Tag Archives: parasite

Malaria, Climate Change, and the Next Top Model

by Joseph DeSisto

Both malaria and climate change are complex global problems that scientists are working hard to understand. Malaria, a mosquito-borne disease, kills roughly 600,000 people every year, mostly in Africa. Attempts to control malaria have been massive and relatively well-funded, but the disease continues to pose a serious threat to human life in the tropics. Climate change has no body count (yet), but will dramatically change our planet, altering weather patterns, sea levels, and human life.

These two problem have another thing in common: scientists use mathematical models to try and predict how they will change in the future. For example, climate scientists use past and current data on temperature, carbon emissions, and many other factors, to predict how the earth’s climate will change. Most models anticipate a global temperature increase of 2̊ to 6̊  by the year 2100 (e.g., Paaijmans et al. 2014). Those predictions may not be perfectly accurate, but that is the nature of predictions, and in the absence of time travel, it’s the best we can do.


Anopheles minimus, a malaria-transmitting mosquito. Photo by James Gathany, in public domain.

Malaria, the microbes that cause it, and the mosquitos that carry it, are all affected by temperature. Scientists have tried developing models to predict how malaria infection rates will change as the world gets hotter. In general those predictions have been bleak: models tend to show an increase in the amount of people affected by malaria (e.g. Pascual et al. 2006, Paaijmans et al. 2014), on the basis that warmer temperatures are most hospitable to malaria-carrying mosquitos. They also show malaria spreading to areas that used to be too cool, like South Africa and the southeastern United States. Yet other models show a decrease in malaria on a global scale (Gething et al. 2010), and some suggest an increase in some areas but a decrease in others (Rogers and Randolph 2000).

Before looking forward, it’s important to look back – how has malaria changed in the last few years? The maps below show malaria infection rates (the percentage of people infected) across Africa, in 2000 (left) and 2010 (right). Darker colors mean more malaria.



Photo by Noor et al. (2014), licensed under CC BY 3.0.

The two maps may not look all that different, but look closely. The red arrow points to West Africa, where the darkest area has grown smaller. Malaria is still common there, but has declined. In Kenya and Tanzania (purple arrow), malaria has declined even more. Even though efforts to completely eradicate malaria have failed, improved mosquito control programs have been successful in reducing the threat of malaria in these regions of Africa.

A model is basically an equation, and in the case of malaria it serves to calculate R0: the basic reproduction number. The basic reproduction number is the answer to the question, for each person already afflicted with malaria, to how many other people will their infection spread? This gives us an idea of how many people are likely to contract malaria in a given year.

For example, if R0 is 15, that means every person with malaria is likely to pass it on to fifteen other people – that’s about the R0 value for measles. If R0 is 0.5, then on average, half of all malaria patients will pass their infection on to another. For all diseases, when R0 is less than 1.0, the disease will decline, and if R0 is greater, the disease will increase in the population. Very high values of R0 can lead to pandemic.

Malaria is a complicated disease, affected by all three players: humans, microbes (more on them later), and mosquitos. In turn, each of these players are affected by temperature, moisture, population density, and other local conditions. As a result, malaria’s basic reproductive number varies depending on where you are. In 2007, a group of scientists attempted to calculate R0 for 121 different human populations in Africa (Smith et al. 2007). The study had two important results.

First, R0 varied wildly across Africa. Although the average value was near 115, many populations had values below 10 and many more had values over 1,000. Second, some R0 were extremely high, approaching 10,000. Remember what this means – on average, each person with malaria has the potential to transmit their malaria to 10,000 other people!

In some cases the R0 value was greater than the human population, suggesting that everyone in the population had malaria when of course this was not the case. Is there a problem with the model? The scientists in question didn’t think so. Instead they pointed out that although malaria’s R0 is much higher than for most other diseases, malaria also takes much more time to spread (on average, 200 days per generation) because of its complex life cycle.

The parasites that cause malaria are single-celled, apparently simple, but with strange and complicated lives. I say parasites, plural, because there are at least five species that cause malaria, but all are protozoans in the genus Plasmodium. The cycle, of course, has no “beginning,” but we will start with the oocyst, a kind of “egg sac” containing multiple Plasmodium cells and surrounded by a protective membrane.


A Plasmodium sporozoite (purple). Image by Ute Frevert; false color by Margaret Shear, licensed under CC BY 2.5.

The oocyst lives in the body of a female Anopheles mosquito, and by the time the mosquito lands for a blood meal, the membrane bursts. Single-celled Plasmodium parasites surge forth, down the mosquito’s mouthparts, and into the blood-stream of an unsuspecting human. The freed cells are called sporozoites; they are worm-like and active, and waste no time snaking their way through the host’s body until they reach the liver.

Here the parasites eat, grow, and change shape. They divide and transform into masses of globular cells, similar to oocysts, once again enveloped in a bag-like protective layer. As time goes on, liver becomes tiresome, and the parasites crave blood. The bag of cells bursts, and the Plasmodium cells return to the blood stream. Now they have a new mission: find a red blood cell.

