Parasite Life Cycles
Despite their diversity, parasites do have some common features: all of
them have unique life cycles and each major parasite group (arthropods,
nematodes, cestodes, trematodes and protozoa) includes a set of well-defined
and recognizable stages unique to each group.
The multicellular nematodes, for example, have life cycles which include
adult males and females. They reproduce sexually and eggs are laid by the
female. Larvae develop, in these eggs, hatch and progress through a series
of developing larval stages until they reach adulthood as sexually mature
males and females and the cycle begins again. Most nematodes follow this
basic life cycle pattern, although some may also include variations and
Click here to view the life
cycle of the small strongyles of horses, an uncomplicated cycle. Remember to
use the back button of your browser to return to this page.
Click here to view the life cycle
of Dirofilaria immitis, the heartworm of dogs. This pattern is
complicated by the inclusion of required intermediate hosts, mosquitos. Use
the back button of your browser to return directly to this page.
The protozoa are unicellular organisms. Most of them have a phase of sexual
reproduction in their life cycles and also include an asexual reproductive
phase. They lack recognizable male and female adult stages as we see in the
nematodes but they do have male and female stages that are analogous to eggs and
sperm. For example, the coccidia have the following stages in their life cycles:
oocysts, sporozoites, schizonts, merozoites, microgamets, macrogametes, zygotes.
In a simple coccidian life cycle, a host is infecte by an oocyst containgin
sporozoites which leave the oocyst and infect host cells. Asexual multiplication
occurs resulting in an intracellular schizont containing many merozoites.
Asexual reproduction (schizogony) may be repeated several times and ends with
the beginning of sexual reproduction. Merozoites transform into micro or macro
gametocytes. Macrogametocytes are female and grow to occupy a single host
cell when they are usually called macrogametes. Microgametocytes are male
and divide to produce microgametes which leave the host cell in search of a
macrogamete. The latter is penetrated by a microgamete, fusion of their nuclei
occurs and a zygote is formed. A cyst wall is formed around the zygote resulting
in an oocyst which sporulates, forming sporozoites. This sporulated occyst is
the infective form for the host. Click here
to view the life cycle of Eimeria bovis, a coccidian of cattle. (Use the back
button of your browser to return to this page).
The arthropods have different life cycles but like the others they also have
distinct and recognizable life cycle stages.