Sanitation Prevents Intestinal Worm Infections

A pit latrine in Haiti. Even a simple design
such as this will do much to reduce
contamination of the environment, and result
in fewer intestinal parasites.
Image by Rémi Kaupp. CC BY-SA 3.0

A paper in PLOS Medicine (January 24, 2012) reports that “sanitation is associated with a reduced risk of transmission of helminthiases to humans.” The authors looked at 36 previously published studies that measured prevalence of intestinal helminths (A. lumbricoides, large intestinal roundworm; T. trichiura, whipworm; and hookworm) compared to availability and use of sanitary facilities. They found that “people who either had or used a latrine were half as likely to be infected with a soil-transmitted helminth as people who neither had nor used a latrine.”

Contaminated Soil and Intestinal Worms

I submit that there are no surprises here. One acquires hookworm by coming in contact with hookworm larvae from feces contaminating the soil. They penetrate skin. Trichuris trichiura and A. lumbricoides eggs, infective a week or so after being deposited in warm moist soil in feces, must be swallowed. Obviously if feces were deposited in a pit latrine, septic system or other sanitary arrangement, instead of on the ground, those eggs and larvae would not be available to infect new hosts.

The fact that intestinal helminthes are much less common, even rare, in developed countries is no mere accident of climate, especially for the tough A. lumbricoides. It is because the majority of people in developed countries don’t defecate outside on the ground.

Parasite Prevention: Sanitation Works

The best point in this paper, though understated, is that periodically treating people for intestinal worms is perhaps not the best long term approach to getting rid of these parasites. Without good sanitation, people will quickly be reinfected due to contamination of their environment. Lets build toilets.

Read the paper:

Ziegelbauer K, Speich B, Mäusezahl D, Bos R, Keiser J, et al. (2012) "Effect of Sanitation on Soil-Transmitted Helminth Infection: Systematic Review and Meta-Analysis." PLoS Med 9(1): e1001162. doi:10.1371/journal.pmed.1001162

Are We Beating Malaria?

No human parasitic disease has ever been eradicated (although we may be close to eradicating Guinea Worm), but if ever there was a target, it would be malaria. Keeping an eye on the news and medical journals convinces me that there has never been more activity in scientific research aimed at understanding the parasites that cause malaria and finding ways to thwart them. Researchers have tried to develop malaria resistant mosquitoes. They’ve uncovered how the parasites invade red blood cells. They’ve investigated enzymes and proteins essential to the parasite’s survival. They’ve developed novel drugs. Hardly a day goes by when there is not something new. One might think we’re on the cusp of success.

Progress in Eradicating Malaria

But the reality is still grim. If you look at the statistics, you see that things are not really changing, at least not yet. Maps released by the Malaria Atlas Project allow us to roughly compare 2007 with 2010, and though there are clearly some changes (WHO statistics do indicate that the total number of malaria cases has dropped over the last decade), in the big picture they are minor changes that could easily reverse themselves.

This is discouraging. I wonder how long it will take for all this new research to provide us with a successful (and likely multi-pronged) approach to loosening the grip of this terrible disease.

Clinical burden of Plasmodium falciparum map in 2007 globally. CC BY 3.0

The spatial limits of Plasmodium falciparum malaria transmission
map in 2010 globally. CC BY 3.0

There are many maps on the Malaria Atlas Project site. Anyone interested in this should have a look.

Parasitic Worms and Wound Healing

Parasites can do you good. I didn’t discuss it at length in Parasites: Tales of Humanity’s Most Unwelcome Guests (I touched on it in chapter six), but I’m fascinated by the growing evidence that at least some of our parasites have good things to contribute to our health. The latest piece of that puzzle is research showing that hookworms can initiate an immune response in the host that actually speeds healing of tissue damage.

The business end of a hookworm.
You may not like the idea of hosting a few
hookworms, but they might return the favor
with better health. CDC; CC BY-SA 3.0

In their paper “An essential role for T_H2-type responses in limiting acute tissue damage during experimental helminth infection,” Fei Chen et al report that in mice “IL-17 initially contributed to inflammation and lung damage, whereas subsequent IL-4 receptor (IL-4R) signaling reduced elevations in IL-17 mRNA levels, enhanced the expression of insulin-like growth factor 1 (IGF-1) and IL-10 and stimulated the development of M2 macrophages, all of which contributed” to healing (Nature Medicine, published online 15 January 2012). In other words, there was an inflammatory response to the worms at first, then inflammation was suppressed while healing was enhanced.

