Isospora suis Toxonomy and Life Cycle
Some 248 Isospora species have been identified, but only Isospora suis is considered to cause neonatal piglet diarrhoea. The life cycle of each species is unique, and knowledge of it is important for the diagnosis, treatment, prevention and control of the parasite.
Isospora suis is part of the coccidia order of intracellular parasites that goes through stages of development inside the host animal as well as in the external environment. Its oocysts (which represent the exogenous phase of the life cycle) are spheroid to spherical, measuring 19.4 to 22.5 µ in diameter, with a smooth single-layer capsule 1.5 µ thick. Each oocyst contains two sporocysts (disparic) with four sporozoites each (tetrazoic).
This parasite targets the small intestine, where it develops in the mucous tissue. The various stages of development ultimately produce a microscopic egg, or oocyst, that is excreted with faeces. Under proper conditions, the oocyst will develop to form a sporulated oocyst within 1 to 3 days. After being eaten, the oocyst releases the four sporozoites contained in each sporocyst in the intestinal lumen.
Each sporozoite is capable of entering the pig’s intestinal cells, dividing many times and producing many offspring that in turn may enter other intestinal cells. This cycle may be repeated twice. The rapid multiplication at this stage of the parasite life cycle results in the destruction of large number of intestinal cells. Eventually, the cycle stops and sexually differentiated cells are produced. The male fertilises the female to produce an oocyst, which detaches from the intestinal cells and is passed into the environment through faeces.
Some authors have broken down the coccidia life cycle into three phases:
Extra-intestinal stages of Isospora suis (as occur in dogs and cats) have not yet been confirmed in piglets, although some studies by Henriksen et al. (1992) support this hypothesis.
Sporogony Phase
This is the exogenous phase of the Isospora suis life cycle, during which the oocysts develop from the unsporulated, non-infectious stage to the infective stage.
The oocysts are excreted in faeces; sporulation to the infectious stage takes place outside the host animal. The sporulated oocysts contain two sporocysts with four sporozoites each. Sporulation is dependent upon humidity, temperature and oxygen.
Isospora suis oocysts sporulate rapidly at temperatures between 20° C and 40° C (Lindsay et al., 1982). In current swine production practices, where supplemental heat between 32° and 35° C is provided to newborn pigs, oocysts sporulation may occur between 12 and 16 hours. Once the oocysts are sporulated they are resistant to most disinfectants.
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Excystation Phase
The excystation phase occurs immediately after the infectious oocysts have been ingested. During this phase, the sporozoites become active and are released into the intestinal lumen.
Passage through the stomach alters the oocyst wall, allowing bile salts and digestive enzymes to activate the sporozoites. The active sporozoites then penetrate the villous epithelium, which starts the endogenous phase of parasite multiplication.
The greatest concentration is seen in the middle part of the jejenum, but the ileum is also infected. In severe infections the cecum and colon can also be involved.
Endogenous Development
The endogenous phase of the Isospora suis life cycle occurs mainly in cells of the small intestine, although in severe infections, the colon and cecum may also be involved. These stages usually take place in the distal portions of the villi, although in severe cases, they may also be located in the crypt region of the intestinal cells.
During this phase, the sporozoites enter the intestinal cells and become binucleated type-1 meronts. The meronts divide within 24 hours to produce two type-2 daughter merozoites. Several divisional cycles may occur to produce many type-1 merozoites that in turn produce type-2 meronts and type-2 merozoites. Type-2 merozoites are multinucleated and smaller than type-1 merozoites, and can be seen as early as 4 days post-inoculation.
The cells of the intestinal epithelium are destroyed with the release of the merozoites from the intestinal cells. According to Tubbs (1987), asexual reproduction repeats three times. Other authors stipulate that the number of asexual reproductions varies.
Following the final asexual stage, the sexual stages of the I. suis life cycle begin to form: macrogamete (female) and microgamete (male). The microgamete is a biflagellate structure that fertilises the microgamete, producing a zygote that develops cell walls and becomes an oocyst. The oocyst is released into the lumen of the intestine and shed with faeces into the environment. These sexual stages may also be seen 4 days post-inoculation, whereas oocysts are first seen in faeces 5 days post-inoculation (rarely 4 days).
The prepatent period for Isospora suis is 5 to 7 days, and the patent time generally ranges from 5 to 16 days.
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