What do we know about the easing of malaria burdens?

by on November 10, 2013 at 7:30 am in Medicine, Science | Permalink

There are some new and interesting results from Lena Huldén, Ross McKitrick and Larry Huldén.  Here is the abstract:

Malaria has disappeared in some countries but not others, and an explanation for the eradication pattern has been elusive. We show that the probability of malaria eradication jumps sharply when average household size in a country drops below four persons. Part of the effect commonly attributed to income growth is likely due to declining household size. The effect of DDT usage is difficult to isolate but we only identify a weak role for it. Warmer temperatures are not associated with increased malaria prevalence. We propose that household size matters because malaria is transmitted indoors at night, so the fewer people are sleeping in the same room, the lower the probability of transmission of the parasite to a new victim. We test this hypothesis by contrasting malaria incidence with dengue fever, another mosquito-borne illness spread mainly by daytime outdoor contact.

The gated published version is here.  A six-page author summary is here.

For pointers I thank Aaron C. Chmielewski and Gregory Rehmke.

prior_approval November 10, 2013 at 8:46 am

‘Warmer temperatures are not associated with increased malaria prevalence.’

Well, possibly depending on how you measure it –

‘Temperature is an important determinant of malaria transmission. Recent work has shown that mosquito and parasite biology are influenced not only by average temperature, but also by the extent of the daily temperature variation. Here we examine how parasite development within the mosquito (Extrinsic Incubation Period) is expected to vary over time and space depending on the diurnal temperature range and baseline mean temperature in Kenya and across Africa. Our results show that under cool conditions, the typical approach of using mean monthly temperatures alone to characterize the transmission environment will underestimate parasite development. In contrast, under warmer conditions, the use of mean temperatures will overestimate development. Qualitatively similar patterns hold using both outdoor and indoor temperatures. These findings have important implications for defining malaria risk. Furthermore, understanding the influence of daily temperature dynamics could provide new insights into ectotherm ecology both now and in response to future climate change.’ http://www.nature.com/srep/2013/130218/srep01300/full/srep01300.html

And there remains this little mystery about malaria’s prevalence, and how it was ‘eradicated’ – ‘Plasmodium vivax is the most common of four human malaria species, with a worldwide distribution within approximately 16 to 20 north and south of the summer isotherms. Before its unexplained disappearance from Europe, P. vivax was probably present as far north as Moscow.’ – ‘Geographic Subdivision of the Range of the Malaria Parasite, Plasmodium vivax’

Ray Lopez November 10, 2013 at 9:37 am

Population size and malaria decrease seems like a spurious correlation. Probably smaller household sizes are associated with richer countries that simply drain wetlands and rid the land of mosquitoes. In Messolonghi Greece, where the Philhellene Romantic poet Byron died of malaria in 1824, there were and are tons of mosquitoes since it’s wetlands, but gradually as the population got wealthier they drained and sprayed and by 1950 Greece was malaria free. Too bad about another casualty of increased wealth and urbanization: the Nemean lion (yes it existed outside of Herculean myth: Wikipedia: “However according to Herodotus, lion populations were extant in Ancient Greece, until around 100 BC when they were extinct.”

Axa November 10, 2013 at 10:06 am

Brazil has lots marshes and wetlands and not so much of malaria or yellow fever. Brazil is also in the Equator. Perhaps the article author is right about his model results.

Roy November 10, 2013 at 11:31 am

Brazil also eliminated malaria and yellow fever in the 1930s-40s. Malaria was basically removed from the western hemisphere then. Of course this form of malaria control, much of which occurred before DDT has been “discredited” by us in this more enlightened age.

http://www.ncbi.nlm.nih.gov/m/pubmed/12383612/

Ricardo November 10, 2013 at 9:23 pm

Before suggesting that a research result is a spurious result that occurs before the authors failed to control for variable X, it’s a wise idea to actually skim the paper and the statistical results to check whether the authors did, in fact, fail to control for variable X. In the description of the paper Tyler posted, here is their description of independent variables they include in their regressions: “Explanatory variables include Gross Domestic Product (GDP) per capita, average household size, female literacy, urbanization and slums, latitude, mean temperature, forest coverage, Muslim population, national DDT usage, population density and national mean temperature (over the 1980-2008 interval).”

