Disease transmission and personality

Thanks Jack and Jonathan for a push to get back in the saddle! Experiments have become manageable again (at least for a short period of time) and Zach and my parents have completed their summer visit. Time for SCIENCE!

Feral_CatOne topic that I find particularly interesting concerns how personality traits influence patterns of disease.  For example, people who are frequently stressed out are at a greater risk of becoming obese or acquiring heart disease. We often look at disease transmission at a population or community level, but I think individual level differences in behavior are a crucial and understudied area of research.

When diseases are transmitted from one individual to another, then social behaviors are an important factor in transmission dynamics. While it’s often difficult to study this phenomenon in humans, numerous social animals provide ideal study systems.

Natoli et al. 2005 is 0ne of the coolest studies that I’ve read on personality traits and disease. This research group examined 3 colonies of feral domestic cats in Europe and collected data on how male personality traits correlated with FIV (feline immunodeficiency virus) infection. According to the article, FIV infections in feral cats are usually lethal within approximately 5 years, at which point the cat’s immune system is too weak to fend off otherwise nonlethal infections.

FIV transmission occurs when one cat bites another, transmitting the virus from its saliva into the other cat’s bloodstream. Bites usually occur when two males are battling over territory or dominance rank or when a male bites a female during sex.

Feral_Cat_2In order to get a handle on the “personality” of the feral cats in each colony, the researchers logged hundreds of hours taking notes on the frequency with which each cat engaged in aggressive, submissive, affiliative, territorial, display or mating behaviors. They aggregated all of these measures and came up for a single score for each individual that indicated how “proactive” or “reactive” that individual was. Proactive cats more regularly marked their territory, were the most aggressive (often winning aggressive interactions) and often affiliated with other members of the colony. Reactive individuals rarely displayed aggressive behavior and were frequently submissive towards other members of the colony.

When the behavioral results were compared with information about which cats were infected with FIV, a clear patterns emerged. The males at the top of the hierarchy (i.e., the most proactive, dominant males) were the most likely to be infected with FIV. Their aggressive demeanor meant that they incurred bites more frequently than more submissive males, increasing the probability that aggressive males contracted the disease.

This finding brings up a logical question. If all of these aggressive cats are contracting terminal diseases, why are aggressive cats still around? Well, the first answer to that question lies in that fact the cats don’t succumb to the illness quickly. The disease has a long asymptomatic period during which the cats continue to strut around as if nothing is wrong.

Importantly, aggressive behavior also maintains access to the ladies. Paternity tests revealed that the proactive cats were fathering a lot more of the kittens than the reactive males. Because aggressive dads produce more offspring than submissive dads, we might not expect aggressive behavior to be diminishing anytime soon.

This was a particularly nice study because it examined multiple populations (lending support to the generality of the results), measured lots of behaviors and included a direct measure of evolutionary fitness. It’s rare to find so comprehensive a study.

When dealing with socially transmitted diseases, personality plays a large role in the number of times an individual may be exposed to an infectious agent. In the study described above, aggressive behavior was the key to transmission. In other systems the important relevant measures may be frequency of sexual behavior, energy put into hygiene, number of individuals with which one associates, etc. Personality plays a large role in determining the diseases to which one is most susceptible.


Smart Mites

Parasitologists commonly observe species in which one sex is more heavily parasitized than the other. For example, territorial male impalas carry a much higher tick load than bachelor or female impalas (paper by Mooring and colleagues found here).  Territorial males spend less time grooming to remove ticks and instead spend their time watching for intruding males and wandering females.

Most of the explanations I’ve read for why one sex is more heavily burdened by parasites have focused on how host behavior, physiology and immune status influences their infection status. Few studies have examined if males or females have higher parasite loads because parasites are actively choosing one sex over another.

I was excited yesterday to come across this paper which examined whether or not parasitic mites are capable of preferentially infesting one sex. These researchers had previously observed that female bats from the genus Myotis are often infested with more mites than are males and so they decided to examine whether part of this pattern could be explained by the parasites “preferring” females.

Mouse-eared_BatsMites can not survive very long on their own and require a host for food and energy. It’s in the mite’s best interest to try to stay in areas of high bat density. This way they have plenty of other hosts when they reproduce and their offspring need to find a host of their own. It’s also a good idea to have other alternatives nearby in case the bat that the mite is currently living on falls ill and the mite needs to abandon ship.

