Rocinela sp.

The bonefish is a popular recreational species for catch-and-release fishing. It is targeted by anglers using fly rods or light tackle, and requires some skills to do so as they're easily startled, and once hooked can put up quite a struggle. But if you are wading on a beach while fly fishing for bonefish, you might in turn become the target, because one of the bonefish's parasites may have its eyes on you too.

Left: Rocinela isopod feeding on a bonefish just above its right eye, Centre: Rocinela isopods on bonefish at the base of the dorsal fin and left flank, Right: Rocinela isopod dorsal view.
Photos from Figure 2 of the paper.

This blog has previously featured Cymothoidae isopods, which tend to be somewhat picky about what types of fish they parasitised But the isopod being featured in today's post isn't picky at all, in fact,  when it comes to its next meal, and it doesn't always have to be a fish. Rocinela is a genus of isopods that belongs to the Aegidae family, and unlike the cymothoids which tend to stay on their hosts for extended periods of time, these isopods are temporary blood feeders, rather like land-dwelling leeches or bed bugs. On rare occasions, they can even feed on human blood. But adopting this kind of free-wheeling blood-sucking can open yourself up to becoming an unwitting carrier of many microscopic passengers.

The study we're looking at in this post investigated the health and microbes of bonefish at Belize. The scientists in this study captured bonefish around Ambergris Caye, and examined each fish for scars and ectoparasites (such as Rocinela), then collected some blood samples for genetic analyses. The scientists also analysed the blood present in the gut of the isopods they collected, to identify what kind of fish they had been feeding on. Genetic analyses of blood-suckers' meals have previously provided valuable insights into the hosts of ectoparasites.

Two of the three sites the scientists sampled from were frequented by Rocinela, and about 70 percent of the isopods they found on the bonefish had plump bellies that were full of blood. As expected, most of the isopods were filled with bonefish blood, but one of the Rocinela also had blood from a type of small killifish called the mangrove rivulus, and somewhat alarmingly, there was an isopod in the sample which had fed on human blood at some point.

What's even more interesting were the plethora of virus sequences that were found. Possibly because of its indiscriminate feeding habits, Rocinela has inadvertently picked up about 11 different types of viruses. Most of those were viruses that usually infect arthropods. One of them, XKRV-2, is related to a group of viruses which have been previously reported from a range of crustaceans, including parasitic isopods, so its presence was to be expected. 

But one of the Rocinela also carried a less expected virus called XKRV-1, which is more related to a common genus of fish virus called Aquareovirus. None of the bonefish sampled had XKRV-1 in their blood, which means Rocinela has picked up the virus from one of other fish species that it had fed on. And rather than just being a transient, XKRV-1 has been persisting in the isopod's belly for a while - which is a common adaptation for vector-borne viruses such as those found in ticks and mosquitoes.

Given Rocinela can feed from a variety of fish, its payload of viruses may disembark into one of its hosts during feeding, so it could be transferring viruses between different species at sea. While Rocinela is also known to feed on humans, the likelihood of those fish viruses jumping into us is comparatively low - viruses that jump into humans tend to come from mammals and other warm-blooded animals, especially those that are evolutionarily closer to us, such as non-human primates. But a much bigger concern is that since Rocinela harbours so many different viruses and it is so indiscriminate about the type of hosts that it feeds on, it might end up acting like a transmission hub for viruses to jump from wild fish into aquaculture species.

Most studies looking at vector-transmitted diseases focus on land-dwelling arthropods such as ticks, fleas, and mosquitoes, but crustaceans like Rocinela and other parasitic isopods might be overlooked vectors that are providing a taxi service for pathogens under the waves.

Reference:

Goldberg, T. L., Perez, A. U., & Campbell, L. J. (2024). Isopods infesting Atlantic bonefish (Albula vulpes) host novel viruses, including reoviruses related to global pathogens, and opportunistically feed on humans. Parasitology 151:1386–1396.

