And Why it Doesn’t Happen More Often . . .
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# 5106
Malaria, like dengue fever, is a mosquito borne illness that had all but been eliminated in Florida over the past half century.
At least . . . locally acquired cases.
But each year, we get scores of imported cases of both diseases courtesy of travelers and tourists coming to Florida (or Floridians returning from a vacation) from someplace where these diseases are endemic.
Since both diseases can be transmitted by mosquitoes, the danger has always been that an actively infected person could come to Florida, be bitten by a local mosquito, and that mosquito could go on to bite someone else . . . spreading the disease.
A bit surprisingly, that doesn’t happen as often as you might think.
It probably take numerous opportunities, and just the right set of conditions, for either of these diseases to reestablish themselves.
In fact, despite millions of visitors each year, it wasn’t until 2009 that locally acquired dengue was detected in Florida – after an absence of more than 50 years (see MMWR: Dengue Fever In Key West).
Through aggressive mosquito control efforts, malaria was pretty much eradicated in Florida in 1948. No locally acquired cases were confirmed for the next 42 years.
Since 1990 there have been a few scattered suspected (and confirmed) cases of locally acquired malaria in Florida, including:
In June 1990, a woman with no history of foreign travel, blood transfusion, or IV drug use was diagnosed with P. vivax malaria. She and her family had recently been camping in the panhandle’s Gulf County, and reported the mosquito activity as `intense’ (see MMWR report February 15, 1991 / 40(6);106-108).
In 1996 (2 cases) and again in 2003 (8 cases) of locally acquired P. vivax malaria were detected in Palm Beach County (see Multifocal Autochthonous Transmission of Malaria --- Florida, 2003).
Today we learn of another suspected locally acquired case of malaria, this time in Duval (Jacksonville) county.
First the press release and a couple of media links, then I’ll be back with more about the anopheles mosquito and the malaria parasite.
FOR IMMEDIATE RELEASE
Contact: DCHD Communications
December 1, 2010 (904) 253-1004Duval County Mosquito-Borne Illness Awareness
DUVAL COUNTY- The Duval County Health Department (DCHD) has identified a case of Plasmodium falciparum malaria in a 31-year old female resident of Jacksonville with no history of international travel.
Although the risk of ongoing transmission is low as the weather becomes cooler and drier, the health department would like health care providers and residents to be aware of additional malaria cases, and requests that any suspect cases be immediately reported to DCHD Epidemiology Department at 904-253-1850.
An investigation is underway to determine whether this case of malaria was possibly locally acquired. The DCHD is working with City of Jacksonville (COJ) Mosquito Control, the Florida Department of Health (DOH) and the Centers for Disease Control and Prevention (CDC) to better understand where this infection originated.
Florida has reported 111 cases of malaria since the beginning of 2010, all imported from other countries. Duval County has reported 5 imported cases of Malaria this year. The sixth case in Duval County appears not to be connected to recent travel making this possibly the first case of locally acquired malaria in the last ten years for the county.
A couple of local media links on this story include:
Jax Woman, 31, Contracts Malaria – News4Jax.com
Malaria case found in Duval County – Florida Times-Union
The good news is, the first really good cold snap of the year has just arrived in Florida, and the temperatures this morning in Jacksonville are down to 37 °F. (3 °C).
While mosquitoes can be found year round in Florida, they are greatly suppressed during the winter . . . at least in central and northern parts of the state.
The potential for additional cases in Duval county is not particularly high, but it isn’t zero either.
There are four microscopic protozoan parasites in the genus Plasmodium (P. vivax, P. falciparum, P. malariae and P. ovale) that cause malaria in humans around the world.
The type of malaria detected in this Jacksonville case is Plasmodium falciparum, which is regarded as more serious than the P. Vivax parasite reported in the cases in Palm Beach and Gulf counties.
Malaria is spread by the Anopheles mosquito, and of the roughly 70 species of mosquitoes that inhabit Florida, more than a dozen are of the genus Anopheles . . . which means there are ample natural vectors available locally.
But it isn’t easy for malaria to become established in a new area, particularly from infrequent, widely scattered, imported cases.
First, an infected person must arrive in the region and be bitten by a female Anopheles mosquito. That, unfortunately, can happen fairly often.
But that mosquito must then survive long enough for completion of sporogony ( the extrinsic incubation period - EIP), which while temperature dependent, usually takes at least 7 to 14 days (cite Malaria Journal).
Until then, the mosquito is thought incapable of passing the parasite on to another host.
Although the lifespan of the adult mosquito varies among species, it is a tough world out there for these little winged blood suckers.
Between birds, insecticides, cold snaps, slapping hands, and and other natural hazards . . . very few live beyond a couple of weeks time.
Which reduces the odds of ongoing transmission.
So it takes just the right combination of an imported case providing a blood meal to the right kind of mosquito . . . that manages to survive long enough to incubate the parasite . . . and then that mosquito then goes on to feed on a new human host.
Like I say, the odds are not zero.
But were it an easy thing to happen, with more than a hundred imported cases of malaria each year, we’d already be hip deep in locally acquired infections here in Florida.
And we aren’t.
But the modest ongoing spread of Dengue (which has a similar EIP of 8-12 days) in south Florida shows that if you have enough introductions, a competent vector, and just the right conditions . . . a vector borne disease can become reestablished.
Something to think about when considering budget requests for your local health departments and mosquito control agencies. They are literally the thin line that protects us from more frequent outbreaks of diseases like these.
For more on vector borne illnesses, you may wish to check out:
ASTMH: Dengue and Insect-Borne EIDs In The US
MMWR: Dengue Epidemic In Puerto Rico
MMWR: Dengue Fever In Key West
Eastern Equine Encephalitis (EEE)
