By Ken Child
Most of the time, a
mosquito bite is nothing more than a minor irritant that swells up, itches something fierce, then fades and is quickly forgotten. That is, unless that
mosquito is carrying a bug of its own.
Mosquitoes kill more than a million people across the globe every year through the transmission of dangerous viruses and parasites. A female mosquito lights on an infected person or animal, sucks up the diseased blood and passes it on to the next victim she bites.
As simply as that, mosquito-borne illnesses like West Nile virus, eastern equine encephalitis, malaria and even canine heartworms can spread throughout a population. The infected don't even realize they've picked up a disease until the
symptoms begin to show.
According to the Maryland Department of Agriculture, "for the first time in nearly 50 years endemic cases of dengue fever and malaria are in the United States. Improvements in world transportation now allow a person infected with a disease to be on a different continent each day. This enables mosquito-borne diseases to travel from one nation to the next."
That ability to spread sickness and death all over the planet is why the Smithsonian National Zoological Park has declared the female Anopheles mosquito - a malaria carrier - the deadliest animal in the world.
So, if you've ever wondered what diseases a mosquito can carry - and how they are transmitted - then read on.
Drawing blood with a
mosquito proboscis
First, you need to understand how a mosquito takes your blood.
For the most part, only female
mosquitoes feed on the blood of people and animals. They need the protein in blood to help their eggs develop, so they usually will feed before laying each batch. A female mosquito can lay up to three batches of eggs before she dies.
The mosquito uses a serrated proboscis to pierce the skin and reach a capillary. Through a tube inside the proboscis, the mosquito injects saliva containing a mixture of painkiller and blood thinner. Many people are allergic to the saliva, which is what causes the swelling and itching around the bite.
Once the saliva is in, the mosquito begins to draw blood through a second tube in the proboscis. The female usually takes about 0.001 to 0.01 milliliter of blood, according to the American Mosquito Control Association.
If the mosquito is carrying a disease, it can be transmitted through the tiny bits of blood remaining on the proboscis from the mosquito's last victim, as well through the saliva, which may contain viruses or parasites.
A single bite from the right mosquito at the right time, and you're infected.
That's why mosquito experts are always encouraging people to check their yards for standing water where the insects can breed, to avoid being outside at nightfall when they're hunting, and to use repellent, mosquito traps and even netting, if necessary, to keep from being bitten.
Mosquitoes bring malaria
Malaria is caused by a parasite that hitches a ride when an Anopheles
mosquito drinks an infected person's blood. Only the Anopheles can transmit malaria, according to the Institute of Food and Agricultural Sciences at the University of Florida.
Ronald Ross, a British entomologist, was the first person to make the connection between mosquitoes and malaria, discovering the parasites in 1897.
The parasites live in human red blood cells, reproducing asexually for two to three days until they burst the cell and flood the bloodstream with new parasites. Some develop into male and female gametocytes, which the mosquito takes in during feeding.
The gametocytes reproduce inside the mosquito over a period of one to three weeks, and create sporozoites that migrate to the insect's salivary glands. When the mosquito injects saliva into a person, it also passes along the sporozoites, infecting the person with malaria, the Florida researchers report.
Malaria symptoms mimic the flu, causing fever, chills and nausea. Left untreated, it can be fatal.
According to the U.S. Centers for Disease Control, nearly 500 million people around the world contract malaria each year, and more than one million die. Most of the deaths occur among children on the African continent.
The CDC reported that there were 63 outbreaks of malaria in the United States from 1957 to 2003. In each case, the outbreaks started with someone who had contracted the disease in a country where it is common, then brought it back to the U.S.
At least two species of Anopheles mosquitoes capable of transmitting malaria are prevalent in this country.
The West Nile virus and mosquito bites
The West Nile virus is a relatively mild infection that can sometimes lead to severe encephalitis. It was discovered in the blood of a woman living in Uganda in 1937, and is common throughout Africa, Asia and the Middle East.
It is spread when mosquitoes - primarily Culex mosquitoes - feed on infected birds such as crows, then pass it to humans through the injection of saliva at the next feeding. The virus enters the bloodstream and begins to multiply.
Symptoms can begin to show within three days to two weeks, and in some cases, the virus crosses into the brain, where it can cause inflammation and disrupt neurological functions, possibly leading to permanent damage to the nervous system.
Those older than 50 are most at risk.
But the good news is that about 80 percent of those who contract West Nile virus from mosquito bites never develop symptoms. A little less than 20 percent will come down with fevers, headaches, nausea and sometimes swollen lymph nodes.
And only about one out of every 150 people infected - less than 1 percent - develop encephalitis, the most severe form of the disease. The first signs of encephalitis are often flu-like symptoms and neck stiffness, leading to high fever, disorientation, seizures, blindness, paralysis and possibly death, according to the CDC.
The mosquito-borne illness first showed up in the United States in 1999 with an outbreak in New York, New Jersey and Connecticut. Within four years, it had had spread to most of the Midwest and killed a reported 23 people.
In 2007, there were West Nile virus infections in nearly every state. The CDC reported 3,598 illnesses and 121 deaths.
Mosquitoes carry other forms of encephalitis
Eastern equine encephalitis is a cousin to the West Nile Virus, spread the same way - by mosquitoes that have fed on infected birds.
The symptoms are similar and usually emerge about three to 10 days after transmission from a mosquito bite. However, it is a much more serious illness, proving fatal for 30 to 50 percent of those infected, especially among kids and the elderly.
"Because of the high mortality rate, (it) is regarded as one of the most serious mosquito-borne diseases in the United States," the CDC reports.
Fortunately, it is also relatively rare. Only 220 cases were reported between 1964 and 2004, primarily in Florida, Georgia, Massachusetts, and New Jersey.
Another particularly dangerous disease in the same family is St. Louis Encephalitis, common along the eastern seaboard, the Gulf Coast, and parts of the Midwest. Until West Nile arrived, it was considered the most serious mosquito-borne illness in the country.
As with the other forms of encephalitis, mosquitoes pick it up from birds and pass it to humans, primarily in the late summer to early winter. It doesn't harm either the birds or the insects, but can be brutal to people.
Slightly more than half of those infected develop brain inflammation and the accompanying neurological problems, and 5 to 30 percent will die, depending on the age of the person infected. Even those who survive sometimes suffer permanent memory loss or paralysis.
According to the CDC, there have been nearly 5,000 cases reported in the last 40 years.
Yeah, but what about mosquitoes and HIV?
Short answer: Doesn't happen.
On top of the diseases already mentioned, mosquito bites can transmit, in rare instances, the deadly yellow and dengue fevers to humans, and can pass heartworm larvae to your pooch through their saliva - by the way, once infected the dog is infected for life, according to the University of Florida agriculture extension office.
But researchers at Rutgers University say the one illness mosquitoes cannot carry from person to person is HIV.
For one thing, the virus that causes AIDS does not live in a mosquito's body, the way encephalitis does. Mosquitoes actually digest the virus along with the blood within a day or two of feeding. It never gets a chance to replicate and migrate to the salivary glands.
And, there just aren't enough HIV particles in the tiny amount of blood that remains on a mosquito's proboscis after feeding to cause infection. While encephalitis bugs circulate at a high volume in the blood stream, HIV traces are relatively minute.
The bottom line is that a person would have to be bitten by 10 million mosquitoes to even have a chance of transmission. And that's statistically improbable, the Rutgers researchers say.
Of course, as you've seen, mosquito-borne illnesses are deadly enough without HIV. Maybe now you understand why those backyard pests are considered one of the most dangerous creatures alive.
