Epidemiology of Equine Strongyle Infections


The large and small strongyles are cosmopolitan parasites. They appear to have adapted to a wide range of climatic conditions and, in the absence of good parasite control programs, are common in horses, donkeys and mules throughout the world. The biological characteristics of the preparasitic stages of the small strongyles and of Strongylus vulgaris are well known and are presumed to be similar for the other two important species of large strongyles in equines, Strongylus equinus and Strongylus edentatus.

Most of our current information comes from studies conducted in temperate climate zones in the Northern Hemisphere with pronounced seasonal changes in weather conditions.

Hatching and development

Eggs of equine strongyles will only hatch between 8'C (46'F) and 38'C(100'F). This means that in most temperate areas of the Northern Hemisphere eggs will hatch only from March through October.  However, within this temperature range there are constraints on the number of hatched larvae that will develop through to infective third stages (L3s). Laboratory studies have shown that few hatched larvae will survive at the upper and lower extremes of this temperature range and within this range, the number of hatched larvae that will reach the L3 stage is also directly dependent on the prevailing temperature.  The table below illustrates this point.

Effects of temperature on development of strongyle larvae from hatching to infective L3s


% hatched larvae developing to L3s

Time for hatched larvae to develop to L3s

<8'C (46'F)



10'C (50'F)



12'C (54'F)



15'C (59'F)



20'C (68'F)


8 days

22'C (72'F)


6 days

25'C (77'F)


5 days

28'C (82'F)


4 days

30'C (86'F)


3 days

35'C (95'F)


2 days

>38'C (100'F)



The highlighted row in the table shows the optimum temperature(25'C/77'F) at which the maximum number of hatched L1s(68%)   develop to L3s within the shortest time (5 days).

These figures suggest that in the United States transmission of equine strongyles will occur in most regions from mid April through mid November.

In the northeast, mid west and mid atlantic states, transmission of equine strongyles  will generally follow that pattern. However, in the hot summer months of July and August when temperatures often exceed 85'F and may sometimes reach 100'F, transmission will decline. Temperature patterns in this region suggest that transmission will be optimal in May and June will fall off  in July and August and will rise to optimal levels again in September and October. Minimal transmission will occur from November through March.

In the southeast states,  temperature patterns suggest that transmission of equine strongyles may be significant throughout the year except for the very hot summer months of July and August when transmission will be minimal because prevailing temperatures often exceed 90'F and few larvae will survive for long on pastures at these temperatures.

In northern New England, the Pacific Northwest (especially the coastal areas of Washington and Oregon) and northern California, transmission of equine strongyles is likely to follow the European pattern with  transmission occurring  from May through mid October  and maximum transmission during the summer months of June, July and August. There will be negligible transmission in the winter months of December, January and February.

Arrested development

Some species of cyathostomes are able to undergo hypobiosis   (arrested development) as exsheathed, encysted early third stage larvae (EL3s) in the intestinal mucosa of infected horses. As with Ostertagia, Haemonchus  and other ruminant nematodes hypobiosis in the cyathostomes stops the life cycle in the host at a time when prevailing environmental conditions are hostile to the survival of eggs and preparasitic larvae. In north temperate areas of the world this occurs in winter and hypobiosis is clearly a mechanism of survival for these nematodes. Little is known about the biology of hypobiosis in cyathostomes but it is presumed that the signal for arrest is received by pasture L3s and that the signal is a falling prevailing temperature in fall and early winter. L3s thus stimulated will undergo hypobiosis if they are subsequently ingested by grazing equines before winter ensues.

Synchronous emergence of large numbers of previously arrested L4s occurs in early spring and is the cause of the clinical syndrome known as larval cyathostomosis. However, it is also known that larval emergence from hypobiosis may occur throughout the year and it is theorized that hypobiosis may  be used by the cyathostomes as a mechanism of controlling the numbers of adult worms in the gut.  This hypothesis is supported by the observation that treating horses with an anthelmintic to eliminate adult cyathostomes from the gut will stimulate the development of hypobiotic EL3s to repopulate the cecum and colon with adult worms.

Hypobiois in cyathostomes is an important consideration in developing parasite control programs because these arrested larvae are refractory to treatment with many of the anthelmintics used in horses, including Ivermectin and Moxidectin.



Parasites and Parasitic Diseases of Domestic Animals
Dr. Colin Johnstone (principal author)
Copyright 1998 University of Pennsylvania
This page was last modified on January 24, 2000