Red blood cells are hollow and doughnut-shaped, like inflatable inner tubes. You use them to transport oxygen and carbon dioxide into and out of your body, with every breath. The blood cell’s membrane is thin, almost fluid, and easy for a Plasmodium cell to invade. Once inside, the parasite has two options. It can grow and divide, forming a new mass of cells (like the oocyst). If it does so, these cells will ultimately break out of their shelter to find new red blood cells of their own. More ambitious parasites, however, refrain from dividing. Instead they metamorphose, transforming into either male or female cells.

For the Plasmodium life cycle to complete, a second mosquito is required. The new mosquito lands on a malarial host, sucking up blood and with it, lots of red blood cells. Some of these are empty, but others, if Plasmodium is lucky, contain either male or female hitchhikers. In the mosquito’s digestive system, red blood cells burst open to release their passengers. The freed Plasmodium cells, male and female, meet and unite. Once they do, they are able to multiply and grow into a multi-celled oocyte, full of wriggling sporozoites ready to enter a new human host at the mosquito’s next meal.


A mosquito in the genus Anopheles, capable of transmitting malaria. Photo by Jim Gathany, in public domain.

For this system to work, mosquitos not only have to be infected with Plasmodium, but sporozoites have to be fully developed and ready to pounce when opportunity (i.e., a bite) comes along. Lots of mosquitos need to be biting people, so that at least some will slurp up the male and female cells when they are ready. Mosquitos need to have reasonable lifespans – those that meet their end in a frog’s belly or a spider’s web are of no use to Plasmodium. The mosquitos, in turn, have requirements of their own: they need optimal growing temperatures, pools of water in which to lay their eggs, and plenty of warm-bodied hosts from which to drink.

All of these factors (and many more) are variables that might appear in an equation to calculate R0. Smith and colleagues (2007) used them to calculate the R0 for all those 121 African populations. More recently, a group of scientists and mathematicians got together to predict how R0 will change with the earth’s climate (Ryan et al. 2015).

In general, both mosquitos and Plasmodium can only develop at temperatures between 63̊ and 93̊ F. That’s good, useful information, but not enough — each of Smith’s variables (mosquito life span, number of bites per person, etc.) is affected by temperature, but each in a slightly different way. Only by combining all of these factors, and considering how each will change under future climate conditions, can we accurately predict how the threat of malaria will change over time. That’s what Ryan and colleagues did, and published this month in Vector-Borne and Zoonotic Diseases.

It turns out that if you combine all those variables, you get a much more complex picture of how climate change and malaria interact. Although some areas will get warmer and more suitable for malaria, others will actually get too hot, so malaria will decline.

This particular study shows a decrease in malaria in West Africa, but an increase in East Africa. In other words, the hot-spot for malaria will shift east over the next six decades.


From Ryan et al. (2015), licensed under CC BY 4.0.

In the maps above, purple indicates the greatest increase in malaria transmission rates, while the palest tone indicates a decline in malaria.

Caution is important. When different models produce vastly different results, it usually means that some of those models are better than others. This model happens to consider more factors than many others, which suggest it may be more accurate, but as I have tried to show, malaria is a complex disease that will be affected by temperature in complex, hard-to-predict ways.

Having an idea of what the future might hold can inform us – where should we concentrate efforts to control malaria? Where will malaria pose the greatest threat to human health? This latest model suggests our focus will have to shift as Plasmodium, mosquitos, and malaria follow their optimal temperatures in an eastward march across Africa.


Gething P.W., D.L. Smith, A.P. Patil, A.J. Tatem, R.W. Snow, and S.I. Hay 2010. Climate change and the global malaria recession. Nature 465(7296): 342–346.

Noor A.M., D.K. Kinyoki, C.W. Mundia, C.W. Kabaria, J.W. Mutua, V.A. Alegana, I.S. Fall, and R.W. Snow. 2014. The changing risk of Plasmodium falciparum malaria infection in Africa: 2000-10: a spatial and temporal analysis of transmission intensity. The Lancet 383(9930): 1739-1747.

Paaijmans K.P., J.I. Blanford, R.G. Crane, M.E. Mann, L. Ning, K.V. Schreiber, and M.B. Thomas. 2014. Downscaling reveals diverse effects of anthropogenic climate warming on the potential for local environments to support malaria transmission. Climate Change 125: 479-488.

Pascual M., J.A. Ahumada, L.F. Chaves, X. Rodó, and M. Bouma. 2006. Malaria resurgence in the East African highlands: temperature trends revisited. Proceedings of the National Academy of Sciences U.S.A. 103(15): 5829–5834.

Rogers D.J. and S.E. Randolph. 2000. The global spread of malaria in a future, warmer world. Science 289: 1763–1766.

