Wednesday, 28 August 2013

The Decline of the Screwworm Fly – Edward Knipling vs Cochliomyia hominivorax

Hundreds of screwworm larvae can infest
a single wound, doing unspeakable damage
and often causing death. Image courtesy
of the US Agricultural Research Service.
In July 2013 a British woman came to the attention of both the medical community and the media when she returned from Peru with maggots in her ear. The fly larvae had created a cavity connected to the ear canal and were feeding on healthy tissue in Rochelle Harris's head.

According to a Medical Daily story by John Ericson, Harris endured “unbearable pains emanating from one side of her face, pains that were eventually accompanied by menacing scratching sounds from inside her head” ("British Woman Discovers Flesh-Eating Maggots Inside Her Ear," July 16, 2013).

This is a horrific story, and many people reading it will have heard of the dreaded New World screwworm fly – Cochliomyia hominivorax - for the first time. In North America, too, we're unfamiliar with the fly - we've forgotten that it once infested the southern United States, spreading northward each summer as far as Canada. If not for entomologist Edward Knipling, it might still plague us, as it does parts of South America.

Edward Knipling Meets the Screwworm Fly

As a boy in the early 20th century, Edward Knipling worked long hours on his parents' Texas farm. There he encountered all manner of insect pests, including the maggots of the screwworm fly, which infested healing umbilical cords of newborn animals and any other open wound that the female fly could find.

Left alone, screwworm maggots feed on healthy flesh, quickly turning a small wound into a large, spreading one, and the odor generated by their activities attracts more female flies to lay their eggs. Infested animals often die. Humans aren't immune, with infested wounds and nasal sinuses most often reported.

According to biographers Adkisson and Tumlinson, Knipling “decided at an early age that he wanted to make a bigger contribution to agriculture than treating screwworm infested calves or pulling a sack down a cotton row” ("Edward F. Knipling, 1909 – 2000; Biographical Memoirs"). But Knipling likely had no inkling that his contribution would be to discover a way to eradicate C. hominivorax from the United States, Central America, and even Libya, in Africa, where it was accidentally introduced in the 1980s.

Knipling and the Idea of Screwworm Eradication

Edward Knipling combined scientific
knowledge and innovative thinking
to pioneer the sterile insect
technique. As a result, the screwworm
fly has been eradicated from much of
its range. Image courtesy of the US
Agricultural Research Service.
Edward Knipling sought a science career in entomology, ultimately earning a PhD in entomology from Iowa State University. During his studies, the United States recorded thousands of cases of screwworm infestation in livestock annually; some years saw hundreds of thousands of cases. The occasional human infestation occurred as well. Screwworm caused serious agricultural losses and horrible human illness.

By 1937 Knipling was studying the life cycle of C. hominivorax with colleague R. C. Bushland, and  the two made an important observation: female screwworm flies mate only once. Knipling knew that  he might be able to turn this against the pest, but the puzzle of just how one might use it to advantage had to wait until the end of WWII, when he had time to return to it.

Adkisson and Tumlinson write that Knipling thought “that if male flies could be produced in large numbers, sterilized, and released into the environment they might out-compete... the wild fertile males in breeding with females... If a sufficient number of sterile males could be released into the wild population," Knipling thought, the sterile males might “breed the screwworm population into extinction.”

How to Sterilize a Screwworm Fly

Knipling's idea raised two questions: how do you sterilize a male screwworm fly without damaging it in other ways, and how do you rear large numbers of screwworm flies in the laboratory? He knew that radiation could render insects sterile because of work done on fruit flies.

Now working in Washington DC, Knipling couldn't do the experiments himself, but Bushland, still in Texas, took it on. With scarce resources himself, Bushland had to innovate: he used X-ray equipment at the Brooke Army Hospital in San Antonio, Texas, to investigate sterilization, by trial and error. Meanwhile, Knipling worked on determining just how many sterile male flies were needed.

Bushland had already devised a laboratory diet for raising screwworms; now he, and others, worked to modify it to make it cheaper and better suited to mass production. The final test would be trials to see if  the sterile male approach actually worked.

