Ormasýkingar í hrossum og lyfjaónæmi tengdar þeim

Deborah van Doorn started teaching parasitology at the Faculty of Veterinary Medicine in Utrecht. "I asked to be able to do research, because it is fun to transfer that information on to others." In her research at the department of Infectious Diseases & Immunology she studied the development of the resistance of horse worms from the family of Cyathostominae to anthelmintics. She received her doctorate for her thesis 'Macrocyclic lactone resistance in cyathostomin species of horses' on the 27th of May 2014.


There are approximately fifty species of worms in the Cyathostominae family, which colonise the large intestines and the appendix in horses. They are also called 'small strongylids'. Standard, however, these worms occur in horses when they walk in the meadow", Deborah van Doorn explains. "Larvae can always be found in the meadow." Most horses, especially the older ones, do not become sick after ingesting the larvae. They can however excrete worm eggs years after they have ingested the larvae. "For young animals though it means that they sometimes ingest large quantities of the larvae", Deborah explains. "In that case winter cyathostominosis can occur in the winter. The larvae in the intestinal wall of the horse are released en masse. This causes diarrhoea and in the worst case scenario even death." The older animals on the other hand develop an immunity to these worms. "But that is not enough to prevent a part of the horses from continuing to excrete eggs, although generally to a lesser degree."

RESISTANCE. Resistance to anthelmintics occurs all across the world. "Due to the high treatment frequency it has become a major problem in sheep and horses in the Netherlands", says Deborah. "In horses there is a resistance amongst roundworms and amongst the small strongylids." Multiple genes are involved in the origin of resistance to macrocyclic lactones (ML), but exactly which mutations it concerns, is unknown. "If we know the mutations, we can improve the diagnostics." Deborah developed a test with which in the laboratory worm larvae could be selected based on ML sensitivity and could be differentiated by species. "It appears that the various species significantly differ from one another with respect to the in vitro sensitivity to ivermectin." She also checked if resistance to anthelmintics occurred in the field. "We did tests at seventy businesses. Fourteen days after the treatment we did not observe a seriously high egg excretion at any of the businesses, which according to the guidelines would mean that there was no resistance present." Deborah did however notice that the period in which the product remained effective, the 'egg-reappearance period' (ERP), grew shorter. "It has quite a lot of consequences when an anthelmintic is effective for only six weeks instead of fourteen." Further studies showed that the shorter ERP was indeed caused by resistance. "We researched which species of strongylids were involved in the reduced ERP", Deborah said. "It appeared to primarily concern species from the genus Cylicocyclus." These worms quickly reappeared after treatment with ivermectin or moxidectin, but did not reappear as rapidly when treated with another anthelmintic. "In an interval study in which we first administered moxidectin, then pyrantel, then moxidectin again, the Cylicocyclus species did not reappear as fast with pyrantel as it did with moxidectin."

CYCLE BREAK THROUGH. According to Deborah individual horses are still often treated with anthelmintics at this time, even without Cyathostominae egg excretion, which only furthers the occurrence of resistance. "For example, foals are very frequently treated with anthelmintics. Treatment is justified when it concerns roundworms, but for the rest the care of foals should be focused on preventing that they can become so heavily infected with other worms." Therefore the best way to administer anthelmintics is not to wait until disease occurs, but to determine with the help of diagnostics in which animals the egg excretion can be reduced. "By treating the animals that are responsible for the largest part of the egg excretion, fewer larvae end up in the meadow", Deborah explains. "Thus the cycle can be broken. Foals will not be able to ingest enough larvae to develop winter cyathostominosis as a one-year old." Other options to break through the cycle are mowing the meadow and the removal of faeces. Deborah: "If you do not treat the horses that excrete few eggs, not all worms are exposed to the selective pressure against anthelmintics." This course of action is in line with the decision of the government to categorise anthelmintics as URA (prescription only) products. Shortly after the revision of the law Deborah and her colleagues did a survey amongst horse veterinarians and again four years later. "We found that a part of the veterinarians did indeed start to apply more diagnostics", Deborah reports. "Regretfully diagnostics are still not required. Veterinarians therefore have to raise this subject with the client themselves." She understands that due to the costs this may be a problem for the client. On the other hand there is a real risk that the products will lose their effectiveness. "One possibility to inform one another about the use of diagnostics is, for example, the initiation of a 'peer-to-peer' system."