Epsilon Archive for Student Projects

Abstract

Infection with Parascaris equorum is common among young horses in Sweden as
well as in the rest of the world. The parasite has a direct lifecycle and reproduces in
the small intestine. The eggs are passed out with faeces and become infective after
approximately two weeks. The horses get infected when they ingest eggs that are
dispersed in the environment. The symptoms seen in horses usually include
retarded growth and loss of weight. There are several anthelmintics registered for
deworming against P. equorum, however in recent studies treatment failure with
macrocyclic lactones has been demonstrated.

The study started when two foals died on a stud farm in the south of Sweden. It
was shown that both foals were heavily infected with P. equorum, although they
had been dewormed according to the routines at the stud farm, with fenbendazol at
6 weeks of age and ivermektin at 14 weeks. A partnership was initiated between
the stud farm and SVA in order to develop a new control program based on
individual faecal egg counts (FEC). The aim of the present study was to evaluate
this program during one grazing season.

In 2007, the study included 14 foals that had been born at the stud between the end
of February and the end of May. Individual faecal samples were collected every
week from 10 weeks of age for approximately 23 weeks. The samples were
analyzed by a modified McMaster method with a lowest detection level of 50 epg.
When a FEC was > 200 epg P. equorum, the foal was dewormed with 7.5 mg
fenbendazole per kg bodyweight. The following year, 2008, 28 foals born between
mid March and the beginning of June, were dewormed with fenbendazole at 13
weeks of age. Individual faecal samples were then analyzed every second week.
When the FEC was > 200 epg P. equorum the foal was dewormed as in the
previous year.

All the foals were infected with P. equorum during 2007. The FEC pattern was
difficult to predict and many foals started to shed thousands of egg from one week
to another. However, a similar explosive rise in FEC was not observed when the
foals started to shed eggs as a result of the second generation of worms, after the
first deworming. In 2008, when all the foals received anthelmintic treatment at 13
weeks of age, the egg shedding was more moderate (maximum 900 epg) compared
to the previous year (maximum 9500 epg). Interestingly, the egg reappearance
period (erp) varied a lot both years, but especially in 2008. This can possibly be
explained by lack of efficacy of fenbendazole against larval stages, mistakes in the
deworming procedure, beginning resistance against fenbendazole or eggs passing
through the digestive system.