Naegleria fowleri, Terror of Swimmers

It’s two thirds of the way through a long hot summer (in some places) and numerous swimming holes have had lots of time to warm up to temperatures well above average. Tragically, this sometimes results in deaths due to infection with Naegleria fowleri, especially in warmer regions such as the southern United States. Naegleria, sometimes referred to as the “brain-eating parasite,” is an environmental amoeba that can, given the opportunity, gain access to the human brain through the back of the nose.

Naegleria Thrives Where Water is Warm

Naegleria does not habitually parasitize humans, but it does multiply in warm waters, and if swimmers draw water up the nose, infection can follow. (This year, one victim reportedly infected himself using a neti pot – a device used to rinse the sinuses.) Once infected, very few people survive the dreadful illness that Naegleria causes. Victims, of course, are typically those who enjoy water sports and games – the young, fit, and healthy.

Deaths due to N. fowleri are often widely reported, especially now that we have the internet. After such a death, people call for disclosure of the bodies of water involved, more public education, and surveillance. This is entirely understandable, but these demands really do miss the point. Naegleria is an environmental organism, widely distributed in nature, and well known for multiplying in warm bodies of water. It could be in any warm water, even the stuff in your hot water tank (as in the neti pot story). We can’t eradicate it, or even pinpoint where it will turn up next except in the most general terms.

If you really want to avoid this risk (and it is a small small risk) don’t swim in water warmer than 80F (26.7C). Never draw warm water up your nose unless it has been sterilized. If there is a risk of drawing water up the nose, use nose plugs or a nose clip.

Naegleria Deaths in Perspective

The reality is that any activity carries a certain risk, even sitting at home. For comparison, the number of fatalities from N. fowleri in the United States in the decade from 2001 to 2010 is thought to be about 30, an average of three per year:

• In that same time period, National Geographic reports that more than 400 people were killed by lightening.


• In 2004, an average of nine people accidentally drowned per day in the United States (Poseidon).


• In 2004, almost 4000 people died in fires, mostly residential.


• Each year, more than 33,000 people are killed in automobile accidents in the United States (NHTSA).

Deaths due to N. fowleri are quick, horrifying, and tragic, but this disease shouldn’t be blown out of proportion. It’s rare. Exercise reasonable caution while swimming (submerged hazards, drowning, pathogens, dangerous aquatic animals etc.) and enjoy the summer.

Is Babesia Spreading or Not?

Babesia microti was first recognized as a potential cause of human infection in New England in 1969. Before this, human cases of babesiosis involving other Babesia species were recognized in other places, chiefly Europe. The appearance and persistence of Babesia microti have been associated with high deer populations, increased human contact with the deer tick, Ixodes scapularis, and the presence of Lyme disease.

[caption id="attachment_316" align="alignleft" width="300" caption="Ixodes scapularis, image by Stuart Meek"][/caption]

In 2003, a paper published in the American Journal of Tropical Medicine and Hygiene noted “Human babesiosis generally is detected in sites where Ixodes ticks are endemic only after Lyme disease has become well established” (Krause et al, “Increasing Health Burden of Human Babesiosis in Endemic Sites," 68(4)). Why?

It’s tempting to explain away the late appearance of babesiosis by saying, well, physicians aren’t familiar with it, no one is looking for it, and most cases are mild anyway, so it’s simply being missed. As well, no one’s collecting data on diagnosed cases, so the incidence is unknown.

A current article published on SouthCoastTODAY.com discusses the work of Stephen Rich at the University of Massachusetts Amherst's Laboratory of Medical Zoology (Clark, “Much to Learn About Babesia’s Spread”). Rich’s research on Ixodes scapularis suggests that Babesia microti is spreading inland at a much slower rate than Borrelia burgdorferi, the organism that causes Lyme disease.

Rich’s work tells us that the organism truly is not well established in new locales – there’s not as much of it there. This likely explains why cases of babesiosis lag behind cases of Lyme disease, but it still raises a big question mark. If ticks and Lyme are spreading, why is Babesia not spreading just as quickly? The answer isn’t clear, but it appears that something is slowing down transmission of Babesia from host to host.

