Overview: Gregarines are protozoans that infect invertebrates-mainly worms, grasshoppers and beetles. Damselflies, a type of dragonfly, are also infected by several genera of gregarines. The infective stage of the gregarine is a waterborne oocyst that must be ingested by the damselfly host. Immature damselflies are aquatic and referred to as nymphs. Both the immature and adult are obligate predators, the nymphs feeding on benthic invertebrates and the adults feeding on small flying insects. Since it is predacious and does not drink (water balance is accomplished through the abdomen) the damselfly nymph encounters the oocysts in its prey. This is unlike other gregarine hosts which are herbivorous or detritovores and can be expected to encounter contaminated food. The damselfly prey must ingest the oocyst first. It is simple to determine what the nymph has fed on, as the gut is easily dissected and the prey in the foregut are in good condition and can be identified. Further, the gregarine trophozoites inhabit the midgut and are also easily seen, so that a single dissection can yield much useful information.
Giardia and Cryptosporidium are two genera of pathogenic protozoans that infect man and many mammals, including cattle. Both have waterborne infective stages, Cryptosporidium being spread by means of oocysts and Giardia by means of cysts. These stages are difficult to detect in the aquatic environment as they must be concentrated first by means of filtration, centrifugation or a combination of both and then differentially stained by immunofluorescent antibodies. Polymerase chain reaction (PCR) detection has been developed, but is not yet in routine use. Currently, there is no good way to quantify these protozoa in the environment and research into their ecology is blocked.
Because the two vertebrate protozoa and the damselfly gregarines are spread from host to host via waterborne cysts and oocysts that must be ingested by the host for infection to occur, and because both groups of infective propagules are similar in size (4-15 microns), then it is reasonable to assume that physical properties of the water and watershed will act on both in a similar fashion. Both should settle in the water column at similar rates, be moved by currents in comparable fashions, be incorporated into sediments similarly, and will likely be scavenged by the same groups of aquatic arthropods and benthic macroinvertebrates. It is therefore feasible that the presence of high numbers of gregarines in a damselfly population may indicate and aquatic environment conducive to the maintenance and distribution of the infective propagules of Giardia and Cryptosporidium as well.
A preliminary study has begun at the Stephenville Center in an attempt to determine whether or not gregarines are present in damselfly populations and if infection prevalence changes over time. Three genera of gregarines, Hoplorhynchus, Steganorhynchus and Nubenocephalus, have been found infecting 4 genera of damselflies in two families. Figure 1 shows the prevalence of these gregarines in adult damselflies during the summer and early fall of this year. In four embanked ponds filled with well water and rainwater, infection began at a low level and increased through the sampling period, while infection in creek-fed ponds remained constant. The three creek-fed ponds are connected in series by Smith Springs creek, which drains nearly 2000 acres of cropland and rural septic systems, a cheese processing plant, a 450 count cow dairy, and a beefmaster cattle herd.
Click here to view a picture of gregarines up close!
Aquatic biology articles:
"Sampling Statistics for Higher Aquatic Taxa"
"Biology of Dragonfly Naiads in Relation to Habitat and Water
Quality"
"Population Biology of Dragonflies in Central Texas"
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