Also noted in the summary Tyler posted, “An interesting aspect of this history is that the disease disappeared in many countries that made no special efforts to eradicate it, while remaining prevalent in other countries that tried. Numerous explanations for the global pattern of eradication have been suggested, such as a change in the feeding pattern of the insects, draining of wetlands, or intensive use of the insecticide DDT (dichlorodiphenyltrichloroethane). Despite superficial plausibility, such explanations begin to fail upon close examination.”

jdm November 10, 2013 at 10:20 am

“so the fewer people are sleeping in the same room, the lower the probability of transmission of the parasite to a new victim”

I’m not sure why the authors appear to consider this claim to be self evident. Can’t mosquitos fly from one room to another? And if there are fewer people in the room, isn’t the chance of a given person being bit by an infected mosquito higher? Would the incidence of malaria drop if every person had their own room (as opposed to their own net), all else being equal? This isn’t obvious, at least to me.

Ray Lopez November 10, 2013 at 12:42 pm

Here in the tropics (in Philippines now), there is dengue fever, which is spread by a daytime mosquito (as the author states). Malaria is a night time mosquito, and since I hate mosquito bites, my protocol is: (1) strong electric fan just over your bed–a strong breeze from said fan will blow the mosquitoes away from you. (2) screens on windows. I had to custom order them. Almost all houses outside the USA lack screens on windows, unless they are luxury homes. No kidding. (3) mosquito net (sprayed weekly with insecticide). A very good deterrent, though it will not stop tiny ‘no-see-up’ biting gnats. (4) spray at the first sign of a mosquito in your room. You cannot slap them, they are too quick. Spray, go outside for a half hour, and come back, all flying bugs will be dead. And, if I was in a malaria-infested country, I would also take anti-malarial drugs, which I’ve read are sometimes ineffective (mosquitoes have developed resistance).

dearieme November 10, 2013 at 2:40 pm

“Almost all houses outside the USA lack screens on windows”: untrue. Australia.

Ray Lopez November 11, 2013 at 5:26 am

Australia does not count, it’s an outlier for Asia. They btw are #1 for OECD countries for violence. When I was there, I felt a fistfight was going to break out any minute. I felt the Aussies were trying to be like the Californians, but without the cool factor. Maybe I was wrong but I would not want to live there–nobody lives there and it’s in the middle of nowhere on the planet. Sorry Peter Carey.

ChrisA November 10, 2013 at 11:57 pm

Ray. A more effective and safe remedy is to use air conditioning and keep the windows and doors always closed (and instruct your staff to do the same). Some people are “macho” about the use of air conditioning in developing tropical countries, but to me it is more about avoiding mosquito bites than comfort. And if you are prepared to use chemical sprays then what is your problem with using AC? We almost never have mosquitoes problems if we follow this process. If you like to experience no AC for a while, go for a walk outside.

Ray Lopez November 11, 2013 at 5:23 am

I do use AC. That’s a given in the tropics if you are a white man. But if you assume that AC, because it cools the room, will make the insects bite less, I doubt that. Like I say the electric fan blowing air onto you at night will drive away the mosquito 90% of the time. In fact, for some people that’s their only defense; they don’t even use nets like I do.

As for the study, since data is sparse (only 150 some countries in the world) I doubt you can “control” for all the variables the authors purport to control for. This paper is just another example of bogus social science research.

ChrisA November 12, 2013 at 1:49 am

Ray – I don’t assume that AC stops insects from biting, but that if you use AC you never need to open your bedroom window or doors for more than a very short period of time. Which means that very few insects ever get into your bedroom. You mentioned window screens, you don’t need them if your windows are always closed. You also need to train any maid etc in this policy.

Rahul November 10, 2013 at 11:15 am

If indeed it spreads intra-room, shouldn’t we observe time-proximal familial clusters in malaria epidemiology.

Do we?

mkt November 10, 2013 at 1:08 pm

Those effects might be swamped by micro-locational effects … yes there’d be family clusters of victims, but at the same time the families living next to the bog would also be the ones getting infected, and at about the same time. Plus I wonder if such a detailed data set exists anyway.