Rich doesn’t say that Babesia is absent in places where ticks and Lyme have appeared, he says there is a “much lower incidence” (qtd. in Clark). I suspect that incidence will rise over time – it may take years – and eventually make babesiosis a significant health concern over a much larger geographical area. I hope surveillance and medical knowledge stay ahead of it. If nothing else, the risk of contamination of the blood supply should fuel increased interest in this organism.

Where Did Toxoplasma gondii Come From?

Toxoplasma gondii, one assumes, evolved as a parasite of cats. Is this a safe assumption? All the texts tell us that the cat is the only animal in which T. gondii completes the sexual phase of its life cycle, which is strong evidence for the cat being the original host. It is possible that there are other hosts in which T. gondii can produce gametocytes and reproduce sexually – maybe we just haven’t found them yet. Maybe we haven’t looked exhaustively either. Nonetheless, in the absence of evidence to the contrary, I’ll stick with the assumption that T. gondii evolved in cats.

T. gondii was first discovered in Tunisia in 1908. By coincidence, Northern Africa is also one of the places where the cat is thought to have been domesticated (there or present day Iraq) and this explains my somewhat illogical but long held assumption that T. gondii probably evolved in Africa. There is actually no reason why the parasite couldn’t have evolved in some other feline species and spread to domestic cats later.

One theory has it that T. gondii evolved in South America.

[caption id="attachment_309" align="alignleft" width="162" caption="Did prehistoric jaguars have T. gondii?"][/caption]

The paper “Globalization and the Population Structure of Toxoplasma gondii,” reports an odd distribution of genotypes: one is found worldwide, one is found everywhere but South America, and several more are found only in South America. The authors' (Lehmann et al, PNAS July 25, 2006) interpretation is that there was an early split, originating in South America and leaving two populations to evolve in isolation from each other. One evolved in the “old world” and today is found everywhere but South America. The other is found everywhere, but apparently only spread very recently from its origins. Several more never left South America.

So maybe Toxoplasma gondii came from a prehistoric cat species in South America. It's a place to start.

(Image by Simon Burchell. Creative Commons)

Must We Hate Worms?

Is the revulsion we feel toward intestinal worms (in fact, anything called a parasite) innate or learned? If we hadn’t been surrounded by the “yuck factor” all our lives, grossed out by anything that wriggles or crawls, would we view them with disgust or curiosity? I think it would be more in the realm of curiosity.

In his book Parasites and Parasitic Infections in Early Medicine and Science, R. Hoeppli describes early attitudes toward parasites. Even within the last few hundred years, many people believed they arose spontaneously from intestinal contents, blood, even dust and ashes. And “in China,” he writes, “there existed from ancient times the widespread belief that one should have at least three worms in order to remain in good health” (p. 164).

Robin M. Overstreet has investigated the deliberate ingestion of parasites for various reasons by humans and found that parasites are often deliberately eaten and sometimes even regarded as delicacies (“Flavor Bugs and other Delights,” Journal of Parasitology: 89(6)). Overstreet describes a boy “open[ing] the intestine (of a possum) where a lump existed to allow the tapewoms to squirt out, remov[ing] the feces from the worms, and drop[ping] each writhing organism straight into his mouth.”

A former co-worker described an encounter with a small boy who was playing with a rather large roundworm, whirling it around while holding on to one end. When asked where he got it, he calmly indicated that it had come out of his nose.

These things suggest that a horror of worms is not a natural characteristic of humans. Similarly, Hans Zinsser writes that “as wise a man as Linnaeus suggested that children were protected by their lice from a number of diseases” (Rats, Lice and History, p. 139).

[caption id="attachment_301" align="alignleft" width="249" caption="Portrait of Carolus Linnaeus by Alexander Roslin, 1775"][/caption]

I’ve often suggested that if it were possible to keep mosquitoes out of your back yard, it would be socially unacceptable to have any there. I believe that’s what’s happened to parasites. If they were unavoidable, we’d accept them as part of life, like mosquitoes and mud puddles. In the handful of decades since we’ve been able to avoid having parasites in developed countries, we’ve also learned to abhor them.

Getting Parasites from Animals

An article by Peter Michael on couriermail.com.au (“Dingo Poo Spreading Deadly Parasites to Humans,” July 13) interests me for several reasons. First, the “deadly parasite” involved is Ecinococcus granulosus, which is by no means a new parasite for people in many parts of the world. It was once a big problem anywhere sheep were raised because sheep dogs could pass the worm’s eggs to people. Today, mostly because sheep dogs typically get better preventative care, the parasite is slowly losing ground.