The screwworm fly, Cochliomyia hominivorax is
aptly named: hominivorax means man eater.Image
courtesy of The Mexican-American Commission
for the Eradication of the Screwworm.
Screwworm Eradication in the United States

The first screwworm eradication trial took place on Sanibel Island, off the coast of Florida. Researchers released 39 sterile males per square kilometer each week for eight weeks, and saw the screwworm population drop off to virtually zero; however, screwworm flies continued to migrate from the mainland, so the island wasn't rendered screwworm free.

A second trial on the island of CuraƧao in 1954 used four times as many sterile flies, and eradicated C. hominivorax in just three and a half months. The method worked.

The years that followed saw the construction of large facilities devoted to producing millions of sterile screwworm flies, and eradication programs, beginning in Florida. Cochliomyia hominivorax was eradicated in Florida in 1959; the process took longer in southwestern states because of continual reintroduction from Mexico, but the US was free of screwworm by 1982.

Screwworm Eradication in Central America

By 1984, there were no screwworm flies north of the Isthmus of Tehuantepec in southern Mexico, and today Central America is screwworm free all the way to the Panama - Columbia border.

Affected countries in South America are now using integrated pest management, including active surveillance, insecticides, chemical attractants and traps, and the sterile male technique to control and, one day, eliminate C. hominivorax from the last of its range.

If Rochelle Harris returns to Peru when she's older, she might not have to worry about another encounter with the screwworm fly, but if she does meet it again, she'll be ready. John Ericson quotes her, after her ordeal: "I'm no longer as squeamish as I was about bugs," she [says] "How can you be when they've been inside your head?"

Additional Reading

Adkisson, P., Tumlinson, J. 2003. "Edward F. Knipling, 1909 – 2000." Biographical Memoirs. 83.

Ericson, John. Jul 16, 2013. "British Woman Discovers Flesh-Eating Maggots Inside Her Ear." Medical Daily.

Mastrangelo, T., Welch, J. B. 2012. "An Overview of the Components of AW-IPM Campaigns Against the New World Screwworm." Insects. 3.

Novy, J. E. 1991. "Screwworm Control and Eradication in the Southern United States of America." In: New World Screwworm Response to an Emergency. World Animal Review. Special issue.

Tuesday, 27 August 2013

Cryptosporidium: A Parasite That Gets Into Drinking Water and Swimming Pools

Swimming pools become contaminated with infective
oocysts of Cryptosporidium when ill people have
accidents in the water.
Image courtesy of the US Dept of State.
Cryptosporidium spp. are tiny parasites that cause outbreaks of diarrhea. Contaminated drinking water is a common source of cryptosporidiosis, but there are others, including swimming pools and food.

At least two species of Cryptosporidium infect humans, and they are increasingly familiar causes of outbreaks. In recent years, online searches have constantly turned up swimming pool closures and boil water orders attributed to Cryptosporidium. Such outbreaks may indeed be more common; we may be getting better at identifying the parasites in outbreaks; and it may be that Internet reports mean that more people hear of it – all three factors likely play a part in the raised profile of these parasites.

When Cryptosporidium contaminates a municipal water supply, it can make many people sick at once, and this happens relatively easily because of the parasite’s small size and its ability to survive chlorination.

An oocyst (pronounced oo-oo-cyst) of  Cryptosporidium sp., the infective stage of the organism, is spherical and only about three to five one-thousandths of a millimetre wide. Environmentally resistant, it survives cold, chlorination, and salt water. It’s found in surface waters all over the globe - municipalities that use surface water supplies must do more than chlorinate water to avoid an outbreak. Most rely on filtration.

In the summer of 2013, an outbreak of cryprosporidiosis in Baker City, Oregon highlighted the risks of unfiltered water supplies, even when the watershed appears pristine. Even municipal water filtration systems can famously fail - more than 300,000 people got cryptosporidiosis in Milwaukee in 1993 due to inadequate treatment and filtration. In terms of numbers, a contaminated water supply is the most common source of human infections, but how does Cryptosporidium get into the water, and how else can we catch it?

