Kathie - doxy and malaria reply
 

This is the answer I got from Steve about why doxy doesn't stop transmission of malaria.

"Kim,

Malaria parasites undergo changes to different life stage forms (eg. sporozoite, merozoite, gametocytes, etc.). Its the asexual merozoites that make us sick. Doxycycline (and other tetracyclines) kill these merozoites (by blocking replication of their apicoplasts). But the forms that are important for transmission via the mosquito are not the merozoites, but rather the gametocytes. Interestingly, the gametocytes are far smaller in number and tend not to be responsible for disease, but without first becoming gametocytes, the parasites will not passage through the mosquito. Doxycycline is less effective at killing gametocytes, so while it make everyone feel better by killing the parasites that make us sick, it does not kill the parasites that may go on to make someone else sick. This is why Doxycycline is typically used in combination with agents that have gametocytocidal activity."

Now that I know exactly what I'm looking for, I can find all sorts of documentation this. This is probably why you need to take it for so long after leaving the malaria risk area...there needs to be time enough for all of the plasmodium to go through an entire life cycle. And, doxy only works in the blood, so any latent or growing parasites in the liver are untouched by doxy.

From http://www.rxlist.com/cgi/generic/doxycyc_wcp.htm
"Doxycycline offers substantial but not complete suppression of the asexual blood stages of Plasmodium strains.

Doxycycline does not suppress P. falciparum’s sexual blood stage gametocytes. Subjects completing this prophylactic regimen may still transmit the infection to mosquitoes outside endemic areas."

From http://www.doxycycline.com/prescribing.htm
"Doxycycline is effective against the asexual erytrocytic forms of the malaria agent, Plasmodium falciparum, but not against the gametocytes. Prophylactic use is recommended when travelling to tropical zones (malarious areas), especially where there are strains resistant to cloroquine and primaquine/sulfadoxine."

I just read a fascinating article on Malaria in the most current National Geographic...

Kim,

I'm leaving for the weekend, so only have time for a quick reply.

The data i've been able to access indicates (as we know) that malarone is effective against all stages of the malarial parasite, which is why it is used in treating malaria - it even kills off the liver stage. BUT - While malarone is effective in inhibiting the development of parasite in the mosquito, what has been studied has been the treatment dose of malarone, not the preventive dose. So the data I (and my malaria expert) was able to access was about treament doses. So it seems that all this is less important in practice than it is in theory in terms of a public health issue.

Ask your friend whether he has data bout the preventive dose also inhibiting the development of the parasite in the mosquito.

Interesting discussion, Kim.

Kim, one other note, when he says "This is why Doxycycline is typically used in combination with agents that have gametocytocidal activity." he is talking about treatment of malaria, not prevention.

OK...I've asked him about Malarone as prophylaxis and preventive dosages effectiveness on development of the parasite in the mosquito.

He has also said that "As a prophylactic, doxycycline may prevent malaria transmission by preventing fulminating infections." And then goes on to say that it's all a numbers game, and that doxy usually kills enough of the parasites so that the immunce system can get the rest...but that this would not be the case with using doxy as treatment, which is why if it is used as treatment, it is combined with another drug, like primaquine.

I'm still not clear about doxy as a prophylaxis. It seems like it may be effective most of the time, but not all of the time...which we know. I know that I'd prefer not to inadvertantly spread malaria...