Equine Parasite Lifecycle Stage 1
Equine Internal Parasites
Internal parasites of horses live in one of two places: in the horse's environment as larvae (typically pastures) or inside the horse. Once inside the horse, the larvae mature and may migrate to various areas of the horse, such as the stomach, intestine, lungs or blood vessels. Each parasite has a specific life cycle, but in general they follow the pattern below:
There are a lot of different internal parasites, but some are especially important because they could be a significant health threat to horses. The nature and extent of damage varies with the type of parasites. They can cause anemia, weight loss, colic, diarrhea, reduced reproductive and athletic performance, poor growth, and immune system suppression.
The most common and important types of internal parasites in horses:
• Nematodes, primarily small strongyles, cyathostomins. (Due to traditional parasite control programs involving rotational treatment, large strongyles (S. vulgaris) are now rare in managed horse populations)
• Tapeworms (Anoplocephala perfoliata)
• Roundworms (Parascaris spp.) in foals and weanlings
Environmental measures are aimed to break the life cycle of internal parasites. Following good management practices to control the frequency and spread of internal parasites can be as important as deworming.
Remove manure daily from stalls and run-ins and weekly (or more frequently) from paddocks and pastures
Be sure pastures and paddocks are well-drained and not overpopulated
Compost manure rather than spreading it on fields where horses graze
Use a feeder for hay and grain and avoid feeding on the ground
Implement fly control programs
Keep water troughs and feed bins clean
Routinely examine horses for telltale signs of infestation
Set up a biosecurity program for all new horses that includes a fecal egg count and larvicidal deworming prior to turn-out with resident horses
Establish a parasite prevention and monitoring program with your veterinarian. This may include regular manure checks and deworming programs tailored to the needs of your horses
The aim of today’s equine parasite-control program isn’t to eliminate all worms. What is needed are properly timed treatments with effective anthelmintics to control parasite egg shedding, to help keep horses healthy and to avoid development of resistance as much as possible.1
To achieve these goals, it is important to know the magnitude of egg shedding of individual horses and of detecting resistance of farms by performing periodic fecal egg count (FEC) surveillance and fecal egg count reduction tests (FECRT).
Parasite-control programs are best viewed as a yearly cycle starting at the time of year when parasite transmission to horses changes from negligible to probable.1 There is no such thing as a “one size fits all” horse deworming schedule. Each farm with veterinary guidance should develop its own program tailored to their specific needs. All adult horses should benefit from one or two treatments per year. Low-egg shedding horses with naturally strong immunity may not need additional treatments. Moderate and high egg shedders will need a third and sometimes a fourth treatment. Any additional treatments would be given on an “as needed” basis. Young horses (less than 3 years of age) require special attention due to their higher susceptibility to parasite infection and to being more at risk for developing clinical illness.
Treatments should be timed to control the level of egg shedding into the environment. This relies on the use of deworming medications that are effective for their intended use.1 The 3 common classes of horse dewormers are macrocyclic lactones (ivermectin, moxidectin), benzimidazoles (fenbendazole, oxibendazole), and tetrahydropyrimidines (pyrantel pamoate). Small strongyles and roundworms are the greatest resistance threats. Small strongyles have developed resistance to two of the three major dewormer classes (benzimidazoles and tetrahydropyrimidines). 2 Ivermectin, the active ingredient in Zimecterin® Gold (ivermectin 1.55%/ praziquantel 7.75%) Paste and Zimecterin® (ivermectin paste) 1.87% remains effective against benzimidazole-resistant small strongyles.3 Ivermectin and excreted ivermectin residues may adversely affect aquatic organisms. Do not contaminate ground or surface water.
ZIMECTERIN GOLD controls more species and stages of parasites than any other product (47 species and stages of equine parasites, including tapeworms).4 ZIMECTERIN GOLD Paste may be used in horses two months of age or older.
(ivermectin paste) 1.87%
Kills important internal parasites with a single dose (bots and arterial stages of S. vulgaris). Not for use in humans. This product should not be used in other animal species as severe adverse reactions, including fatalities in dogs, may result.
(ivermectin 1.55%/praziquantel 7.75%) paste
Controls more species and stages of parasites than any other horse dewormer. ZIMECTERIN GOLD has not been tested in breeding, pregnant or lactating mares, or breeding stallions.
ZIMECTERIN GOLD IMPORTANT SAFETY INFORMATION: ZIMECTERIN GOLD Paste may be used in horses two months of age or older. ZIMECTERIN GOLD has not been tested in breeding, pregnant or lactating mares, and breeding stallions. There have been rare reports of swelling and irritation of the mouth, lips, and tongue following administration of ZIMECTERIN GOLD. These reactions have been transitory in nature. This product should not be used in other animal species as severe adverse reactions, including fatalities in dogs, may result. Not for use in humans. Keep this and all drugs out of reach of children. Refrain from smoking and eating when handling. Wash hands after use. Refer to the package insert for complete product information.
ZIMECTERIN IMPORTANT SAFETY INFORMATION: Not for use in humans. Keep this and all drugs out of reach of children. Do not use in other animal species as severe adverse reactions, including fatalities in dogs, may result. Ivermectin and excreted ivermectin residues may adversely affect aquatic organisms. Do not contaminate ground or surface water. Dispose of the syringe in an approved landfill or by incineration.
1. AAEP Parasite Control Guidelines Available at: https://aaep.org/sites/default/files/Documents/InternalParasiteGuidelinesFinal5.23.19_0.pdf. Accessed Nov. 16, 2020.
2. Kaplan RM, et al. Prevalence of anthelmintic -resistant cyathostomes on horse farms. J AM Vet Med Assoc. 2004;225(6):903-910.
3. McFarlane D, Hale GM, Johnson EM, Maxwell LK. Fecal egg counts after anthelmintic administration to aged horses and horses with pituitary pars intermedia dysfunction. J AM Vet Assoc. 2010; 236(3):331.
4. ZIMECTERIN Gold Product Label.
ZIMECTERIN® is a registered trademark of the Boehringer Ingelheim Group. ©2021 Boehringer Ingelheim Animal Health USA Inc., Duluth, GA. All rights reserved. US-EQU-0111-2021