If you’re like me, then when you imagine what bats you envision a cave wall with bats crammed in there nose to nose. The bats that roost in these large groups are females with their young. The males on the other hand, are loners. Instead of roosting with the group, they find a place to hunker down on their own. This means that, given a choice, mites should prefer to infest females who will surround themselves with other tasty bats over males encounter other bats far less frequently.

MyotisResearchers decided to test whether or not mites were capable of preferentially infecting females given a choice between both sexes. They placed a male and a female bat into an enclosed arena where their movements were limited and released a set number of mites into the enclosure. After keeping track of the mites’ choices, the researchers released the bats into an outdoor arena. Ten days later, the bats were recaptured and mite survival was quantified.

I know lots of people who don’t seem to know what’s good for them, but the mites seem to have it all figured out. The mites choose adult females significantly more often than they choose adult males AND their survival on female bats was much higher than on males.

The exact mechanism by which the mites differentiate between the sexes is unknown, but it’s likely that they’re using hormonal cues.

The more I learn about parasites the more amazed I am at how good they are at keeping themselves alive. This study showed that they’re capable of making good decisions when picking a host and I’ve discussed in a previous post how parasites are able to alter the behavior of hosts that they’ve successfully infected. I wouldn’t be surprised to find that the decisions made by parasites are driving differences in parasite loads between the sexes in lots of other species as well.

My Beef With Bottled Water

Some of the comments on my “Don’t Be a Sucker 101” post accused me of assuming that most people buy bottled water because they wish to avoid the fluoride that is added to tap water. I’m sorry if I was unclear, but I was not making that assumption. I don’t give regular bottled water drinkers THAT much credit.

Here’s the problem.

Old man giardia

1) TAP WATER IS FINE IN MOST CASES. Tap water is regularly tested and local municipalties have to abide by super stringent water quality standards. Lots of research has gone into establishing these standards and confirming that the levels of compounds in the water are safe for human consumption.

According to a 4-year National Resource Defense Council report, bottled water companies frequently are not held to the same standards. For example, bottled water companies are not required to test for waterborne parasites like giardia (a parasite that looks like an old man and will give you a nasty case of diarrhea) while local water municipalaties are required to do so.

In fact, the same NRDC report that I mentioned above identified contamination (bacteria, arsenic and synthetic organic chemicals) in nearly 1/3 of the bottled waters that they tested. I tried to read the rebuttal argument to the NRDC report from Bottled Water Web (the “definitive bottled water site”), but only paying members are allowed to view the document.

2) LOTS OF BOTTLED WATER CAME FROM THE TAP TO BEGIN WITH. Between 25-40% of the bottled water found on shelves didn’t come from pristine mountain springs, but from the local water utility near bottled water production plants. On the plus side, this at least means that the water has met FDA standards…

For example, Coca-Cola’s Dasani and Pepsi’s Aquafina come from local water sources and pass the water through a few additional filters before marking it up 10,000% and sticking it on the shelves.

If you’ve examined the research and have decided that you personally believe the levels of certain compounds should be even lower than recommended, then fine. Go find a bottled water company that filters more of this compound out than local municipalities do. I am going to go out on a limb here and say that probably no more than 10% (I think this is a conservative estimate) of bottled water consumers have looked at the research and made an educated decision about buying their water.

3) BOTTLED WATER IS RIDICULOUSLY EXPENSIVE. I’m not convinced that tap water which is already totally safe for human consumption is worth a huge mark-up after being run through a few more filters. During my days in the food service industry (thank god those are over!) I remember selling bottled water to customers for $2.16. I would always offer to give them a glass of tap water, but many preferred the bottled water instead. I found this particularly amusing as we were selling Dasani, which I’ve already noted is simply refiltered tap water anyway. Anyway, if you consider that they could have gotten tap water for free then they’re essentially paying infinitely more for bottled water.

