Do mosquitoes prefer certain blood types? Plus: Do tall men get more skin cancer?

August 28, 2009

Dear Cecil:

Do mosquitoes favor certain blood types over others? I have noticed while out working that mosquitoes love me while my brother never gets bit. Why is my blood so tasty?

Cecil replies:

Sibling rivalry thing going on there, Brian? I've got good news for you. We still don't know why mom preferred your brother, but we're starting to home in on why mosquitoes prefer you.

A study in Nature (Wood and Dore, 1972) found that mosquitoes were more likely to bite people with type O blood than other potential victims, while people with type A got the fewest bites. A follow-up experiment examined whether this had anything to do with the fact that some people secrete saccharides — sugarlike chemicals related to blood type — through their skin. Results:

  1. Generally speaking, skeeters favor people with type O blood over those with types A and B blood, confirming the earlier research.
  2. They really favor type O secretors over type O nonsecretors and type A secretors.
  3. They don't have a strong preference between type O secretors and type A non-secretors, a distracting result I'll ignore.

Conclusion: to quote a favorite line from the Straight Dope Message Board, if you're a type O secretor, to a mosquito you look like caramel-covered crack.

Other studies have cast doubt on this phenomenon, claiming that factors such as sweatiness are more likely to influence victim selection. But a Japanese study (Shirai et al., 2004) seems convincing. Researchers exposed 64 volunteers to a swarm of hungry female mosquitoes, each of which had had its proboscis — the part it bites with — amputated. (One can imagine the outraged e-mail this surely elicited from PETA.) Since the mosquitoes couldn't bite, and thus couldn’t drink their fill, the researchers were able to compare how often they landed on the skin of different volunteers. They found type O secretors were twice as attractive to mosquitoes as type A secretors.

How do mosquitoes know about chemicals on your skin? Smell, probably. Mosquito repellents ranging from garlic to DEET seem to work by blocking their sense of smell.

Now for the grim part. Some research suggests being infected with malaria changes your body odor or breath to attract more of the mosquitoes who infected you in the first place. Does this mean type O secretors who travel to malarial parts of the world are doomed to help spread the illness? Not necessarily. A 1980 study by the World Health Organization showed Indian malaria victims were more likely to have type A blood than any other type. That's not because people with type A get bitten more, the researchers speculated, but rather because when they do get bitten, their type A blood makes them more likely to contract the disease.

Dear Cecil:

I just read this interesting statistic: taller men have a higher incidence of melanoma than shorter men. What would account for this? Being a foot closer to the sun shouldn't be a big factor as far as UV radiation goes. Is it from standing in crowds, where the taller men get the sun and the shorter people get their shade? Are taller men balder and so get more sun on their heads?

Cecil replies:

Who knows? While some health issues are clearly connected to height — banging your head on doorways, say — melanoma (skin cancer) isn't one of them. The major risk factors are sun exposure and ethnic background, with Celtic types having red hair and freckles in greatest danger. However, a few studies claim to show a link between increased height and weight and skin cancer risk — more specifically, between cancer and body skin area (BSA). The idea is that the more skin you've got, the more chances you have to develop skin cancer.

For example, a study of farmers in Iowa and North Carolina found that a six-foot-tall person weighing 200 pounds (BSA = 23 square feet) had a 40 percent greater risk of developing skin cancer than a five-foot-tall person of the same weight (BSA = 21 square feet). Another study conducted in Washington State showed height, weight, and BSA were all risk factors for skin cancer, and that men in the tallest 25 percent of the population were twice as likely to get skin cancer as those in the next quarter down. Australian researchers reviewing a collection of women's skin cancer studies concluded height was more likely a factor than skin area.

But why? Maybe taller people spend more time outside playing sports, or just eat more as kids. (High calorie intake in childhood is thought to be a risk factor for some cancers.) Maybe it has something to do with hormones — height has been found to correlate with the incidence of other cancers besides melanoma.

Then again, let's not forget the Longer Name Hypothesis: In the 22 U.S. presidential elections between 1876 and 1960, the candidate with more letters in his last name won the popular vote 20 times. Interesting? Sure. Meaningful? No. Likewise, the seeming height/cancer connection may just be a statistical fluke.

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References

Shirai, Yoshikazu et al. “Landing Preference of Aedes albopictus (Diptera: Culicidae) on Human Skin Among ABO Blood Groups, Secretors or Nonsecretors, and ABH Antigens” Journal of Medical Entomology 41.4 (2004): 796-799.

Gupta, Madhu and Chowdhuri, A.N. Rai. “Relationship between ABO blood groups and malaria.” Bulletin of the World Health Organization 58.6 (1980): 913-915.

Lacroix, Renaud et al. “Malaria Infection Increases Attractiveness of Humans to Mosquitoes.” PLOS Biology 3.9 (2005): 1590-1593.

Wood, Corrine Shear, and Dore, Caroline. “Selective Feeding of Anopheles gambiae according to ABO Blood Group Status.” Nature 239 (1972): 165.

Wood, Corrine Shear. “”ABO Blood Groups Related to Selection of Human Hosts by Yellow Fever Vector” Human Biology 48.2 (1976):337-341.

Zahar, A.R. (1984) Vector Bionomics in the Epidemiology and Control of Malaria. WHO/VBC/84.6 – WHO/MAP/84.3

Dennis, Leslie K. et al. “Cutaneous Melanoma and Obesity in the Agricultural Health Study.” AEP 18 (2008): 214-221.

Olsen, Catherine M. et al. “Anthropometric factors and risk of melanoma in women: A pooled analysis.” International Journal of Cancer 122 (2008):1100-1108.

Shors, Andrew et al. “Melanoma risk in relation to height, weight, and exercise (United States)” Cancer Causes and Control 12 (2001): 599-606.

Freedman, D. Michal et al. “Risk of melanoma in relation to smoking, alcohol intake, and other factors in a large occupational cohort.” Cancer Causes and Control 14 (2003): 847-857.

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