Mosquito-Borne Diseases

Mosquito-borne Diseases

Within New Jersey a widespread mosquito research and control network has been in place since the early 1900’s and is one of the most comprehensive in the country. This has served to keep the incidence of mosquito-borne disease at relatively low levels. Mosquito research and control efforts are carried out by a coordinated effort between Rutgers University, mosquito control agencies in each of New Jersey ‘s 21 counties, and the State Mosquito Control Commission.

Throughout New Jersey routine surveillance is conducted to measure mosquito populations as well as to capture adult mosquitoes to be tested for the presence of viruses that can be transmitted to humans or livestock. When significant mosquito populations and/or mosquito-borne viruses are detected, appropriate control actions are taken.

The following is an excerpt from:

“Mosquito-Borne Viruses of New Jersey ” produced and distributed by:

NJ State Department of Health and Senior Services, NJ State Mosquito Control Commission, NJ State Health Officer’s Association, NJ State Department of Environmental Protection, NJ State Department of Agriculture, NJ Mosquito Control Association and Monmouth County Mosquito Extermination Commission

Mosquito-borne Viruses – Medically Important In New Jersey

Disease Transmission

Many types of viruses have been isolated from mosquitoes in nature. Fortunately, only several are of concern in the United States and only 3 are of significant health importance in New Jersey. It should be noted that mosquitoes cannot transmit all viruses. For example, the human immuno-deficiency virus (HIV), which causes AIDS in humans, doesn’t survive in mosquitoes, and therefore cannot be transmitted when they bite. In general, mosquito-borne virus transmission cycles are complicated and may involve birds and/or other animals. For example, eastern equine encephalitis (EEE) virus is maintained in a bird-mosquito cycle. The virus is transmitted among birds, principally by Culiseta melanura, a mosquito species that feeds exclusively on birds. Normally, viral transmission is limited to this natural cycle. However, in years with large populations of Culiseta melanura, the amount of viral infection in birds ‘amplifies’ and the virus gets transmitted to mammals via mosquitoes that serve as “bridge vectors”. Bridge vectors are mosquitoes with broad feeding preferences that might bite a bird for one meal, and later take another blood meal from a mammal. Human cases of mosquito-borne viruses typically occur late in the season; it takes time for mosquito populations to increase, for the virus to build up in bird populations to the point where it might be picked up by bridge vectors, and finally for infected mosquitoes to transmit the virus to mammals.

Eastern Equine Encephalitis (EEE)

EEE reaches its highest levels in the southern half of the state. EEE is a severe disease with headache, stiff neck, high fever, stupor, disorientation, tremors, convulsions, and coma. It has a high fatality rate (usually 50% or greater) and persons who recover are often left with permanent disabilities. The virus circulates between wild birds and mosquitoes in freshwater swamp environments. The birds involved in this cycle have adapted to the virus so they do not become ill from it. When viral activity levels are high, the disease may spread out from the swamp and become a threat to man, horses, and certain species of domestic birds which are susceptible to the virus. This virus caused New Jersey’s first recorded outbreak of mosquito-borne disease in 1959, causing 32 cases with 21 deaths. A smaller outbreak (12 cases with 6 deaths) occured in 1968. Sporadic cases have occured since 1968, including a suspect case in 1989. However, we know the disease still exists in New Jersey. In fact, EEE virus is isolated every year in mosquitoes and a year without a horse case is rare. There is a vaccine that will protect horses from the disease. All New Jersey horse owners are urged to have their horses vaccinated every year in late spring.

Saint Louis Encephalitis (SLE)

SLE is generally a less severe disease than EEE, causing anywhere from a mild fever and headache to symptoms similar to EEE. People over age 60 are the most likely to become ill, but the case fatality rate is lower (5 to 10%) than that of EEE. The SLE virus also circulates between wild birds and mosquitoes. However, it is more commonly detected in urban and suburban settings. The primary mosquito vector, Culex pipiens, is called the northern house mosquito or the rain barrel mosquito. This mosquito likes to lay its eggs in standing water, no matter how stagnant it may be. These mosquitoes reach their highest densities in stagnant water that is polluted with organic waste, such as catch basins. There are 2 recorded outbreaks of SLE in New Jersey, the first in 1964 resulted in 97 cases with 10 deaths, and the second in 1975 resulted in 29 cases with 3 deaths. No animals other than man are known to become ill from this virus, and with the absence of human cases since 1975, it is thought that this virus exists only in extremely low levels in New Jersey in most years.

