Equine Parasitology

What are the Cyathostomin species?

Considered the equine parasite of most epidemiological and clinical significance.

Over 50 different species identified.

Direct, non-migratory life cycle

Ingested larvae develop (L3→L4) within mucosal crypts of the large colon

Early L3 larvae can arrest their life cycle when conditions outside the host are suboptimal for development.

Mass eruption of the EL3 larvae can result in clinical disease – Larval
Cyathostominosis

What is Larval Cyathostominosis?

Emergence of larvae from mucosal crypts causes localised mucosal inflammation.

Mass emergence will lead to widespread disruption of mucosal barrier and loss mucosal function.

Acute disease:

Colitis and systemic inflammatory response syndrome (SIRS)

Diarrhoea, colic, pyrexia, tachycardia, cardiovascular compromise

Chronic disease:

Low grade colitis, altered digestive function, loss of protein from GIT

Clinical signs- low grade or recurrent colic, weight loss

What are the main methods of parasite prevention?

Remove/Reduce exposure

Eradicate disease in population

Isolation

Vaccination

Prophylactic therapy

Prevent development of clinical disease

Prevent infection/infestation in an individual

What is interval dosing?

A blanket, indiscriminate approach to prevention of equine parasites

E.g treat every horse for roundworms every 8-13 weeks during the grazing season

What are the benefits of interval dosing?

Very effective in reducing some parasite burdens

Non-discriminatory, non-selective pattern allows for control of different parasites with the same regimen.

Simple for owners to follow

What is the problem with interval dosing?

There is no evidence to say that prophylactic therapy will prevent the onset of clinical disease.

It may in some cases hasten the onset of clinical disease.

Development of anthelmintic resistance.

What is a “wormer”?

Worming, Deworming, Drenching

Large number of products on the market

But only three classes of anthelmintics:

Benzimidazoles- Fenbendazole
Macrocyclic Lactones- Avermectins
Tetrahydropyrimidines- Pyrantel

How is anthelmintic resistance in the equine industry?

Anthelmintic Resistance (AR) is considered one of the greatest threats
facing the equine industry

Widespread, indiscriminate use of anthelmintics has created a positive
selection pressure for the development of AR
• Selective pressures for AR include;
• Under-dosing - Very Common Problem
• Prolonged use of single agent
• Regular rotation of products – Selecting for resistance genes in parasites
• Removal of parasite refugia

Worldwide resistance to BENZIMIDAZOLES reported

Resistance to the macrocyclic lactones is rapidly developing

Parascaris equorum- Resistance to ivermectin reported

There are no new classes of anthelmintic in development Anthelmintic Resistance

What are some simple control methods that can be utilised to prevent parasites?

Removing faeces from a paddock

Individual grazing paddocks

Rotating paddock grazing with livestock or resting over-grazed paddocks

What are the aims of strategic targeted dosing?

To reduce exposure by targeting high shedders

To preserve anthelmintic efficacy by actively discouraging the development of anthelmintic resistance.

What are the methods of strategic targeted dosing?

Establish drug effectivity

Target the high shedders- those requiring treatment

Monitor the efficacy of the treatment prescribed

Maintain parasite refugia

What do faecal egg counts tell us?

Provide an estimation of the level of egg excretion?

What are the limitations of faecal egg counts?

Don’t differentiate between large and small strongyles

No information on the number of larval stages present

Sensitivity is generally poor for individuals

Not suitable for detecting:
Tapeworm (A.perfoliata), Lungworm, Bots, Pinworm, Fluke

How should cyathostomin larvae be controlled?

Treat high shedders

Reduce infective ‘load’ which susceptible individuals will be exposed to should be reduced.

Strategic Treatment
• Moxidectin is only recommended treatment effective in killing the larval stages of cyathostomin spp
• Some advocate treatment of high risk horses in late-Autumn or winter
• Potential concern though about inducing re-emergence
• Is likely contributing to resistance to moxidectin

ELISA has now been developed and is reaching commercial application
• Detects IgG to three antigens, including the encysted larvae

How is tapeworm diagnosed and controlled?

Diagnosis- faecal egg count is not a useful diagnostic tool, ELISA is available, confirms exposure not infection

Control- Strategic evidence based dosing, single annual treatment at the end of risk period