4) PACKAGING AND PROCESSING CAN INTRODUCE CONTAMINANTS. In the past, processing and packaging the tap water introduced bacteria, resulting in water that was less safe to consume than when it first left the faucet (references here and here). The last study that I read on this topic was published in 1998, so I’ll concede that the industry may have cleaned up its act by now. That doesn’t change the fact that the water has encountered a whole lot more surfaces and has had a lot more time to sit and culture bacteria than would water that came directly from the tap.

BottledWater5) WASTE. This one is pretty obvious, so I won’t harp on it for long. You can probably imagine how much plastic is wasted in packaging bottled water, but lots of water is wasted as well. For example, producing 1 liter of bottled water requires the usage of 3 liters of water in the production process. Here are some other fun facts.

To be fair, there are certainly some cases where it may be smart to drink bottled water. For example, if you have a kidney condition and need to avoid fluoride, then researching which bottled water companies remove fluoride and then purchasing water from that company may be a good idea.

If you’re ever in an area where you know the tap water is contaminated or are outside of the US in countries where the tap water isn’t well regulated, then again it might be a good idea to drink bottled water.

To reiterate, if you live in a town in which the EPA’s yearly water quality report has informed you that you have high, unsafe contaminant levels in your drinking water, then I’m NOT talking about you. But you’re in the minority, so I’m talking about nearly everyone else.

There certainly are times when spending a lot of money on bottled water makes sense, but the majority of bottled water sales are purchased outside of these circumstances. Part of the taxes I pay go to ensuring the quality of the tap water I consume. As I live in a city that boasts no water quality problems, I stay away from bottled water as much as possible.

Epigenetics and delivery

In earlier posts I discussed how studies on rats have revealed that early life experiences modify the epigenome, which has long term consequences for stress responsiveness, reproductive behaviors, and obesity rates. It’s often difficult to do epigenetic studies on humans due to ethical concerns, but some pretty clever work has been done lately revealing that the results from the rat studies may be broadly applicable.

ChildBirthI found this article recently that describes a study comparing the methylation patterns of children delivered naturally to children delivered by C-section. They found that the section of the epigenome which codes for white blood cells is more heavily methylated in children born by C-section, perhaps because this procedure is more stressful for the child than natural birth.

We already know that children delivered by C-section are at a greater risk for diabetes, allergies and leukemia and the researchers who ran this study postulate that these greater risks may be the result of changes in methylation patterns during delivery.

It’s unclear whether the methylation patterns is caused by the procedure per se or by the mother’s stress over whatever complications are resulting in the need for a C-section in the first place. Further studies will be needed, but (as I mentioned in a previous post) some studies have already concluded that the fetus’ epigenome is altered when mothers are stressed out or depressed during gestation.

If researchers can convincingly make the claim that C-sections change methylation patterns (and therefore patterns of gene expression) in babies and that these changes in gene expression result in a greater risk for particular diseases, then perhaps we’ll see the trends in C-section rates reverse. At the moment, the percentage of deliveries that are done by C-section is steadily increasing worldwide.

I was under the impression that most C-sections were done when there were complications that put the mother and/or the baby at risk, but it looks like there are plenty of less serious reasons for why women get this procedure done. Some mothers elect to have C-sections so that they can “maintain vaginal tone” (i.e., they don’t want to become “loose”) or because they don’t want to experience the pain associated with childbirth.  Doctors are also inclined to perform C-sections at the slightest hint of a complication because this could save them from dealing with expensive malpractice suits.

I’d be interested in seeing if differences in the prevalence of particular diseases between countries could be correlated with country specific C-section rates. For example, Brazil has the highest rate of C-section delivery and I would be interested in finding out if they also have high rates of diabetes, allergies and leukemia as well.

Hopefully clever research will teach us more about the link between C-section, the epigenome and the risk of contracting particular diseases. If a clear link is found then we may see a big change in the way the public perceives non-emergency C-sections.

Don’t Be a Sucker 101

Bottled water sales are a sure sign that there are a lot of suckers out there. The world is full of people who can be talked into buying or believing almost anything, but who would make far better decisions if they had a very basic understanding of subjects like chemistry, logic, and statistics. I would love to see a course entitled “Don’t Be a Sucker 101” taught as an after school program or as a college elective course. The course would cover these topics:

1) Ridiculously basic chemistry.

Concentration matters.