Echinophthirius horridus

Lice are common parasites of mammals. Humans alone are host to three different species of lice, and it's not just humans or land mammals that can get infected with lice. Pinnipeds - seals and sea lions - also have to contend with being covered in those ectoparasites. Unlike many other ectoparasites in the sea which have been bestowed with the name of "lice" such as salmon lice, tongue-biter lice, or whale lice (all of which are crustaceans), seal lice are true lice, in that they are parasitic insects belonging to the order called Phthiraptera.

Left: An adult seal louse, Right: two opened seal lice eggs (nits) glued to a strand of seal fur hair
From Fig. 1 of the paper

When the ancestors of modern pinnipeds took to the sea some time in the Oligocene about 30 million years ago, the lice followed them into the water, and in the process, they have to deal with all the challenges associated with living on an aquatic host. Seal lice belong to a family of lice called the Echinophthiridae and they have some specialised adaptations for living on hosts that spend most of their time immersed in sea water. This include elongated spiracles (the opening insects use to breathe) with mechanism for closing, a dense covering of spines and scales, and stout clamp-like claws that allow them to grip tightly onto their hosts' fur.

Blood-sucking arthropods such as ticks, fleas, and lice are often responsible for transmitting a wide variety of parasites and pathogens. And it seems that seal lice can also play a similar role in the sea. While performing routine diagnostics on 54 harbour seals and a very heavily-infected grey seal pup that were hospitalised at the Sealcentre Piteterburn (a seal rehabilitation and research centre in Netherlands), a group of scientists were able to use that opportunity to collect a massive number of seal lice from those marine mammals. They ended up collecting 200 lice from the harbour seals, and another 1000 from that one very heavily infested seal pup.

Those researchers divided the lice into batches of 1-20 lice, based on the individual host that they came from (the lice from the heavily infected seal pups were divided into multiple batches of 15 lice), then grind them up, and examined the lice slurry by subjecting it to polymerase chain reactions that amplified the DNA of known seal parasites and pathogens.

The DNA analyses showed that the seal heartworm (Acanthocheilonema spirocauda) was the most commonly found parasite, with it being detected in about one-third of the lice samples. While most people would associate "heartworm" with the dog heartworm (Dirofilaria immitis), that species is just one out of many different filarial roundworms that live in the heart of mammals. The findings of this study corroborates with previously published research which have found heartworm larvae dwelling in the gut of seal lice, demonstrating that these ectoparasitic insects play a key role in the transmission and life cycle of these nematodes.

Alongside the heartworm, there were also some bacterial pathogens lurking in those lice. Some of the lice from the grey seal pup were also carrying Anaplasma phagocytophilum, the bacteria which causes tick-borne fever and as their name indicates, are usually carried by ticks. Additionally, a few of the lice from that seal pup and some of the harbour seals were also carrying a species of Mycoplasma bacteria. This microbe is commonly found in seals and other marine mammals, but when it gets transmitted to humans, it is also associated with a disease known as "seal fingers". However, unlike the heartworm, it is unclear if the lice actually play a role in the transmission of these bacterial pathogens, or if they were incidental infections that simply came with living on a seal host.

It is worth noting that while pinnipeds had retained an heirloom of their terrestrial ancestry in the form of lice, another group of marine mammals - the whales - have acquired their own unique suite of ectoparasites which are unlike that of any other mammals. They have "whale lice" which are actually crustaceans in the same group as sandhoppers, along with pennellid copepods - a family of parasitic copepods that usually infect fish, with the exception of one species which has evolved to parasitise whales.

So why are there no "true lice" on whales? Well, for all their adeptness at clinging to their host, lice ultimately depend on the presence of hair or similar structures to hang onto their host. When a seal dives underwater, the layer of fur forms a covering that the lice can shelter underneath. But no such shelter exists on the smooth, hair-free surface of a whale. As a result, while whales have escaped the lice (and have picked up other parasites in the process), pinnipeds have kept their fur, and along with it, their lice and the worms that they carry.

