Mosquitoes & Public Health

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 1950’s, 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. See Links below for more information

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 victims 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 bloods in through her proboscis. If the victim’s blood contains disease-causing organisms, they too get sucked into the mosquitoes 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.

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.

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 wide-spread 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.