West Nile Encephalitis (WNV)

WNV causes a very similar disease to SLE and also affects primarily elderly people. Approximately 10 to 15% of the clinical cases are fatal. WNV was first discovered in Uganda in 1937. Since that time there have been sporadic outbreaks throughout Africa, Eastern Europe and parts of Asia. Similar to SLE, it appears to exist in a mosquito-bird cycle, with the same type of mosquito vectors (Culex pipiens) involved. WNV was first recorded in North America in the fall of 1999, causing illness in 63 people in New York City and the surrounding area. No documented human cases occured in New Jersey in that year, but the virus was isolated from 73 dead crows and 2 groups (or pools) of mosquitoes collected in New Jersey. Crows in particular, have a high mortality rate when exposed to WNV. Generally, WNV is not lethal to birds; however, many of the bird species native to North America have not previosuly existed with the virus. Therefore, the virus causes disease in them, much like it does in humans. Introduced birds, such as the European starling, have a “built-in” immunity to the virus and consequently show no signs of illness when infected. It is these types of birds that are thought to be important in the natural transmission cycle of WNV.

La Crosse Encephalitis (LACV)

LACV is a mosquito borne virus transmitted primarily by Aedes triseriatus, a mosquito that utilizes natural tree holes and artificial containers for egg laying and larval habitat.  There are approximately 80-100 cases reported each year in the United States.    LACV is generally a mild illness; initial symptoms include fever, headache, nausea, vomiting and fatigue.  Severe disease symptoms include encephalitis and can lead to seizures, coma and paralysis.  Rarely, long term disability or death can result.  Most severe cases occur in children 16 and under.   The Warren County Mosquito Commission has undertaken surveillance for the virus due to the historic presence of this virus in New Jersey and its recent appearance in New York State.

Chikungunya (CHIKV), Dengue (DENV) and Zika (ZIKV) 

Humans can serve as a reservoir for chikungunya, dengue, and Zika viruses.  Therefore, a mosquito can actually acquire enough virus from an infected person (while obtaining a blood meal) to pass that virus on to another person. Unlike WNV, these viruses do not require a bird reservoir.  Aedes aegypti and Aedes albopictus are both able to transmit the virus.  Aedes aegypti is not present in Warren County but Aedes albopictus is slowly becoming more abundant. 

Dengue virus is rarely transmitted in the Continental United States but is now endemic in Puerto Rico and in Latin America (and other common travel destinations).  Worldwide, it is one of the most important mosquito-borne viruses infecting over 400 million people annually.   Imported cases of dengue virus in domestic U.S. are common.   Chikungunya virus was first detected in the Americas in 2013.  Previously, the virus caused outbreaks in Africa, Asia and Europe.  Now chikungunya virus is found in parts of the United States, Central and South America, Asia, Europe, Africa, and the Pacific Islands.  Before 2015, Zika virus was thought not be an important mosquito-borne virus.  However, outbreaks in 2015 in Brazil received massive media attention due to the ability of the virus to cause birth defects and also to be sexually transmitted.  Zika virus is now thought to be endemic in Central and South America with local transmission occurring in parts of the Continental United States.  

To date, there have not been any locally transmitted cases of CHIKV, DENV, or ZIKV in New Jersey; however, there have been travel related cases of all three viruses in the state.  Since, humans can become viremic enough to infect a mosquito; it is theoretically possible for local transmission to occur in Warren County in areas where Aedes albopictus are abundant (when travel related cases are present).

Countrywide surveillance maps for diseases (current and historical) can be found at 

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