It takes 5 to 15 days after the bite of an infected mosquito to develop symptoms of LACV disease. Symptoms include nausea, headache, vomiting in milder cases and seizures, coma, paralysis and permanent brain damage in severe cases.

LAC encephalitis initially presents as a nonspecific summertime illness with fever, headache, nausea, vomiting and lethargy. Severe disease occurs most commonly in children under the age of 16 and is characterized by seizures, coma, paralysis, and a variety of neurological sequelae after recovery. Death from LAC encephalitis occurs in less than 1% of clinical cases. In many clinical settings, pediatric cases presenting with CNS involvement are routinely screened for herpes or enteroviral causes. Since there is no specific treatment for LAC encephalitis, physicians often do not request the tests required to specifically identify LAC virus, and the cases are reported as aseptic meningitis or viral encephalitis of unknown cause.

As with many infections, the very young, the very old and the immunocompromised are at a higher risk of developing severe symptoms.

The virus can infect the brain (encephalitis), the meninges (meningitis) or both (meningoencephalitis).

In general, mortality is 1% to 2%, with deaths occurring 5 to 7 days after the onset of neurologic signs.

In dogs, the disease also manifests as a neurological disorder with signs varying from tremors to seizures and death.

In ruminants, neurological disease is also present, and animals may refuse to eat, appear lethargic, and also develop respiratory signs.

The majority of infections result in mild illness, including fever and headache. When infection is more severe the person may experience headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, occasional convulsions and spastic paralysis. Fatality ranges from . Aged people are more likely to have a fatal infection.

There are two ways in which the virus can progress, systematic and encephalitic, depending on the person's age. Encephalitic involves swelling of the brain and can be asymptomatic while the systemic illness occurs very abruptly. Those with the systemic illness usually recover within one to two weeks. While the encephalitis is more common among infants in adults and children it usually manifests after experiencing the systemic illness. Symptoms include high fever, muscle pain, altered mental status, headache, meningeal irritation, photophobia, and seizures, which occur three to 10 days after the bite of an infected mosquito. Due to the virus's effect on the brain, patients who survive can be left with mental and physical impairments such as personality disorders, paralysis, seizures, and intellectual impairment

The incubation period for WNV—the amount of time from infection to symptom onset—is typically from between 2 and 15 days. Headache can be a prominent symptom of WNV fever, meningitis, encephalitis, meningoencephalitis, and it may or may not be present in poliomyelitis-like syndrome. Thus, headache is not a useful indicator of neuroinvasive disease.

- West Nile fever (WNF), which occurs in 20 percent of cases, is a febrile syndrome that causes flu-like symptoms. Most characterizations of WNF generally describe it as a mild, acute syndrome lasting 3 to 6 days after symptom onset. Systematic follow-up studies of patients with WNF have not been done, so this information is largely anecdotal. In addition to a high fever, headache, chills, excessive sweating, weakness, fatigue, swollen lymph nodes, drowsiness, pain in the joints and flu-like symptoms. Gastrointestinal symptoms that may occur include nausea, vomiting, loss of appetite, and diarrhea. Fewer than one-third of patients develop a rash.

- West Nile neuroinvasive disease (WNND), which occurs in less than 1 percent of cases, is when the virus infects the central nervous system resulting in meningitis, encephalitis, meningoencephalitis or a poliomyelitis-like syndrome. Many patients with WNND have normal neuroimaging studies, although abnormalities may be present in various cerebral areas including the basal ganglia, thalamus, cerebellum, and brainstem.

- West Nile virus encephalitis (WNE) is the most common neuroinvasive manifestation of WNND. WNE presents with similar symptoms to other viral encephalitis with fever, headaches, and altered mental status. A prominent finding in WNE is muscular weakness (30 to 50 percent of patients with encephalitis), often with lower motor neuron symptoms, flaccid paralysis, and hyporeflexia with no sensory abnormalities.

- West Nile meningitis (WNM) usually involves fever, headache, and stiff neck. Pleocytosis, an increase of white blood cells in cerebrospinal fluid, is also present. Changes in consciousness are not usually seen and are mild when present.

- West Nile meningoencephalitis is inflammation of both the brain (encephalitis) and meninges (meningitis).

- West Nile poliomyelitis (WNP), an acute flaccid paralysis syndrome associated with WNV infection, is less common than WNM or WNE. This syndrome is generally characterized by the acute onset of asymmetric limb weakness or paralysis in the absence of sensory loss. Pain sometimes precedes the paralysis. The paralysis can occur in the absence of fever, headache, or other common symptoms associated with WNV infection. Involvement of respiratory muscles, leading to acute respiratory failure, can sometimes occur.

- West-Nile reversible paralysis, Like WNP, the weakness or paralysis is asymmetric. Reported cases have been noted to have an initial preservation of deep tendon reflexes, which is not expected for a pure anterior horn involvement. Disconnect of upper motor neuron influences on the anterior horn cells possibly by myelitis or glutamate excitotoxicity have been suggested as mechanisms. The prognosis for recovery is excellent.

- Nonneurologic complications of WNV infection that may rarely occur include fulminant hepatitis, pancreatitis, myocarditis, rhabdomyolysis, orchitis, nephritis, optic neuritis and cardiac dysrhythmias and hemorrhagic fever with coagulopathy. Chorioretinitis may also be more common than previously thought.

- Cutaneous manifestations specifically rashes, are not uncommon in WNV-infected patients; however, there is a paucity of detailed descriptions in case reports and there are few clinical images widely available. Punctate erythematous, macular, and papular eruptions, most pronounced on the extremities have been observed in WNV cases and in some cases histopathologic findings have shown a sparse superficial perivascular lymphocytic infiltrate, a manifestation commonly seen in viral exanthems. A literature review provides support that this punctate rash is a common cutaneous presentation of WNV infection.

Saint Louis encephalitis is a disease caused by the mosquito borne Saint Louis encephalitis virus. Saint Louis encephalitis virus is related to Japanese encephalitis virus and is a member of the Flaviviridae subgroup. This disease mainly affects the United States. Occasional cases have been reported from Canada and Mexico.

Symptoms manifest within 7–10 days and include fever, headache, partial paralysis, confusion, nausea and even coma.

Japanese encephalitis (JE) is an infection of the brain caused by the Japanese encephalitis virus (JEV). While most infections result in little or no symptoms, occasional inflammation of the brain occurs. In these cases symptoms may include headache, vomiting, fever, confusion, and seizures. This occurs about 5 to 15 days after infection.

JEV is generally spread by mosquitoes, specifically those of the "Culex" type. Pigs and wild birds serve as a reservoir for the virus. The disease mostly occurs outside of cities. Diagnosis is based on blood or cerebrospinal fluid testing.

Prevention is generally with the Japanese encephalitis vaccine, which is both safe and effective. Other measures include avoiding mosquito bites. Once infected there is no specific treatment, with care being supportive. This is generally carried out in hospital. Permanent problems occur in up to half of people who recover from encephalopathy.

The disease occurs in Southeast Asia and the Western Pacific. About 3 billion people live in areas where the disease occurs. About 68,000 symptomatic cases occur a year with about 17,000 deaths. Often cases occur in outbreaks. The disease was first described in 1871.

Tick-borne encephalitis (TBE) is a viral infectious disease involving the central nervous system. The disease most often manifests as meningitis, encephalitis, or meningoencephalitis. Although TBE is most commonly recognized as a neurological disorder, mild fever can also occur. Long-lasting or permanent neuropsychiatric consequences are observed in 10 to 20% of infected patients.

The number of reported cases has been increasing in most countries.

The tick-borne encephalitis virus is known to infect a range of hosts including ruminants, birds, rodents, carnivores, horses, and humans. The disease can also be spread from animals to humans, with ruminants and dogs providing the principal source of infection for humans.

TBE, like Lyme disease, is one of the many tick-borne diseases.

Adult patients with encephalitis present with acute onset of fever, headache, confusion, and sometimes seizures. Younger children or infants may present irritability, poor appetite and fever.

Neurological examinations usually reveal a drowsy or confused patient. Stiff neck, due to the irritation of the meninges covering the brain, indicates that the patient has either meningitis or meningoencephalitis.

Arbovirus encephalitis refers to encephalitis that is caused by arbovirus infection.

There are many types of arboviral encephalitides found in the United States.

Examples include:

- California encephalitis

- Japanese encephalitis

- St. Louis encephalitis

- Tick-borne encephalitis

- West Nile fever

- Murray Valley encephalitis

The signs shown depend on the horse's age, the strain of the infecting virus, the condition of the horse and the route by which it was infected. Most horses with EVA infection don't show any signs; if a horse does show symptoms, these can vary greatly in severity. Following infection, the first sign is fever, peaking at , followed by various signs such as depression, nasal discharge, "pink eye" (conjunctivitis), swelling over the eye (supraorbital edema), urticaria, and swelling of the limbs and under the belly (the ventral abdomen) which may extend to the udder in mares or the scrotum of male horses. More unusual signs include abortion in pregnant mares, and, most likely in foals, severe respiratory distress and death.

Encephalitis lethargica is identified by high fever, headache, delayed physical response, and lethargy. Individuals can exhibit upper body weakness, muscular pains, and tremors, though the cause of encephalitis lethargica is not currently known. From 1917 to 1928, an epidemic of encephalitis lethargica occurred worldwide.

Viral encephalitis is a type of encephalitis caused by a virus.

It is unclear if anticonvulsants used in people with viral encephalitis would prevent seizures.

La Crosse encephalitis is an encephalitis caused by an arbovirus (the La Crosse virus) which has a mosquito vector ("Ochlerotatus triseriatus" synonym "Aedes" "triseriatus").

La Crosse encephalitis virus (LACV) is one of a group of mosquito-transmitted viruses that can cause encephalitis, or inflammation of the brain. LAC encephalitis is rare; in the United States, about 80–100 LACV disease cases are reported each year, although it is believed to be under-reported due to minimal symptoms experienced by many of those affected.

