Re: Why do some people seem more prone to ticks and chiggers?

Thank you for your interesting question. 

You observed that you and your wife can go out in the woods together, 
wearing the same type of clothing and using the same repellants, yet she 
will come home with far more ticks and chiggers on her arms and legs as I.
You ask if some people are more prone to these parasites. 

Yes this is absolutely the case.

I can provide two examples stating scientific data illustrating your 
observation on a broader scale:

1.) There are 84 species of ticks in the United States (excluding Hawaii). 
In a recent survey by Merten and Durden (July 2000) a total of 44 tick 
species was recorded as parasites of humans, consisting of 11 species of 
soft ticks (Argasidae) and 33 species of hard ticks (Ixodidae). Four of the 
hard tick species are not native to the U.S, and were removed from 
travelers returning from foreign destinations. Therefore, 40 of the 84 
species of U.S. ticks are ectoparasites (means: living on the skin) of 
humans.

2.) There are numerous diseases transmitted by ticks, e.g. borreliosis, 
erlichiosis, babesiosis, and tick paralysis. On the latter one there is a 
recent report by Dworkin, Shoemaker and Anderson (Dec. 1999). They counted 
33 human cases in Washington State during the 50 years from 1946 to 1996. 
Most of the patients were female (76%), and most cases (82%) occurred in 
children aged <8 years.

These data show very well that tick select the species, that they bite and 
within a species they prefer one sex over the other or they differentiate 
between age groups. For mosquitoes (but not for ticks) it has been shown, 
that one person might be much more attractive for them than another person 
even if both are the same sex and age.

You ask: Is it a biochemical matter? Are there any indications that one 
person rather than another would be more susceptible to these insects?
What is the reason for such preferences?

Yes, it is – at least in part - a biochemical matter. Mosquitoes and ticks 
are attracted to ammonia, carbon dioxide, and fatty acids. But there are 
numerous other chemicals emitted by humans and animals, e.g. hexanal, 2-
heptenal, nonanal, furfural, benzaldehyde, and 2-hydroxybenzaldehyde, 
heptanal, 2-, 3-, and 4-methylbenzaldehyde, 2-nitrophenol, 4-methyl-2-
nitrophenol, gamma-valerolactone, and many more. All named above can be 
detected by ticks (and mosquitoes) and influence its behavior. 
Now add perfumes, hair spray, soaps, washing detergents, deodorants, 
repellents, …. Everything that causes us to smell differently can cause 
different attraction to mosquitoes and ticks.
Therefore it can not be easily predicted who gets most bites. It is not 
"the more you have the more often you get bitten", instead certain 
combinations of these chemicals seem to be viewed as "very delicious" by 
some mosquitoes and ticks.
The attractivity of a person for these blood suckers depends on age, sex, 
if a female then also what time of the monthly cycle, and on the bacterial 
flora you have on the skin. All these factors influence which and how many 
chemicals you emit. 

In addition being bitten by a tick it is also depended on physical 
properties, mainly: size, color, movements, body temperature, and humidity 
in the exhaled breath. Even the best smelling object will never be bitten 
if it does not have around 37 degree Celsius.

And finally the number of bites depends on behavioral factors. A person 
that slowly walks along a path but never leaves it and never stops gets 
much less bites than one who leaves the path two or three times to observe 
something, stopping for several seconds or minutes, especially if lots of 
leafs and grass are around – favorite spots for ticks to wait for a host.

What can be done against being bitten? Wear long sleeved clothes, show 
little bare skin. Use repellents. Avoid being outside at dawn - go for your 
walk in the morning. Be careful with perfumes; some really attract insects 
and ticks. Have a shower before you start. This reduces body odors and 
keeps attraction fo arthropods to a minimun.(Works well up to two hours.)

I hope this answer helps.

Here are the references for the reports cited: 

Merten HA, Durden LA, A state-by-state survey of ticks recorded from humans 
in the United States JOURNAL OF VECTOR ECOLOGY 25: (1) 102-113 JUN 2000

Dworkin MS, Shoemaker PC, Anderson DE. Tick paralysis: 33 human cases in 
Washington State, 1946-1996 CLINICAL INFECTIOUS DISEASES 29: (6) 1435-1439 
DEC 1999

If you want to dig deep into these questions, there is a good book:
Olfaction in mosquito-host interactions. Ciba Foundation Symposium 200, 
1996, John Wiley and Sons, Chichester, New York, 331 pages

If you would like more information about ticks, there are numerous Internet 
sites.
Just a few links as starting points:

 
tick related links 
 
tick transmitted diseases 
 
The tick research laboratory

Have fun and thanks again for your interesting question.
Dr. J Ziesmann