Introduction to Parasitology

The Importance of Life Cycles

Life cycles of parasites may provide two important pieces of information.

  1. Information that has predictive value with respect to the pathogenic importance of each particular parasite.
  2. Information of epidemiological significance that is indispensable to developing  effective control programs.


We can make generalizations about the epidemiology and control of many parasites from information about their life cycles, in particular the requirements for transmission.

For example the strongyles of horses are transmitted directly by ingestion of infective third stage larvae from pasture while horses are grazing. The survival of these infective larvae as well as their development for eggs is controlled primarily by temperature combined with humidity. This is true, not only of the horse strongyles, but also of all nematodes with direct life cycles. 

Below 8'C, horse strongyle eggs will not hatch and above 38'C, they die. The image to the right shows that between these two extremes, the proportion of eggs developing through to third stage infective larvae is directly dependent on temperature with the maximum number of L3s developing at 26'C. Not only is 26'C the temperature at which the maximum number of L3s will develop from eggs but it is also the temperature at which eggs will develop to L3s in the shortest possible time. In the case of horse strongyles this will be approximately 72 hours.

% of infective stage larvae developing from viable eggs at various temperatures

As a result of this influence of temperature  and moisture on the survival and development of preparasitic life cycle stages, the strongyles of horses show seasonal patterns of development in temperate areas of the world with distinct seasonal changes in weather. These effects are shown in this graph of seasonal changes in strongyle eggs in the feces of grazing horses
This graph shows that maximum transmission of horse strongyles  occurs in spring, summer and early fall(autumn) when temperatures are optimum for rapid development of infective L3s from strongyle eggs passed in horse feces. In winter minimum development occurs and the majority of eggs and larvae  will not survive.  The great majority (95%) of eggs passed are from the cyathostomes with only a small
small contribution from the large strongyles such as Strongylus vulgaris. This reflects the  number of cyathostomes (tens of thousands) present compared with the large strongyles (hundreds) in naturally infected horses. These cyclical patterns occur regularly from year to year, from grazing season to grazing season and are predictable because of the effects of temperature and moisture on the life cycles of these nematodes. Their predictability is valuable because it allows us to focus control programs at strategic times of the year with the maximum possible benefits.




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