"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift
January 31, 2010
In 1993, almost 25% of the residents of Milwaukee, Wisconsin came down with severe stomach cramps, fever, and diarrhea. Over 100, mostly elderly or immunocompromised residents, died. The cause? The most common water-borne disease in the developed world: Cryptosporidium. Cryptosporidium parvum is one of many species of this group of apicomplexan parasites, distant relatives to those that cause malaria and toxoplasmosis. Water supplies may be tainted with the oocysts of these parasites, which are then consumed by people. In the small intestine, the parasites attach to the villi and begin to asexually divide. Eventually they will produce gametocytes - macrogametocytes are female, microgametocytes are male. These stages fuse and then produce two types of zygotes. Some have thin walls only - these serve to keep the infection going in the same host. Others, though, develop thicker walls and are released into the environment to infect new hosts. There isn't a very good treatment for those that become infected except for supportive therapy (fluids, etc) until the immune system can get it under control. The complete genome of C. parvum was completed in 2004 and it is unusual among apicomplexans in that neither its mitochondria nor its plastid organelles contain DNA.
January 30, 2010
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 29, 2010
This chytrid fungus (Chytriodiomycota: Chytridiales) is the causative agent of chytridiomycosis, an emerging infectious disease of amphibians. This fungus has been implicated as the cause of amphibian declines and extinctions of more than 250 species of frogs across six continents (the widespread distribution of this disease the likely consequence of anthropogenic effects). B. dendrobatidis can infect both larval and adult amphibians. Infections in larvae cause a reduction in grazing efficiency, food intake, and survival. Infections in adults cause thickening of the skin which might interfere with osmoregulation or ion balance. This fungus has two parts of its life cycle: one part in the host and one part outside of the host (a motile zoospore stage). Recent studies have shown that B. dendrobatidis can survive for long periods of time outside of the host, increasing its ability to drive host populations extinct. Although this fungus is believed to have originated in Africa, B. dendrobatidis was first reported in North and Central America and Australia in 1998, coinciding with massive amphibian declines. However, this fungus has been around since at least 1938 (documented in museum specimens), therefore researchers are trying to determine if B. dendrobatidis is truly a novel emerging disease or a long-term endemic pathogen (where population declines are the result of changes in pathogen virulence, host susceptibility, environmental change, or a combination of these factors). What kind of disease B. dentrobatidis actually is may help to determine how to stop the rampant spread of this devastating fungus, if possible. Findings currently point to B. dendrobatidis being a novel pathogen (laboratory experiments, wave-like declines of amphibians, etc), however much more work needs to be done to be sure.
Contributed by Jessica Light.
See this paper or this one (which just came out today) for more information.
January 28, 2010
Seems like everyone's been kind of gaga over vampires lately, so thought I would use a vampire as today's parasite. Vandellia cirrhosa is a relatively small catfish that lives in the murky waters of the Amazon river basin. It swims into the gills of a fish, slices open a wound with its teeth and engorges itself with fish blood. It then drops off, hides out, and waits until it's hungry again to go seek out a new victim. These little fish also have another sinister reputation - there has been one documented report of a candiru, as they are commonly called, mistaking the human urethra for a fish gill where it became lodged in the man's penis and had to be removed via surgery. Although fairly widespread in the folklore of the region, these invasions of man appear to be rare and their primary targets are indeed fish. My favorite quote about these parasitic fish was stumbled upon on a website devoted to aquarium lovers who keep catfish. Under "husbandry", the description says, "For obvious reasons this fish should be kept alone; smaller fish are not ignored by this fish and can be killed outright by one parasitic visitation."
January 27, 2010
Blame Borrelia burgdorferi for one of the most common vector-borne diseases in the United States: Lyme disease. Discovered in 1982 by NIAID zoologist Willy Burgdorfer, this species belongs to a phylum of corkscrew-shaped bacteria known as spirochetes. Spirochetes are quite at home in the guts of humans and other mammals, bivalves, and insects. In the United States, black-legged ticks (also known as deer ticks) ingest Borrelia burgdorferi from an infected animal during a blood meal. When the ticks bite other suitable hosts, they pass along the spirochete through their saliva.