For example, there are currently a lot of people who think that fluoride is dangerous to human health, and there is even a group actively working to get officials to stop adding fluoride to our drinking water. For those who don’t know, fluoride is added to our drinking water because it decreases the rate of tooth decay. In fact, studies have shown that water defluoridation results in an 18-40% decrease in children’s cavities. For poor families who can’t afford good dental care, water fluoridation makes a big difference.

But the critics of fluoride have a point. It CAN be dangerous. The key point, however, is that it is only dangerous in high concentrations. Like many other compounds (e.g., niacin, Vitamin A, and copper), fluoride is beneficial to the human body at low concentrations but is toxic or even fatal at high concentrations. The levels at which we encounter fluoride in our day-to-day lives are typically within the healthy range.

2) Basic logic.

I think that this part is particularly important. Politicians and advertisers are constantly making illogical statements in an attempt to sway public opinion. Here are a few that I think people fall for pretty often:

Ignoratio elenchi. These are arguments that may be true, but are not relevant to the question at hand. For example, I’ve heard people argue that evolution can not be true because, if it were true, then life would have no meaning. The potential ethical implications of evolution are not relevant to the question of whether or not evolution is “true” and thus should not be included in arguments against evolution.

This type of logical fallacy seems particularly popular with politicians, who often avoid directly answering a question by diverting attention to an argument that they feel more comfortable making. If more people were watching out for ignoratio elenchi fallacies then perhaps we could hold more politicians accountable for answering the question they were actually asked.

Ad hominem. An ad hominem attack is when someone attempts to discredit a statement made by an opponent by attacking the person rather than addressing the claim. A good example of this can be found in a previous post, where the author tries to discredit the claims of a group of scientists by noting that these scientists once collaborated with a scientist who now has a bad reputation. They should have attacked the scientist’s methods and claims.

It definitely is important to take into consideration who is making a particular claim. If the person making the claim is an established liar, then you certainly want to take their arguments with a grain of salt. But if you’re arguing about the best way to deal with the financial crisis, for example, then the fact that your opponent smoked pot once in 10th grade probably isn’t relevant.

Appeal to authority. Check out my post on the worst advertisement ever for my favorite example. In essence, this type of argument hinges on the hope that the listener will believe a statement is true simply because it was made by a prominent figure. Lots of money is spent and poor decisions are made because people are more than willing to do what an authority figure directs them to do.

A particularly heinous way in which appeals to authority are used is in commercials with doctors to sell products. A doctor’s opinion on a particular product or procedure means nothing unless that doctor has conducted peer-reviewed research to back his opinion up. Unfortunately, many of us are quick to trust anything that comes out of a doctor’s mouth without carefully scrutinizing her statements.

3) Basic statistics.

People rarely question the statistics that they encounter on a day-to-day basis. For example, when you hear that 50% of people believe X, you should immediately ask yourself 1) What was the sample size on that poll? and 2) What sample population was polled (e.g., a call-in poll would have very different results if the viewers were calling in while watching Bill O’Reilly as opposed to Keith Olbermann)?

Many people also fail to put statistics into perspective. Hearing that 332 peopled died from swine flu sounds impressive, but it means nothing unless you also know how many people were infected total (and did not die) as well as information about the people that did die (e.g., were these 332 people immunocompromised or elderly?). I’m certainly not indicating that the swine flu or other emerging diseases should not attract our attention (or more importantly the attention of epidemiologists), but I do think that the publicity surrounding swine flu caused unnecessary hysteria in the United States.

Here is another good blog post on the misuse of statistics by creationists and AIDS denialists.

4) Home economics.

This course would cover the absolute basics, including what to expect when you get your first credit card, buy your first house, or take out your first loan. Who knows, maybe we could have avoided some of the current housing crisis if people understood what they were getting into.

Don’t Be a Sucker 101 wouldn’t even have to be a particularly long course. If the course met for 2 hour sessions once a week it could probably be done in a month’s time. Anyway, I’m looking forward to hearing about what you think belongs in a Don’t Be a Sucker course.

A Critical Look at the STATS Article Covering the BPA Debate

I was recently directed by my fiancé to this article by a non-profit organization called STATS. The article is called “Science suppressed: How America became obsessed with BPA” and focuses on the controvery over health impacts of Bisphenol-A (BPA).