It’s a bitter twist, then, that wild canines that have lost their fear of humans and come into human communities are now a source of infection. Therein lies the other thing that interests me. A few of the people who commented on that article have it right: it’s not that these wild animals are “encroaching” on us; it’s that we have encroached on them, and we’ve been doing that ever since the first human settlements with the beginning of agriculture and domestic animals.

[caption id="attachment_296" align="alignleft" width="200" caption="Dingos by Joshposh, Creative Commons 3.0"][/caption]

Before we grew our own food and raised animals, how often would we have come in close enough contact with an animal to catch a disease? Sporadically: when an animal was killed for food, when we shared the same cave perhaps, and by accident. Now we breed them and invite them in by carelessly providing food: cattle, pigs, birds, dogs, cats, fish, raccoons, dingos, rats, mice, and lots more. Meanwhile, we destroy their habitat and oblige them to adjust. Civilization is bad for our health: so many devastating diseases would be rare if we had not settled down and brought animals, and their parasites, into our space, one way or another.

Of course, without civilization, people would be rare as well. Most of us wouldn’t be here, I wouldn’t be writing this, and there wouldn’t be an internet. So one wonders, was it all worth it, and where does it stop?

Babesia in the Blood Supply

Blood transfusions have saved a lot of lives, my own included, but gone are the days when we thought that the chief concern with blood transfusions was transfusion reactions. Units of blood today are subjected to a growing battery of tests to detect infectious agents, because many pathogens are found in the blood stream and can easily be passed from donor to recipient, setting up a new infection. Blood transfusion can pass along HIV, hepatitis, malaria, trypanosomiasis, and a host of other things. Now, Babesia is recognized as being common enough to be of concern for the blood supply.

Babesia is a tiny blood parasite usually transmitted by a tick bite. The tick responsible is the same species that transmits Lyme disease, and babesiosis appears to be following in the footsteps of Lyme, appearing in places where Lyme has emerged, notably the northeastern United States.

[caption id="attachment_290" align="alignleft" width="300" caption="Babesia in ticks, mice, and humans"][/caption]

In some places, studies show, about one in ten people have antibodies to Babesia,  indicating that they have been infected. Because the diagnosis is often missed, and because, while fatal in some cases, the infection goes unnoticed in many people, the infection is likely considerably more common that we think.

It looks as though we’ll need to add a screen for Babesia to the processing of blood units for transfusion, at least in places where Lyme disease is common. Though babesiosis is mild in many cases, anyone receiving a blood transfusion is likely more vulnerable to serious illness. Reliable, cost effective, large scale screening methods, however, take time to develop and implement.

Maps of Dog and Cat Parasites in the US

The Companion Animal Parasite Council (CAPC) website, www.capcvet.org, has posted maps of the United States showing the prevalence of dog and cat parasites based on lab results at two commercial laboratories. If you’ve ever wondered how common the common dog and cat parasites actually are, and you live in the US, these maps are enlightening. There are maps for tick borne diseases, hookworm, whipworm, Toxocara (roundworm), and heartworm.



One should take care drawing conclusions from the results. This data is not based on a survey of all dogs and cats, or even a survey of a representative sample of all dogs and cats. It is data based on lab results of dogs and cats (presumably dogs and cats with owners and homes) whose samples were submitted to two specific commercial laboratories. We can assume that submission of these samples was done for a reason, even if it was just a routine health check; we don’t know whether parasites were suspected in some or most of these animals, or what the average age of the animal was.

You can look at two presentations of the data: traditional and three-dimensional. The traditional is best if you are only interested in looking at data from one state. If you want to compare, however, I’d suggest the three-dimensional. It gives you an immediate visual comparison between neighbouring states and various regions based on percentage positive. The map for Lyme disease clearly – and unsurprisingly – shows a concentration in the northeast, while hookworm's stronghold in the southeast is equally obvious.

I wish this data included Canada, although I understand why it doesn’t. Looking at the results for the northern states may provide clues to prevalence in southern Canada (and what the heck is going on with Toxocara in North and South Dakota?), but there may be differences in veterinary care and other variables that can’t be taken into account.  I still think these maps are very interesting, even with all the unanswered questions.