Ryan S.J., A. McNally, L.R. Johnson, E.A. Mordecai, T. Ben-Horin, K. Paaijmans, and K.D. Lafferty. 2015. Mapping physiological suitability limits for malaria in Africa under climate change. Vector-Borne and Zoonotic Diseases 15(12): ahead of print.

Smith D.L., F.E. McKenzie, R.W. Snow, and S.I. Hay. 2007. Revisiting the basic reproductive number for malaria and its implications for malaria control. PLOS Biology 5(3): e42. doi: 10.1371/journal.pbio.0050042


Two New Species of Ant-Decapitating Fly

by Joseph DeSisto

An ant-decapitating fly. Photo by Scott Bauer, in public domain.

An ant-decapitating fly. Photo by Scott Bauer, in public domain.

Today saw the description of two new species of South American flies, both fire ant parasites that decapitate their victims. The two new species were discovered in Brazil and Argentina, associated with fire ant mounds in their native territory (Plowes et al. 2015).

Ant-decapitating flies, as might be expected, have unusual and macabre life histories. The adults are tiny, just a few millimeters in length, with the general appearance of fruit flies. Instead of hovering around rotten bananas, however, female ant-decapitating flies hang around ant mounds. When the time is right, a fly soars down to meet her victim, using a hooked, needle-like ovipositor to inject an egg into the ant’s head (Porter 1998).

With egg laid, her work is done. The fly departs, ant still intact and seemingly healthy.

A fly attacking a fire ant, hoping to lay its egg on the ant's head. Photo by Sanford Porter, in public domain.

A fly attacking a fire ant, hoping to lay its egg on the ant’s head. Photo by Sanford Porter, in public domain.

Not all is well, however. From the fly’s egg emerges a maggot that, as it grows, eats away at the inside of the ant’s head. At the same time, the maggot secretes chemicals that cause the ant to go mad, fleeing its colony and finding shelter in moist leaf litter. With its host almost spent, the maggot severs the ant’s head, and forms a cocoon or pupa inside the now-hollow shell. Some weeks later, a new fly emerges and takes off in search of a new ant for her offspring.

This all sounds very sinister, but it also could be very useful to humans. As it happens, many of these flies specialize in decapitating fire ants and, in enough numbers, can seriously impact fire ant colonies. Now that invasive fire ants are well-established in the southern U.S., the Department of Agriculture is looking to ant-decapitating flies to control the ants’ march north.

The remains of a fly's victim. Photo by Sanford Porter, in public domain.

The remains of a fly’s victim. Photo by Sanford Porter, in public domain.

Whether the two new species of ant-decapitating fly will be useful in controlling fire ants remains to be seen. Multiple fly species have already been introduced in states like Texas (Gilbert and Patrock 2002) and Alabama (Porter et al. 2011), where fire ants are a serious problem for agriculture and human health. In these states the flies have become established and already have a tangible impact on fire ant populations.

However, not all species fare equally well. In order to be useful in controlling fire ants, the flies must be able to adapt to the southern U.S. climate, as well as all the new predators they may not have faced in South America.

Plowes and colleagues (2015) suggest that many more unknown species of ant-decapitators live in remote regions of South America. Discovering new species may help scientists figure out which flies will be most successful at colonizing the U.S., and which species will have the biggest impact on fire ant populations.


Gilbert L.E. and R.J.W. Patrock. 2002. Phorid flies for the biological suppression of imported fire ant in Texas: region specific challenges, recent advances and future prospects. Southwestern Entomologist Supplement 25: 7-17.

Plowes R.M., P.J. Folgarait, and L.E. Gilbert. 2015. Pseudacteon notocaudatus and Pseudacteon obtusitus (Diptera: Phoridae), two new species of fire ant parasitoids from South America. Zootaxa 4032(2): 215-220.

Porter S.D., L.F. Graham, S.J. Johnson, L.G. Thead, and J.A. Briano. 2011. The large decapitating fly Pseudacteon litoralis (Diptera: Phoridae): successfully established on fire ant populations in Alabama. Florida Entomologist 94(2): 208-213.

Porter S. D. 1998. Biology and behavior of Pseudacteon decapitating flies (Diptera: Phoridae) that parasitize Solenopsis fire ants (Hymenoptera: Formicidae). Florida Entomologist 81(3): 292-309.

Shrimp-like Amphipods found in Sea Anemones

by Joseph DeSisto

Amphipods are crustaceans, similar to tiny shrimp. There are around 10,000 known species, with more being discovered each year. Most amphipods are marine and live as scavengers or predators, swimming or scuttling after tiny particles of food or plankton. A few are predatory, and use mantis-like front legs to snap up smaller creatures.

Stenothoe marina, related to the newly described amphipods. Photo by Hans Hillewaert, licensed under CC BY-SA 4.0.

Stenothoe marina, related to the newly described amphipods. Photo by Hans Hillewaert, licensed under CC BY-SA 4.0.