Background on the BPA debate

BPABPA is chemical found in many products, including medical supplies, dental fillings, water bottles, baby bottles, CD’s, DVD’s, and food containers. BPA leaches out of these products over time and is frequently ingested.

When bisphenol-A is found in the blood, it acts as a weak endocrine disruptor, which means that its structure is similar enough to natural hormones (in this case, estrogen) to be capable of binding to hormone receptors. Studies on animal models suggest that BPA consumption can lead to numerous negative outcomes, including reproductive health problems, the early onset of puberty, obesity, increased rates of mammary and prostate cancers and neurological problems (to name a few).

The article written by STATS was particularly interesting because it presented a side of the debate about which I wasn’t familiar. It pointed out that numerous regulatory agencies including the European Food Safety Authory as well as agencies in Japan, Australia, New Zealand and the United States have reviewed the available literature and determined that BPA does NOT pose a significant health threat.

PET_bottlesThe literature on the health impacts of BPA is divided. Independent researchers often determine that BPA has harmful health effects while industry scientists decide that there are no harmful health effects at the levels in which people are currently encountering BPA.  So how did the government agencies decide which body of literature to base their findings on?

The agencies determined that the independent researchers often used experimental methods that were unsound.  For example, the route of BPA introduction for many of these studies was by injection rather than by ingestion. This is a significant problem because when BPA is ingested it is processed by the gastrointestinal tract.  In the liver, enzymes add a sugar molecule to BPA, which makes it easy to pass from the body and (most importantly) makes it unable to bind to estrogen receptors (so it is no longer an endocrine disrupter). Because most ingested BPA is made inert by this process, studies which inject BPA will dramatically overestimate the negative health effects associated with consuming the same concentration of the chemical orally.

The STATS article also points out that many of the discarded studies had low sample sizes, which means that there were too few animals in the study to draw any reliable conclusions.

A final complaint against the independent studies was that they only administered one dose concentration to the animals in their study.  Administering different dose concentrations (for example, using a control saline solution, a low dose, and a high dose) gives you a better idea of how the compound effects human health.  The impacts could be linear, meaning that the more you get the the worse the health effects or non-linear, meaning that small doses may cause no ill effects but passing a threshold concentration could induce serious effects (an all-or-nothing kind of effect).

An additional problem with administering only one dose is that you might mask an impurity in your BPA solution. If you administer two different doses and your BPA solution was well-manufactured, then there should be a difference in the magnitude of health effects if BPA is causing problems.  If the solution was not made well and some impurity in the solution is causing health problems, then hopefully the concentration of the impurity will be the same in the two doses and the health effects will be of the same magnitude in each treatment.

Based on these concerns, government agencies threw out much of the independent research and made their decisions based on industry research.

Commentary on the STATS article

BabyBottleThe remainder of the STATS article discusses how The Journal Sentinel reported only one side of the story and began America’s hysteria over the use of BPA in food and beverage containers.  I’m more interested in the actual science on BPA than I am about how reporters misrepresented the story (which is clearly important as well), so I won’t be commenting on that aspect of the STATS article.

The website claims that the STATS organization is non-partisan, which I took to mean that they would present both side of the argument in a balanced manner. Perhaps the point of this article was to scold The Journal Sentinel and not to present both sides of the debate, but the tone of the paper was definitely anti anyone who thinks BPA has negative health effects.

Frederick vom Saal, a prominent researcher on the negative health effects of BPA, is set up at the beginning of the article as being an overzealous fool. Many government agencies decided did not consider vom Saal’s findings in their official decisions because of unsound scientific methods (likely for the reasons outlined above) and vom Saal has taken a fairly extreme stance on the BPA debate.  He has likened BPA to the “biological equivalent of global warming” and his extreme stance sets him up to look like an unreliable source.

It should be noted, however, that vom Saal’s research is funded by the National Institute of Health. This means that a number of prominent medical researchers read vom Saal’s experimental protocol and determined that it was sound enough to be funded. His findings were then published in high-end, peer-reviewed journals, again attesting to the fact that vom Saal’s standing in the scientific community is sound.

