Mosquitoes & Public Health

HISTORY

Historically, mosquito-borne disease presented a serious problem for early western settlers. In the upper Midwest, in the late 1800’s it was necessary to hire two crews for each logging camp because it was not unusual for half of the workers to be out at any one time with malaria. This mosquito-transmitted blood parasite continued to be a problem in many parts of the U.S. into the early 1950s when it was brought under control by eliminating the human sources of infection. Humans infected with the parasite are the only source of malaria infection, and with the appearance of effective drugs, window screens, and a better understanding of mosquitoes and the disease, human malaria in the U. S. was eliminated. Malaria is not the only mosquito-borne disease that has caused problems in the past, even into the early 1940’s hundreds of cases of equine encephalitis were reported in the mid-west and west each year. It wasn’t until the 1950’s however that the first human cases of mosquito-borne encephalitis were recognized. Even today human and equine cases of encephalitis are not rare occurrences. In Colorado in 1987, 45 horse and 30 human cases of Western Equine Encephalitis (WEE) were diagnosed. Also, in the same year, 6 human cases of St. Louis Encephalitis (SLE) were reported. In 1991, 1 human and 1 horse case of WEE was reported in the State. In 2002 everything changed, West Nile Virusarrived in Colorado.

HOW MOSQUITOES SPREAD DISEASE

Mosquitoes can spread disease only when they bite their victim. Although it is commonly called a “bite”, the process is actually a piercing-sucking action. Only the female mosquito bites, and takes a blood meal. The blood protein is needed to complete the mosquito’s egg production cycle. During the feeding process, the female pierces her victim’s skin with her proboscis, (a long straw-like structure with a sharp end) injects her saliva (which contains anti-coagulants), and then sucks the victim’s blood in through her proboscis. If the victim’s blood contains disease-causing organisms, they too get sucked into the mosquito’s stomach. These organisms are then maintained within the mosquito and eventually may be injected into the next victim’s bloodstream. In this way, the mosquito can spread disease from animal to animal, animal to man, or even from person to person.

MALARIA

Malaria Heat Map

Malaria is a parasitical disease. The parasite spreads to humans through the bite of an infected mosquito. People who contract malaria usually feel very sick with a high fever and shaking chills, death is also possible with this disease. It may take up to a year for the symptoms of malaria to appear. The disease can lie dormant in the liver for an extended period of time.

While the disease is uncommon in temperate climates, malaria is still common in tropical and subtropical countries. Each year nearly 290 million people are infected with malaria, and more than 400,000 people die of the disease.

ENCEPHALITIDES

In the United States, there are now about seven primary mosquito-borne viruses that are capable of crossing the blood-brain barrier in humans and other animals, causing an acute infection of the central nervous system. These include Western Equine Encephalitis (WEE) and St. Louis Encephalitis (SLE) which have been known to occur in Colorado

ZIKA VIRUS

The Zika virus is most often contracted in people through mosquito bites. This happens primarily in tropical and subtropical areas of the world. Most people infected with the Zika virus have no signs or symptoms. Some people have a mild fever, rash, and muscle pain. In rare cases, the Zika virus may cause brain or nervous system complications, such as Guillain-Barre syndrome, even in people who never show symptoms of infection.

WEST NILE VIRUS

In 2003, Colorado recorded the first large-scale human epidemic of mosquito-borne disease on record. Although the exact reasons for this widespread epidemic are not entirely clear, Colorado’s wet spring and hot summer certainly played a critical role. These conditions created an abnormally large and much earlier than normal hatch of Culex tarsalis mosquitoes. This hatch was approximately one month earlier than normal based on Colorado Mosquito Control trapping records over the previous nine years on the northern Front Range. We feel that this early and large hatch of Culex mosquitoes allowed the virus to replicate and spread rapidly through the bird and existing adult mosquito populations, which in turn infected the rapidly increasing Culex populations and eventually allowed the virus to spread to other animals and humans.