Arboviruses are a group of diseases spread to humans through the bites of infected mosquitoes, ticks and sandflies. These viruses cause fever, polyarthralgia and encephalitis syndromes.
Vaccines are available for some arbovirus diseases, but others can only be prevented by avoiding contact with insect vectors. Prevention methods include community-based initiatives such as spraying breeding sites and reducing stagnant water pools, and personal protection with clothing, bed nets and insect repellents.
Symptoms
Infections with arboviruses can range from asymptomatic to life-threatening. Arboviruses are found worldwide, and are transmitted by a variety of arthropods (insects with jointed legs, such as mosquitoes and ticks) that bite infected mammals or birds to acquire the virus from their blood. These viruses can cause disease in humans and animals, and often lead to encephalitis, meningitis, or flaccid myelitis. The risk of infection with neuroinvasive arboviruses increases with travel to endemic areas, age, pregnancy, and immunosuppression.
Symptoms of an arboviral infection vary depending on the type of virus and the individual host, but can include mild symptoms of fever, headache, muscle or joint pain, a skin rash, or eye inflammation (conjunctivitis) that resolve without any major health problems. More severe infections can produce a rapid onset of symptoms, such as high fever, confusion, tremors, seizures, paralysis, or coma.
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Some arboviruses can also cause viral hemorrhagic fever, which causes severe bleeding and shock. The most common example is Dengue virus. Others include Rift Valley fever, Crimean-Congo haemorrhagic fever, and Zika virus. These diseases are notifiable to CDC using specific case definitions, and health-care providers should maintain a high index of suspicion for exotic or unusual arbovirus infection in travelers, especially those from endemic areas.
The most commonly encountered symptom of arboviruses that target the central nervous system is headache and fever. Other symptoms associated with these viruses may include a rash, myalgias, arthralgias, vertigo, vomiting, or paresis. Virus-specific IgM antibodies in cerebrospinal fluid or serum are typically not available, but brain imaging is usually performed to rule out encephalitis. The risk of arboviral encephalitis is highest for patients traveling to endemic areas, but the virus can also be acquired in the United States from imported mosquitoes and ticks. Practicing infection prevention through the use of insect repellant and wearing long-sleeved clothing in endemic areas can help reduce this risk.
Transmission
Many arboviruses are maintained in complex transmission cycles among viruses, vectors, and vertebrate hosts. These cycles vary in time and space as a result of species assembly and interactions between host, vector, and virus, as well as ecological factors. Some arboviruses are maintained through a specific transmission cycle involving one vector and host species, while others, such as the chikungunya and dengue virus, are more generalist and use multiple vectors and hosts.
Often, human infections are the result of a mosquito, midge, or tick bite. Most of the diseases that are classified as arbovirus are transmitted by these insects, including West Nile fever, Rift Valley fever, and Crimean-Congo haemorrhagic fever.
Mosquitoes spread most arbovirus diseases, but midges and sand flies also may transmit them to humans. Infections from these arthropods range from mild to severe and can include a flu-like illness, encephalitis, or other complications such as arthritis and joint pain. Most arbovirus diseases can be divided into two groups, based on the symptoms they cause: neuroinvasive and non-neuroinvasive. Neuroinvasive diseases can cause a serious, life-threatening inflammation of the brain called encephalitis or a neurological disorder such as paralysis and coma. Non-neuroinvasive diseases can cause a wide variety of symptoms, from mild flu-like symptoms to rashes and other skin problems.
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Most arboviruses are maintained by a biological transmission between the vector and the vertebrate host, with the virus circulating as a reservist or amplifying invertebrates before entering the naive vertebrate. The virus is then transmitted by the vector, which infects a new host when it bites. Alternatively, certain arboviruses are maintained by horizontal transmission between infected humans, who act as dead-end hosts for the virus.
The emergence of new arbovirus diseases and expansion of existing ones occurs through various factors, such as changes in climate, human activities that disturb the ecology, and the movement of people and cargo by air, sea, and land. As a result, the transmission patterns of these diseases can change, even between regions that have previously been free from disease. This phenomenon is known as spillover. This is true of the reemergence of diseases such as Rift Valley fever, dengue, and chikungunya.