Last week’s new species are even more bizarre: they are “associates” living on the bodies of larger animals, ranging from sea anemones to mussels to hermit crabs (Krapp-Schickel and Vader 2015). So what exactly are they doing there?

Anemone-dwelling amphipods spend their time clambering over the tentacles of their host. Sea anemone tentacles are covered in microscopic stingers, to which the amphipods (like clown fish) are immune. Being surrounded by venomous tentacles might protect the amphipods from larger predators.

In turn, amphipods probably scavenge bits of detritus, such as uneaten prey, off the body of the anemone. One amphipod, however, does things a little differently. Instead of scavenging, it lives as a parasite, feeding on the flesh of its sea anemone host (Moore et al. 1994). Regardless, all of these amphipods have grasping, hook-like front legs called gnathopods (silent G), which they use to climb over and within the bodies of other animals.

Sea anemones. Photo by Francois Guillon, licensed under CC BY-SA 4.0.

Sea anemones. Photo by Francois Guillon, licensed under CC BY-SA 4.0.

Mussel-associated amphipods are more peaceful, but not entirely welcome, guests. Mussels are filter-feeders that extract plankton from the water, so instead of eating the mussel, amphipods steal some of the daily plankton catch (Tandberg et al. 2010). They are kleptoparasites, animals that steal their food.

Mussel-dwellers belong to the genus Metopa, and have surprisingly sophisticated lives (Tandberg et al. 2010). Each mussel is home to a single mating pair of adult amphipods, who defend their home against intruders. Most species spend their entire adult lives within the shell of a single mussel. In this time they work hard to raise multiple generations, which grow together until they are ready to head out to sea and find molluscan homes of their own.

A common hermit crab, host to a unique amphipod. Photo © Biopix: N Sloth, licensed under CC BY-NC 3.0.

A common hermit crab, host to a unique amphipod. Photo © Biopix: N Sloth, licensed under CC BY-NC 3.0.

Still other amphipods are described as hermit crab associates, some living on the crab’s acquired snail shell (Krapp-Schickel and Vader 2015). At least one species eats invertebrates that land and begin to grow on the shell. Others, however, live on the bodies of the crabs themselves.

This seems like a risky move. Hermit crabs are rigorous self-cleaners, constantly scrubbing the insides of their shells with specialized, brush-like hind legs. Perhaps the amphipods, by scavenging bits of uneaten food from the crab’s body, are helpful maids to their type-A hosts. If this is the case, hermit crabs may spare their amphipod companions on purpose. We still don’t know if the crab-dwelling amphipods are helpful cleaners or true parasites — if the latter is true, they must be well-adapted to avoid detection by their hosts.


Krapp-Schickel T. and W.J.M. Vader. 2015. Stenothoids living with or on other animals (Crustacea, Amphipoda). Zoosystematics and Evolution 91(2): 215-246.

Moore P.G., P.S. Rainbow, and W. Vader. 1994. On the feeding and comparative biology of iron in coelenterate-associated gammaridean Amphipoda (Crustacea) from N. Norway. Journal of Experimental Marine Biology and Ecology 178: 205-231.

Tandberg A.H.S., C. Schandler, and F. Pleijel. 2010. First record of the association between the amphipod Metopa alderi and the bivalve Musculus. Marine Biodiversity Records 3, e. 5, 2 pp.

Rat Lungworm Disease: How it Works

by Joseph DeSisto

Rat lungworm disease — even the name sounds awful. But to understand the disease, we first have to understand the life cycle of the worm that causes it which, incidentally, is as fascinating as it is terrifying.

The rat lungworm (Angiostrongylus cantonensis) is a kind of parasitic roundworm or nematode which, unsurprisingly, is mainly a parasite of rats. It’s favorite host is the brown “Norway” rat, now found throughout the world where it has been spread by human travels. The worms enter a rat as larvae less than a millimeter long, first entering the bloodstream and then migrating, like salmon up a stream, to the host’s brain. Here they gorge themselves with brain tissue until they become sub-adults. The brain-filled, sub-adult lungworms are almost half an inch in length — and still not done growing.

The brown rat, common in cities, is the primary host of the rat lungworm. Photo by  Ian Kirk, licensed under CC BY 2.0.

The brown rat (Rattus norvegicus), common in cities, is the primary host of the rat lungworm. Photo by Ian Kirk, licensed under CC BY 2.0.

Before they reach adulthood, the worms migrate again via the circulatory system, this time stopping when they reach the heart’s right ventricle, or the pulmonary arteries. In the heart tissue, the worms finally mature, mate, and lay their eggs — but the eggs, too, must migrate. The eggs are laid directly into the bloodstream, and because the pulmonary arteries lead directly to the lungs, that’s where the eggs end up. Hence, the name “lungworm,” even though they might as well be called “heartworms” or even “brainworms.”

Even now, the lungworms have yet to finish their ricochet across your — I mean, the rat’s — body systems. When the eggs hatch, baby worms travel up the respiratory tract, leaving the lungs and entering the esophagous, where they enter the digestive system.

