> Large strongyles are now rare, and cyathostomins (small strongyles) are now the major parasite of concern in adult horses. Parascaris equorum is still considered the primary parasite infecting foals and weanlings.
> Anthelmintic resistance is highly prevalent in cyathostomins and P. equorum and should be factored into treatment regimens.
> Adult horses vary greatly in their innate susceptibility to cyathostomin infection and level of strongyle egg shedding and thus require individualized control programs.
> Horses younger than 3 years old require special attention as they are more susceptible to parasitic infection and are at greater risk of developing disease.
Because decades of frequent anthelmintic use have selected for high levels of anthelmintic drug resistance in cyathostomin and P. equorum populations, formerly popular control methodologies are not sustainable and new strategies are needed.
As for the frequency of treatment, less is more, says subcommittee member Ray Kaplan, DVM, PhD, DACVM, DEVPC, professor at the University of Georgia College of Veterinary Medicine. "Frequent treatments are unnecessary and often counterproductive," he explains. Instead, veterinarians need to emphasize properly timed treatments with effective anthelmintics administered at the appropriate time of the year—i.e. during epidemiological transmission cycles and based on the relative parasite burdens in individual horses. "Periodic, properly timed treatments can best benefit the health of the adult horse," Kaplan says.
Parasite control goals
The goal of parasite control is to limit parasite infections so animals remain healthy and avoid clinical illness. Treatments effective against adult parasite stages also help prevent further environmental contamination with infective stages. It's especially important to prevent egg shedding in individual horses with high egg-shedding capacity.
Simply stated, effective parasite control should minimize the risk of parasitic disease, control parasite egg shedding, maintain the efficacy of antiparasitic drugs and attempt to prevent further development of resistance. To achieve these goals, it's important to know the magnitude of an individual horse's egg shedding by performing periodic fecal egg counts.
Parasite control methods
To achieve optimal results with parasite control, it's critical to balance the right environmental conditions with effective treatments.
Keep it clean. Keep pastures as free of manure as possible. Picking up feces or using a mechanical vacuum can be beneficial.
Compost. Since eggs hatch and develop into infective larvae under conditions of moderate temperature and moisture, either extreme cold or excessive heat can help reduce numbers of larval parasites on pasture. Composting, which increases fecal temperature, will kill the nematode eggs and developing larvae and is a good practice.
Time your treatments. The AAEP subcommittee recommends performing anthelmintic treatments at times of year most optimal for larval development (these vary by climate and will be different in different regions of the country). Doing so will reduce pasture contamination, decreasing the number of new infections. This is also a time when refugia are present, so selection pressure for anthelmintic resistance is lessened.
Use the most effective treatments. The efficacy of alternative or natural remedies has never been demonstrated in controlled studies. Effective anthelmintic classes include:
> Heterocyclic compounds
> Macrocytic lactones
Here are the AAEP Parasite Control Subcommittee's recommendations:
For mature horses. Concentrate on control of cyathostomins. One or two yearly treatments are sufficient to prevent recurrence of large strongyles. Also, consider a treatment that's effective against encysted cyathostomins at the time when the mucosal burden is at its peak.
For deworming. When choosing which anthelmintics to use in a worm control program, be sure to evaluate dewormer efficacy using the fecal egg count reduction test. Include a basic foundation of one or two anthelmintic treatments to target the broad assortment of parasites that commonly infect horses, and offer further treatments targeting horses with high strongyle contamination potential. Be sure to focus anthelmintic treatments during seasons of peak transmission (i.e. usually spring and fall) when refugia peak.
In general. Design a parasite control program within the framework of the farm management regimen, which includes stocking density, the amount of time horses spend on pasture, the age of the horses, the presence of transient horses coming to and from the farm and the farm's ability to clean up the environment. Here are some other general considerations:
> Keep in mind that cyathostomins, large strongyles and tapeworms are acquired only via pasture; ascarids and pinworms are acquired in confinement as well as on pasture.
> Use fecal egg counts to determine egg-shedding status.
> Do not underdose.
> Decrease treatments when climate conditions are adverse (i.e. in extremely hot or cold weather).
> View worm control programs as a yearly cycle beginning at the time of year when parasite transmission to horses changes from negligible to probable.
> All general anthelmintic treatment recommendations in the AAEP document are made within the context of a preventive program in healthy horses, where fecal egg count surveillance is being performed.
> In contrast, if presented with a horse showing clinical signs of parasitic disease during a time of the year when treatments are not recommended (i.e. summer in the south, winter in the north), treat that horse. If the horse is showing overt symptoms of intestinal disease consistent with larval cyathostominosis, the best treatment is moxidectin, the agent of choice to kill encysted mucosal larvae.
Given the changes in the biology and fauna of the parasites over the years, the veterinary profession needs to reevaluate its role in parasite control, Kaplan says. "Practitioners need to become more actively involved in parasite surveillance, including periodically performing fecal egg counts to monitor levels of egg shedding, as well as monitoring the effectiveness of the various treatments they are administering."
The AAEP guidelines provide principles to follow but do not prescribe any one particular parasite control program, since the optimum program is individualized. "These recommendations are meant to be applied for a general, overall preventative program so that horses remain healthy and parasitic disease does not occur," Kaplan says. "On farms where parasite control has failed and horses are showing symptoms of parasitic disease, a very different approach will be required. There is no single prescribed program that is going to be optimal for every farm. This is why the veterinarian needs to play an active role in tailoring programs that are most appropriate for their circumstances."
Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author and editor on nutrition, physiology and veterinary medicine with a background in horses, pets and livestock. Kane is based in Seattle.
AAEP Parasite Control Guidelines. Lexington, Ky: American Association of Equine Practitioners, 2013. Available at: http://www.aaep.org/info/parasite-control-guidelines|~http://www.aaep.org/info/parasite-control-guidelines .