Laboratory Testing
As arboviruses continue to expand their range, the need for accurate and timely laboratory testing becomes increasingly important. Several methods can be used to detect arbovirus infection, including immunologic assays and nucleic acid detection. These tests are often best suited for patients with clinical suspicion of illness and can help to differentiate between different arbovirus infections, such as West Nile virus (WNV) and dengue.
In most cases, arbovirus infection is diagnosed based on the clinical presentation of the patient. In the presence of a neuroinvasive disease, such as meningitis or encephalitis, direct detection of viral nucleic acid in affected tissue is needed. In the absence of this, diagnosis is based on serologic evidence or a combination of clinical and laboratory findings.
A negative result on a first-line serologic assay indicates that no antibodies against the specific virus are present in the serum or cerebrospinal fluid (CSF). Due to cross-reactivity, a positive test suggests infection, but not the specific virus involved. Confirmation requires either a neutralization assay or seroconversion, or a 4-fold increase in IgM or IgG antibody titers between paired sera.
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Molecular assays can confirm arbovirus infection using direct detection of viral nucleic acid in serum or CSF. These assays can be very fast and are ideal for use in resource-limited settings, where other diagnostics are difficult to obtain. The sensitivity of molecular assays is comparable to serologic methods and is much higher than traditional virology methods, such as plaque reduction neutralization.
BCDC’s Arbovirus Laboratory provides serological and virologic assays for mosquitoes, ticks and other vectors. It also conducts a comprehensive tick-borne disease surveillance program, which includes Powassan virus, and has the capacity to test for other emerging diseases.
Local health departments may request testing for the presence of vector-borne viruses and bacteria, such as rickettsial diseases, through the state’s Division of Vector-borne Diseases (DVBD). Testing is free to all state agencies. To submit a specimen, contact your county health department. All mammals suspected of being infected with a tick must be submitted to the NYS Rabies Laboratory, which must perform rabies testing before the animal can be tested for arboviruses.
Treatment
Many arboviruses cause neuroinvasive disease which can lead to aseptic meningitis, encephalitis, or acute flaccid paralysis (AFP). These illnesses are typically characterized by the rapid onset of fever with headache, polyarthralgia, and stiff neck. Neurological symptoms may include altered mental status, limb weakness, disorientation, seizures and cerebrospinal fluid (CSF) pleocytosis. Infection with certain viruses can also lead to peripheral demyelinating disease including anterior (“polio”) myelitis, post-infectious peripheral neuropathy and/or Guillain-Barre syndrome.
The prognosis for those with neuroinvasive disease varies among the different virus infections and ranges from 1% up to 30% mortality. Those who recover often have long-term neurological sequelae. Currently, there is no specific treatment for neuroinvasive arbovirus infection. However, supportive care is necessary and should be started as soon as possible. This includes fluid replacement, blood pressure control and symptomatic medications such as acetaminophen (Tylenol).
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There are no vaccines or drug treatments that can prevent neuroinvasive arboviral infections. Prevention of these infections should focus on vector control, personal protective measures such as wearing long clothing and using insect repellent and considering vaccination when available if traveling to areas where the infection is common.
In addition, laboratory diagnostic work is important to identify an arbovirus infection as soon as possible. Typically, CSF pleocytosis and a fourfold or greater change in virus-specific IgM antibody titers between acute and convalescent serum specimens provide strong clinical and laboratory evidence that the illness is due to an arboviral infection.
The majority of people who are bitten by an infected mosquito will have no symptoms or only mild ones such as a headache and fever. However, 1 percent to 2 percent of those who are bitten will develop recognizable symptoms. Symptoms of WNV, SLE and La Crosse encephalitis include a severe headache, high fever, aches in the muscles and joints, a stiff neck, problems with muscle coordination, confusion and sometimes seizures and coma.
Anyone can get infected with an arbovirus but young children and the elderly appear to be at higher risk for developing a more severe illness. Those who travel to areas where certain arboviral infections are common such as chikungunya, Powassan virus and Eastern equine encephalitis virus may also be at higher risk of developing a more severe illness.
