Lecture 10- Treatment & Control of GI Nematodes in Horses

What type of animals are horses in terms of their evolutionary history with parasites?

Herd animals with a long evolutionary history of success in managing parasitic challenges.

How do feral/nomadic horses interact with infective pastures?

Feral and nomadic horses typically have little contact with infective pastures.

What is the general parasite challenge level for wild horses throughout their life?

Low-level parasite challenges throughout their life, contributing to natural resistance.

How do horses naturally reduce their exposure to parasites?

They have a natural aversion to grazing fecally contaminated areas within pastures.

How do the grazing behaviors of horses help prevent parasite infestations?

Horses avoid grazing on contaminated areas, such as roughs and lawns, to reduce parasite exposure.

How does pasture contamination affect parasite control in horses?

Pastures with low stocking density (1-2 horses per acre) lead to minimal pasture contamination and reduced parasite burden.

How does a horse's fecal egg count (FEC) relate to pasture infectivity?

Horses with low FECs have minimal effect on pasture infectivity, reducing the spread of parasites.

What is the relationship between parasite challenge and immunity in horses?

Parasite exposure helps build immunity, though young animals with undeveloped immunity are more susceptible to infection and disease.

Which group of horses is associated with high fecal egg counts (FEC)?

Young animals, such as foals, are associated with high FEC and are more susceptible to infections like Parascaris equorum.

How can pasture hygiene and management reduce parasite risk?

Removing feces (optimal management) and cross-species grazing can help reduce parasite load in pastures.

What is the role of grazing on hay aftermath in parasite control?

Grazing on hay aftermath helps reduce the risk of parasite exposure, as it minimizes contact with fresh fecal contamination.

What is the role of pharmacologic solutions in managing poor pasture management?

There are no pharmacologic solutions for poor management; effective control relies on good pasture hygiene and management.

What does "resistance" refer to in the context of parasitic infections in horses?

Resistance refers to the ability of parasites to survive drug treatments due to the selection pressure that increases the frequency of naturally occurring resistance genes.

How do treatment-resistant parasites reproduce differently?

Treatment-resistant parasites can reproduce without competition from susceptible worms, leading to increased frequency of resistance.

What management factors contribute to the development of drug resistance in parasite populations?

Exclusive use of a single drug and frequent use of treatments without giving susceptible worms the opportunity to reproduce.

How does the concept of refugia help in managing parasite resistance?

Refugia refers to the portion of the parasite population that remains untreated, allowing susceptible worms to reproduce and maintain the genetic diversity needed for effective drug treatments.

What happens when there is no refugia in a parasitic population?

Without refugia, resistant worms will continue to reproduce and spread, eventually leading to the failure of drug treatments.

What is "Refugia" in the context of parasite control?

Refugia refers to the portion of the worm population not subject to drug selection pressure at the time of treatment, including encysted stages in the host and free-living stages on pasture.

How does refugia help maintain parasite genetic diversity?

Refugia allows resistant and susceptible parasites to coexist and reproduce, preserving genetic diversity and reducing the negative effects of drug resistance.

What is the role of pyrantel in refugia and drug selection?

Pyrantel only affects adult worms and has no effect on encysted stages, allowing encysted parasites to survive and contribute to refugia.

How do different anthelmintics affect refugia?

- Pyrantel: Targets adults only, no effect on encysted stages.

- Ivermectin: Targets late L3 larvae and adults.

- Moxidectin: Targets early and late L3 larvae, and adults.

How does selective treatment maximize refugia?

By treating only high-risk animals or using targeted deworming, the amount of refugia is maximized, which mitigates negative selection pressure and supports a mixed population of susceptible and resistant parasites.

What is the disease risk in foals and juveniles regarding parasitic infections?

Foals and juveniles are more susceptible to parasitic infections due to undeveloped immunity, contributing to pasture contamination.

How do adult horses (over 3 years old) manage parasite infections?

Adults have acquired immunity and premunition, leading to a stable population of worms and making them less susceptible to hyper-infection.

What is the main goal of treatment in foals and juveniles?

The goal is to reduce pasture contamination, inhibit the intensity of infections, and prevent disease.

What is the recommended deworming schedule for foals in their first year of life?

- 8 weeks: Deworm with Fenbendazole (FBZ) to target Parascaris.

- 16 weeks: Deworm with Pyrantel for Parascaris and Ivermectin (IVM) for Cyathostomes.

- 24-32 weeks: Deworm with a Macrocyclic lactone (ML) for Small Strongyles, and possibly Parascaris.

- 52 weeks: Deworm with a Macrocyclic lactone for Small Strongyles.

How should parasite control be managed in the second year of a horse's life?

- Treat at 10-12 week intervals during high transmission seasons (Fall, Winter, Spring).

- Ensure the largest possible refugia by leaving worms untreated in certain individuals.

What is the deworming schedule for horses over 3 years old?

Treat at least once annually for Large Strongyles and Tapeworms using Macrocyclic lactones and Praziquantel. Identify and treat high FEC shedders with selective deworming. No more than 3 treatments per year at 12-week intervals (Fall, Winter, Spring).

How does selective deworming help manage parasite control in adult horses?

By identifying high FEC shedders and selectively treating them, it reduces unnecessary treatments, helps preserve refugia, and minimizes drug resistance.