When Resistance Is A Good Thing

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When faced with the prospect of deworming a horse known to be heavily parasitized, or one that has not been dewormed for a long time, people fear reactions. While these reactions are rarely serious enough to be life-threatening, varying degrees of colic, diarrhea and distress may occur. There are several things you can do to minimize reactions.

Pretreatment.Veterinarians frequently recommend flunixin meglumine (brand name Banamine) before and at regular intervals after deworming horses at risk for reactions. This prescription anti-inflammatory drug is particularly effective with abdominal pain, combats reactions to bacterial toxins and can help with the horse's intestinal motility problems.

Because the immune system?s reaction to parasites involves eosinophils and mast cells, which release histamine, antihistamines may also be recommended to help with reactions. Antihistamines are much less effective when given after a reaction has started, so these too should be given before (or simultaneous with) the deworming.

Tapeworms.Tapeworms can cause colic as a result of damage to the nerve supply of the distal small intestine and the cecum. Keeping them at bay is a must. Diagnosis by fecal examination is hit and miss, and tapeworm risk also varies by geographical region and management (it's higher for horses on pasture). If the horse has a large tapeworm burden, there is a risk of impaction of the ileocecal valve, the link between the large and small intestine, by dead tapeworms. You can approach this in two ways:

  • Regular dose pyrantel pamoate (Strongid paste) kills only the egg-laying segments of the tapeworm, without causing the upper segments to let loose. Doing a regular dose first can help reduce the volume of tapeworm tissue. This is then followed in two weeks by a double dose of pyrantel pamoate (two doses, given at the same time) or a combination dewormer that contains the drug Praziquantel.
  • A gentle way to remove tapeworms is a month of the daily dewormer pyrantel tartrate (Strongid C or equivalent product). Although not FDA-approved for this use, research confirms this treatment removes tapeworms in 30 days.

Small strongyles. Because of the widespread resistance to multiple dewormers, small stronglyes can easily get the upper hand. The mass die-off of large numbers of encysted forms can cause colic and even damage to the intestinal wall and protein leakage.

Research has shown that moxidectin (Quest) actually produces far less tissue damage than our recommended five-day double-dose fenbendazole (Panacur Power Pack) treatment. However, if the horse is very thin, there may be a higher risk of drug side effects since moxidectin is normally sequestered in, and slowly released from, fat.

One approach that avoids killing encysted forms is to deworm with ivermectin at three-week intervals. This prevents the parasites from reaching an egg-laying stage and also catches the tissue larvae after they have matured to their next life stage. Three treatments with three weeks between treatments should be followed by a fecal check in one month.

Some people are concerned that removing all adults efficiently with ivermectin will trigger encysted larvae to emerge in large numbers. This can cause tissue damage and diarrhea. In those instances, parasite resistance actually can work in your favor.

Using regular-dose paste pyrantel pamoate, fenbendazole or oxibendazole (Anthelcide EQ) will result in decreased kill rate of the resident adults and theoretically will slow the emergence of encysted forms. However, during this period the fecals will continue to be positive and reinfection will be possible. After one or two treatments with the less effective dewormer, you can use Quest (moxidectin) or the ivermectin series.

Roundworms in foals.Deworming foals heavily parasitized with roundworms is risky, so be sure you fully understand. These parasites are very large and a massive die-off can cause impaction. Once again, resistance can actually work in your favor.

Since decreased activity of ivermectin has been documented in many parts of the country, this can be a good choice for a partial removal as the initial step.

Another option is to use a reduced dose of dewormer. However, this can quickly cause selection for drug resistance. To avoid worsening the current known resistance issues, use reduced-dose ivermectin followed by full-dose pyrantel pamoate, piperazine (Farnam Alfalfa Pellet Wormer), fenbendazole or oxibendazole.

Bottom Line.Gentle is key when dealing with horses who are battling heavy pasasite infestation. it's important to follow drug recommendations and give a dose correct for the horse's weight. Get a weight tape, if you're not sure what the horse weighs.

Underdosing is actually worse than over-dosing. Choose a milder drug instead, if you're concerned.

The term ?double dose? means give two of the recommended weight-adjusted doses at the same time, one right after the other.

Article by our Veterinary Editor, Dr. Eleanor Kellon.