By Justin Bauer
The widespread North American leech Myzobdella lugubris is part of the Piscicolidae family. Due to its small size (1-2mm) and its marine habitat, it is not very well researched. While is maintains a commensal relationship with crustaceans it is a parasite to fish and more research must be conducted to fully assess its role in the aquatic community. Before understanding the danger these leeches carry, it is important to explain how these leeches were able to spread so far. Myzobdella lugubris’s reproductive strategy at first seems to make little sense. Secreting high numbers of cocoons each only containing one egg instead of many embryos in less cocoons seems to be less efficient. However, scientists have determined that this might give the leech an advantage. Given that the probability of all of the eggs adhering to the crustacean host’s exoskeleton is low, and the hatched juveniles need to successfully transfer to the alternate fish host, it makes evolutionary sense that there is a high number of eggs rather than a low one (Saglam, 2018). The surface area to volume ratio additionally favors stronger adherence for a smaller cocoon.
These leeches have several different impacts on fish: In largemouth bass M. lugubris causes oral ulcerations (Noga,1990), in logperch, brown bullhead catfish, it causes lesions and dermal erosions (Appy, 1982). A specific study focused on direct health impacts of M. lugubris on largemouth bass in Back Bay, Virginia. The results indicated that leeches did not substantially impact stress or hematological parameters (Pomposini, 2019). However, the leech attachment sites were associated with ulcerations that occasionally exposed underlying bone and cause hemorrhaging. Additionally, the attachment sites were colonized by several bacteria whose severity still needs to be determined.
Another study focused on M. lugubris in Lake Erie, Michigan. This leech species is the dominant one, mostly preying on channel catfish (Ictalurus punctatus) and freshwater drum (Aplodinotus grunniens) (Schulz, 2010). Since leeches commonly contain harmful bacteria (Kikuchi, 2005), bacteria from M . Lugubris’s viscera were extracted to fully understand the bacterial community found in this leech. Multiple bacterial groups were identified, but few were of importance. Most importantly, the Flavobacterium psychrophilum was identified, a causative agent that causes high mortalities in salmonids and increases a fish’s susceptibility to other diseases (Nematollahi et al., 2003). Further research might be directed at whether these leeches transmit this bacterium to other fish. Certain environmental bacteria detected in the leeches, were also found in the aquatic environment, suggesting that those bacteria are not resident to leech internal organs and instead are contaminants from the fish mucous or outer membrane which the leech would have to make contact with to attach. Ultimately the bacterial community was not very diverse compared to other organisms from Lake Erie (Winters, 2008), and the difference in bacteria between catfish and drum varies very little.
However, an emerging disease in the Lake Erie Watershed has been potentially tied to M. Lugubris. Viral Hemorrhagic Septicemia virus (VHSV) is deadly to fish populations, but little is known about it. Other leech species have already been confirmed as potential vectors for fish viruses, such as Piscicola salmositica for Infectious Hematopoietic Necrosis Virus in the sockeye salmon (Mulcahy, 1990), or P. geometra for Spring Viraemia of Carp Virus for the common carp. (Ahne, 1985). As previously mentioned, Myzobdella lugubris is the dominant species in Lake Erie, and due to its wide host range, it would be the ideal leech for the pathogen to spread between host species. Indeed, in the study (Faisal, 2009) 57 out of the 91 leech samples were positive by cell culture for VHSV and 66 out of 91 were positive by RT-PCR for the VHSV. While the leech hasn’t been directly confirmed to transmit VSHV, it is the first time this disease has been isolated from a leech species and more research needs to be done in order to confirm Myzobdella lugubris’s role.
This tiny organism needs to be investigated further. It is difficult enough to secure funding for wildlife studies but researching and understanding this organism might shed important light on the role it plays transmitting diseases. Due to its growing numbers in the US, this leech has the potential to attach to millions of fish if left unchecked, causing both physical wounds and transmitting diseases that could wipe out whole populations, potentially destabilizing local ecosystems. In order to avoid any future problems, it is imparative that this leech be researched in more depth.
References:
Saglam N, Saunders R, Lang SA, Shain DH. Phylogeny and cocoon production in the parasitic leech Myzobdella lugubris Leidy, 1851 (Hirudinidae, Piscicolidae). Acta Parasitol. 2018 Mar 26;63(1):15-26. doi: 10.1515/ap-2018-0002. PMID: 29351062.
Appy, R. G., & Cone, D. K. (1982). Attachment of Myzobdella lugubris (Hirudinea: Piscicolidae) to logperch, Percina caprodes, and brown bull‐head, Ictalurus nebulosus. Transactions of the American Microscopical Society.
Noga EJ, Bullis RA, Miller GC. Epidemic oral ulceration in largemouth bass (Micropterus salmoides) associated with the leech Myzobdella lugubris. J Wildl Dis. 1990 Jan;26(1):132-4. doi: 10.7589/0090-3558-26.1.132. PMID: 2304195.
Pomposini A, Blubaugh J, Boyce RC, Gauthier DT. Leech (Myzobdella lugubris) infestations in largemouth bass (Micropterus salmoides) in Back Bay, Virginia, USA. J Fish Dis. 2019 May;42(5):739-749. doi: 10.1111/jfd.12978. Epub 2019 Mar 8. PMID: 30972838.
Schulz C, Faisal M. The bacterial community associated with the leech Myzobdella lugubris Leidy 1851 (Hirudinea: Piscicolidae) from Lake Erie, Michigan, USA. Parasite. 2010 Jun;17(2):113-21. doi: 10.1051/parasite/2010172113. PMID: 20597437.
Kikuchi Y. & Fukatsu T. Rickettsia infection in natural leech populations. Microbial Ecology, 2005, 49, 265-271
Nematollahi A., Decostere A., Pasmans F. & Haesebrouck F. Flavobacterium psychrophilum infections in salmonid fish. Journal of Fish Diseases, 2003, 26, 563-574. Winters A. Microbial communities associated with the zebra mussel (Dreissena polymorpha) in the Laurentian Great Lakes Basin (USA) [thesis]. East Lansing (MI): Michigan State University, 2008, 94 p.
Winters A. Microbial communities associated with the zebra mussel (Dreissena polymorpha) in the Laurentian Great Lakes Basin (USA) [thesis]. East Lansing (MI): Michigan State University, 2008, 94 p.
Faisal M, Schulz CA. Detection of Viral Hemorrhagic Septicemia virus (VHSV) from the leech Myzobdella lugubris Leidy, 1851. Parasit Vectors. 2009 Sep 28;2(1):45. doi: 10.1186/1756-3305-2-45. PMID: 19785752; PMCID: PMC2761889.
Mulcahy D, Klaybor D, Batts WN. Isolation of infectious hematopoietic necrosis virus from a leech (Piscicola salmositica) and a copepod (Salmincola sp.), ectoparasites of sockeye salmon Oncorhynchus nerka. Dis Aquat Org. 1990;8:29–34. doi: 10.3354/dao008029.
Ahne W. Argulus foliaceus L. and Piscicola geometra L. as mechanical vectors of spring viremia of carp virus (SVCV) J Fish Dis. 1985;8:241–242. doi: 10.1111/j.1365-2761.1985.tb01220.x.
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