EQUUS - - Eq In Brief -

The opos­sum is the de­fin­i­tive host for Sar­co­cys­tis neu­rona, mean­ing that the pro­to­zoa can ma­ture and re­pro­duce within its body.

1. The Opos­sum Ex­cretES The Par­a­site eggs, called oocysts, in its fe­ces.

2. The oocysts re­lease a sec­ondary stage, called sporo­cysts, which may con­tam­i­nate feed or wa­ter and be con­sumed by other an­i­mals.

3. The horse may in­gest sporo­cysts. Horses are con­sid­ered aber­rant hosts be­cause, so far, no ev­i­dence has been found that the pro­to­zoa com­plete their life cy­cles in horses.

4. In some cases, the pro­to­zoa may cross into the horse’s cen­tral ner­vous

/ or brain, caus­ing equine pro­to­zoal myelo-en­cephali­tis (EPM).

5. Other an­i­mals—in­clud­ing rac­coons, skunks, cats and ar­madil­los— may in­gest the sporo­cysts and be­come in­ter­me­di­ate hosts.

6. Once in­side the in­tes­tine of an in­ter­me­di­ate host, the sporo­cysts hatch and go through other life stages. Even­tu­ally, they in­vade the mus­cle tis­sue and form sar­co­cysts, which con­tain par­a­site spores.

7. When the in­ter­me­di­ate host dies, its car­cass may be scav­enged by an opos­sum, which in­gests the sar­co­cysts. The par­a­sites ma­ture in the opos­sum’s in­tes­tine, and the cy­cle be­gins again.

Note: The life cy­cle of Neospora hugh­esi is less un­der­stood, but it ap­pears that horses do not have to eat in­fected food or wa­ter to con­tract it: Mares who carry the or­gan­ism can pass it to their off­spring dur­ing ges­ta­tion. This means that EPM may be a pos­si­bil­ity even in ar­eas where opos­sums are not found.

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