Parasite control programs in Horses (Equis) | Vetlexicon
equis - Articles

Parasite control programs

ISSN 2398-2977


Synonym(s): Parasite control strategies

Podcast: Parasite control programs

Factors to consider when designing a program to control worms

  • Drug resistance - mostly in cyathostomins and primarily to benzimidazoles - fecal egg count reduction test required.
  • Stocking density - can be deceptive, some paddocks may be heavily contaminated.
  • Age:
  • Pasture management   Pasture management  .
  • Worm egg count assessment knowledge of parasite population.
  • Climate - severe winters and long dry summers reduce larval survival on pastures.
  • Repeated (years) use of same drug selects for resistant populations.
  • Use of different drugs in rapid rotation within a year selects for resistance.
  • Establish a parasite control program for all horses on the premises.
  • Recall that the age of horses will determine to some degree the kinds and numbers of parasites they harbor.
  • A highly effective worm control program might prevent the development of naturally acquired immunity.
Classes of anthelmintics Major classes  Therapeutics: parasiticides
  • Macrocyclic lactones (avermectins/milbemycins - ivermectin   Ivermectin  , moxidectin   Moxidectin  ) - effective against nematodes, bots, some ectoparasites; no against activity tapeworms and flukes.
  • Benzimidazoles and probenzimidazoles (fenbendazole   Fenbendazole  , oxibendazole   Oxibendazole  , febantel   Febantel  , etc) - effective against nematodes with possibly some low activity against tapeworms; no activity against bots, flukes.
  • Pyrimidines (pyrantel   Pyrantel  ) - effective against nematodes and tapeworms, no activity against bots, flukes.
  • Organophosphates (dichlorvos   Dichlorvos  ) - not commonly available now - effective against nematodes and bots.
  • Piperazines - not commonly available now - effective against nematodes not marked. Also marketed as a combination with benzimidazoles to reduce rate of development of resistance.
  • Isoquinoline-pyrazines (praziquantel Praziquantel) - effective against tapeworms and flukes. Also marketed in combination with avermectins or milbemycins.
  • See also Therapeutics: parasiticides   Therapeutics: parasiticides  .
Egg reappearance period
  • Interval dosing with anthelmintics is based on the egg reappearance period, this being the time after dosing that eggs will reappear in the feces of susceptible horses. This varies from anthelmintic to anthelmintic being based primarily on the age of parasites that are killed in the horse, ie effective only maturing adults and adults, or effective against stages as young as L3.
  • See graph for presentation of estimated efficacy of commonly used anthelmintics   Anthelmintics: efficacy - graph  .
Anthelmintic resistance
  • Benzimidazoles and probenzimidazoles: resistance among the cyathostomin small strongyles to these is increasingly common in stables.
  • 5-day fenbendazole   Fenbendazole   and pyrantel   Pyrantel  : cyathostomin resistance now has been recognized and will spread to other stables unless biosecurity measures are in place.
  • Macrocyclic lactones: resistance of Parascaris equorum  Parascaris equorum   to ivermectin   Ivermectin   is being described and a possible shortening of the egg reappearance period of strongyles after ivermectin   Ivermectin   treatment might be an early manifestation of resistance.
  • Before designing an anthelmintic program carry out a fecal egg count reduction test particularly with benzimidazoles, testing before and 7-14 days after treatment. Reductions of >95% (90% for pyrantel   Pyrantel  ) required. Efficacy of 50% of macrocyclic lactones could be tested to possibly detect resistance early resistance.
Beware of the development of triple resistance.
  • As resistance among horse strongyles to benzimidazoles is widespread and pyrantel   Pyrantel   resistance now is apparent, it seems likely that resistance in horse stables will follow the pattern that has occurred on sheep farms.
  • Trichostrongyles on sheep farms show high levels of resistance to benzimidazoles and resistance to levamisole and the macrocyclic lactones is developing.
  • TRIPLE resistance to ALL three groups now has been described on sheep farms.
  • Unable to cure parasitic disease in their sheep, farmers have had to depopulate sheep farms in the UK and other countries.
  • Development of triple resistance in horses must be retarded.
Biosecurity
  • Resistant parasites must not be imported into the stables nor exported. Recommendations in place for sheep farms could be followed, although biosecurity will be complicated where stables have large numbers of horse movements.
  • Horses should be wormed, confined for 48-72 h (all feces disposed of) before being turned out.
  • Preferably, fecal egg count before, and 7-10 day after treatment, confining the horses and disposing of feces from the stable and when exercising. Repeat treatment if required.
  • Turn out onto dirty grazing any resistant parasites remaining in the horse will be diluted by interbreeding with the parasites at the stable.
  • Treatment during quarantine should be moxidectin   Moxidectin  . Alternately, following new recommendations for sheep, 2 drugs moxidectin   Moxidectin   followed by pyrantel   Pyrantel    should be used to produce redundant killing little information is available as to how closely these can follow each other in horses but different classes have been used one day later. This will produce redundant killing and kill nematodes resistant both to benzimidazoles as well any resistant to macrocyclic lactones (killed by pyrantel   Pyrantel  ) and pyrantel   Pyrantel   (killed by moxidectin   Moxidectin  )
Remember that horses can acquire parasites if allowed to graze at other stables visited for shows and rallies even for a day. Feces from horses visiting a property should be picked up and disposed of carefully. As moxidectin is not fully effective against immatures (developing and arrested L3) some horses could pass resistant eggs from worms that developed months or more after the treatmentPrint off the Owner factsheets All about worms, Pasture management and Worm control to give to your clients.

