Can you give a horse dewormer twice in a row?

The frequency for deworming horses depends on their age, their environment and their own natural resistant to parasites.

Horses under 18 months of age need to be dewormed frequently because they are more susceptible to parasites, the parasites have different life cycles and because parasites have been developing resistance to the commonly used deworming drugs. This means that in order to treat all the likely parasites infecting young horses, different dewormers need to be used at different times to effectively control infections.

Adult horses tend to be more resistant to parasites and 50% or more of adults require deworming only twice a year (spring and fall), for small strongyles, bots and tapeworms as well as less common parasites in certain situations. Other adult horses do not develop as strong an immunity and may need to be dewormed 3-4 times a year.

Some environments and management practices also help to reduce the chances of reinfection with parasites. Very hot (over 85 degrees Fahrenheit ), dry climates reduce the survival time of infective small strongyle larva in the environment. Stalls are also poor environments for parasites and horses kept in stall are less likely to be reinfected. Removing feces from paddocks/pastures at least every other day reduces the number of parasite eggs and larva in pastures, thus reducing reinfection risks. Rotating horses off of pastures for 6 months or more also reduces reinfection risks for small strongyles because infective larval stages don't survive indefinitely.

Ascarids are the exception to hot, dry environmental conditions and time significantly reducing reinfection. Ascarid eggs can survive for 10 years or more in the environment and area concern for young horses and very old horses.

Horse owners need to base deworming decisions off of the results of fecal egg counts, environmental conditions, horse age and seasonal parasite risks. It is also important to know what deworming products are effective against what parasites and what products have resistance issues in a particular area. An equine veterinarian should be consulted to assess each horse and determine an appropriate deworming regimen.