Dr Eugene Lyons headed the team from the Gluck Equine Research Centre in Kentucky, which conducted the investigation on a farm in the same US state.
The study found that the number of small strongyle eggs returned twice as quickly as when the drug was first marketed in the early 1980s.
"Whatever the cause, at least some degree of resistance of ivermectin against small strongyles has occurred," the study team said.
Small strongyles - a common parasite in horses - have already shown resistance to a range of deworming agents.
Dr Lyons and his team said the first reports of small strongyle resistance came in the late 1950s and early 1960s, to phenothiazine (PTZ).
Thiabendazole (TBZ), which came on the market in the early 1960s, was the first of the benzimidazole class of compounds marketed. Soon after, in the mid-1960s, TBZ-resistant small strongyles were reported in a study.
Resistance was reported by the late 1980s for all of the compounds on the market in the United States - PTZ, benzimidazoles, pyrantel pamoate and piperazine - except for ivermectin.
Recent reports had already shown that small strongyle eggs are returning quicker than initially had been the case after ivermectin and moxidectin treatment.
The Gluck study was undertaken to determine the current status of ivermectin's effectiveness against small strongyles in a herd of about 250 animals, mostly mixed light horses, which had been treated with ivermectin about four times a year since 1990.
Since that date, the yearlings and older horses had been treated exclusively with ivermectin about four times a year. Occasionally, the foals were given fenbendazole, oxibendazole, and pyrantel pamoate in addition to ivermectin.
Ivermectin's performance was evaluated by determining strongyle egg counts before treatment and up to 4 to 6 weeks after treatment. Results were compared to those found previously in this horse herd and to those from other studies.
The researchers argued the need for further testing to better understand the situation.
"Critical-controlled tests in the future should provide a better understanding of the situation."
An added alarming situation, the researchers said, is the resistance of ascarids (large roundworms, or Parascaris equorum) to ivermectin, as identified in a 2007 study, and probably moxidectin.
"Apparently, no new classes of compounds are forthcoming on the market in the near future, so current control of species of equine internal parasites against several drugs that formerly were effective is a real dilemma."
The study, entitled 'Field studies indicating reduced activity of ivermectin on small strongyles in horses on a farm in Central Kentucky', was carried out by Dr Lyons with S C Tolliver and S S Collins, both of whom are also from Gluck; M Ionita of Faculty of Veterinary Medicine of Bucharest; and A Lewellen, from the Department of Natural Sciences, Asbury College, Wilmore, Kentucky.
The study will be published in the upcoming issue of the Parasitologica Research journal.
The four common internal parasites that affect horses are bots, strongyles, ascarids and pinworms.