Pasture management

  • The most effective means for control of the important gastrointestinal parasites of horses.
  • Feces should be removed, preferably daily or twice weekly (even in cool temperate climates a few larvae will have reached the grass by 1 week). Manual removal or mechanical sweeper/vacuum cleaner.
  • Gives excellent control of the small cyathostomin and large strongyles and Trichostrongylus axei  Trichostrongylus axei  . Will control Parascaris equorum  Parascaris equorum  , but as eggs very sticky and produced in large numbers, some may remain.
  • Will remove many of the Anoplocephalaeggs before mites are able to eat them (daily removal preferable). Will remove Eimeria oocysts.
Advantages
  • No anthelmintics required cost saving and does not promote anthelmintic resistance.
  • Increases the area of grazing by preventing formation of the roughs in paddocks.
Disadvantages
  • Time consuming.
  • Must be carried out consistently.
  • Bots will not be controlled.
  • Preferably cut off or pluck out hairs with eggs while grooming or drug treat in the late autumn/early winter.
Other management factors
  • Rotate stock to clean pastures at a time when larval survival is expected to be minimal or one week after treatment. If you worm at this time consider anthelmintic resistance problems.
  • Moving horses to new pasture every 3-4 weeks should reduce numbers of strongyles (used only once in a year or rested for 6 months following other harrowing, alternate grazing).
  • Harrow pastures to scatter manure during dry periods and when the grass is short to decrease survival of larvae weeks or months to kill larvae depending on climate.
  • Alternate grazing with ruminants consider Trichostrongylus axei  Trichostrongylus axei   and fluke   Fasciola hepatica  .
  • Do not over-graze as the horses will eat the roughs containing maximum L3 numbers.
  • The above techniques can improve pasture hygiene, but are not consistently effective.

Targeted dosing with anthelmintics

Worm the wormy horses
  • Based on the principle that some horses carry few worms and are consistent low egg shedders (</=200 epg for adults, </=100 epg for foals and yearlings) while other horses are susceptible and carry heavy burdens (>200 epg).
  • Preferably begin in late winter.
  • Initially consider fecal egg counting every 2-3 weeks.
  • Low egg shedders: </=200 epg re-examine 2-3 weeks later. After several counts these can be identified as consistent low egg shedders and can be examined less frequently (every 3-6 months spring and autumn).
  • High egg shedders:
    • >200 epg treat (carry out a fecal egg count reduction test particularly for benzimidazoles at the first treatment).
    • Fecal egg count at egg reappearance period for the drug (4-5 weeks benzimidazoles and pyrantel; 6-8 weeks ivermectin   Ivermectin  ; 10-13 weeks 5-day fenbendazole and moxidectin).
    • >200 epg treat and re-examine at egg reappearance period.
    • d200 epg re-examine at 2-3 weeks intervals (some horse even though susceptible will have a longer egg reappearance period, young horses usually have a short egg reappearance period.
  • Table (below): example of low, variable and high egg shedders over a period of 2 years.

Egss per g of feces in month

Year 1

Year 2

Horse

Feb

May

Aug

Nov

Feb

May

Aug

Nov

1

0

0

0

0

0

0

50

100

2

0

0

0

0

0

0

0

0

3

0

50

0

50

0

0

50

0

4

50

150

50

0

150

0

100

100

5

50

0

200

50

0

50

0

0

6

0

150

200

50

150

350

100

300

7

100

0

200

100

100

50

200

150

8

100

100

200

50

50

50

400

200

9

250

650

850

650

750

0

750

1750

10

400

600

450

500

900

1600

1400

500

11

100

800

700

550

200

750

850

1250

12

1250

100

1050

450

1200

800

800

700

13

1250

100

1100

800

1450

150

3150

1000

14

350

1500

2500

100

1700

2050

2300

1000

When starting this program remember that the proportion of low egg shedders may depend on the previous control program used within the herd. A very efficient control program would have produced low pasture challenge that will manifest as low egg counts even in susceptible horses for several months to a year or more after that program has ceased. Table (below): example of effect of previous control program on proportion of low egg shedders in the year following withdrawal of that program.

Previous program

Proportion of low egg shedders

No control

19%

Anthelmintic given 2 times/year

27%

Anthelmintic given 5-7 times/year

70%

Advantages
  • Fewer anthelmintic doses required:
    • Reported to be cost effective in adult horses fecal egg count costs paid for by reduced anthelmintic usage.
    • Should reduce rate of development of anthelmintic resistance.
    • Parasites in untreated horses are in refugia, unselected for anthelmintic resistance and available to dilute emerging resistant genes
Disadvantages
  • Anthelmintics are used in susceptible horses so promoting worm resistant genes.
  • Bots will not be controlled - preferably cut off or pluck out hairs with eggs while grooming or drug treat in the late autumn/early winter.
  •  Anoplocephalatapeworms will not be controlled - anticestode treatments may be targeted after blood samples (taken late autumn/early winter) have identified individuals using the ELISA antibody test. However, serologic tests may only confirm exposure.
  • Time consuming - requires regular quantitative fecal egg counts by knowledgeable, trained personnel.