But fine, I’ll buy that vom Saal’s opinion shouldn’t be the final word on the BPA debate. After setting vom Saal up to look like a zealous loony, they then attempt to discredit numerous other BPA researchers by noting any association with vom Saal that they may have had in the past.

For example, National Public Radio aired a Living on Earth episode which relied heavily on the input of Ana Soto. The STATS article is quick to note that Ana Soto collaborated with vom Saal in 1993 on one of the first papers to propose that BPA may be an endocrine disrupter (which it is). This is mentioned as a way to insinuate that Soto is as unreliable as vom Saal, which is an unfair conclusion to draw. First of all, collaborators can agree on results but disagree on their broader implications, suggesting that Soto could take a more down to earth stance on the BPA debate than vom Saal has taken.  Additionally, Soto has not collaborated with vom Saal again in 16 years. A lot can happen in a decade and a half and it’s possible that they haven’t collaborated again in that span of time because of differences in opinion.

The STATS article also tries to discredit a large group of independent researchers who signed a document called the Chapel Hill Consensus by pointing out that, you guessed it, vom Saal played a big role in the development of the document (he was the lead author).  You can’t discard the opinion of 38 prominent researchers because one researcher with whom you disagree was involved in the drafting of the document. All 38 of these researchers hail from respectable institutions across the country where they run successful research labs.

A big general problem with the BPA debate is that independent researchers often determine that BPA has negative effects on human health while industry researchers (with a vested interest in keeping BPA on the market) find that BPA is not harmful to human health (at least at the levels in which we currently ingest it). The STATS article rightly points out that immediately discrediting industry research is foolish and that lots of high quality research is conducted by industry groups. This still, however, doesn’t reconcile the problem with the fact that a large group of independent researchers are not coming to the same conclusions as the industry scientists.

Mouse_pupsThe article writes off this discrepancy by refering to the experimental method issues described above (e.g., injecting and not feeding BPA to animal subjects). While it is certaintly true that some experiments may not be very informative due to experimental design issues, there are plenty of experiments conducted by independent researchers that are finding harmful effects of BPA when conducting sound research.  For example, I mentioned an experiment in an earlier post in which BPA was fed (not injected) to a pregnant mouse, having a significant impact on the pup’s epigenome (see slide show explaining the research here).

If there is a discrepancy between industry and independent research findings, then we should give money to independent researchers who are employing sound experimental methods. Because BPA has the potential to be such a huge public health issue, it’s imperative that we get independent researchers to run the same test that industry researchers are running and compare their results. With so much at stake, we can’t afford to rely entirely on the findings of a group with vested interests.

Additionally, it seems to me as though there is a lot about potential effects of BPA that we don’t yet understand. For example, perhaps BPA is safe in adults and safe in children, but can have major effects on a developing embryo.  The tone of the STATS article would lead you to believe that everyone should conclude that BPA is totally safe at current exposure levels, but clearly there is a lot of work left to do.

I would certainly suggest reading the STATS article (if you have time, it’s 24 pages long) to get the often unheard other side of the story. It makes a good case that our panic over the use of BPA may be overblow, but I think that we still have a long road ahead of us before we can say with certaintly that BPA really isn’t a health hazard.

Conversion Troubles

Challenger1983Despite the fact that NASA’s spokesperson has admitted that NASA needs to make the jump to using the International System of Units (SI units), it will be building its next space shuttle using Imperial units. The international community is complaining that this will make their shuttle difficult to use by other countries because of the conversions that will be necessary for them to use it. Additionally, it shows that NASA hasn’t learned from the incident back in 1999 when it lost an unmanned probe due errors in unit conversions between metric software and SI systems on board. NASA argues that it would cost entirely too much to convert their equipment and blueprints to SI units instead of the archaic Imperial units they currently use.

It has always really frustrated me that the system of measurements I learned in school is not universal and, more importantly, is not the system being used by most scientists. We’re at enough of a disadvantage in science due to the fact that many of us speak different languages, can’t we at least have a universal measurement system?  I understand that it would take a lot of money to make the switch, but we’d pay to make the switch once and from now on everyone would be on the same page. Heck, Canada has already done it (mostly)!

For an article on NASA’s decision to stick with Imperial measurements, check this out.