Learn more about wildlife hosts of B. burgdorferi in this new Science Bulletins video from the American Museum of Natural History. A second video highlights new imaging techniques that can detect B. burgdorferi's influences on the human brain.
Contributed by Laura Allen at Science Bulletins, AMNH
Image by Jeffrey Nelson, North Park University
January 26, 2010
Cymothoa exigua is a parasitic isopod with a very odd and gruesome life cycle. Juveniles first attach to the gills of a fish and become males. As they mature, they become females, with mating likely occurring on the gills. The female then makes its way to the fish’s mouth where it uses its front claws to attach to the fish’s tongue. It begins to feed on the blood in the tongue until the tongue eventually atrophies completely. The isopod then takes the place of the tongue in the mouth, attaching to the floor of the mouth and the stub of what is left of the tongue with its hind pereopods. There it lives out the rest of its life – even letting the fish use it like its old tongue, holding prey against its teeth. Although these parasites were long thought to be restricted to the Gulf of California and environs, recently one turned up in the mouth of a fish that was caught off the coast of the U.K.
January 25, 2010
Leeches are a class of annelids that has many members that feed exclusively on blood and are often found ectoparasitic on their hosts. Branchellion torpedinis is a piscicolid leech that infects a wide range of elasmobranchs (sharks, rays and skates). The individual shown here came from The Georgia Aquarium collection, but they can be found on cownose rays and sawfish along the Atlantic coast. The frilly gills on the side help with respiration. Unlike many leeches, which glue a handful of cocoons containing eggs to the substrate, this species is a supreme egg layer and instead casts large quantities of eggs into the water, more like a monogenean parasite.
Contributed by Alistair Dove.
January 24, 2010
Let's say you are dissecting some sea cucumbers and you come across this thing (see photo) - what do you think it is? Is it a worm? No - In fact, it is a snail! You have just found Enteroxenos oestergreni - a species of parasitic gastropod that lives inside the body cavity of sea cucumbers. Evolution has done away with all its apparently superfluous organs like its digestive system, gills, heart, nervous system, and reducing it down to just the bare essentials of a parasite - reproductive organs. The only morphological clue that this organism is a gastropod are its larval stages (veligers), which look like tiny, delicate snails - just like that of many other marine snails. However, after it enters the sea cucumber host, it eventually transforms into the worm-like adult form. An adult E. oestergreni is essentially a long stringy sac of eggs, just floating in the coelomic fluid of its host.
See the original paper here.
Contribution by Tommy Leung.
January 23, 2010
Plants can be parasitic, too and some of the best known are in the genus Rafflesia. These really unusual plants don’t have stems or leaves or even proper roots. Instead, they have structures known as haustoria, which penetrate the cells of their hosts, Tetrastigma vines, and steal their nutrients and water. The only part of the plant that ever is visible is the flower – and it’s a doozy! The flowers of Rafflesia arnoldii can be almost a meter in diameter, making them the largest in the world. And if the size wasn’t enough to get it noticed, it also has a very distinctive smell – like rotting flesh - that attracts the insects that pollinate it. Despite these factors, Rafflesia is very hard to find. Part of this is because the flowers are very short-lived, but sadly the other reason is that they are restricted to forests of Borneo, Malaysia, and the Philippines, which are disappearing.