I was skeptical of their claims at first, but the 6-page summary made it sound more convincing, both on the basis of theory and evidence.

babar November 10, 2013 at 5:53 pm

not sure about that ‘drain the swamp’ stuff in places like kenya. i lived there a while, a pothole in the road or puddle in farm field would breed plenty mosquitoes

mulp November 10, 2013 at 6:10 pm

Declining household size is certainly correlated with better housing which is tighter and has screened doors and windows to keep out bugs.

The life cycle requires female getting blood meals to reproduce, but it requires 10 days from being infected with a malaria parasite to infect a host, so a house that keeps out most mosquitos, will disrupt the cycle because the female will need to survive 10 days or more flying around in a house, or get out of the house for 10 days or more and then get back in a house to find a host to infect. While humans can be bitten outside, the human is much more likely to react and kill it, and if killed before or within ten days of infection, the spread is prevented.

Reduce the number of human’s infected and that reduces the number of mosquitos inflected reducing the odds new people will be infected. Increase the odds of the mosquito being killed within ten days of being infected and getting to another human, and that reduces the number of humans invested over time.

“the fewer people are sleeping in the same room, the lower the probability of transmission of the parasite to a new victim”

If there were ten people in a room and one had malaria, in the ten days a few mosquitos had to fly around bothering people in order to be able to spread the parasite, aren’t the odds higher they will be killed than if there were only two people in the room?

Infected mosquitos do become more attracted to humans than uninfected mosquitoes, so 10 humans will be more attractive than one or two, but if the windows are screened, they will be stuck buzzing outside the house.

Treated bed nets are an attempt at a free lunch, because the mandated sacrifice of mandated capitalism is too hard for anyone to accept these days. No way can government set minimum building codes. No way can government step in to ensure even the poorest are forced capitalists by investing in subsidized housing to obtain higher tax revenue in the future from the higher productivity of capitalists freed of the productivity loss of malaria.

Malaria wasn’t ended in the US by eliminating all the bugs, but by creating the housing and workplaces that interrupted the lifecycle of the parasite, and the buildings are still designed to interrupt the life cycle so individuals coming into the US infected are not going lead to many infections even though the Anopheles is still around.

And screens keep out pollinators and other useful pest predator bugs that are needed for gardening and farming, while pesticides kill them all, good and bad, and thus reduce the productivity of nature on which we depend on.

Marie November 11, 2013 at 8:32 am

This has got to be the big “bingo”.
As societies become modernize you see both a strong reduction in family size and a strong reduction in time spent outside.
Since with malarial infection we’re talking about a good reduction in infection rates if you can get a good amount of disruption of transmission, this seems like a very clear answer. If you could, for example, have a ten year period where nobody in the world went outside at all, you’d probably take malaria out entirely.

The Anti-Gnostic November 10, 2013 at 6:42 pm

And of course, the author is Canadian. The perfect example of the liberal, educated idiot.

K-selected populations are more forward-thinking and apt to use things like mosquito nets, insect repellent and removing sources of standing water.

God help us if people stop eradicating rodents and fleas and bubonic plague starts showing up again. Epidemiologists will be frantically trying to figure out the correlation to government funding for pre-school.

Axa November 11, 2013 at 4:14 pm

Science, if it were easy everybody will do it. Pretty sure that James in 1926 was not the average MR reader.

“Early experiments with Plasmodium vivax showed that an infective mosquito will bite 30–40 times (James 1926). For a new person to be infected, a mosquito carrying the mature parasite back to its feeding location must find a victim who is not already infected. Therefore the more people who are sleeping together in the same room, the higher the probability of spreading the infection to a new person. Reinfection is thus a stochastic process, and below a certain threshold number of persons sleeping together, Plasmodium infection success rates drop below the replacement rate and it begins to disappear from the human population, even without other control measures. This study indicates that the threshold is likely crossed when average household size drops below somewhere between 4.0 and 4.5 persons.”

The Anti-Gnostic November 12, 2013 at 9:00 am

It’s still so bizarrely the answer to the wrong question. Most of the world has always had plenty of people living cheek by jowl in small spaces. But malaria isn’t a problem. Instead of giving him grant money to run his computer models to answer questions nobody’s asking (and solutions nobody’s going to implement), we’d literally get more bang for the buck by buying him a plane ticket and sending him off to spray insecticide and dump the water out of old tires.

Comments on this entry are closed.

Previous post:

Next post: