encyst and develop in the intestinal wall.

What is the primary cause of disease in Small Strongyle infections?

Tissue damage due to mass emergence of larvae from the colonic wall.

What diagnostic method is commonly used to detect Strongyle infections?

Fecal flotation to detect eggs.

Are Large and Small Strongyle eggs morphologically distinct?

No, they are indistinguishable on fecal flotation.

What does ERP stand for in equine parasitology?

Egg Reappearance Period.

How short can the ERP be for some species of Cyathostomes?

As short as 28 days.

What is the typical ERP range for most Small Strongyles?

6 to 12 weeks, depending on the drug used.

What are the ERP durations for Large Strongyles?

6, 8, and 12 months.

What are Fecal Egg Counts (FEC) useful for?

- Monitoring drug effectiveness

- Assessing pasture contamination

- Identifying high egg-shedding ("wormy") animals

Can Fecal Egg Counts (FEC) predict actual worm burdens in horses?

No, they cannot be used to determine total worm load.

What are the three major families of equine deworming drugs?

1. Benzimidazoles

2. Imidazothiazole/Tetrahydropyrimidines

3. Macrocyclic Lactones

What is the mode of action for Benzimidazoles?

Inhibition of β-tubulin synthesis.

What is the mode of action for Imidazothiazole/Tetrahydropyrimidines?

They act as nicotinic agonists, leading to neuromuscular paralysis of the parasite.

What is the mode of action for Macrocyclic Lactones?

They act on gated glutamate chloride channels, causing paralysis and death of the parasite.

How does resistance vary among Strongyle species?

- No known resistance in Large Strongyles.

- Small Strongyles show responsiveness to Macrocyclic Lactones, but shorter ERPs (Egg Reappearance Periods) are emerging.

What is the most common deworming program used by horse owners?

Interval dosing.

How does interval dosing work?

All horses are treated at fixed intervals throughout the year without diagnostics.

How does the treatment interval compare to the Egg Reappearance Period (ERP)?

Treatment occurs at intervals shorter than ERP (every 2 months or 8 weeks).

What is the goal of interval dosing?

Suppressive deworming to minimize pasture contamination.

How does interval dosing contribute to anthelmintic resistance?

Frequent treatments increase selection pressure, driving resistance.

How does strategic dosing differ from interval dosing?

It targets all horses at specific times of the year instead of fixed intervals.

When is strategic dosing applied?

During active grazing season when horses are at higher risk.

How does strategic dosing consider parasite lifecycle biology?

It aligns treatment with times when parasites are in hypobiosis, reducing active populations.

How does strategic dosing impact resistance development?

Fewer treatments reduce selection pressure, lowering resistance risk.

What drug is used in continuous daily treatment for Strongyles?

Pyrantel tartrate (Strongid C).

How does continuous daily treatment work?

Daily administration kills L3 larvae before they can invade mucosal tissues.

What are the effects of continuous daily treatment on adult Strongyles?

It inhibits emerging adults and reduces egg counts.

What unintended consequence has continuous daily treatment caused?

It has contributed to resistance in Pyrantel-based compounds like Strongid T.

What is selective deworming based on?

Treating only horses that exceed a set fecal egg count (FEC) threshold.

What is the major benefit of selective treatment?

It reduces overall treatment intensity, slowing resistance development.

What is refugia in parasite control?

The portion of the parasite population that is not exposed to drug selection pressure.

How does refugia help in resistance management?

It maintains a population of drug-susceptible parasites, reducing the spread of resistance.

Why is targeting adult nematodes ineffective for Strongyle control?

It does not stop larval invasion (Large Strongyles) or mass emergence (Small Strongyles), which are the major pathogenic events.

What is a limitation of interval treatment with Pyrantel or Benzimidazoles?

These drugs have limited extra-intestinal activity against late-stage larvae and adults, allowing for reinfection.

Why do parasites quickly reinfect horses after deworming with short-acting drugs?

Low persistence in tissues allows continuous parasite population turnover.

What happens even if a drug is 99% effective at killing parasites?

1% of worms survive, reproduce, and contribute to reinfection.

What are the consequences of sexually maturing worms in horses?

- Hazardous pasture contamination

- High rates of reinfection

- Accumulation of significant worm burdens

- Increased incidence of parasitic disease

How long do adult Strongyle worms typically live?

35 to 55 days.

What environmental conditions inhibit parasite reproduction and transmission?

Seasonally hostile conditions, such as high temperatures and low moisture.

How does summer weather in Tennessee affect Cyathostome transmission?

Warmer temperatures and low moisture availability inhibit transmission.

How do parasites survive unfavorable environmental conditions?

By entering arrested development (hypobiosis) within the host.

When do parasites emerge and reproduce in the Southern US?

During Fall and early Spring (March), when pasture conditions are favorable.

What is the primary treatment goal for Large Strongyles?

Target migrating larvae with an annual Macrocyclic Lactone treatment that has persistent extra-intestinal activity.