Cryptosporidium in Livestock

Dairy and beef cattle suffer from and spread Cryptosporidium parvum. Young calves catch it and suffer severe diarrhea, while older cattle continue to carry the parasite and spread it. Runoff from pastures into rivers and wells after heavy rain is an important source of Cryptosporidium in surface waters. Ranched elk and bison also spread the parasite.

Cryptosporidium in Human Sewage

Untreated sewage from human communities often contains oocysts of Cryptosporidium. When sewage effluent is discharged into bodies of water without proper treatment, as it frequently is, especially after rainfall when treatment plants are overwhelmed, oocysts are discharged with it.

Canada Geese and other water birds could potentially
spread Cryptosporidium from cattle pastures to
distant surface waters.
Image by Robert Lawton; CC BY-SA 2.5.

Cryptosporidium in Wild Animals

Many species of wild animals can be infected with Cryptosporidium parvum, one of the species that infects humans. Dogs, cats, goats and mice are among them. Although this does not appear to be a significant source of water contamination, migratory birds may be a different story.

Cryptosporidium parvum is known to pass unharmed through the gut of a Canada goose without making the bird sick. Thus a goose can ingest millions of oocysts while pecking corn kernels from cow dung in Maryland, and discharge them into a watershed in Pennsylvania. It’s not clear how much geese and other migratory birds contribute to the spread of Cryptosporidium.

Direct Person to Person Spread of Cryptosporidium

Oocysts of Cryptosporidium are infective as soon as they are passed in stool. Thus, an infected person can pass on the parasite with dirty hands or objects contaminated with feces. Likewise, infected animals can pass the infection directly to other animals or to humans.

Cryptosporidium in Swimming Pools

It’s fairly common for swimming pools to become contaminated with Cryptosporidium - sometimes people go swimming and have minor “accidents” in the water, or feces work their way out of leaky diapers. Chemical treatment of swimming pools must reach high concentrations in order to kill the oocysts and pool filtration systems cannot remove them —or at least not fast enough to prevent some swimmers from swallowing some with a mouthful of water.

Food and Cryptosporidium


Food items can potentially be contaminated with oocysts of Cryptosporidium, particularly produce that has been irrigated with contaminated water. Because of this, and other disease-causing organisms that may be present, produce that will be eaten raw should be thoroughly washed.

Oocysts of Cryptosporidium have been found in oysters along the eastern seaboard of North America where human sewage effluent and runoff from agricultural lands flows into the ocean, probably because . oysters feed by filtering nutrients from the water around them. Eating raw oysters or other raw shellfish is, therefore, a potential source of cryptosporidiosis.

Though it is more common in warm climates,  Cryptosporidium is found in surface water everywhere - never drink untreated water and heed any boil water advisory issued by your local water utility.


Alberta Government. “Relationship Between Beef Production and Waterborne Parasites (Cryptosporidium spp. and Giardia spp.) in the North Saskatchewan River Basin.” Agriculture, Food and Rural Development. Apr 2006.

Graczyk, T. K. et al. “Giardia sp. Cysts and Infectious Cryptosporidium parvum Oocysts in the Feces of Migratory Canada Geese (Branta canadensis).” Applied and Environmental Microbiology 1998 Jul; 64(7), pp. 2736-8.

Roberts, Larry S. and John Janovy Jr. Foundations of Parasitology 8th Ed. Boston: McGraw Hill, 2009.

Terrey. Lynn. “Goats Not Behind Baker City Parasite Suspected of Sickening Thousands, Officials Say.” Oregon Live: The Oregonian; Aug 21, 2013 Accessed Aug 21, 2013.

Friday, 23 August 2013

Angiostrongylus vasorum - Canine Lungworm Larvae in Snails, Slugs, and Slime

Is this the face of a killer? This slug, Arion rufus, has
been shown to carry Angiostrongylus vasorum larvae.
Image by Guillaume Brocker; CC BY-SA 3.0.

The roundworm Angiostrongylus vasorum goes by many common names: canine lungworm, canine heartworm, French heartworm – all descriptive labels acknowledging the fact that the adult worms reside in the heart and pulmonary artery (the blood vessel that carries blood from the heart to the lungs) of domestic dogs and wild canine species.

