disease known as microsporidiosis

Glugea hertwigi develops in the cytoplasm of host cells. It causes nuclear division without cytokinesis, resulting in one large cell up to 0.5–1 mm in diameter filled with many nuclei. Parasitic spores mature in a central vacuole in the enlarged host cell, undergoing schizogony, or asexual reproduction, many times. Once the spores have finished reproducing, the host cell ruptures and the parasites are eventually released into the water column to be ingested by new hosts. In new hosts, they attach to the digestive tract or other organs. The sporoplasm is then discharged from the spore case at high pressure, entering the host cell. The sporoplasm reorganizes before, during, and after ejection, and the resulting mass is larger than it was inside the spore case. Spore discharge can be triggered by a large change in pH to highly alkaline conditions (Delisle 1972, Scarborough-Bull and Weidner 1985, Wellings et al. 1969).

The number of parasitic cysts in smelt populations typically peak between August and September (Pekcan-Hekim et al. 2005), although die-offs occur in Quebec in both spring and fall (Delisle and Veilleux 1969). Cyst growth may increase with increasing water temperature (Delisle 1969). However, as spores frequently enter both juvenile and adult fish at maximum water temperatures, the largest cysts may not appear until after this time (Pekcan-Hekim et al. 2005). Younger smelt appear more vulnerable to infection, while older smelt may have increased immunity (Pekcan-Hekim et al. 2005).

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental	Socioeconomic	Beneficial

There is little or no evidence to support that Glugea hertwigi has significant environmental impacts in the Great Lakes.
Potential:
In the Great Lakes, G. hertwigi exhibits host specificity for Rainbow Smelt (Osmerus mordax) (Muzzall and Whelan 2012), a species which competes with and feeds on several important native and non-native species in the Great Lakes food web. Glugea hertwigi is known to damage the mesentery, intestinal organs, and gonads in Rainbow Smelt. Such parasitism by G. hertwigi can be lethal or non-lethal (Pekcan-Hekim et al. 2005). Infections in the stomach and intestine can cause starvation and intestinal poisoning (Chen and Power 1972, Delisle 1972, Mills et al. 1993, Pekcan-Hekim et al. 2005, Scarborough and Weidner 1979). Die-offs of Rainbow Smelt as a result of G. hertwigi infection have the potential to indirectly cause significant cascading food web effects in the Great Lakes.

Great Lakes native Atlantic Salmon (Salmo salar), Lake Trout (Salvelinus namaycush), and other salmonids are known to prey heavily upon Rainbow Smelt (EPA 2008). Osmerus mordax is believed to have aided the growth of landlocked Atlantic Salmon in Maine (Havey 1973). Die-offs of O. mordax as a result of G. hertwigi could have adverse health effects on Great Lakes salmonids.

While there is no evidence of native coregonid infection in the Great Lakes, G. hertwigi is known to parasitize Coregonus species in northern Russian lakes (Dykova 1995).

There is little or no evidence to support that Glugea hertwigi has significant socio-economic impacts in the Great Lakes.
Potential:
Glugea hertwigi has been implicated in mass mortalities of the introduced Rainbow Smelt (O. mordax) in Lake Erie (Nepszy and Dechtiar 1972, Nepszy et al. 1978) and Lake Ontario (A. Dechtiar, unpublished data). Mortality events are believed to be seasonal and directly related to parasite prevalence (Dechtiar and Nepszy 1988). Glugea hertwigi cysts in ovaries of mature Rainbow Smelt females have also caused greatly reduced egg production (Chen and Power 1972). However, it should be noted that the presence of G. hertwigi in Rainbow Smelt in other dieoff events had no significant effect on fecundity or condition, although it did cause growth to slow somewhat (Nsembukya-Katuramu et al. 1981). While economic impacts of G. hertwigi on the commercial smelt (see Pflieger 1997, Smith 1985) and salmonid (which rely on smelt) fisheries have not been realized, significant impacts could be realized if Rainbow Smelt mortalities were to increase as a result of infection.

Glugea hertwigi poses the largest threat (e.g., high infection rates and mortalities) to Lake Erie Rainbow Smelt populations as compared to any other parasite (Muzzall and Whelan 2012; Nepszy and Dechtiar 1972).

There is little or no evidence to support that Glugea hertwigi has significant beneficial effects in the Great Lakes.

Potential:
Declines in Rainbow Smelt as a result of G. hertwigi infection has the potential to improve resource availability for native Great Lakes species. Several of these scenarios are proposed below.

As Rainbow Smelt preys heavily upon Bloater (Coregonus hoyi), Alewife (Alosa pseudoharegus), Slimy Sculpin (Cottus cognatus), Emerald Shiner (Notropis atherinoides), Burbot (Lota lota), and opossum shrimp (Mysis relicta) (Brandt and Madon 1986, Creaser 1925, O’ Gorman 1974, Stedman and Argyle 1985, Van Oosten 1940), its diet is believed to affect prey population numbers and can be an important component to the total mortality of yearlings (Creaser 1925, O’ Gorman 1974). In turn, commercially valuable native and non-native salmonids rely on several of these prey species. Die-offs of Rainbow Smelt as a result of G. hertwigi may increase prey fish populations, thus increasing feeding opportunities and improving the health and value of the salmonid fisheries. However, such effects have not been realized in the Great Lakes.

Additional potential food web effects include those on Lake Trout (Salvelinus namaycush). Juvenile Lake Trout tend to eat Slimy Sculpin, while adults consume Rainbow smelt (Brandt and Madon 1986). Direct competition for Slimy Sculpin between juvenile Lake Trout and Rainbow Smelt has been observed (Brandt and Madon 1986). Lake Trout may therefore be a keystone predator in the relationship between Rainbow Smelt and Slimy Sculpin (Brandt and Madon 1986). Mortalities in Rainbow Smelt by G. hertwigi could decrease competition and stress in juvenile Lake Trout and could have significant beneficial effects on the Great Lakes food web.

Rainbow Smelt compete with Lake Herring (Coregonus artedii) and Yellow Perch (Perca flavescens) and may be partially responsible for the decline of Great Lakes whitefish (Coregonus spp.) (Becker 1983, Christie 1974, Hrabik et al. 1998, Todd 1986). Increased G. hertwigi infections and die-offs of Rainbow Smelt could lead to a decrease in competition and stress on Lake Herring and Yellow Perch and may increase the commercially valuable whitefish population.

Rainbow Smelt contributed to the extinction of Blue Pike (Sander vitreus glaucus), a subspecies prevalent in the Great Lakes until the 1980s. Rainbow Smelt have also affected imperiled species outside the Great Lakes (EPA 2008). Because of competition between Rainbow Smelt and native species, other imperiled species in the Great Lakes could benefit if Glugea hertwigi limited Osmerus mordax populations.

Regulations (pertaining to the Great Lakes)
There are no known regulations for this species.

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.

Control
Biological
There are no known biological control methods for this species.

Physical
There are no known physical control methods for this species.

Chemical
There are no known chemical control methods for this species.

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.