A large German study, looking at 2,000 horses on 76 farms over a two-year period, investigated the influence of parasite resistance and management factors on level of parasitism. The study reinforced the influence of age and management on levels of parasitism.
More frequent dewormings, over three per year, decreased strongyle burdens in mature horses but not in yearlings or weanlings.
Yearlings on high-traffic breeding farms were twice as likely to have strongyle burdens, and foals on farms where manure was spread on the fields had a significantly higher risk of roundworm burdens than on farms where manure wasn?t used. Yearlings on stud farms were at higher risk of showing incomplete parasite removal as evidenced by fecal egg counts.
While incomplete parasite removal following deworming in yearlings on stud farms could indicate resistance, it wasn?t observed in the older horses so likely represents the higher parasite exposure on the breeding farms.
More frequent dewormings didn't influence parasite burdens in foals and yearlings, but the average number of dewormings in foals was 4.52 times per year and in yearlings 3.26 times per year, which is too infrequent for good control in young animals. Clearly, management factors are important here.