Summary of Imported and Locally Transmitted Mosquito Borne Viruses in the United States

Stephanie L. Richards, PhD, Medical Entomologist

The Centers for Disease Control and Prevention (CDC) and the United States Geological Survey provide maps of some mosquito borne diseases in the United States (US) as a tool to track historical cases. For some diseases (such as Zika), the CDC provides more up-to-date data on their website. Below is a summary of some key mosquito borne illnesses of concern in the United States. Local transmission means that mosquitoes in the US are infected with the virus and have transmitted it to someone that has not traveled. Imported cases are people that have traveled to a virus-endemic area and come back to the US infected.

Chikungunya virus

Local chikungunya virus transmission was first detected in the Americas in 2013. There have been about 28 imported cases/year in the United States from 2006-2013. In 2014, a dramatic increase in imported cases occurred and Florida experienced 12 cases of local transmission. Imported cases have been found in 49 states and the District of Columbia. Chikungunya virus is maintained in a mosquito-human-mosquito cycle. Humans are considered the reservoir for this virus and, during the viremic period, can infect mosquitoes. Common symptoms are fever and joint pain; however other symptoms can occur (e.g. headache, muscle soreness, swollen joints, rash). Joint pain may last for several months, depending on the individual immune response. Primary vectors include Aedes species of mosquitoes, i.e. Aedes aegypti, Aedes albopictus. Chikungunya is not a nationally notifiable disease.

  • 12 locally transmitted cases

  • 3,490 travel-associated cases

  • Number of deaths not yet reported by CDC

  • Travel-associated imported cases: No

United States Territories - Number of human cases 2014-2015

  • 4,861 locally transmitted cases

  • 51 travel-associated cases

  • Number of deaths not yet reported by CDC

Dengue virus

The United States experienced a dengue epidemic (attributed to imported cases leading to local transmission) in Florida in the 1930s. There have been focal outbreaks of local dengue virus transmission in Texas (2005, 2013), Florida (2009, 2010, 2011, 2012, 2013, 2014, 2015), and Hawaii (2001, 2011, 2015) in recent years. There has also been one case of local transmission in New York (2013). It has also been detected in imported cases from all 50 states and the District of Columbia. Humans are considered the reservoir for this virus and, during the viremic period, can infect mosquitoes. Dengue virus is maintained in a mosquito-human-mosquito cycle. Common symptoms of a dengue infection include high fever, headache, pain behind the eyes, joint/muscle/bone pain, rash, minor bleeding, and low white blood cell count. In some infections, more severe symptoms may be experienced such as abdominal pain, vomiting blood, clammy skin, and/or difficulty breathing. Primary vectors include Aedes species of mosquitoes, i.e. Aedes aegypti, Aedes albopictus. Dengue became a nationally notifiable disease in 2009.

Continental United States - Number of human cases 2014-2015:

  • 269 locally transmitted cases

  • 1,401 travel-associated cases

  • Number of deaths not yet reported by CDC

United States Territories - Number of human cases 2014-2015:

  • 605 locally transmitted cases

  • 0 travel-associated cases

  • Number of deaths not yet reported by CDC

Zika virus

Zika virus was first imported into the United States in 2015. It has since been detected in imported cases from 45 states and the District of Columbia. It is maintained in a mosquito-human-mosquito cycle, but can also be transmitted between humans through sexual contact. Humans are considered the reservoir for this virus and, during the viremic period, can infect mosquitoes. Most (approximately 80%) people infected with Zika virus will not show symptoms. Those who experience symptoms may have fever, rash, joint, pain, conjunctivitis, muscle pain, and/or headache. Zika virus infection during pregnancy can cause birth defects. A small percentage of infected people may experience Guillain-Barre syndrome, where a person’s immune response to infection damages the nervous system. Primary vectors include Aedes species of mosquitoes, i.e. Aedes aegypti, Aedes albopictus; however, limited laboratory studies have indicated that some Culex species may also be implicated in transmission. Zika became a nationally notifiable disease in 2016.

Continental United States (as of August 17, 2016) - Number of human cases 2015-2016:

 

  • 43 locally transmitted cases

  • 2,920 travel-associated cases

  • 1 laboratory-acquired

  • Total 2,964 (includes sexually transmitted, Guillain-Barré syndrome and pregnant)

  • Total number of deaths not yet reported by CDC

United States Territories (as of September , 2016) - Number of human cases 2015-2016:

 

  • 15,809 locally transmitted cases

  • 60 travel-associated cases

  • Total 15,869 (includes Guillain-Barré syndrome and pregnant)

  • Total number of deaths not yet reported by CDC

Eastern equine encephalitis virus

Eastern equine encephalitis virus has been detected primarily in the Atlantic, Gulf Coast, and Great Lakes regions. Like West Nile and St. Louis encephalitis viruses, it is maintained in a mosquito-bird-mosquito cycle with spillover into human populations. Humans and horses are considered “dead end” hosts for this virus, i.e. an infected human cannot infect mosquitoes. Infected people experience chills, fever, joint pain, and/or muscle pain. In severe cases where the central nervous system is affected, people may experience irritability, restlessness, drowsiness, anorexia, vomiting, diarrhea, blue skin/lips, convulsions, and/or coma. Eastern equine encephalitis causes death in approximately 33% of cases. People that survive the illness are often left with debilitating mental and physical issues. The primary vector that maintains the virus in bird populations is Culiseta melanura; however, this species only feeds on birds (not humans). Primary vectors that “bridge” the virus to other mammals (including humans) include Culex species of mosquitoes, e.g. Culex pipiens, Culex quinquefasciatus and other species such as Aedes vexans and Coquillettidia perturbans, among others. The highest number of human cases occur in summer and fall.