Continuous treatment (not available in the UK)

Treatment with single agent
  • Continuous administration of one drug.
  • Low-dose feed additive pyrantel tartrate   Pyrantel   (not available in the UK).
  • Advantage: easy to administer.
  • Disadvantages: potential for rapid development of resistance; delays development of natural acquired resistance.

Annual rotation with interval treatments

Annual rotation using the same class of drug for a year
  • The same class of anthelmintic is used regularly throughout a year, then substituted with a different class in the following year - alternate years macrocyclic lactone/pyrantel or 3 year rotation macrocyclic lactone/pyrantel/benzimidazole if benzimidazole resistance absent.
  • The treatment interval will vary with the class of drug (4-5 weeks benzimidazoles and pyrantel   Pyrantel  ; 6-8 weeks ivermectin   Ivermectin  ; 10-13 weeks 5-day fenbendazole   Fenbendazole  and moxidectin   Moxidectin  ).
  • Advantages: slows development of multiple drug-resistance but not as much as other methods; easy to implement; user-friendly.
  • Disadvantages: doesn't take into account efficacy of drugs against different worm populations. May have to include additional drugs for parasites not removed by selected drug, eg bots and benzimidazoles/pyrantel   Pyrantel   - may have to add macrocyclic lactone that year - preferably manually remove bot eggs while grooming.
  • Tapeworms and macrocyclic lactone - may have to add double dose pyrantel   Pyrantel   or praziquantel Praziquantel that year - preferably blood sample for tapeworm ELISA to see if treatment required.
Rapid rotation with more than one class of drug within the year (should not be used as it promotes multiple anthelmintic resistance)
  • Different anthelmintic classes are used throughout a year at intervals of 1-2 months and will cover all types of parasites.
  • Requires frequent post-dosing fecal egg count reduction tests to monitor resistance (but as the test is insensitive, resistance will already have established).
  • Disadvantage: likely to select for multiple drug resistance.
  • Advantage: all parasites are targeted during the same year. Classes of drugs may be selected for their activities at certain times of the year, eg encysted small strongyles, bots.

Seasonal/strategic dosing

  • Use of anthelmintics at specific times of the year, aimed at disrupting the seasonal cycle of transmission, ie at turn-out and at intervals through spring and summer in northern temperate climates.
  • Advantages: reduces pasture contamination; most effective on farms where few new horses are introduced.
  • Disadvantages: problems may arise with abnormal seasons (eggs can develop in mild winters in northern temperate climates); breakdown if heavily infested horse in introduced; does not take into account age groups.

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Stratford C H et al (2014) A questionnaire study of equine gastrointestinal parasite control in Scotland. Equine Vet J 46 (1), 25-31 PubMed.
  • Lester H E et al (2013) Cost comparison of faecal egg counted-directed anthelmintic delivery versus interval programme treatments in horses. Vet Rec 173 (15), 371 PubMed.
  • Matthews J (2010) Clinical forum: Drug resistance in cyathostomins. UK Vet 15 (3), 9-17 VetMedResource.
  • Fritzen B et al (2010) Endoparasite control management on horse farms - lessons from worm prevalence and questionnaire data. Equine Vet J 42 (1), 79-83 PubMed.
  • Lloyd S (2009) Effects of previous control programmes on the proportion of horses shedding small numbers of strongyle-type eggs. Vet Rec 164 (4), 108-111 PubMed.
  • Matthews J B (2008) An update on cyathostomins: Anthelmintic resistance and worm control. Equine Vet Educ 20 (10), 552-560.
  • Lloyd S, Smith J, Connan R M et al (2000) Parasite control methods used by horse owners: factors predisposing to the development of anthelmintic resistance in nematodes. Vet Rec 146 (17), 487-492 PubMed.
  • Herd R P & Gabel A A (1990) Reduced efficacy of anthelmintics in young compared with adult horses. Equine Vet J 22, 164-169 PubMed.
  • Herd R P (1986) Epidemiology and control of equine strongylosis at Newmarket. Equine Vet J 18, 447-452 PubMed.

Other sources of information

  • Little D & Gardner S Y (2003) Current Therapy in Equine Medicine 5. Ed: Robinson N E. W B Saunders. pp 161-164. ISBN: 072169540X.
  • Lloyd S (1997) Gastrointestinal parasites of equines and their control. In: Traction Animal Health and Technology. Universities Federation for Animal Welfare, Potters Bar. Ed: Hall S J G. pp 51-66.
  • Klei T R (1997) Current Therapy in Equine Medicine 4. Ed: Robinson N E. W B Saunders. ISBN: 0721626335.