An adult female rat lungworm. Photo from Lindo et al. (2002), in public domain.

An adult female rat lungworm. Photo from Lindo et al. (2002), in public domain.

Despite being microscopic, the baby lungworms are extremely tough. They have to be, because they are going all the way, from throat to stomach to intestines and beyond. Finally, when the rat defecates, its feces are loaded with baby lungworms, all ready to start infectious lives of their own.

If they are lucky, the scent of rat dung will catch the nose of a passing snail or slug. Land snails are usually scavengers that will eat almost any non-living biological material, from dead leaves to carrion to, yes, dung. Should a snail care to take a bite, it will quickly become infected with hordes of developing lungworms.

As any city-dweller will attest, rats will eat almost anything, including, it just so happens, snails. The life cycle of the rat lungworm continues with a rat eating an infected snail or slug, and with the worms travelling up to the rat’s brain to eat, grow, and make wormy babies of their own. For a more technical description of the rat lungworm and its strange life cycle, I recommend Cowie’s 2013 review paper.

The gray garden slug, which has been recorded as an intermediate host for the rat lungworm. In other words, if you eat them, cook them first. Photo by Bruce Marlin, licensed under CC BY-SA 3.0.

The gray garden slug (Deroceras reticulatum), which has been recorded as an intermediate host for the rat lungworm. In other words, if you eat them, cook them first. Photo by Bruce Marlin, licensed under CC BY-SA 3.0.

Rat feces aren’t just eaten by snails, and snails aren’t just eaten by rats. As a result, rat lungworms accidentally infect animals they aren’t supposed to, such as flatworms, shrimp, frogs, birds and, yes, humans. People all over the world eat snails, both on purpose and by accident. Rat lungworm disease in China is usually attributed to eating market-bought raw snails (Lv et al. 2008). During a 2002 outbreak in Jamaica, where snails aren’t as popular, infections were the result of contaminated vegetables (Lindo et al. 2002).

Eating cooked snails is fine, since the cooking process kills the lungworm larvae — it’s raw escargot that can cause problems. Snails and slugs in gardens can also leave a trail of worm larvae in their slime, so washing vegetables in lungworm-inhabited areas can be important.

What happens when a person accidentally eats a rat lungworm? In a human, the worm follows the same cycle as it does when in a rat, going from circulatory to nervous to circulatory to respiratory to digestive systems, and back out to be eaten again by snails.

A summary of the rat lungworm life cycle (click to enlarge). Figure from the Centers for Disease Control and Prevention, in public domain.

A summary of the rat lungworm life cycle (click to enlarge). Figure from the Centers for Disease Control and Prevention, in public domain.

Medical problems come from the sub-adult worms, as they eat away at the host’s brain tissue. Worms are pretty big things to have squirming around in your head, and as they burrow through nervous tissue, they can cause enough damage that the brain becomes inflamed. The result is eosinophilic meningitis, a series of symptoms of which lungworms are just one possible cause. In some cases the damage can cause behavioral changes in the host — one victim developed severe photophobia, and was terrified of light (Ramirez-Avila et al. 2009).

Rat lungworm disease is not common but can be serious, and potentially fatal. Most cases occur in the tropics, especially in Southeast Asia and the Pacific, where the worm is native. Recently, however, lungworms have become more common across the world, as rats and certain snails have been introduced by humans (Kliks and Palumbo 1992).

A small outbreak in Hawaii occurred only a decade ago (Hochberg et al. 2007) and made the news. Global trade in food has also been a factor — the contaminated vegetables that caused the outbreak in Jamaica may very well have been grown halfway around the world (Lindo et al. 2002). As the world becomes economically smaller, strange local diseases can become worldwide problems.

And yet, for all this gloom and doom, the reason I wrote this article in the first place is that the rat lungworm is actually a pretty cool animal. It’s easy to view wormy parasites like nematodes as simple and unsophisticated creatures. But if the rat lungworm can teach us anything, it’s that even “simple” animals can have incredibly complex and, yes, amazing life cycles. And maybe, just maybe, even the most nightmarish of animals can be, in its own twisted way, sort of, well … beautiful.

Have a lovely and parasite-free day.

(Disclaimer: My interest is in science education. I am not a doctor, and nothing in this article should be interpreted as medical advice. If you are here because you’re worried you might actually have rat lungworm disease, please stop browsing the Internet and talk to a real doctor. Thank you.)


Cowie R.H. 2013. Biology, systematics, life cycle, and distribution of Angiostrongylus cantonensis, the cause of rat lungworm disease. Hawai’i Journal of Medicine & Public Health 72(6): 6-9.

Hochberg N.S., S.Y. Park, B.G. Blackburn, J.J. Sejvar, K. Gaynor, H. Chung, K. Leniek, B.L. Herwaldt, and P.V. Effler. 2007. Distribution of eosinophilic meningitis cases attributable to Angiostrongylus cantonensis, Hawaii. Emerging Infectious Diseases 13(11): 1675-1680.

