Inodosporus fujiokai

A few years ago, rainbow trout at a trout farm in the Shiga prefecture, Japan, were being struck down by a mysterious illness. The flesh of the dead fish were speckled with red dots and white cysts. It turns out the disease was caused by a type of previously unknown microsporidian parasite. Microsporidians have been reported from other farmed fish in Japan, where they are locally called "beko disease". It was suspected that the trout might be getting infected from their food, and during feeding trials it was found that trout fed with fresh or chilled prawns developed the disease, while those fed frozen prawns stayed healthy. This shows that prawns were somehow involved in the life cycle of this parasite.

Left: Prawn infected with Indosporus fujiokai (indicated by red arrow), Centre: Electron microscopy of spores from muscles of an infected prawn (top), and a spore from the muscles of an infected trout (bottom). Right: An infected trout showing signs of hypoxia associated with infection by I. fujiokai (top), muscles of infected trout with red specks and white cysts of the parasite as indicated by arrows (bottom).
Photos of prawns + spores from Fig. 1, 7, and 9 of the paper, Photos of infected trout + their flesh from Fig. 3 and 6 of this paper

Microsporidians are single-celled parasites which are related to fungi. There are 1500 known species, though the actual number of microsporidians out there is likely to be much higher. For most of them, relatively little is known aside from how they look like and what they infect. About half of all known microsporidians are parasites of aquatic animals (and their parasites), and their life cycles can vary considerably between different species. Despite their importance as parasites of fish and crustaceans in aquaculture, the life cycles of many microsporidians are unknown. 

In the study featured in this blog post, researchers set out to find samples of the Shiga trout farm parasite out in the wild - and they found it amidst some prawns from Lake Biwa. Microsporidian-infected prawns are easy to spot because in contrast to healthy prawns which are translucent, infected prawns become opaque white as the parasite proliferates in their muscles. But surprisingly, despite the numerous spores filling up their flesh, infected prawns seemed rather healthy and were able to live for several weeks in the lab. Some of them even managed to produce eggs despite being parasitised! This is in stark contrast to the effect that this parasite has on its trout hosts.

The researchers named this microsporidian Indosporus fujiokai - after a parasitologist who, back in 1982, suggested the involvement of prawns in the transmission of microsporidian parasites. But that is not the entire story, because those prawns were harbouring a lot more than just I. fujiokai. The researchers actually found FOUR different types of microsporidians in those prawns, including the one that they eventually named Indosporus fujiokai. These microsporidians all differ in their spore sizes and shapes, and all of them were entirely new to science. Three of the microsporidians, including I. fujiokai, belong to a group called "Marinosporidia'' which are usually found infecting fish and aquatic invertebrates - this was to be expected since they were examining prawns. However, one of the microsporidians was more unusual, as it hails from an entirely different part of the microsporidian tree called "Terresporidia", which is composed of species that usually infect insects.

The results of this study suggests that prawns and other crustaceans could be harbouring a rich array of microsporidian parasites that are currently unknown to science, and there might be many more of them out there which are infecting fish by the way of crustacean hosts. While the researchers in this study were able to resolve the life cycle for I. fujiokai, mysteries continue to surround the life cycles of the three other microsporidians that they found - what hosts they might infect in the next stage of their respective life cycles are anyone's guess at this point.

As is often the case with parasites, just as you manage to answer one question, three (or more) others pop up in the process. So if life gives you a raw prawn, you should examine it for parasites.

Reference:

Yanagida, T., Asai, N., Yamamoto, M., Sugahara, K., Fujiwara, T., Shirakashi, S., & Yokoyama, H. (2023). Molecular and morphological description of a novel microsporidian Inodosporus fujiokai n. sp. infecting both salmonid fish and freshwater prawns. Parasitology 150: 1-14.

Encephalitozoon cuniculi

This is the second post in a series of blog posts written by students from my third year Evolutionary Parasitology unit (ZOOL329/529) class of 2015. This particular post was written by Brenda Cornick and it is about an outbreak of a microsporidian parasite that causes rabbits to look like they were being animated by Shaft Studio (you can read the previous post about a parasitoid that commandeer a spider to weave a tangled web for it here).

