Parasite of the month

Microphallus sp.

The life-cycle of trematodes typically involves asexual reproduction in molluscs and the use of a secondary intermediate host for transmission (fish, amphibians...) to the final host where sexual reproduction occurs (birds). The production of cercariae larvae in the mollusc is considered critical for these parasites. Indeed, this production of thousands of clones determines an important part of the parasite's success to complete its life-cycle.

The trematode Microphallus sp. is widespread in New Zealand (although undescribed). It reproduces in the gonads of the aquatic snail Potamopyrgus antipodarum (see picture). Kopp and Jokela (2007, Oikos) showed that the presence of the great pond snail Lymnea stagnalis, following its introduction at the end of the 19th century, alters the transmission of Microphallus sp.: the presence of the introduced species reduces the ability of the parasite to enter in contact with its native host.

Indeed, the parasite has evolved to infect the native snail but the introduced species is not compatible with the parasite (i.e. Microphallus sp. can't reproduce efficiently in the introduced snail). Therefore, these introduced hosts constitute decoys for the parasite. This in turn decreases the probability of infecting the native P. antipodarum, a phenomenon called dilution effect. Overall, the study by Kopp and Jokela highlights the need to consider infection rates in a more realistic context, taking in account other species in the community as well as their interactions.


source for picture: http://www.indiana.edu 

Guess who's on the faculty's facebook page?

Yep - that might be me :)

My project was selected to go on our faculty's facebook page to celebrate  Säätiöpäivä (i.e. Foundation Day). Like many research projects conducted at the University of Helsinki, my Ph.D. work is supported by a grant from a private foundation. Since its creation in 1972, the Maj and Tor Nessling Foundation aims at advancing Finnish science and culture. Nowadays, it supports scientific research concentrating on environmental problems and their solutions.







On the picture: setting minnow traps to catch stickleback

Parasite of the month

Ribeiroia ondatrae

This trematode reproduces asexually in ram's horn snails before infecting amphibians where it causes limb malformation. This in turn is supposed to enhance predation by the final hosts, i.e. birds.

Human-induced eutrophication (artificial enrichment of water bodies with nutrients, typically nitrogen and phosphorus) has been shown to facilitate its transmission by altering the interactions between the parasite and the snail (Johnson et al. 2007 PNAS). Indeed, eutrophication promotes the growth of algae and in turn favours populations of grazers such as ram's horn snails. In these conditions, snails occur at high densities and grow bigger. This provides more resources for the parasite and thus facilitate its transmission to and development in the snail. Moreover infected snails were better able to tolerate infection for longer periods of time. This has caused the production of cercariae (stages infective to amphibians) to double in eutrophied waters, generating outbreaks in amphibian populations.

Recently, the impact of global warming has been investigated concerning the transmission of this parasite (Paull & Johnson 2014 Ecology Letters). Using mesocosms, researchers showed that increases in temperature extend the time window for parasite transmission from snails to tadpoles. Yet, the study also revealed that less cercariae are produced during the summer at warmer temperatures compared to under normal conditions. This generates an asynchrony in the presence of cercariae and of amphibian hosts and lowers the overall transmission of the parasite in heated mesocosms. These results indicate that global warming might not be beneficial to all parasites.

It would be interesting to investigate the joined effects of eutrophication and global warming. Indeed, eutrophication is often associated with higher water temperatures. The negative effects of global warming on parasite transmission could then be reduced by the positive effects of eutrophication on the snail-trematode interaction.


source for picture: http://www.science-art.com

Parasite humour

Parasite of the month

Lepeophtheirus salmonis

Salmon lice are parasitic copepods that feed on the skin and blood of salmonids. Over their life-cycle, they alternate between free-swimming, stationary and mobile life-styles. They attach to salmonids during the thrids life stage and grow up to 12mm for adults females (30mm with egg strings) and 6mm for adult males. Mean generation time is about 6 weeks at 12°C.


In small amounts, lice cause little damage to the fish but high intensities of infection can reduce growth and even lead to death of the host. Symptoms include skin erosion, constant bleeding and open wounds, which can facilitate infections by pathogens. Moreover, salmon lice are also vectors of a few pathogens, such as infectious salmon anemia.


In the recent years, their numbers have been increasing due to the growth of the aquaculture industry. High densities of hosts at fish farms was shown to increase transmission and to trigger the evolution of higher virulence of L. salmonis, as discussed in Mennerat et al. (2012 J. Evol. Biol.). This can have dramatic consequences for wildlife as (1) infestations of sea lice in salmon farms increases the number of lice in the rest of the surrounding water and because (2) lice act as a vector for diseases between wild and farmed salmon, affecting the evolution of virulence of parasites of wild fish.


Recently, a study by Losos et al. (2010 Behaviour) found stickleback to feed on female salmon lice - towards new management practices at fish farms?


(source for picture: http://www.bears-and-more.de/grafiken/2010-12-03_1.jpg)

Parasite of the month

Cystidicola spp.

Cystidicola are parasitic nematodes infecting many fish species throughout the northern hemisphere. Their life-cycle  involves two hosts: an amphipod as first intermediate host and a fish as definitive host. Amphipods become infected by accidentally feeding on the eggs of the parasite. The eggs hatch inside the host and undergoes two molts in the hemocoele before becoming infective to fish. When a fish feeds on an infected amphipod, the parasite migrates from the gut to the swim-bladder where sexual reproduction will take place. The eggs are then produced, enter the gut via the pneumatic duct and ultimately released into the water with the faeces of the host.

Black (1983 Can J Fish Aquat Sci 40: 643-647) discussed the possibility that intensive fishing of the lake trout Salvelinus namaycush in the Great Lakes of North America may have caused a decline in Cystidicola stigmatura infections over the 20th century. In particular, inspection of museum specimens revealed that trouts caught before 1925 were more often infected with the nematode than trouts caught after 1925. This is particularly interesting for stock management as maintaining fish populations below a threshold could limit transmission for some parasite species and improve filet quality. More about this here.

(source for picture: http://img-fotki.yandex.ru/get/9810/133242081.6a/0_ed45e_dbb3331_XL.jpg)

Parasite of the month

Pelseneeria spp.

Pelseneeria are small parasitic snails belonging to the Eulimidae family. They feed on sea urchins by the use of a proboscis which extends into the body cavity of their host.

In their study, Sonnenholzner et al. (2011 Ecology 92: 2276-2284) highlight how over-fishing can deeply affect food webs and thereby parasites.

Pelseneeria snails are preys of the crab Mithrax nodosus in the Galapagos. Increased fishing pressure on the predators of the crab has thus benefited the crab populations, leading to a decrease in the prevalence of the parasitic snail. Under these circumstances, sea urchins are benefiting from over-fishing.

Accordingly, the authors reveal in another study (Sonnenholzner et al. 2009 Mar. Ecol. Prog. Ser.) the impact of sea urchins on algae and anemones. Hence, environmental disturbances can have drastic cascading effects on the whole ecosystems in which parasite can play a crucial role.


(source for picture: http://www.conchology.be)