Mi­crobes or Not, Par­a­sites All

by Elio

Par­a­sites pose a prob­lem for the se­man­ti­cally-ori­ented mi­cro­bi­ol­o­gist. There is no ques­tion that uni­cel­lu­lar par­a­sites such as Gi­a­r­dia, Plas­mod­ium, or Tox­o­plasma are mi­crobes, thus we can ap­pro­pri­ate them with im­punity. But what about par­a­sitic worms? They are clearly not mi­cro­scopic* and are tax­o­nom­i­cally apart from par­a­sitic pro­tist. Yet par­a­sitic pro­tists and meta­zoans are taught along­side one an­other in the mi­cro­bi­ol­ogy class­room and they are of­ten con­sid­ered in uni­son by epi­demi­ol­o­gists and oth­ers. For all their phy­lo­ge­netic di­vide and ap­par­ent dif­fer­ences, they all share the gen­eral traits of a par­a­sitic life style, in­clud­ing un­der­go­ing the same steps in patho­gen­e­sis (en­counter with the host, en­try, sur­vival, mul­ti­pli­ca­tion, caus­ing dam­age) and elic­it­ing com­plex im­mune re­sponses.

The adult pro­to­zoon, Spiro­chona gem­mi­para, on the gills of a crus­tacean. Source

Par­tic­u­larly fas­ci­nat­ing is that par­a­sites have evolved in­tri­cate mech­a­nisms to sur­vive in­side the host and en­sure their trans­mis­sion. Many par­a­sites, whether uni­cel­lu­lar or mul­ti­cel­lu­lar, go through life cy­cles that are ul­tra-baroque in com­plex­ity. Con­sider, for ex­am­ple, the malaria plasmodia's con­tor­tions both within the mos­qui­toes and the hu­mans. Plas­modia in­ter­act with at least three kinds of cells in the in­sect host and go through some six dif­fer­ent stages of dif­fer­en­ti­a­tion in the hu­man host (in­clud­ing sex). Or pon­der about cer­tain flukes that go through as many as four dis­tinct hosts in their life cy­cle!

A ne­ma­tode, Strelkovimer­mis spic­u­la­tus, par­a­sitiz­ing a mos­quito larva. Source

So, some­where along the line, us mi­cro­bi­ol­o­gists should con­sider nuz­zling up to the meta­zoan par­a­sites, at least to learn more about them. The In­ter­net is a good place to start, with a num­ber of fine blogs, ar­ti­cles, and other forms of so­cial me­dia that are both in­for­ma­tive and ex­cit­ing. Vis­it­ing them is more than just worth­while. Here is a par­tial list:

This Week in Par­a­sitism is an ASM-spon­sored pod­cast hosted by Vin­cent Racaniello and Dick­son De­spom­mier where a wide range of is­sues of par­a­sitism are art­fully dis­cussed.

Par­a­site of the Day presents a pa­rade of un­usual and un­ex­pected par­a­sites, from some that are found in tar pits to oth­ers that feed on mos­quito lar­vae.

Pretty Pro­to­zoa deals with uni­cel­lu­lar par­a­site species in ways that sat­isfy both the eye and the mind.

10 As­ton­ish­ing Ex­am­ples of Bizarre Par­a­sitic Life Cy­cles presents a small pa­rade of par­a­sites, from those that make their crab host dance, to one that eat the fish host's tongue, to those that make their fly host dive in a lake.

The Art­ful Amoeba has ex­quis­itely writ­ten ar­ti­cles, oc­ca­sion­ally on par­a­sitic forms.

*A hu­man tape­worm can reach 10 me­ters in length, that of a whale, 40 me­ters.