To compare the efficacy and egg reappearance period of a single active anthelmintic containing Ivermectin with a dual active anthelmintic containing both Ivermectin and Pyrantel in horses.
An initial faecal worm egg count (WEC) and larval differential was conducted on all horses over the age of two years on a small horse property to identify horses with a WEC over 200 eggs per gram (epg). From a group of 21 horses, 12 horses were identified that met this criterion. The worm species identified were 100% small strongyles. These horses were divided into two treatment groups of six horses each so that there was a similar average WEC for each group. In addition, as the horses on the property were managed in a number of smaller groups, horses were allocated into treatment groups so that if two horses with a sufficiently high WEC were paddocked together, each would receive a different treatment.
Management groups of these horses were as follows:
All horses from both treatment groups were treated on the same day with either of the two anthelmintics being compared. Group A received the Ivermectin (Razor, Virbac) treatment at 1ml per 40kg body weight, and Group B were treated with Ivermectin + Pyrantel (Elevation, Virbac) at 3.3ml per 100kg bodyweight. The bodyweight of each horse treated was estimated based on height and body condition score.
Faecal worm egg counts were then conducted at 14 days, 28 days, 42 days, 56 days and 70 days to determine the efficacy of each drench at 14 days post treatment, and to determine if there was any difference in the egg reappearance period (ERP) between the two treatments. Faecal worm egg count kits supplied by New South Wales Department of Primary Industries were utilised and the samples were processed at the Elizabeth Macarthur Agricultural Institute by the modified McMaster worm egg count technique.
The initial WEC identified 12 horses with counts above 200epg, which ranged from 240 – 840epg. The Razor group had an initial average WEC of 523.3epg and the Elevation group had an initial average WEC of 626.7epg. Both groups showed marked reduction in the average WEC at 14 days post treatment, the Razor group average being 3.3epg or 99% efficacy, and the elevation group being 0epg or 100% efficacy. Repeat WEC conducted at 28 days resulted in 0epg for both groups. Another test at 42 days resulted in the reappearance of worm eggs in both groups, with an average WEC for the Razor group of 110epg, and in the Elevation group of 50epg. A faecal collection was carried out at 56 days, but the samples were waylaid and did not arrive at the testing laboratory until 10 days post collection rendering them unsuitable for testing. A repeat sampling was then undertaken at 70 days, the Razor group returning an average WEC of 420epg and the Elevation group returning and average WEC of 156.7epg, less than the Razor group. These results are summarised in Table 1 and Figure 1.
|Horse Name||Age (yrs)/Breed||Initial WEC||WEC 14D||Efficacy||WEC 28D||WEC 42D||WEC 70D|
In this study the efficacy of both Ivermectin (Razor) and Ivermectin + Pyrantel (Elevation) at 14 days post treatment against intestinal cyathostomins was found to be excellent with results of 99% and 100% respectively. Only one horse produced worm eggs at 14 days in the Razor group with a count of 20epg. This egg count in fact represents only one egg identified, and given that this horse in subsequent tests had 0epg, this finding probably represents an aberrant egg remaining in the gastrointestinal tract, rather than an egg produced by a parasite that had survived treatment. It would therefore be reasonable to conclude that both treatments were 100% effective.
In addition, the egg reappearance period (ERP) for both treatments was found to be 42 days, consistent with previous studies of both active ingredients. A number of studies have found that there is growing resistance to both Ivermectin and Pyrantel Embonate on a number of horse farms, both in Australia and overseas. A recent study of resistance of the cyathostomin population to Ivermectin in Australian horses found that there was no resistance in the populations sampled (Beasley, 2015), but studies in the United Kingdom and Brazil have found the presence of resistance, while there have been reports from the United States of decreased sensitivity to both actives (Kaplan, 2009).
The egg reappearance period of cyathostomins treated with Ivermectin was originally considered to be 56 days. This has reduced in recent times and is now accepted as 42 days, although in some horse populations this has reduced further to as little as 28 days indicating increasing resistance (Beasley, 2015). In a 1989 study by Lumsden, et al the ERP of cyathostomins treated with Pyrantel was found to be 39 days.
What has been surprising in this study is that while the ERP of both treatments is consistent with previous studies, the average WEC for each group differed at 70 days, with the Razor group having 2.7 times the egg count of the Elevation group. In addition, when the initial WEC for each group is compared to the 70 day WEC, the Razor group has had an increase back to 80.3% of the initial count, while the Elevation group has had an increase of only 25% of the original count. The question arises, is this a real finding or a coincidental one, and what could account for this?
It is possible that individual horse susceptibility to worms may account for this result as most adult horses, around 80%, show natural resistance to worms, while 20% of the population have greater susceptibility to high worm burdens (Beasley, 2015). In this case this is unlikely, as the initial WECs of the groups showed that the Razor group had a lower average (523.3epg) than the Elevation group (626.7epg). In addition, some horses in the Razor group returned WECs as high or higher than their initial WEC, whilst horses in the Elevation group returned WECs of only half their initial count, or less, at 70 days.
Has the combination of Pyrantel with Ivermectin resulted in some synergistic effect, or had some residual action, which has not been seen when the actives are used on their own, been responsible? Further studies are required to determine if this is the case and a repeat of the study on different populations of horses is recommended.
Another interesting finding in this study was that some horses who had initial counts of over 200epg had very low counts at the end of the study, while others reverted to moderate to high counts. This effect was seen across both treatment groups. The reason for this is explained by the management of the different groups of horses. Those that completed the study with a very low WEC were moved during the study from an area of short pasture onto a paddock sown down to an oat crop with additional access to short pasture, which had been rested for several months over the preceding summer. In contrast, those horses that completed the study with higher WECs were continuously de-pastured in smaller holding paddocks with only short green pick. This finding demonstrates that the use of anthelmintic is not the only tool that should be used in the management of horses to reduce worm burdens. It has been shown in studies of intestinal parasitism of sheep, that grazing pastures when they are over 10cm in length significantly lowers the worm burden when compared with grazing pastures of less than 5cm. (Callinan & Westcott, 1986)
Both Ivermectin (Razor) and Ivermectin + Pyrantel (Elevation) showed a very high level of efficacy in the treatment of internal parasitism by cyathostomins in horses aged over two years, with 99% efficacy of Ivermectin and 100% efficacy of Ivermectin + Pyrantel at 14 days. Both drenches would be suitable as part of a deworming program.
Egg reappearance periods of both treatments were found to be 42 days, consistent with previous studies. The level of internal parasitism at both 42 days and 70 days post treatment was different between the two treatment groups with the Elevation group showing a much lower level of egg shedding compared with the Razor group. Further investigation is required to determine if this finding can be replicated.
Finally, grazing horses on pasture that has been spelled from horses over summer and / or on pasture over 5cm in length significantly reduces reinfestation by internal parasites. These strategies can be used as part of an integrated worm management program to reduce reliance on anthelmintics in deworming programs thereby reducing the likelihood that worms will develop drench resistance.
Virbac Animal Health for providing the drench products and funding for the worm egg counts.