An Encephalitozoon cuniculi spore
From Figure 7 of this paper

For those with pet rabbits, Calicivirus and Myxoma virus are generally thought to be the main dangers to bunny's health. However, there is another nasty lurking within our little furry friends that you may not be aware of - the parasite Encephalitozoon cuniculi. The vast majority of rabbits that carry this parasite show no symptoms at all, and can live a normal healthy life. But for the unlucky few that are affected, the symptoms are particularly unpleasant, and usually fatal. There was an outbreak of E. cuniculi in a rabbit colony at a Japanese zoo between 1999-2001 that claimed the lives of 42 rabbits. But before we look at the study surrounding this outbreak, a summary of how this parasite operates would be helpful.

Encephalitozoon cuniculi is a type of microsporidian, a single-cell parasite equipped with a structure called a polar tube, which is curled inside the infective spore. Spores are the infectious stage, and are either inhaled or consumed by the host. When it comes into contact with a host cell, the spore discharges its polar tube and penetrates the cell membrane, allowing the parasite to enter. It is an intracellular parasite that lives inside its host's cell, and this species also attacks the host's central nervous system. The most common means of transmission is from the urine of an infected rabbit.

Dat Shaft head-tilt
From Figure 1 and 2 of this paper

In rabbits that develop disease from E. cuniculi infection, clinical symptoms include head tilt, loss of balance, weakness in the hind legs, depression, stunted growth, and lesions results from inflammation caused by the rupturing cells releasing spores. Rabbits showing some or all of these symptoms, can have nodules and cysts on their internal organs such as brain, heart, liver, and kidneys. This parasite has also been known to be transmitted to humans with compromised autoimmune systems, such as those suffering from AIDS, and was listed by the World Health Organisation as an emerging infectious agent. Encephalitozoon cuniculi spores are able to survive pretty well in the external environment, but can be eradicated with the use of standard disinfecting routines.

In Japan, this nasty little parasite has also been found in squirrel monkeys and domestic dogs living in close quarters with humans. The E. cuniculi outbreak at the Japanese facility prompted the study featured in this post, which involved clinical and pathological examinations, and biosecurity countermeasures. The alarm was first raised when two young bunnies showed signs of a central nervous system problems. Blood tests were conducted, and those bunnies were diagnosed with encephalitozoonosis. Following these cases, biosecurity measures were put in place included monitoring, isolation, and transport limitation. Any rabbits even suspected of harbouring E. cuniculi were humanely euthanized. Despite these measures, periodic infections were still occurring, leading to the entire rabbit colony being euthanized. In total, 32 out of the 42 (76.2%) rabbits were found to be infected with E. cuniculi.

Following this incident, the facility was closed and all the equipment, such as cages, feeders, floors were thoroughly sterilized using burners, 70% ethanol solution, and boiled water. New rabbits were introduced back into the facility two months after this procedure. and there has been no recurrence of E. cuniculi outbreaks.

It became clear during this study that the original infection had come from eight rabbits that were introduced to the colony with no quarantine period. Due to the lack of simple biosecurity measures, the act of introducing new bunnies became a death sentence for the whole colony. For this particular facility, the rabbits were a popular interactive attraction for visitors, many of whom were infants or the elderly whose immune systems may not be as strong as others. This highlights the importance of adequate biosecurity and husbandry techniques when dealing with readily transmissible parasites that can be harboured by multiple host species, and can have such devastating effects.

Reference:
Fukui, D., Bando, G., Furuya, K., Yamaguchi, M., Nakaoka, Y., Kosuge, M., & Murata, K. (2013). Surveillance for an Outbreak of Encephalitozoon cuniculi Infection in Rabbits Housed at a Zoo and Biosecurity Countermeasures. Journal of Veterinary Medical Science, 75(1), 55-61.

This post was written by Brenda Cornick

Edhazardia aedis

When two different parasites find themselves in a small host animal like a mosquito, there is only so much of the host to go around. So there is a pretty good chance that those co-occurring parasites are going to fight it out, and there's no guarantee that there will be a winner out of this conflict.