Life Cycle of Angiostrongylus vasorum

The life cycle of A. vasorum is complex:
  • Adult worms produce eggs that are carried to the lungs by the blood circulation.
  • The eggs hatch in the lungs, releasing larvae, which break through into the air space of the lungs, travel up the airways to the throat and are swallowed.
  • Passing through the intestine, larvae are deposited in the environment with feces.
  • Slugs and snails feeding on dog feces either ingest the larvae or the larvae penetrate the mollusk’s foot and undergo further development in the mollusk’s tissues.
  • Infective larvae are released in the slime secretions left behind by snails and slugs, or are ingested with the mollusk when it is eaten by something (frogs, dogs, etc.)
  • It is thought that dogs swallow the larvae and become infected when they eat snails and slugs, when they eat frogs or other animals that have eaten infected snails and slugs, or when they eat or lick things that are contaminated by slug or snail slime.
  • Larvae migrate to lymph nodes, where they develop further, and then move to the heart and pulmonary artery.

Dogs Eat Snails and Slugs

Dogs eat slugs and snails, and frogs. While many dog owners will readily agree with this statement, it leaves others, like me, scratching our heads.

Would my dog eat a slug or snail? She has never shown the slightest interest in doing so (and there are lots where we live).

Would she eat a frog? Sniff it maybe. Play with it maybe. Step on it by accident; yes, I’ve seen this. Eat it? No.

Would she eat a food item left lying on the ground and possibly crawled over by slugs and snails (or with slugs and snails still on its surface)? Definitely.

And that is one of the reasons why I was particularly interested in this video of the movements of snails in a British garden at night.

I had no idea they traveled so far and so fast.

Risk of Angiostrongylus vasorum Infection

A lot is still unclear about A. vasorum.  We don’t know what the most common route of infection is: snails, slugs, slime, frogs etc. We don’t know how many dogs have the parasite but exhibit no symptoms. We don’t know how many larvae a dog would need to ingest to become ill.

And of course, different species of slugs and snails are likely to differ in the distance they travel and the number of larvae they leave in their wake. Clearly, however, wherever A. vasorum occurs, all dog owners should take the threat seriously. As Eric Morgan et al write, the infection “is associated with coughing, dyspnoea [shortness of breath], exercise intolerance, weight loss, vomiting, abdominal pain, lumbar pain, neurological signs, heart failure, bleeding diathesis [tendency to bleed], and sudden death.”



Barcante, Thales Augusto, et al. "Angiostrongylus vasorum (Baillet, 1866) Kamensky, 1905: emergence of third-stage larvae from infected Biomphalaria glabrata snails." Parasitology Research 91.6 (2003): 471-475.

Fletcher, Damien. "Snails Can Travel at One Metre an Hour and Piggy-back on Others' Slime to Save Energy." Mirror News. Aug 23, 2013.

Morgan, Eric R., et al. "Angiostrongylus vasorum: a real heartbreaker." Trends in Parasitology 21.2 (2005): 49-51.

Thursday, 22 August 2013

Bed Bugs in New Products, Retail Stores, Imported Goods

In his Aug 22, 2013 blog post for the Huffington Post, Andrew Rennie describes a bed bug encounter. “Plainly visible on our brand-new sheet set, purchased at a big-box retail chain,” he writes, “was a bed bug. Dead, thank God, but its tiny, contorted legs twisting skyward still filled me with… dread” ("Parasites and Profiteering," Huffpost Living). 

Yes, that would fill me with dread as well. Suddenly we’ve gone from “don’t buy (or otherwise salvage) used furniture” to “nothing is safe.” Even brand new products, apparently, straight from the store, can be infested with bed bugs. 

At its actual size – about 5 millimeters long – this female bed bug would resemble other insect species to the untrained eye. Image by Gilles San Martin from Namur, Belgium. CC BY-SA 2.0

Where Bed Bugs Come From

Rennie’s post was not about bed bugs: it was about lousy customer service, and on
that topic I agree with him one hundred percent. By the time I finished reading, however, I was more focused on the whole bed-bug-in-the-new-sheets thing. How ironic – that an adult bed bug would be found neatly packaged in sheets that had never seen a bed! Sort of like getting a free sample of dishwasher detergent with your new dishwasher... only, not.