Reference:

Hirzmann, J., Ebmer, D., Sánchez-Contreras, G. J., Rubio-García, A., Magdowski, G., Gärtner, U., Taubert, A & Hermosilla, C. (2021). The seal louse (Echinophthirius horridus) in the Dutch Wadden Sea: investigation of vector-borne pathogens. Parasites & Vectors 14: 96.

Plasmodium relictum (revisited)

This is the third post in a series of blog posts written by students from my third year Evolutionary Parasitology unit (ZOOL329/529) class of 2013. This particular post was written by David Rex Mitchell on a paper published just this year on how an avian malaria parasite might make its bird host more attractive to mosquitoes which are the parasite's vector (you can read a previous post about toxic birds and their lice here and a post about bees protecting themselves against fungal parasites by lining their hives with resin here).

Photo of Culex pipiens
by Joaquim Alves Gaspar

One of the aspects of parasites that people tend to find a little more disturbing is the idea that they can control the minds of other animals. Although this may seem like the stuff of science fiction, this is indeed sometimes the case. For those parasites that live inside other animals, there are often several stages to their lives and each of these stages may require the use of a different type of animal. This presents a challenge in getting from one animal to the next and so if a parasite can influence the behaviour of one animal in some way, making it easier to reach the next, this is incredibly advantageous.

Many parasites have evolved abilities to do just this. For example, some blood-sucking insects infected with certain parasites are known to bite more frequently than when uninfected, helping to spread the disease to more animals. This is seen in malaria-infected mosquitoes, tsetse flies infected with sleeping sickness, and plague-infected fleas. But is it possible that a parasite can also influence a healthy, uninfected animal’s behaviour? The paper featured today attempts to address this question. Researchers used a species of avian malaria (Plasmodium relictum - a parasite that has been previously covered on the blog by this post here) and its natural mosquito carrier (Culex pipiens) to find out if malaria-infected animals are more attractive to mosquitoes than healthy, uninfected animals. This species of malaria is spread among birds via its mosquito carriers and thus the researchers chose canaries to carry out the experiment.

Photo of canaries by 3268zauber

Pairs of canaries, one infected with the parasite and one uninfected, were exposed to uninfected mosquitoes to see which bird they would prefer to feed on. The mosquitoes mostly fed on only one animal per sitting, so the blood inside their bellies could be removed and the DNA analysed to determine which bird it fed upon. The experiment was carried out on the day the birds were injected with the parasite, as well as 10 days and 24 days after injection, so as to monitor any changes as the parasites matured inside them.

From this experiment the researchers discovered that, not only did the mosquitoes clearly prefer to feast on the malaria-infected canaries, but also this behaviour became more prominent as the malaria parasites mature within the canary and become capable of crossing into a mosquito. The researchers suggest that the malaria parasite influences the mosquito’s decision to feed on the infected animal, assisting its transfer to said mosquito – the next stage in its life-cycle. The mechanism used to achieve this has not yet been determined but the researchers suggest that the parasite may alter the odours that are emitted from the host animal, enticing the mosquitoes to choose its infected animal over other uninfected animals. If these odours can be identified and reproduced, they may prove very useful in control of malaria in the future, for example in mosquito traps.

So is this an example of crazy sci-fi mind-controlling by parasites? Ok, so mosquitoes may not exactly be renowned for their calculated decision making skills. But the results of this experiment were still able to show us how the malaria parasite can influence a healthy mosquito’s decisions, offering further insight into the awesome manipulative powers of parasites.

Reference
Cornet S, Nicot A, Rivero A, & Gandon S (2013) Malaria infection increases bird attractiveness to uninfected mosquitoes. Ecology Letters 16: 323 – 329.