January 22, 2010
What do parents dread their children bringing home from school more than a bad report card? Pediculus humanus capitis, better known as head lice. These wingless insects, also called sucking lice, have parasitized humans for thousands of years and are now are common worldwide, infesting millions of school children every year. Head lice are entirely dependent on their hosts for their survival (there are no free-living stages), are found on the head and attach their eggs to the base of hair shafts. These parasites are surprisingly nimble, moving quickly among the hairs and can transfer quickly to a new host should the opportunity present itself. Head lice reproduce rapidly and treatment can be expensive and time consuming. Many common colloquialisms resulting from human parasitism of lice include “nit-picking”, “going over with a fine-toothed comb”, “nitwit”, and “lousy”.
Contribution by Jessica Light.
January 21, 2010
Heartworm, Dirofilaria immitis, is a filarial nematode parasite that usually infects dogs or other canines, but can also infect cats, foxes, ferrets, and sea lions. The adult worms live in the right ventricle of the heart, hence their common name. After the adults mate, they produce microscopic larvae known as microfilariae, which are ingested by mosquitos when they bite the mammalian host. The larvae complete their development in the mosquito and then are transmitted to a new host when she feeds again. Although most dogs do not show signs of infection, in some cases worms can cause issues and need to be treated. However the treatments are sometimes risky because the dead worms can get carried to the lungs and cause respiratory distress or failure. Preventative therapies, such as giving dogs the drug ivermectin, are effective and safe. Heartworm are also hosts to the endosymbiotic bacteria, Wolbachia (see January 12th).
Image from this website.
January 20, 2010
Neoechinorhynchus emyditoides is a species of acanthocephalan, or thorny-headed worm. These parasites often have very complex life cycles involving multiple trophic levels. The vertebrate host of this species is a turtle and, as the picture shows, a single turtle can have hundreds of worms – in some cases, more than 1000! - filling its intestine. The acanthocephalan eggs are expelled in the turtle’s feces and are eaten by ostracods, tiny crustaceans, where they develop into a stage called an acanthella. When fish eat the ostracods, the acanthella travel to the fish’s liver and await the fish’s ingestion by a turtle. There are 10 species in this genus and they are extremely difficult to tell apart. This photo likely contains a mix of both N. emyditoides and Neoechinorhynchus pseudemydis. The host in this case was a red-eared slider (Trachemys scripta) collected from Reelfoot Lake, Tennessee.
Nomination and photo by Mike Barger.
January 19, 2010
Trypanosoma brucei is the causative agent of sleeping sickness, a disease found in sub-Saharan Africa. Trypanosomes are flagellated unicellular eukaryotes that belong to the kinetoplastids, named for their distinctive organelles called kinetoplasts. T. brucei is vectored by tsetse flies (Glossina), but can also be spread by blood transfer. Trypanosomes are masters of immune evasion due to their ability to rapidly change their coats – their variable surface glycoprotein (VSG) coats, that is. By changing the proteins on the surface of their cells, they are able to thwart the antibodies of the specific immune system from recognizing them again. Sleeping sickness is a very serious disease which progresses from infection of the blood and lymph nodes (producing characteristic swollen lymph nodes known as Winterbottom’s sign), to eventually infecting other tissues including the nervous system. At this point, in addition to confusion and loss of coordination, the sleep cycle may be disrupted, giving the disease its name.
January 18, 2010
Cataloguing biodiversity is an important task – but so is understanding what causes it. Recently, scientists working on the parasitoid wasp, Diachasma alloeum, showed that speciation in the hosts has resulted in the formation of incipient species of the wasp. The hosts in this case are parasites themselves – apple maggots in the genus Rhagoletis. The D. alloeum parasitoid wasps use volatile compounds that the fruits emit to locate them and then they probe the fruit to find the maggot fly larvae within. The female wasp lays her eggs into the maggots, which eventually crawl out of the fruits when they fall to the ground. When the maggot pupates, the wasp larva consumes the maggot, pupates and then overwinters to wait for the next season’s fruits. Recently, the maggot flies have switched from hawthorns as their fruits of choice to domesticated apple trees. Genetic tests of the wasps that use maggot flies in apples vs. those that parasitize the hawthorn maggot flies (as well as in other recently diverged host races) shows that the wasps are following the speciation events of the flies and even show behavioral preferences for the volatiles of the new fruits. This process has been termed a speciation cascade.