Kliks M.M. and N.E. Palumbo. 1992. Eosinophilic meningitis beyond the Pacific Basin: the global dispersal of a peridomestic zoonosis caused by Angiostrongylus cantonensis, the nematode lungworm of rats. Social Science and Medicine 34(2): 199-212.

Lincoln M. 15 April 2015. Rat lungworm disease spreads fear across Hawaii Island. Hawaii News Now. Retrieved from http://www.hawaiinewsnow.com/

Lindo J.F., C. Waugh, J. Hall, C. Cunningham-Myrie, D. Ashley, M.L. Eberhard, J.J. Sullivan, H.S. Bishop, D.G. Robinson, T. Holtz, and R.D. Robinson. 2002. Enzootic Angiostrongylus cantonensis in rats and snails after an outbreak of human eosinophilic meningitis, Jamaica. Emerging Infectious Diseases 8(3): 324-326.

Lv S., Y. Zhang, P. Steinmann, and X. Zhou. 2008. Emerging angiostrongyliasis in mainland China. Emerging Infectious Diseases 14(1): 161-164.

Ramirez-Avila L., S. Slome, F.L. Schuster, S. Gavali, P.M. Schantz, J. Sejvar, and C.A. Glaser. 2009. Eosinophilic meningitis due to Angiostrongylus and Gnathostoma species. Clinical Infectious Diseases 48(3): 322-327.

Mega-Diverse Megaselia: Flies in the News

by Joseph DeSisto

The phorid fly genus Megaselia contains around 1600 known species, but there are estimates that the genus may contain as many as 30,000 species in total. These flies made the news this year when 30 new species (and 16 already-known species) were discovered in the city limits of Los Angeles (Hartop et al. 2015). This is amazing but not unheard of for Megaselia — an earlier study in Cambridge, England revealed 53 species in a single garden (Disney 2001).

A female Megaselia aurea, scavenging on a dead cricket. Photo by Brian V. Brown and Wendy Porras, used under CC BY 4.0.

A female Megaselia aurea, scavenging on a dead cricket. Photo from Brown and Porras (2015), licensed under CC BY 4.0.

Diversity in Megaselia, however, goes much deeper than a simple species tally. In terms of behavior and ecology, diversity can be awe-inspiring, even within a single species.

Megaselia adults are great, and some of the largest phorid flies around. but the larvae can be just as interesting. Many species are scavengers, and some feed on carrion;  Others are parasites of vertebrates (like bot flies), and still others live inside other insects. Most recently, scientists in Mexico discovered that one species, Megaselia scalaris, is a parasitoid of the tarantula Brachypelma vagans (Machkour-M’Rabet et al. 2015). The tarantula examined contained more than 500 fly larvae which, had the spider not been killed for study, would have ultimately eaten their host alive.

Megaselia scalaris, a fly with incredibly diverse ecological roles. Photo by Charles Schurch Lewallen, licensed under CC BY 3.0.

Megaselia scalaris, a fly with incredibly diverse ecological roles. Photo by Charles Schurch Lewallen, licensed under CC BY 3.0.

Aside from being amazing in itself, this find is significant because M. scalaris already has an incredibly diverse range of lifestyles — the examples here come from a review paper by Disney (2008). In addition to acting as parasitoids on a wide range of arthropods, scalaris larvae have been observed eating decaying plant matter, dung, bacteria and other microorganisms, the leaves and seeds of live plants, already-dead insects, and carrion.

Speaking of carrion, scalaris larvae are well-known for burrowing through up to six feet of soil to feed on human corpses in their coffins. They have also been observed in honey bee hives, scavenging on dead bees, and in amphibian egg masses, feeding on the developing tadpoles. A few specimens were found on the mouthparts of a land crab, where they were apparently eating bits of food the crab’s food.

I could go on … and I will. Sea turtle eggs, shoe polish, a preserved snake specimen recently removed from alcohol, the wounds of living animals from humans to pythons to poison dart frogs, highly toxic millipedes, blue emulsion paint …


Brown, B.V. and W. Porras. 2015. Extravagant female sexual display in a Megaselia Rondani species (Diptera: Phoridae). Biodiversity Data Journal 3: e4368. doi: 10.3897/BDJ.3.e4368

Disney, R.H.L. 2001. The scuttle flies (Diptera: Phoridae) of Buckingham Palace Garden. The London Naturalist 80: 245–258.

Disney, R.H.L. 2008. Natural History of the Scuttle Fly, Megaselia scalaris. Annual Review of Entomology 53: 39-60.

Hartop, E.A., B.V. Brown, and R.H.L. Disney. 2015. Opportunity in our ignorance: urban biodiversity study reveals 30 new species and one new Nearctic record for Megaselia (Diptera: Phoridae) in Los Angeles (California, USA). Zootaxa 3941(4): 451-484.

