In response to this anti-pesticide sentiment, some local governments and institutions have mandated "minimal use" or "non-pesticide use" policies. However, it is important to realize that -- from the public health perspective -- pesticides serve a valuable function in society in preventing/controlling infectious diseases carried by insects and other arthropods.1 In addition, pesticides are not only valuable in disease control. Many areas of the U.S. coastlines would be totally uninhabitable without chemical (pesticidal) mosquito control due to the tremendous nuisance effect of their biting.

The Problem of Emerging or Re-emerging Infectious Diseases
All over the world, infectious diseases are making a comeback after a lull in the years following World War II. The ability of disease germs to adapt to the human defense system and intense pressure from antibiotic use, combined with changes in society, have contributed to this resurgence. Also, there are now several "new" diseases, including Legionnaires' disease, Lyme disease, ehrlichiosis, toxic shock syndrome, and Ebola hemorrhagic fever.

In just the last six or seven years we have seen the appearance of a new strain of bird influenza that attacks humans, a human form of "mad cow" disease, and new drug-resistant forms of Staphlococcus aureus. These new or emerging infectious diseases have raised considerable concern in the medical community about the possibility of widespread and possibly devastating disease epidemics.

It might be argued that at least some of this increase in infectious disease is due to increased recognition and reporting. Maybe we're just getting better at diagnosing these things. Specific disease recognition is certainly made easier by new laboratory technologies such as polymerase chain reaction (PCR). However, changes in society such as population increases, ecological and environmental changes, and especially suburbanization (building homes in tracts of forested lands) are contributing to a true increase in incidence of many of these diseases.

Major Mosquito-borne Diseases
Malaria. Malaria - the number one vector-borne disease worldwide - continues to reemerge in many areas. There are now an estimated 300 - 500 million cases of malaria worldwide each year with 1.5 - 2.7 million deaths.2 Several factors are responsible for the resurgence of age-old enemy: 1) massive environmental changes affecting Anopheles mosquito populations in endemic areas, 2) insecticide resistance in the vector mosquitoes, 3) drug resistance in the malaria parasite, 4) deforestation, 5) human population migrations, and 6) increased travel by non-immune expatriates.2,3 Malaria was once common in the U.S., but was eradicated in the 1940's and 50's. However, the mosquito vectors of malaria are still common in this country and, theoretically, the disease could once again become established here (there are occasional cases of malaria in the U.S., mainly from travelers returning from malarious areas).

Dengue. Since 1975, the mosquito-carried disease, dengue fever, has surfaced in huge outbreaks in more than 100 countries worldwide. Some experts estimate that there may be as many as 100 million cases of dengue each year.4 It is called "break-bone fever" because the classic form is characterized by sudden onset of fever, frontal headache, retro-orbital pain, and severe myalgias. The more dangerous form of the disease, dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS), with internal bleeding and shock, mainly affect children under age 15. DHF/DSS can be a dramatic disease with the patient's condition deteriorating very rapidly.

As is the case with malaria, the mosquito vectors of dengue occur commonly in the U.S., therefore, theoretically, there could be an outbreak of dengue in this country at any time (there are a few cases of dengue on the Mexican-U.S. border each year).

Mosquito-caused encephalitis. In the U.S., perhaps the worst danger from mosquitoes is encephalitis. There are certain viruses - carried by mosquitoes - which cause encephalitis in people and sometimes horses. These diseases are collectively called the encephalitides. They are essentially animal diseases, called "zoonoses", which only occasionally get transmitted to people. Most of them circulate among birds or small mammals by way of mosquito bites. Humans (or horses) get infected by accident - and thus are called "accidental hosts". All encephalitides more or less cause similar symptoms in humans, although with great differences in severity. Some of them are extremely mild with a 1% or less mortality rate, while others may kill as many as half of the people infected. And, making matters worse, even for the survivors, there are sometimes long-lasting effects such as memory loss, personality changes, etc. Below is a breakdown of the common mosquito-carried encephalitis viruses in the United States and some characteristics of each one.