Read more about this here.
Parasite nomination and photo by Andrew Forbes.
January 17, 2010
In 2006, a caver in upstate New York came across bats with an unusual white substance on their muzzles. By 2008, white nose syndrome (WNS), as it was now known, was reported in caves from Vermont to Virginia and thousands of bats were dying, with mortality rates of over 90% in some caves. The fungus growing on the bats has been indentified as Geomyces destructans, related to other soil fungi that are psychrophilic (=like cold temperatures). Bats with WNS often wake up from hibernation and begin to fly around, searching for food, as their fat reserves are low. This has caused a great deal of alarm as some endangered species of bats are at risk of extinction should they be exposed to the disease. But another worry are the trophic effects to the ecosystem if bats, potent insectivores, disappear in large numbers. One estimate, by the US Forest Service, is that there will be an extra 2.4 million pounds of bugs per year without the bats that have been killed by WNS. That will mean more crop pests and more mosquitoes and other disease vectors.
Read more about Geomyces destructans and white nose syndrome here and here.
January 16, 2010
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 15, 2010
Curtuteria australis Allison 1979 (Platyhelimthes: Digenea: Echinostomatidae) infects bivalves (in this case, a cockle, a type of clam) on the mudflats and sandflats of New Zealand. This parasite lodges itself in the foot of the bivalves where it forms a hard cyst. As more and more parasites accumulate, the cockle loses its ability to dig itself into the underlying sediment, leaving it stranded on the surface of the mudflat/sandflat. There, it is exposed to predation by shorebirds such as oystercatchers, which are the parasites' next hosts. These parasites also have a cascading effect on the rest of the ecosystem - as the mudflat is filled with stranded bivalves, the nature of the substrate changes from one consisting largely of mud and sand, to one littered with the hard shells of bivalves. This, in turn, alters the biotic community which inhabits the rest of the ecosystem.
The photo shows a cockle's foot with encysted metacercariae of Curtuteria australis tagged with a fluorescent dye.
Contributed by Tommy Leung.
Read the full paper here.
January 14, 2010
Parasites aren't all squirmy worms or microscopic organisms. Sometimes they are devious birds. The common cuckoo, Cuculus canorus, is the classic example of a brood parasite - an organism that lets another raise its offspring. Female cuckoos lay their eggs in the nests of other birds, tossing out one of the host's eggs (because birds are very good at counting!). The young cuckoos often hatch first, grow larger, and will kick out the baby host birds. Cuckoos have even evolved the capacity to mimic the eggs of most of their hosts.
January 13, 2010
Tapeworms are one of the main groups of parasitic flatworms, which also include the flukes (e.g. Schistosoma mansoni) and the 'monogeneans' (freshwater and marine fish parasites). Species of Hymenolepis, as shown here, are common parasites of rodents and use flour beetles as intermediate hosts. These species are thus easily maintained in the laboratory and have served as models for understanding tapeworm biology since the 1950s. Ongoing efforts to sequence the complete genome of this animal (see this link) will bring this classical model into the age of genomic research and will thus greatly increase the speed with which investigators can elucidate the genetic mechanisms that underly tapeworm biology.
Contributed by Pete Olson
January 12, 2010
Talk about manipulative little buggers - Wolbachia pipientis are alpha proteobacteria that infect a very wide variety of insects and other arthropods. Like mitochondria, they are vertically inherited from mothers to offspring. But, they get very cranky if they don't get their way. In some cases, they kill off infected males, in some they feminize males, in some they allow females to reproduce parthenogenically, and then there are several documented cases of more complicated interactions in the form of cytoplasmic incompatibilities. Some are now investigating their potential use in biocontrol of malaria vectors and other insect pests, taking advantage of these manipulative tendencies.
Thanks to Mike Charleston for this nomination.