This post was written by David Rex Mitchell

October 13 - Nanophyetus salminicola

West of the Cascade Range in the Pacific Northwest, it is an extremely poor idea to feed fish scraps to your dog. This tiny intestinal fluke is the reason. The parasite itself is relatively harmless but it can act as a vector for a rickettsial illness caused by Neorickettsia helminthoeca (“helminth,” as we all should know by now, is Greek for “worm”). When a canid is infected by eating uncooked salmon, the rickettsia can cause symptoms such as vomiting, diarrhea, and dehydration, which can be fatal in as many as 90% of cases if untreated. For unknown reasons, cats, raccoons and bears are not susceptible to the illness.

Contributed by Mike Kinsella.

September 22 - Ambylomma americanum

Yeeee haw! Today's parasite is the Lone Star Tick, Ambylomma americanum, though I must confess that although it is sometimes found in Texas, its name comes from the spot on the back of the females of the species. These ticks primarily feed on cattle or deer as adults. This species of tick is not thought to vector Lyme Disease, but they do transmit other parasites to humans, such as granulocytic ehrlichiosis (Ehrlichia ewingii), tularemia (Francisella tularensis), and a close relative of the Lyme disease bacterium, Borrelia lonestari, which produces Southern tick-associated rash illness (STARI). A. americanum also holds the distinction of being the first species of tick that was described from the United States - back before we were the United States - in 1754.

August 2 - Gnathia trimaculata

TThe parasite for today is a parasitic isopod belonging to the family Gnathiidae - the larvae of this particular species feed upon the requiem shark (Carcharinus melanopterus). There are many different species of gnathiids parasitising many different species of fish, and they have an interesting life-cycle which involve "protelian parasitism" where only the juvenile stages (called a praniza) are parasitic, while the adult stages are free-living. They go through several stages of development, alternating between feeding and non-feeding developing stages (when they are engorged with blood) before reaching sexual maturity.

They are almost like a functional equivalent of ticks for fishes - they wait in ambush for a passing host, and when one arrives, it climbs onboard, sucks blood for a few days until full, then drops off to develop into the next stage. And like ticks, they can also act as vectors which can transmit blood parasites between the fishes they feed upon.

The photo shows a pair of third-stage pranizae, scale bar is 1 mm and it came from this paper:

Coetzee, M.L., Smit, N.J., Grutter, A.S., Davies, A.J. (2009) Gnathia trimaculata n. sp. (Crustacea: Isopoda: Gnathiidae), an ectoparasite found parasitising requiem sharks from off Lizard Island, Great Barrier Reef, Australia. Systematic Parasitology 79:97-112


Contributed by Tommy Leung.

July 2 - Argas persicus

Argas persicus is a species of soft-bodied tick (Argasidae) that is common on chickens and other poultry. They like to hide in the wooden parts of the coops and then crawl onto the birds to feed. They can vector a species of bacteria called Borrelia anserina, which produces avian spirochetosis. Soft-bodied ticks aren't really so soft - they just are called that since they look rather shriveled until they fill themselves up with blood.

The image comes from this site.

May 1 - Ambylomma variegatum

Yesterday you saw the bacteria that are responsible for Q fever – Coxiella burnetii and learned that they can be transmitted by ticks. This tick, Ambylomma variegatum, is one of those ticks – and what a handsome tick that it is (well, at least the males)! These ticks are present in sub-Saharan Africa, but they have been introduced to several islands in the Caribbean as well. They have a wide range of hosts that they take bloodmeals from – as larvae and nymphs they feed on birds, reptiles, sheep or goats and as adults they like cattle, horses, camels, and some antelope as well. These ticks vector not only Q fever but also heartwater and African tick-bite fever (we’ll meet these later), and on top of that, they also have a really nasty bite due to their very long mouthparts. These bites can be painful and sometimes are also sources for secondary infections of both bacteria and screwworms. So, they might be good-looking, but they’re pretty nasty little arachnids.

Photo of male (left) and female (right) A. variegatum ticks, from this recent paper on Q fever in Senegal.