Photo of E. aedis spores from here

Edhazardia aedis is a microsporidian parasite that specialises on infecting Aedes aegypti - also known as the mosquito that can act as the main vector for a variety of viruses include those that causes degnue fever, yellow fever, and Chikungunya. Edhazardia aedis can spread through the mosquito population via two methods; (1) the parasite can proliferation throughout the mosquito's body until it ultimately overwhelms the host, which dies and dissolves into a cloud of infective spores, or (2) if an infected female mosquito survives the ordeal to adulthood and still manage to produce offspring, her mosquito babies will inherit E. aedis from her (gee, thanks a lot mum!).

But E. aedis can sometimes run into a competing species - Vavraia culicis. It is also a microsporidian, but unlike E. aedis, it is a generalist that can infect many different species of mosquitoes. It is also a mosquito-killer which has the same general modus operandi as E. aedis, where the parasite's spores are released when the host finally succumbs. This study found that mosquito larvae which have less access to food and are infected by both parasites tend to die earlier - and when the host dies, the spores are dispersed for both E. aedis and V. culicis - so everyone wins, right? Well not quite.

While host death does release the spores which allow them to infect more mosquito larvae, the parasites get more spores for their bucks by keeping their host alive for longer - so a host that ends up keeling over too early is not very cost effective. This applies to both E. aedis and V. culicis. Even before host death, the cost of co-infection starts manifesting itself. Regardless of whether the host dies sooner or later, both parasites produce less spores in co-infections. If E. aedis is sharing a host, it produces half as many spores as it would have if it had the host all to itself. But co-infection is even more costly for V. culicis, which manages to produce only a bit over a quarter of the spore it would have in single infections.

It is unknown how these two parasites duke it out in the mosquito, or why E. aedis has a competitive edge over V. culicis. Perhaps by being a specialist of A. aegypti, E. aedis has some sort of home ground advantage when it comes to getting the most out of its host. So it seems that some parasites just don't like sharing, and when it comes to living with others, sometimes it pays for a parasite to be a specialist.

Reference:
Duncan, A. B., Agnew, P., Noel, V., & Michalakis, Y. (2015). The consequences of co-infections for parasite transmission in the mosquito Aedes aegypti. Journal of Animal Ecology 84: 498-508.

Nematocenator marisprofundi

Parasitism is the most common mode of life on Earth and it can found everywhere, in all kinds of environments. Even in extreme places such deep sea hydrothermal vents, amidst hellish geysers pouring out hot sulfide or seeping methane, parasitism carries on as usual - the players may change, but the game stays the same. While on this blog most of the nematodes we have featured are the parasites, in this particular case, they play the role of the host.

SEM photo of D. marci from the paper

Laying about 85 kilometres off the coast of Oregon, under about half a mile (800 metres) of water is the Hydrate Ridge methane seeps. These vents are covered in mats of sulfide oxidizing bacteria which are crawling with worms - mostly nematodes from the genus Desmodora. One of these species happens to be a host to the parasite we are featuring today - Nematocenator marsiprofundi - which translates into "nematode eater of the deep sea". It is a microsporidian - a group of single-cell parasite somewhat related to fungi, and taxonomically speaking, N. marsiprofundi lies right near the base of the split between these two groups.

Microsporidians are found in a wide range of animals including vertebrates such as fish and reptiles, as well as invertebrates such as insect, crustaceans, and nematode worms. The host of N. marsiprofundi, a nematode named Desmodora marci (see above), is one of the more abundant animal at methane seeps. There can be as many as twenty worms for each millilitre of carbonate rocks from such locales, and over half of those worms would be infected with N. marsiprofundi. This parasite seems to be common at such vents and were found at sites which are 15 kilometres, so N. marsiprofundi is not localised to just a particular location and/or worm population.

Spores of N. marisprofundi
(image from the paper)

The spores of this parasite (see left) are mostly found in the worm's reproductive tract; in female worms, the spores sit in the uterus next to the eggs, and in the male, the spores lined the worm's sperm duct and cloaca. This led the researchers who found this parasite to suggest that N. marisprofundi is sexually transmitted between its host. However, they also noticed some stages of the parasite were situated in the body wall, where they seem to degrade and digest the worm's muscle tissue, not unlike the microsporidian we featured a two months ago which infects an amphipod that has become invasive in Central Europe.