West Nile encephalitis. West Nile virus (WNV) was identified for the first time in the Western Hemisphere in New York in 1999. Apparently, it was accidentally brought to the U.S. in an infected mosquito (maybe inside an airplane), an infected animal (perhaps a bird), or an infected person (traveling from an international location). By the end of the year, the virus had caused encephalitis in 62 people and numerous horses in and around New York City, resulting in 7 human and 10 equine deaths.10-11 The virus continued to spread in 2000, and now evidence of WNV has been found in at least 12 states and the District of Columbia. WNV will likely eventually occur throughout the eastern United States. As far as severity of the disease, WNV is no more dangerous than SLE (one of our "native" encephalitis viruses), with a mortality rate of 3-20%. Like SLE, WNV is more dangerous to older patients. A lot is yet unknown about the ecology of WNV in the U. S., but we do know the virus causes a bird disease, and is transmitted by mosquitoes. WNV is believed to be transmitted to humans by Culex pipiens, Cx. restuans, Cx. salinarius, and possibly Aedes japonicus.

Eastern equine encephalitis. Eastern equine encephalitis (EEE) is generally the worst strain, being severe and frequently fatal (mortality rate 30 to 60%). It is especially bad in children. Fortunately, large and widespread outbreaks are not common; between 1961 and 1985 only 99 human cases were reported in the U.S.5 EEE occurs in late summer and early fall in the central and northcentral U.S., parts of Canada, southward along the coastal margins of the eastern U.S. and the Gulf of Mexico, and sparsely throughout Central and South America. The life cycle of EEE is poorly understood. The virus circulates in wild bird populations by bird-feeding mosquitoes, but the exact mechanism of spread to humans is largely speculative. It is believed to be transmitted to humans by the mosquitoes, Aedes. sollicitans, Coquillettidia perturbans, and possibly Ae. vexans and Anopheles crucians.

St. Louis encephalitis. St. Louis encephalitis (SLE) produces lower mortality rates than EEE (3 to 20%), but occurs occasionally in large epidemics over much of the U.S. In contrast to EEE, SLE is worse in older people. But, like EEE, most cases occur in late summer. In 1933 there were 1,095 cases in the St. Louis area with more than 200 deaths.6 In 1975-76 there were over 2,000 cases reported from 30 states, primarily in the Mississippi valley.6 SLE is transmitted by Culex tarsalis (western and southwestern U.S.), Cx. quinquefasciatus (central and southeastern U.S.), and Cx. nigripalpus (southeastern U.S.).

Western equine encephalitis. Western equine encephalitis (WEE), occurring in the western and central U.S., parts of Canada, and parts of South America, has occurred in several large outbreaks. There were large epidemics in the north central U.S. in 1941 and in the central valley of California in 1952. The 1941 outbreak involved 3,000 cases. During 1964-1997, there were 639 human WEE cases reported to the CDC, for a national average of 19 cases per year.7 WEE is generally less severe than EEE and SLE, with a mortality rate of only 2 to 5%. Cases appear in early to midsummer, and are primarily due to bites by infected Culex tarsalis mosquitoes.

LaCrosse encephalitis. LaCrosse encephalitis (LAC) is a California group encephalitis which primarily affects children in the midwestern states of Ohio, Indiana, Minnesota, and Wisconsin. Cases have also occurred in the southern states, but certainly not to the extent that they have in the midwestern U.S. The mortality rate of LAC is less than 1%, but infection often leads to seizures. In fact, that is one of the main symptoms - seizures in infants and children. The national average for LAC cases is 73 per year.7 Most cases occur in July, August, and September. LAC is transmitted to humans by the tree hole mosquito, Aedes triseriatus. Interestingly, the virus may be transferred from adult female Ae. triseriatus to her offspring through the eggs. Some amplification of the virus takes place in nature through an Ae. triseriatus, wild vertebrate cycle.

Other California group encephalitis. Although LAC (above) encephalitis is probably the most notorious, several other California group encephalitis viruses exist. North American forms include California encephalitis (CE), Jamestown Canyon (JC), Jerry Slough (JS), Keystone (KEY), San Angelo (SA), Trivittatus (TVT), and others. Viruses in the California serogroup are primarily pathogens of rodents and rabbits. They are transmitted to people by several species of mosquitoes, but especially the tree-hole, floodwater, and snow pool Aedes spp. California group encephalitis viruses generally produce only mild illness in humans (mortality rates 1% or so).

Venezuelan equine encephalitis. Venezuelan equine encephalitis (VEE) is relatively mild in humans and rarely affects the central nervous system, but it will be included here. VEE is endemic in Mexico and Central and South America; epidemics occasionally reach the southern U.S. Cases generally appear during the rainy season. Although the mortality rate is generally <1%, significant morbidity is produced by this virus. In an outbreak in Venezuela from 1962 to 1964, there were more than 23,000 reported human cases with 156 deaths.8 In 1971, an outbreak of VEE in Mexico extended into Texas resulting in 84 human cases.9 There has been a relatively recent outbreak in Colombia and Venezuela during the summer of 1995 with at least 13,000 human cases.