It’s reasonable that a healing response may have evolved to help the host’s body deal with damage done by the worms themselves, but of course the potential exists for us to use that response to help heal tissue damage from other causes. This new report also ties in with previous work that suggests the ability of some of our parasites to suppress inflammation may protect us from autoimmune diseases.

It does make sense to me that organisms that have been with us for millions of years would have a relationship of both give and take with the host. While it’s true that parasitic diseases are some of the worst we face, and hookworm is a nasty parasite, I think we need to set aside the idea that anything parasitic is utterly bad. Let’s get to know them properly before we send them to extinction. (Not that hookworm is in danger of going extinct any time soon.)

I think we still have a lot to learn about our relationship with our parasites, especially our "old friends."

Helping Mosquitoes Fight Off Malaria

Anybody who knows anything about malaria knows that one catches it from a mosquito bite. Mosquitoes don’t just physically carry the parasite from person to person: they are a required host for Plasmodium spp., the agents of malaria. In the mosquito, the parasites multiply sexually, producing tiny forms called sporozoites which are injected into the next person the mosquito bites.

[caption id="attachment_382" align="alignleft" width="300" caption="Anopheles stephensi feeding; CDC, public domain image"][/caption]

Clearly, the mosquitoes are infected just as people are, but we seldom feel sorry for the poor mosquitoes because, well, we hate them for all sorts of reasons. The mosquito, however, does have an immune system which tries to fight off invading Plasmodium sp. parasites; Mosquitoes don’t mean to transmit these dangerous parasites.

Humans have had a long and costly battle with malaria which, so far, we have not won. Though not self-evident perhaps, it makes sense that we might be able to enlist the help of the lowly mosquito to our mutual benefit, and that’s what some researchers at Johns Hopkins University have done. Yuemei Dong et al. have genetically modified the immune system of a mosquito species, Anopheles stephensi, giving it an enhanced ability to fight off invading Plasmodium falciparum, the worst of the malaria parasites in humans.

In order for this research to prove useful in the real world, the modified mosquitoes would have to be released into the wild and allowed to breed with wild populations (and hopefully do better than the wild type). Aside from the obvious need for caution when releasing a genetically modified organism into the wild, at this point we still don’t know whether:

• the resistant mosquitoes will do as well in the wild, faced with different A. falciparum strains


• other Anopheles spp., also malaria vectors, can be similarly modified (there are about 40)


• Plasmodium falciparum will develop resistance to the mosquito resistance


• all other species of Plasmodium infecting humans can be targeted this way

This breakthrough is not the answer to the battle against malaria yet, but it may be part of the answer.

Read the paper:

Dong Y , Das S , Cirimotich C , Souza-Neto JA , McLean KJ , et al. 2011 “Engineered Anopheles Immunity to Plasmodium Infection” PLoS Pathog 7(12): e1002458. doi:10.1371/journal.ppat.1002458

Leishmania donovani Resistance to Drugs and Host Defenses

Leishmania donovani, agent of visceral leishmaniasis, or kala-azar, has always known how to evade the human immune system. This parasite literally uses the cells of the immune system to survive and multiply: when it enters the body via a sand fly bite it is engulfed by a macrophage – part of the immune response – and then it multiplies until the cell is destroyed.

[caption id="attachment_376" align="alignleft" width="300" caption="Leishmania parasites inside macrophages"][/caption]

Nonetheless, the immune system does have some control over the infection and some infections are without symptoms of disease. An interesting paper published in 2011 (Vanaerschot et al.) reports on research that suggests that strains of L. donovani that are resistant to the commonly used antimonial drugs are also able to multiply to greater numbers in the host. So these strains not only fail to respond to antimonial drug treatment, but also cause worse disease than other strains. That is unfortunate.

It seems that antimonials work by enhancing the ability of macrophages to kill the parasites they ingest. Antimonial resistant strains appear to have evolved a way to swing things the other way somehow – they don’t do just as well as the sensitive strains; they actually do better. The authors write “all [sensitive] strains caused a similar parasite burden both in the liver and the spleen… [Resistant] strains displayed… an average 8-fold higher parasite burden in the liver and 3-fold higher parasite burden in the spleen compared to [sensitive] strains” (p. 3). That is an impressive increase, and not good news for the patient.

The authors also looked at whether these strains are also more resistant to the new drug of choice, miltefosine. They didn’t find evidence of this, but note that more study and surveillance are needed to be sure it’s not the case.

Vanaerschot M, De Doncker S, Rijal S, Maes L, Dujardin J-C, et al. (2011) "Antimonial Resistance in Leishmania donovani Is Associated with Increased InVivo Parasite Burden." PLoS ONE 6(8): e23120. doi:10.1371/journal.pone.0023120