Machjour-M’Rabet, S., A. Dor, and Y. Henaut. 2015. Megaselia scalaris (Diptera: Phoridae): an opportunistic endoparasitoid of the endangered Mexican redrump tarantula, Brachypelma vagans (Araneae: Theraphosidae). Journal of Arachnology 43(1): 115-119.7

Honey Bees and Pseudoscorpions: Best of Frenemies

by Joseph DeSisto

This is a pseudoscorpion. Depending on how you look at it, you might describe it as a scorpion without a stinger, or a tick with pincers. In fact, it is neither.

A pseudoscorpion, Chelifer cancroides, commonly found in houses. Photo by Christian Fischer, licensed under CC 3.0.

A pseudoscorpion, Chelifer cancroides, commonly found in houses. Photo by Christian Fischer, licensed under CC 3.0.

Pseudoscorpions are arachnids, like spiders, mites and, yes, scorpions. But unlike scorpions, pseudoscorpions are a) tiny, b) don’t have stingers and c) instead inject venom into their tiny prey through glands in their pincers (Weygoldt 1969).

To be more specific, the pseudoscorpion in the picture above is Chelifer cancroides, commonly called the house pseudoscorpion. This species is cosmopolitan — it often associates with humans and lives in buildings, where it feeds on the other assorted animals that dwell in the forgotten cracks and crevices. They are harmless, and do us a favor by keeping pests in check, although their domestic habits can lead to awkward encounters such as this one:

No children were harmed in the collection of this specimen. Photo by Joseph DeSisto.

No children were harmed in the collection of this specimen. Photo by Joseph DeSisto.

Many pseudoscorpions live on soil and leaf litter, or under the bark of rotting logs. Others have more restrictive habits: there are several species that specialize in living in honey bee hives, where they sneak about among honeycombs and bee larvae. What do they do in bee hives? Some species are beneficial. Others are decidedly not.

A grand total of 15 species of pseudoscorpions have been recorded in honey bee hives, most of them in the tropics (Gonzalez et al. 2008). Many species appear to live exclusively alongside honey bees, but hives have also been found to contain C. cancroides — remember, the one that always seems to turn up in places it shouldn’t.

At least one species, Ellingsenius handrickxi, is definitely not a bee friend — it regularly preys on the bees (Vachon 1954). Another species, Ellingsenius indicus, has been seen travelling about by clinging to the bees’ necks, which may prevent them from gathering nectar and pollen efficiently (Subbiah et al. 1957).

A honey bee hive is a dangerous place to live if you aren't a bee. Photo by Eugene Zelenko, licensed under CC 3.0.

A honey bee hive is a dangerous place to live if you aren’t a bee. Photo by Eugene Zelenko, licensed under CC 3.0.

Most pseudoscorpions don’t eat bees, but instead prey on mites, waxworms, and other invertebrates that live in honey bee hives. This can benefit the bees, since some of these squatters rob the hive of its resources: precious wax and honey. Pseudoscorpions also eat bee parasites, including Varroa mites, which can destroy honey bee colonies and devastate beekeepers.

The big question is, can we use pseudoscorpions to help control the Varroa mite? At least some species can be efficiently bred in captivity (Read et al. 2014), and unlike many other predators, pseudoscorpions are comfortable living in groups — cannibalism is rare (Weygoldt 1969).

Several New Zealand entomologists are optimistic, among them Dr. Barry Donovan. He has published several popular and technical articles touting pseudoscorpions as having potential to control Varroa. His evidence is compelling — pseudoscorpions do eat Varroa mites. Video surveillance reveals they will even remove the mites from bee larvae for an easy snack (Fagan et al. 2012).

These voracious predators can eat up to nine mites per day, and Fagan et al. (2012) estimate that a population of only 25 pseudoscorpions is enough to control Varroa mites in a typical honey bee hive. So, it seems that pseudoscorpions could be an effective way to control Varroa. Donovan and Paul (2006) even suggest modifying commercial hives to provide “breeding sites” for pseudoscorpions.

The devastating parasitic mite Varroa destructor, clinging to the head of a developing honey bee. Photo by Gilles San Martin, licensed under CC 2.0.

The devastating parasitic mite Varroa destructor, clinging to the head of a developing honey bee. Photo by Gilles San Martin, licensed under CC 2.0.

It might not be that easy. A systematic study using the pseudoscorpion Ellingsenius indicus in the Himalayas revealed that although this species may eat Varroa, it prefers to eat bee larvae, non-parasitic lice, and the remains of already-dead bees (Thapa et al. 2013). This doesn’t contradict Fagan et al.’s study showing that pseudoscorpions do eat Varroa mites — Fagan et al used a New Zealand species, not E. indicus, but an unspecified pseudoscorpion.

