Equine Strongyle Worms Weather The Winter



By Nikki Alvin-Smith


Like many horse owners I was under the assumption that freezing temperatures in winter would kill off any small strongyle worm eggs or larvae on horse pastures, and stop the life cycle of these parasites when they are outside of their equine host.


Apparently that is not the case. Surprisingly, the small strongyle can survive not just freezing temperatures, but also freeze and thaw cycles. There is no such thing as a ‘killing frost’ where strongyle worms are concerned, as Dr. Neilsen and Dr. Reinemeyer explain in their wonderful Handbook of Equine Parasite Control 2nd Edition.


While there are environmental factors of moisture and oxygen availability to be considered as critical components in the development life of the strongyle worm, the effect of temperature plays a significant part in its survival.


Optimum temperature for egg and larvae development is in the range of 77-91 degrees Fahrenheit. While non-optimal conditions may slow the rate of hatching and development of the strongyle worm, studies conducted by Dr. Nielsen indicate that unembryonated eggs can survive occasional freeze/thaw cycles up to 97 days.


When you think about the insulating effects of a light layer of snow on animal and plant life on the surface of the soil and the likelihood that worm eggs are often located in equine fecal balls, the microenvironment for the strongyle worm can be kept at a relatively constant temperature close to 32 degrees Fahrenheit. This provides an effective level of protection for the worm eggs from repeated freeze and thaw cycles.


That is not to conclude that long term freezing doesn’t damage strongyle eggs and reduce larval yield significantly. It indicates that the horse owner still needs to be vigilant about their targeted horse worm control program throughout the year.


It is important to remember that worms enjoy a life ‘cycle’ and that their life is not one in stasis. This means that even if you have dewormed your horse following an initial positive F.E.C.T. (fecal egg worm test) and have a negative shedding egg worm count when you conduct a follow up test F.E.C.R.T. (fecal egg count reduction test), it does not mean that the stronglye worm is not present in its host the horse, laying wait in an encysted stage in the intestinal mucosal layer for shedding later. It also means that worm eggs may be present on the pasture that may have survived cold or warm temperatures.


A smart horse owner will therefore take the precaution to pick up manure on a regular basis from the pastures to reduce the degree of contamination of the horse herd from infective parasites and also enact a regular F.E.C.T. program to keep abreast of the levels of strongyles present in the herd and administer appropriate dewormer treatments.


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This article is brought to you courtesy of Horsemen’s Laboratory Inc., Mahomet, IL. –


About Horsemen’s Laboratory: Established in 1993 by John Byrd D.V.M., an experienced lifelong horseman and a graduate of the University of Illinois College of Veterinary Medicine. As an equine medicine practitioner in California for 13 years, Dr. Byrd served as ex-officio member of the board of directors of the Pacific Coast Quarter Horse Racing Association where he also served as the organization’s official sales veterinarian.  In addition, Dr. Byrd frequently officiated, as veterinarian for horse shows sponsored by the management of Orange County Fairgrounds in Costa Mesa, California.  Dr. Byrd’s extensive experience with horses led him to observe how a horse’s health could impact performance leading to the founding of the specialist lab for equine fecal worm egg counts. Please visit https://www.horsemenslab.com/ to find out more about F.E.C.T. services available directly to the horse owner including; advice on equine fecal egg count testing; quick and easy purchase of test kits online; reporting and expert consultation services. Dr. Byrd enjoys sharing his wealth of knowledge of equine parasitology with horse owners from all walks of life, and is available to provide lectures/symposiums for your club, organization or event. Please contact Dr. Byrd via his website for rates and further information