I wonder, was that bug identified as a bed bug by someone who really knows their bugs? An awful lot of insects look superficially like a bed bug (and if those sheets were imported, we’re not just talking North American insects). If the bug had turned up in a box of cereal, would it have been a weevil? Or a grain beetle of some kind? If it had arrived in a flower pot from the local garden center, would Rennie have given it a second glance? What's the probability that new sheets are now coming complete with a bed bug infestation?

It’s not impossible that Rennie’s nemesis was a bed bug. Bed bugs certainly can travel in shipments from other countries. Bed bugs have been found in retail stores. They could be present in a sheet factory, sure, but unless the employees of that factory are sleeping there too, you wouldn’t expect that to be a very happy home for a bed bug – lots of places to hide when the lights are on, granted, but not much to feed on in the dark.

Bed Bugs in Retail Stores and Products

Bed bugs can’t recognize new sheets as pay dirt and plan ahead. They don’t know an unused sheet from a windsock, a pillow from a bag of marshmallows. It’s us they recognize, not items we have never used. A dead bug in a package of new sheets is no more likely to be a bed bug than one in a package of computer paper.

The University of Minnesota tells us where bed bugs are most likely to be found in a clothing store (unsurprisingly, they don’t specifically mention the linen department of a big-box store). High-risk locations include fitting rooms, returns areas, seating areas, cloth bags attached to shopping carts etc. These locations have something in common: they are exposed to the (used) clothing and belongings of numerous people who could potentially bring bed bugs into the store ("Let’s Beat the Bed Bug," 2012).

As David Emery points out in Urban Legends: “Email: Bedbug Infestations Due to Imported Clothing,” imported products have not been associated with bed bug infestations in retail stores (, 2010). Similarly, unused manufactured products are unlikely to introduce bed bugs into our homes. 

Is it a Bed Bug? Be Sure

Anyone who suspects they have found a bed bug should have it properly identified before spending time and money on a problem that may not exist. I suspect Rennie’s “bed bug” was an innocent look-alike, left with “its tiny, contorted legs twisting skyward” when it got caught up in whatever process folds new sheets and jams them into plastic packaging.

Wednesday, 21 August 2013

Manure and Latrines = Farming and Parasites

Pig sties are notoriously smelly places, but aged
pig manure makes excellent fertilizer.
 Image by MontagZen; CC BY-SA 3.0.

Almost twenty years ago, Jared Diamond wrote about the domestication of plants in Discover  ("How to Tame a Wild Plant," Sept 1994). "Human latrines," he wrote, "may have been a testing ground for the first crop breeders."

How so? Many plants get spread around when their seeds are eaten, passed through the digestive tract of the forager intact, and deposited in feces. Because humans tend to use latrines - designated outdoor toilets that they return to again and again - the seeds of the plants that early humans liked to eat got deposited in a concentrated area, and the wastes deposited with them provided fertilizer. Voila! Latrines would have effectively selected the choicest food items of early humans and aided their reproduction.

Because I'm interested in parasites, I couldn't help thinking that this very process also aided the transmission of intestinal parasites: choice food plants growing in areas used as latrines means people gathering food there, with resulting exposure to the eggs and larvae of intestinal worms (and probably protozoa as well).

"Our ancestors' garbage dumps," Diamond pointed out, "undoubtedly joined their latrines to form the first agricultural research laboratories." This, because larger seeds, roots and other plant parts that might reproduce would end up in the midden, or garbage dump, along with all the other food waste (think compost) and animal manure (if they had livestock), gaining a survival advantage in virtually the same way.

And now, in his article "Early Farmers and Manure: Stone Age Europeans Were More Advanced Than We Thought," (Decoded Past, July 3013) Frank Beswick reports on evidence that stone age people used animal manure as fertilizer as long ago as 6000BC. Beswick wonders "is it possible that the use of manure was a precondition of the development of agriculture?"

And I wonder how much it contributed to the parasites we share with domestic animals, particularly cattle and pigs.