Worming drugs

The use of drugs to suppress faecal egg output has been the mainstay of parasite control for many years.

The rationale behind the use of worming drugs was to kill the egg laying adult parasites and thereby minimise pasture contamination.

Wormers

Worming drugs can be used in a number of different ways:

Interval dosing
  • This strategy involves the administration of a specific drug at regular time intervals during the high-risk, summer grazing period.  Some horse owners continue to dose their animals at the same frequency during the lower-risk winter period or when stabled for most of the day.  This is expensive and often unnecessary.  Furthermore, many horse owners use worming drugs at inappropriate intervals, e.g. monthly use of ivermectin which is unjustified scientifically. The over-use of worming drugs increases the speed at which parasites will develop resistance to the drugs. The table indicates the manufacturers recommended dosing intervals
Drug
Tradenames
Recommended frequency of dosing for adult horses
Efficacy against cyathostomes and tapeworms

 

 

 

 

 

 

Fenbendazole

 

 

 

Panacur Equine Granules/Paste (Intervet)

 Panacur Equine Guard (5 day course) (Intervet)

Zerofen (Chanelle)

 

 

 

6 weeks

 

 

6–12 months

6 weeks

Will kill adult small redworms, their eggs and some immature stages

Some small redworms are resistant to this drug

Five day course effective against inhibited mucosal stages of small redworms

No efficacy against tapeworms

 

 

 

Mebendazole

 

 

 

Telmin (Janssen)

 

 

 

6 weeks

Will kill adult small redworms

Some small redworms are resistant to this drug

No efficacy against tapeworms

 

 

 

 

Pyranteol embonate

 

 

Strongid-P Granules/Paste (Pfizer)

 Strongid Caramel (Pfizer)

Paratype P (Intervet)

Provid 44 % Paste (Chanelle)

Exodus (Janssen)

 

 

 

 

4-6 weeks 

 

Will kill adult small redworms. Not effective against encysted mucosal stages

Double dose (38 mg/kg) will kill tapeworms

Use anti-tapeworm dose every 6-12 months. No efficacy against A. mamillana

 

 

 

 

Ivermectin

 

Eqvalan Paste for Horses(Merial)

Panomec Paste for Horses(Merial)

 Furexel(Janssen)

Vectin Horse Paste (Intervet)

Eraquell (Virbac)

Bimectin (Bimeda)

 

 

 

 

8-10 weeks

 

Highly effective against adult small redworms

Limited effect against inhibited mucosal stages

No efficacy against tapeworms.

 

 

Moxidectin

 

 

 

Equest(Fort Dodge)

 

 

13 weeks

High efficacy against adult and developing small redworms

Persistent effect

No efficacy against tapeworms

 

 

Praziquantel

 

 

Equitape (Fort Dodge)

 

 

6-12 months

Efficacy against tapeworms, including A. mamillana

No efficacy against roundworm

 

 

 

Praziquantel + Invermectin

 

 

Equimax (Virbac)

 

Eqvalan Duo (Merial)

 

 

 

6-12 months

High efficacy against tapeworms, including A. mamillana

Highly effective against adult small redworms

 

Praziquantel + Moxidectin

 

Equest Pramox (Fort Dodge)

 

6-12 months

High efficacy against tapeworms, including A. mamillana

Highly effective against adult small redworms including inhibited mucosal stages

Note: Efficacy against large redworms, Parascaris equorum, and  Oxyuris equi, is assumed unless stated otherwise.

Strategic dosing.
  • The use of worming drugs at specific times of year to disrupt the seasonal cycle of transmission has been widely and effectively employed in farm animal practice.  The seasonality of horse parasites suggests that strategic dosing at turn-out, in the middle of the grazing season and again in the autumn is a rational approach to parasite control. Problems can arise with such a system in years with abnormal weather patterns leading to early or late peak pasture larval burdens.  Such a system is susceptible to breakdown if heavily parasitised horses are added to the population.  Also, the fact that horses do not develop a significant degree of protective immunity, and the fact that they graze in mixed age groups, make this method more difficult to institute
Targeted dosing
  • This method of worm control relies upon periodic testing of horses to establish the level of infection in individuals. Treatment is then targeted at animals with significant (e.g. >200 eggs/gram) adult parasite burdens.  Diagnostic limitations mean that mucosal larval parasites can not be detected by faecal egg counts so such a regimen must include larvicidal dosing of horses. Many wormers currently available lack efficacy against mucosal larvae in a state of arrested development.  Therefore, it is important that these stages are not allowed to build up in large numbers in the colon wall of young, susceptible horses.  Anti-cestode treatments can be targeted by identification of significantly infected animals using the tapeworm antibody ELISA