In October 2008, we discussed the current controversy involving proper deworming. Many experts state that the traditional dewormer rotation system remains the best, but we side with the researchers stating that deworming by need (called target deworming) and using ivermectin as the cornerstone of your program is your best bet. The problem with rotational deworming is that too many of the drugs in these systems have documented widespread resistance problems. Dr. Zajac explains why she also recommends target deworming and disapproves of over-deworming.
What’s the current status of anthelmintic (dewormer) resistance'
Resistance is a serious problem, and it is getting worse. Large strongyles used to be the focus of our anthelmintic drugs, but the dewormers have been effective and now our focus has switched to small strongyles. Unfortunately, drug resistance has now developed in the small strongyles. A survey conducted in the southeastern United States found that small strongyles were resistant to benzimidazole anthelmintics on virtually all farms and pyrantel resistance was also present on a substantial proportion (40%) of the farms.
Anthelmintic resistance is not confined to any geographic area or climate and the extensive movement of horses nationally and internationally probably accelerates the spread of resistant parasites. If resistance has not been reported from a specific area of North America or Europe it is most likely because no one has looked.
Why are small strongyles are worrisome'
There are over 40 species of small strongyles in horses, although 10 to 12 predominate in most surveys. Adult worms may be present in enormous numbers, reaching into the millions in heavily infected horses.
Even though adults may be present in huge numbers, the pathogenic effects of small strongyle infection are caused by larval stages. Larvae spend a period of time developing in the wall of the large bowel. The amount of time spent in the intestinal wall varies with species and may be as short as one month or as long as three years. In heavily infected horses with thousands or even a million adult worms in the large intestine there may be several times more larvae in the intestinal wall. In some horses, 90% of the worm burden may be in the form of larvae. These larvae can produce mucosal inflammation and thickening in the cecum and colon.
The ivermectin/moxidectin class of dewormers was thought to be beyond resistance.
Yes, but reports of small strongyle resistance to this drug group are now starting to emerge. In addition, recent reports from several locations have identified apparent ivermectin resistance in equine roundworms.
New drug groups would help. Are there any on the horizon'
One of the big drug companies has a new drug under development that may be available in the United States in five years or so, although it is unlikely that an equine preparation will be available in that time. But it is important to remember that if we overuse this new drug as we have the currently available ones, we may rapidly select for resistant parasites.
Once we have resistant parasites, is there any hope'
Reversion to susceptibility for most anthelmintics may be possible. But it seems to take years, even if the drug is no longer used.
What can we do to combat resistance'
We must slow the development of resistance by not deworming any more often than necessary. Every-other-month treatment of all horses is no longer advised as a routine recommendation. Dewormer use selects for resistant parasites that live and breed. Too frequent treatment can increase the population of resistant parasites unnecessarily.
So, we shouldn’t deworm any more often than necessary.
Correct. We balance protecting our horses today with decreasing the rate of development of resistance in parasites. Some horses will need to be dewormed more frequently than others. Generally, young horses are more susceptible to parasites, but horses vary in their individual immunity and some animals in all age groups will be highly susceptible. We want to target our dewormers to the horses that need them the most.
How can we determine deworming frequency'
It is very helpful to use fecal egg counts (FEC) in your horses. This monitors strongyle and ascarid eggs. Results are reported as the quantity of parasite eggs per gram of manure.
How do we get an FEC done'
Many teaching hospital diagnostic labs, some commercial labs and some government diagnostic labs offer quantitative fecal exams (there are many types of fecal exams, so be sure to specify quantitative). It’s possible for you to do these tests yourself with a microscope and limited equipment. Your vet might teach you.
How do we use the results'
FECs help identify horses that need more intensive treatment. The basic idea is to treat only horses that reach a threshold egg count. Generally, your vet will recommend a level--it might be 200 eggs per gram for adults and 100 eggs per gram for foals. The lower number would be appropriate for more susceptible horses. Other individual factors will also are taken into account (body condition, impending foaling, etc.) Decisions can be made about specific treatments of individual horses or animals can be allocated to groups that are given more or less treatments depending on whether they are more or less susceptible to parasites.
How often should we do the FECs'
Talk with your vet about how often you should be doing FECs to determine horses that require treatment.
How can I tell if a dewormer is still working on my farm and there aren’t too many resistant parasites'
We determine drug efficacy using a Fecal Egg Count Reduction Test (FECRT). This monitors egg counts in horses following treatment.
Typically, some horses are treated and others remain untreated. Fecal samples are taken 10 to 14 days post treatment and FECs determined. The reduction in FEC in treated compared to untreated horses is determined.
Work with your vet to perform this test effectively. The test works best when horses have high enough FECs to easily detect the reduction. It’s also important to be sure that each horse is treated with the full dewormer dose and that you are not using an out-of-date product.
How often should I use a FECRT'
Testing every other year is prudent on farms with substantial introduction of new horses or horse movement and lots of horses with frequent use of anthelmintics. On farms where horses remain isolated and treatment is less frequent, the interval between FECRTs can be extended.
What do you recommend about dewormer changes'
Use the dewormer until the FECRT shows evidence of resistance--the egg reduction falls below what we consider an appropriate level of effectiveness. Then go to another class of dewormer.
Many owners may find that they are using products from different classes during the year anyway, for example to target bots in autumn or tapeworms. But a strict rotation pattern is not needed.
Manure removal and disposal is still important.
Correct. Frequent removal and correct disposal of manure is vital in reducing the frequency of deworming and, thus, resistance. It removes parasites and eggs from the environment and reduces horse exposure to parasites.
Appropriate disposal includes placing manure piles where runoff won’t contaminate pastures with parasite larvae. Avoid spreading fresh manure on pastures.
How do FEC results help us manage pastures better'
FEC provides an approximate way to predict when numbers of parasite larvae on pasture will be greatest. In general, given mild temperatures and moisture, larvae increase a few weeks after an increase in FEC (as the eggs develop to the infective stage). Deworming when pasture levels are down may not be necessary. If feasible, move horses-- particularly more vulnerable youngsters--before an anticipated rise.
If routine manure removal from pastures is not feasible, consider targeted removal. Remove manure during periods of highest egg hatching; remove manure from horse-dense areas; and remove manure from pastures used by more vulnerable horses.
Should we rest a pasture'
Resting pastures will reduce the numbers of parasite larvae. The length of the rest period needed will vary depending on the climate. During very hot, dry weather, larvae may die within weeks or months, whereas in cool, moist weather larvae may survive for many months.
Similarly, harrowing pasture in hot dry weather can expose larvae to desiccation, but harrowing in wet weather merely spreads larvae over the pasture and may enhance transmission.
Avoid overcrowded paddocks. If you have to use paddocks heavily, remove the manure.