What the Himalayan study does tell us is that knowing all the details, including the exact species relationships, is critical. Some pseudoscorpions are beneficial and eat mites straight off the bees, but others cut out the middle-mite and just eat the bees themselves. Most species probably do both. Pseudoscorpions may prove invaluable in the war against honey bee decline, but for now, there’s a lot left to learn.


Donovan, B.J. and F. Paul. 2006. Pseudoscorpions to the rescue? American Bee Journal 146(10): 867-869.

Fagan, L.L., W.R. Nelson, E.D. Meenken, B.G. Howlett, M.K. Walker, and B.J. Donovan. 2012. Varroa management in small bites. Journal of Applied Entomology 136: 473-475.

Gonzalez, V.H., B. Mantilla, and V. Mahnert. 2007. A new host record for Dasychernes inquilinus (Arachnida, Pseudoscorpiones, Chernetidae), with an overview of pseudoscorpion-bee relationships. Journal of Arachnology 35(3): 470-474.

Read, S., B.G. Howlett, B.J. Donovan, W.R. Nelson, and R.F. van Toor. 2014. Culturing chelifers (Pseudoscorpions) that consume Varroa mites. Journal of Applied Entomology 138: 260-266.

Subbiah, M.S., V. Mahadevan, and R. Janakiraman. 1957. A note on the occurrence of an arachnid – Ellingsenius indicus Chamberlin – infesting bee hives in South India. Indian Journal of Veterinary Science and Animal Husbandry 27: 155-156.

Thapa, R., S. Wongsiri, M.L. Lee, T. Choi. 2013. Predatory behavior of pseudoscorpions (Ellingsenius indicus) associated with Himalayan Apis cerana. Journal of Apicultural Research 52(5): 219-226.

Weygoldt, P. 1969. The Biology of Pseudoscorpions. Cambridge, Massachusetts: Harvard University Press.

Vachon, M. 1954. Remarques sur un Pseudoscorpion vivant dans les ruches d’Abeiltes au Congo Belge, Ellingsenius hendriekxi n. sp. Annales du Musbe royal du Congo Beige, N. S. Zool. 1: 284-287.

Tongue-Eating Parasites with Mayonnaise

by Joseph DeSisto

Tongue-eating parasites that are found in cans of tuna eat tuna, not human, tongues. But that’s a boring headline, so most news outlets left that part out when a woman in Britain found a “thing” in her can of tuna. The victim was one Zoe Butler, who opened her can of Princes tuna chunks this week to find an eyed, globular, undeniably cute invertebrate staring back at her.

Since the specimen was sent back with the tuna can to Princes, Stuart Hine of the Natural History Museum in London had to make do with a photograph in making his identification. He postulated that it might be a parasite related to Cymothoa exigua. Unfortunately, he also mentioned the common name, the tongue-eating louse, to reporters, who got right to work making sure the public knew that we were about to take part in a real-life version of The Bay.

What these reporters didn’t appreciate, sadly, is that the actual life history of Cymothoa is far more interesting and twisted than any sensationalist headline. These creatures belong to the crustacean order Isopoda, and are distantly related to woodlice. Unlike woodlice, however, Cymothoa are roly-poly fish nightmares.

Cymothoa exigua, a tongue-eating isopod. Photo by Marco Vinci.

Cymothoa exigua, a tongue-eating isopod. Photo by Marco Vinci.

As free-living larvae, Cymothoa are all males, and spend their time swimming about in search of a fish host. When several individuals “colonize” a fish, they begin feeding on its gills. If C. exigua is the louse, the host will be red snapper (not tuna). Once a fish has been parasitized, some males will change into females, and mating takes place on the gills or in the fish’s mouth.

The females are the tongue-eaters. Either before or after mating, depending on the species, a female invades the fish’s mouth and begins to suck blood. Extracting blood from the tongue eventually causes the tongue to wither away, after which the female persists on bits of mucus and blood remaining in the fish’s mouth. In C. exigua, this is when mating takes place: a few males will migrate from the host’s gills and mate with the female inside the fish’s mouth.

During all this time, the fish is still able to function, because the female Cymothoa functionally replaces the tongue. So with a new segmented, chitinous, leggy tongue, the fish can still eat and reproduce normally. Eventually, the female’s young emerge as free-living males, and disperse to find a new fish.

So, are tongue-eating parasites dangerous to humans? An adult female can bite in self-defense, and revealing a giant isopod while cleaning a snapper might be emotionally scarring. But rest assured, our tongues are safe. Enjoy your tuna.

For more information:

Driscoll, Brogan. “Mystery ‘Crab’ Found in Tuna Could Actually Be A Tongue-Eating Parasite, Claims Expert.” Huffington Post. 2 Feb 2015. Web. 6 Feb. 2015.

Creighton, Jolene. “Meet The Sex-Changing, Tongue-Eating Parasite.” From Quarks to Quasars. 3 Apr 2014. Web. 6 Feb 2015. <http://www.fromquarkstoquasars.com/the-most-horrifying-parasite-cymothoa-exigua/&gt;.