Cause for Future Concern
It appears that we are in a precarious situation. The entire ecosystem - including plant and animal life on earth - is being affected by humans. People once lived in far-removed, relatively isolated groups. Now we are all essentially one large community. Further, things such as population increases, building cities in/near jungles, and widespread and frequent air travel are providing the opportunity for a great plague. For example, the number of international departures from U.S. airports doubled from 20 million to nearly 40 million between 1983 and 1995.12 A person hiking in the Amazon jungles today might be in New York City tomorrow. Should one or more new "emerging" mosquito-borne diseases begin to spread, control of the epidemic would be difficult. If the disease agent is a virus, specific treatments are unavailable (or, at least, untested against the arboviruses). The only way to stop a viral mosquito-borne illness is to kill the mosquitoes to a low enough level to interrupt virus transmission. Since mosquitoes can fly, control of an epidemic is even harder. Compounding all of this, many mosquito species are resistant to many of the currently available (older class) insecticides used to control them.

The Need for Pesticides
The U.S. Environmental Protection Agency (EPA) registration process, requiring many years of product testing and review, helps ensure that EPA-registered products are safe when used according to their label directions. Millions of dollars are invested in testing pesticide products - before they ever reach the consumer - for their relative safety to humans and the environment. Prospective pesticides are tested for harmful effects to adults, children, the unborn, as well as the environment. Some people claim that pesticides are ruining the environment and causing widespread disease (such as cancer) in the human population. But where is the evidence? Wildlife is rebounding after years of decline. There are more deer and wild turkeys in the U.S. now than at the turn of the century. Raptors are back. People are healthier and living longer. Overall cancer incidence rates are decreasing (although there are a few specific categories with increases). We must be doing something right.

Pesticides are extremely important to human survival. They are essentially "environmental medicines" to correct insect imbalances. Not only are they needed to correct insect imbalances (like for crop protection), but they are also needed as public health tools. We need a wide array of pesticides to combat any vector-borne diseases that may arise, or any re-emergence of existing diseases (such as malaria, dengue, etc.). Certainly, integrated pest management and other strategies to reduce pesticide use are in order, but in many cases insect populations explode and are unmanageable by non-chemical methods. We must have pesticides readily available for use.

We don't know what the future holds. But it's probably safe to say that there will continue to be increased human population numbers, plenty of infectious diseases (both old and new), and widespread, frequent air travel. This is a combination bound to lead to disease epidemics. We better keep our pesticides . . .

References
1. Rose RI: Pesticides and public health: integrated methods of mosquito management. Emerg Infect Dis 2001; 7: 17-23.

2. Gratz NG: Emerging and resurging vector-borne diseases. Ann Rev Entomol 1999; 44: 51-75.

3. Molyneux DH: Patterns of change in vector-borne diseases. Ann Trop Med Parasitol 1997; 91: 827-839.

4. Gubler DJ: Epidemic dengue and dengue hemorrhagic fever: a global public health problem in the 21st century. In: Scheld WM, Armstrong D, Hughes JM, eds. Emerging Infections. v. 1. Washington, DC: American Society for Microbiology, 1998:1-14.

5. Morris CD: Eastern equine encephalomyelitis. In: Monath TP, ed. The Arboviruses: Epidemiology and Ecology. v. 3. Boca Raton, FL: CRC Press, 1988:1-20.

6. Chamberlain RW: History of St. Louis encephalitis. In: Monath TP, ed. St. Louis Encephalitis. Washington, DC: American Public Health Association, 1980:3-61.

7. CDC: Arboviral infections of the central nervous system -- United States, 1996-1997. MMWR, 47: 517-522, 1998.

8. Harwood RF, James MT: Entomology in Human and Animal Health. 7th ed. New York: Macmillan, 1979.

9. Anonymous: Venezuelan equine encephalomyelitis, a national emergency. U.S. Department of Agriculture, APHIS-81-1, Washington, DC, 1972.

10. CDC: Outbreak of West Nile-like viral encephalitis -- New York, 1999. MMWR, 48, 845-848, 1999.

11. CDC: Update: West Nile virus encephalitis -- New York, 1999. MMWR, 48, 944-946, 1999.

12. Gubler DJ: Arboviruses as imported disease agents: the need for increased awareness. Arch Virol 1996; 11: 21-32.

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