January 11, 2010
Humans are not the only animals that get malaria. In fact, there are over 100 species of Plasmodium that have been described from lizards worldwide. This is one of the newest species, Plasmodium minuoviride, reported last year by Perkins and Austin (link to paper). Its Latin name means "to draw green blood" - so given to this parasite, because the host in which it lives, the skink Prasinohaema prehensicauda, has green blood due to the presence of biliverdin in it.
January 10, 2010
Many people have heard of grave concerns about the loss of honey bees, key pollinators of our crops and other important plants. One of the reasons for their trouble is the ectoparasitic mite, Varroa destructor. These tiny little arachnids climb onto bees and suck out their hemolymph - insect blood - and can also transmit dangerous viruses from bee to bee.
Blame it on the cat. Or the raw meat. Exposure to either can result in infection with Toxoplasma gondii. Distant relatives of Plasmodium, the parasites that cause malaria, T. gondii often doesn't show many symptoms in its host. Immunocompromised patients and fetuses of pregnant women who become infected can be at high risk from more serious complications. Recently, data have shown that people infected with T. gondii show behavioral changes, too - with differences between sexes. Men with T. gondii are more aggressive, while women seem more intelligent! Another case of parasite manipulation?
A cousin to the parasites that cause the disease sometimes affectionately referred to as "beaver fever" in humans, is Giardia muris. Unlike the parasite that plagues people, this species cannot infect humans, but rather only mice and other rodents. It has thus been developed as a laboratory model for studying the parasite of humans. These parasites and their relatives, the diplomonads are also cool in that they have two nuclei.
Image from the CDC Public Health Image Library.
A great example of a crustacean that parasitizes another crustacean is the barnacle, Sacculina carcini, which is a parasite of crabs. These parasites are favorites of professors as they represent a great example of host manipulation. Sacculina mimics the broods of female crabs, causing her to groom the parasite sac and help the eggs disperse into the water. And if the Sacculina finds itself in a male crab - it just sterilizes it and causes it to act like a female!
Ick! I guess that's a common thing people might say about parasites. Turns out that there's one called that, though it's usually spelled "Ich", short for Ichthyophthirius multifiliis. These parasites are ciliates, and they cause skin lesions and other issues in fish and are a bane of tropical fish hobbyists and aquaculturists alike.
If you liked the movie Alien, then you might be a fan of parasitic wasps, such as Peristenus digoneutis. These insects, sometimes incredibly tiny, capture and kill other insects (in this case little plant bugs) that they then lay their eggs into. The larvae hatch out and eat their hosts from the inside out. Although it's rather gruesome, these parasitoids have been developed as biocontrol agents to combat insect pests.
Did you overindulge at the holidays? Gym's been too crowded? Well, there might be another solution: infect yourself with the beef tapeworm, Taenia saginata! These cestodes can be 5 meters in length and were, in fact, marketed as weight-loss tools...though it's not recommended as they can cause malnutrition and other health problems that offset the removal of holiday pounds. Humans can become naturally infected when eating undercooked beef if it comes from unsanitary conditions.
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?
Ever feel like you're just not quite in control? Try being a snail infected with Leucochloridium paradoxum Carus 1835 (Platyhelminthes: Strigeidida). These trematode parasites alternate between snails, their intermediate hosts, and birds, the definitive hosts. The parasites invade the tentacles of the snails, causing them to look like caterpillars (see photo, which shows a normal and an invaded tentacle), which makes them attractive to birds.
Plasmodium falciparum Welch 1897 (Apicomplexa: Haemosporida) is one of the handful of species of malaria parasites that infect humans as their hosts. These single-celled eukaryotes are transmitted by Anopheles mosquitos. Of all of the species that infects humans, P. falciparum is the most deadly. It is thought that these parasites might kill close to two million people a year, primarily in sub-Saharan Africa. The genome of this species was completely sequenced in 2002.