March 13 – Ixodes scapularis

Ixodes scapularis, the blacklegged or deer tick, is a hard-bodied (ixodid) tick that is common across the eastern and Midwestern U.S. These ticks have become recognized as important vectors of several emerging diseases, most notably Lyme Disease, but also Babesia and Anaplasma. Like many other ticks (see Dermacentor variabilis, for example), the life cycle of I. scapularis involves three stages: a larva, a nymph, and an adult. Larval and nymphal I. scapularis often take their blood meals from deermice, but the adults favor larger mammals, particularly white-tailed deer. Humans (and domestic animals) can become infected when they are accidental hosts to nymphal or adult ticks that became infected when they acquired the bacterial or protozoan parasites from their first or second hosts.

February 11 - Pediculus humanus humanus

In honor of "Fashion Week" in New York this week, six of the posts this coming week will involve parasites that relate to fashion in some way.

Today's: Body lice - I thought it would be fitting to start with these little creatures, which are a little "bonus" that we acquired when humans began to wear clothes.

Body lice, Pediculus humanus humanus (also called clothing lice) are believed to have evolved from head lice, likely invading the body region only recently with the advent of clothing use in modern humans. These wingless insects are found on the body and in clothing and prefer to attach their eggs to clothing rather than body hair. Compared to head lice, body lice are less prevalent parasites, associated mainly with those living in poor conditions in colder climates. Body lice are, however, potentially more harmful because they are known vectors of at least three bacterial pathogens in humans: Rickettsia prowazekii (epidemic or louse-borne typhus), Borrelia recurrentis (louse-borne relapsing fever) and Bartonella quintana (trench fever). Body lice, and the diseases they carry, can be transmitted fairly easily and quickly, especially in crowded situations such as war. In fact, body lice are known to have been prevalent amongst soldiers in Napoleon’s Grand Army and it’s possible that the diseases they carried may have played a role in the French retreat from Russia.

Contributed by Jessica Light.
Image from the CDC Public Health Image Library.

January 30 - Dermacentor variabilis

Ticks are small, blood-feeding arachnids, relatives of spiders and mites. The American dog tick, Dermacentor variabilis has three life stages: larvae (which are 6-legged not 8!), nymphs, and adults. As the ticks pass from each life stage to the next, they require a bloodmeal from a vertebrate. Larvae and nymphs will usually obtain this bloodmeal from rodents, but adults, as their name suggests will seek out larger mammals, particularly dogs, and sometimes humans. These ticks find their hosts using a behavior known as “questing”. They crawl to the top of a blade of grass or other variation and extend their front two legs until a host brushes past, at which point they grab on. The life cycle can take two years to complete, with the ticks overwintering in the soil. D. variabilis is another example of an ectoparasite that can vector other diseases, in this case, Rocky Mountain Spotted fever, tularemia, and ehrlichiosis, which are all bacterial zoonotic diseases.

Image from Insect Images (but remember that ticks are not insects!)

January 16 - Oeciacus vicarious

The swallow bug (Hemiptera: Cimicidae) primarily feeds on the blood of nestling and adult cliff swallows (Petrochelidon pyrrhonota), which are colonial nesters found throughout North America. The bugs are long-lived and remain on the swallows’ mud nests throughout the year, even when the birds have migrated south. Swallow bug infestation can reach up to 2,500 individuals in a single nest and are known to vector Buggy Creek virus (Togaviridae, Alphavirus), which is an arbovirus hosted by birds. This ectoparasite is related to the common bedbug (Cimex lectularius), which is typically found in human environments. Swallow bugs are known to also bite humans when handled.

Contributed by Sarah Knutie.
Photo by Chris Kulhanek.

January 3 - Xenopsylla cheopis

As the quote on the top of the pages says, big fleas have little fleas. Well, the Oriental rat flea, Xenopsylla cheopis Rothschild 1903 (Arthropoda: Siphonaptera) isn't exactly crawling with other fleas - but it is often an important vector for other parasites that infect it. Not only does this jumping insect parasitize rats and other mammals by feeding on their blood, it can also be infected by Yersinia pestis, the bacteria that cause the bubonic plague and serve as the intermediate host for certain tapeworms. Doesn't that make you itch just thinking about it?