  • Number of human cases in 2015: 23

  • Number of human deaths in 2015: not yet reported

  • Number of human cases 2004-2014: 90 (100% neuroinvasive [meningitis/encephalitis])

  • Number of human deaths 1999-2013: 2 (not yet reported for 2014-2015)

  • Local transmission: Yes

La Crosse virus

La Crosse virus is an important mosquito borne virus in the United States that primarily affects children under the age of 16. It was first detected in La Crosse, Wisconsin and an increase in cases in southeastern states have been observed in recent years. It is maintained in a mosquito-small rodent (e.g. chipmunk)-mosquito cycle with spillover into human populations. Humans are considered “dead end” hosts for this virus. Children and adults infected with La Crosse virus can experience fever, headache, nausea, vomiting, and fatigue. In severe cases (primarily children) where the nervous system is affected, seizures, partial paralysis, and mental issues can occur. Primary vectors include Aedes triseriatus and Aedes albopictus. The highest number of human cases occur in summer and fall.

  • Number of human cases 2014-2015: 134

  • Number of human cases 2004-2013: 787 (92% neuroinvasive [meningitis/encephalitis], 8% nonneuroinvasive)

  • Number of deaths 2004-2013: 11 (not yet reported for 2014-2015)

  • Local transmission: Yes

  • Traveler-associated imported cases: No

St. Louis encephalitis virus

St. Louis encephalitis virus has been detected primarily in the eastern and central United States since the 1930s. Like West Nile virus, it is maintained in a mosquito-bird-mosquito cycle with spillover into human populations. Humans and horses are considered “dead end” hosts for this virus. Less than 1% of people infected with St. Louis encephalitis virus show symptoms. Symptoms include fever, headache, dizziness, and nausea. In severe cases, individuals may experience neck stiffness, confusion, tremors, unsteadiness, and/or coma. Older adults generally experience the most severe symptoms. Approximately 5-15% of infected people die from St. Louis encephalitis virus infections. Children and adults infected with La Crosse virus can experience fever, headache, nausea, vomiting, and fatigue. In severe cases (primarily children) where the nervous system is affected, seizures, partial paralysis, and mental issues can occur. Primary vectors include Aedes triseriatus and Aedes albopictus. The highest number of human cases occur in summer and fall.

  • Number of human cases in 2015: 23

  • Number of human cases 2004-2014: 92 (100% neuroinvasive [meningitis/encephalitis])

  • Number of human deaths 1999-2014: 2 (not yet reported for 2015)

  • Local transmission: Yes

  • Travel-associated imported cases: No

West Nile virus

West Nile virus is the most common mosquito borne virus in the United States. It was first detected in the United States in New York in 1999 and has since spread to 48 states and the District of Columbia. It is maintained in a mosquito-bird-mosquito cycle with spillover into human populations. Humans and horses are considered “dead end” hosts for this virus. Most (up to 80%) people do not show symptoms after West Nile virus infection. Symptomatic people may experience headache, body/joint pain, vomiting, diarrhea, and/or rash. In severe cases, encephalitis or meningitis may occur, resulting in symptoms such as tremors, seizures, paralysis, and/or coma. Approximately 10% of severe cases end in death. Primary vectors to humans include Culex species of mosquitoes, e.g. Culex nigripalpus, Culex pipiens, Culex quinquefasciatus, Culex tarsalis. Culiseta melanura is involved in maintaining the virus in bird populations. The highest number of human cases occur in summer and fall.

  • Number of human cases in 2015: 2,060 (66% neuroinvasive [meningitis/encephalitis], 34% nonneuroinvasive)

  • Number of human deaths in 2015: 119

  • Number of human cases 1999-2014: 41,762 (45% neuroinvasive [meningitis/encephalitis], 55% nonneuroinvasive)

The US is faced with many mosquito borne viruses that are public health threats and this landscape is constantly changing as more pathogens are imported into our region through travel outside of the continental US.

The epidemiological cycles of these viruses vary, hence knowledge of transmission cycles and biology of possible vectors helps mosquito control operators target the most dangerous mosquito populations. Some viruses cause more widespread harm (i.e. greater geographic range, more cases), while others may cause deadly illness in a more focal area. However, all mosquito borne illnesses that threaten public health should be monitored and mosquito control agencies should receive continual support for surveillance and targeted control efforts.

 

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