While their presence in the body wall and the effects they have on their host's muscle indicates they can be quite harmful and may transmit through means other than the worms' sexual activities, that is not to say that this parasite might not exploit multiple mode of transmission. Some parasite change their shape and infect different host tissue at different stages of their lives, and it is possible that N. marisprofundi can both be sexually transmitted and also eventually kill their host to allow their spores to disperse from a rotting cadaver

Studies like this shows parasites might be more common in the deep sea that we might have previously suspected, and that even in seemingly extreme environments like hydrothermal vents, there is good living to had as a parasite. Parasitism is everywhere on this planet, and while many people may think parasites are odd freaks of nature, in reality they are just a normal part of life on Earth.

Reference:
Sapir, A., Dillman, A. R., Connon, S. A., Grupe, B. M., Ingels, J., Mundo-Ocampo, M., Levin, L. A., Baldwin, J. G., Orphan, V. J. & Sternberg, P. W. (2014). Microsporidia-nematode associations in methane seeps reveal basal fungal parasitism in the deep sea. Frontiers in Microbiology 5: 43.

Cucumispora dikerogammari

Invasive species can be very disruptive - cane toads, rabbits, water hyacinth, and zebra mussels are just a few well-known examples of species that have been introduced to areas outside of their original geographic range and have caused extensive ecological disruption in their new home. One of the hypotheses for why some introduced species become so successful when they arrive at a new region is called the "enemy release hypothesis". In their new home, introduced species run amok as they are no longer hounded by their usual foes that would otherwise keep their population in check.

Top: A heavily infected amphipod
Bottom: Spores of C. dikerogammari
Photo from here

Dikerogammarus villosus is an amphipod (a little, shrimp-like crustacean) from the Ponto-Caspian that has invaded western and central Europe, and is now also found in the United Kingdom. They might only grow up to a little over an inch long, but they are voracious little predators that eat everything smaller than themselves, including each other. Released from their usual predators and parasites, D. villosus rips through the freshwater life of its new neighbourhood. But they have not completely escaped from their past foes; one parasite has managed to come along for the ride, and it is a microsporidian called Cucumispora dikerogammari.

As far as the parasite goes, Cucumispora dikerogammari is a pretty nasty one. It invades the host's muscles, reproduces prolifically and eventually kills the host by overwhelming it with sheer numbers. There is some concern that this parasite can spill over into the native invertebrates and add insult to injury to the local stream life. But on another hand, a new study shows that this parasite might be one of the few things holding back this voracious invasive amphipod from causing even more destruction.

A group of scientists from France conducted a study to looked at how C. dikerogammari affects the activity levels and appetite of D. villosus. They observed the behaviour of both infected and uninfected amhipods in a water-filled glass tube and noticed that amphipods at a late stage of infection that are visibly "filled to the brim" with parasite spores are actually more active than healthy amphipods or those that are not visibly parasitised because they are at a much earlier stage of the infection.

Close-up of a C. dikerogammari spore from here

Furthermore, they also presented amphipods with midge larvae (also known to some as "bloodworms") to see how many they ate. Both infected and uninfected D. villosus pounced on those insect larvae, but the heavily infected amphipods ate far less than the healthy ones. For whatever reason, this parasite seems to cause D. villosus to lose its appetite, and given this crustacean's reputation of eating everything that it can get its claws around, this may have significant ecological ramifications. It could mean that C. dikerogammari may be subtly reducing the impact these amphipods have on the areas where they have been introduced.

But why would heavily-infected D. villosus, which would have much of their muscle mass already converted to parasite spores by C. dikerogammari, be more active? Well, it could just be an odd manifestation of the disease, but if it is, it is certainly a useful one for this parasite - as it depends upon cannibalism for transmission to new hosts. Dikerogammarus villosus are rather homely creatures and usually prefer to stay under a shelter and wait for potential prey to wander by. By getting their host out and about, C. dikerogammari might increase the chances that its host will either run into one of its cannibalistic buddies, or die out in the open where it can be scavenged by other D. villosus.

It seems that for this little invasive amphipod, no matter how far you go, you can never really run away from your past (foes).

Reference:
Bacela-Spychalska, K., Rigaud, T., & Wattier, R. A. (2013). A co-invasive microsporidian parasite that reduces the predatory behaviour of its host Dikerogammarus villosus (Crustacea, Amphipoda). Parasitology 141: 254-258.