nos-trum. pronunciation: \nos'-trum\. noun. Etymology: Latin, neuter of noster our, ours.
1. a medicine of secret composition recommended by its preparer but usually without scientific proof of its effectiveness.
2. a usually questionable remedy or scheme.
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Sunday, July 18, 2010

Mosquito-Borne Dengue Returns To The US. Why Not Malaria?

Do epidemic infectious diseases ever go away forever?

I saw a note in the USA Today health blog (July 17): about 40 people over the last year and a half have been diagnosed with dengue in Florida—confined to Key West. A survey of the population showed that almost 5% had been exposed over that period.

This is interesting because dengue has been unreported in the US for the last half-century. Yes, there have been scattered cases along the Texas-Mexico border when there were outbreaks in northern Mexico. But it was temporary and not an indication that the virus was returning to the United States. Most people have never heard of dengue though it’s the most common mosquito-borne illness in the world.

The cases from Florida, indicate that the virus is back in the in the continental United States…to stay, for now. Dengue is a febrile illness caused by a virus and transmitted by mosquitoes. No cure, no vaccine. The species of mosquito responsible for transmitting the disease tends to dwell in houses and can bite both in the daytime and at night.

Back when I was in graduate school in parasitology in the 70’s, I recall learning that the mosquito vector of malaria had been reported on the South side of Houston but not in the northern half, implying that the city was the northernmost area where the mosquito could be found. However there had been no cases of malaria in that area, and it was not considered a threat. Why was there no malaria? The opportunity for transmission was there. There was standing water; we all got bitten all the time…but no disease. Unknown.

So dengue is back, and other mosquito borne illnesses are still around. Annual outbreaks of West Nile virus appear to be waning. There are several vector borne infectious diseases that are present at low levels (St Louis encephalitis), but they tend to be rare, and occur in cycles.  Yellow fever hasn't been seen for a long time.

It just goes to show that you can't become too complacent about the spread of infectious disease. Is it possible that malaria could return to the US?  Well, it’s theoretically possible. We no longer spray for malaria like we did when I was a kid. The trucks would come down the street of our neighborhood, spraying what I assume was DDT, and for fun we would run through the fog of the spray. I’ve often wondered whether that exposure imposed any risk. Apparently not…so far.

That method of prevention wouldn’t meet current standards for safety. The threat of malaria and other mosquito-borne diseases is so remote, spraying was only partially effective, and the implications of breathing insecticide have not been fully studied. We were taught as kids to not allow standing water outside: we would empty rainwater out of barrels and old tires.

Again, could malaria, yellow fever, and dengue return in a big way? It’s possible. All it takes is (1) weather that mosquitoes like, (2) the introduction of the virus, (3) enough people around and available for mosquitoes to bite (so the mosquito can get the virus by biting an infected person and then bite someone uninfected) (4) standing water for the breeding of the mosquitoes, and (5) lack of awareness to detect and adopt preventive measures for the disease.

The last of these worries me a little. The detection and identification of H1N1 took several months.

And, I’m not thinking malaria when someone comes in with a fever.

On the whole, though, there are more important risks. You can’t prepare for everything.

Doc D

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