The Importance of Life Cycles
Life cycles of parasites may provide two important pieces of information.
- Information that has predictive value with respect to the pathogenic
importance of each particular parasite.
- 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
|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.