Powassan encephalitis, caused by the Powassan virus (POWV), as flavivirus also known as the deer tick virus, is a form of arbovirus infection that results from tick bites. It can occur as a co-infection with Lyme disease since both are transmitted to humans by the same species of tick. There has been a surge in the number of cases and geographic range in the last decade. In the United States, cases have been recorded in the northeast. The disease was first isolated from the brain of a boy who died of encephalitis in Powassan, Ontario, in 1958. The disease is a zoonosis, an animal disease, usually found in rodents and ticks, with spillover transmission to humans. The virus is antigenically related to the Far Eastern tick-borne encephalitis viruses.

Eastern equine encephalitis (EEE), commonly called Triple E or, sleeping sickness (not to be confused with "Trypanosomiasis") is a zoonotic alphavirus and arbovirus present in North, Central and South America and the Caribbean. EEE was first recognized in Massachusetts, United States in 1831 when 75 horses died mysteriously of viral encephalitis.

Epizootics in horses have continued to occur regularly in the United States. It can also be identified in asses and zebras. Due to the rarity of the disease its occurrence can cause economic impact in relation to the loss of horses and poultry. EEE is found today in the eastern part of the country and is often associated with coastal plains. It can most commonly be found in east and gulf coast states. In Florida about one to two human cases are reported a year although over sixty cases of equine encephalitis are reported. Some years in which there are favorable conditions for the disease there number of equine cases are over two-hundred. Diagnosing equine encephalitis is challenging because many of the symptoms are shared with other illnesses and patients can be asymptomatic. Confirmations may require a sample of cerebral spinal fluid or brain tissue although CT scans and MRI scans are used to detect encephalitis. This could be an indication that the need to test for Eastern Equine Encephalitis is necessary. If a biopsy of the cerebral spinal fluid is taken it is sent to a specialized laboratory for testing.

EEEV is closely related to Venezuelan equine encephalitis virus and Western equine encephalitis virus.

Murray Valley encephalitis virus (MVEV) is a zoonotic flavivirus endemic to northern Australia and Papua New Guinea. It is the causal agent of Murray Valley encephalitis (previously known as Australian encephalitis or Australian X disease). In humans it can cause permanent neurological disease or death. MVEV is related to Kunjin virus which has a similar ecology but a lower morbidity rate. Although the arbovirus is endemic to Northern Australia, it has occasionally spread to the southern states during times of heavy rainfall during the summer monsoon season via seasonal flooding of the Murray-Darling river system. These outbreaks can be "...decades apart, with no or very few cases identified in between".

West Nile fever is a viral infection typically spread by mosquitoes. In about 75% of infections people have few or no symptoms. About 20% of people develop a fever, headache, vomiting, or a rash. In less than 1% of people, encephalitis or meningitis occurs, with associated neck stiffness, confusion, or seizures. Recovery may take weeks to months. The risk of death among those in whom the nervous system is affected is about 10%.

West Nile virus is typically spread by infected mosquitoes. Mosquitoes become infected when they feed on infected birds. Rarely the virus is spread through blood transfusions, organ transplants, or from mother to baby during pregnancy, delivery, or breastfeeding. It otherwise does not spread directly between people. Risks for severe disease include age over 60 and other health problems. Diagnosis is typically based on symptoms and blood tests.

There is no human vaccine. The best method to reduce the risk of infections is avoiding mosquito bites. This may be done by eliminating standing pools of water, such as in old tires, buckets, gutters, and swimming pools. Mosquito repellent, window screens, mosquito nets, and avoiding areas where mosquitoes occur may also be useful. While there is no specific treatment, pain medications may be useful.

WNV occurs in Europe, the Middle East, Africa, India, Asia, Australia, and North America. In the United States thousands of cases are reported a year, with most occurring in August and September. It can occur in outbreaks of disease. The virus was discovered in Uganda in 1937 and was first detected in North America in 1999. Severe disease may also occur in horses and a vaccine for these animals is available. A surveillance system in birds is useful for early detection of a potential human outbreak.

Rocio viral encephalitis is an epidemic flaviviral disease of humans first observed in São Paulo State, Brazil, in 1975. Low-level enzootic transmission is likely continuing in the epidemic zone, and with increased deforestation and population expansion, additional epidemics caused by Rocio virus are highly probable. If migratory species of birds are, or become involved in, the virus transmission cycle, the competency of a wide variety of mosquito species for transmitting Rocio virus experimentally suggest that the virus may become more widely distributed. The encephalitis outbreak in the western hemisphere caused by West Nile virus, a related flavivirus, highlights the potential for arboviruses to cause severe problems far from their source enzootic foci.

The causative Rocio virus belongs to the genus "Flavivirus" (the same genus as the Zika virus) in family Flaviviridae and is closely related serologically to Ilhéus, St. Louis encephalitis, Japanese encephalitis and Murray Valley encephalitis viruses.

Japanese encephalitis is diagnosed by commercially available tests detecting JE virus-specific IgM antibodies in serum and /or cerebrospinal fluid, for example by IgM capture ELISA.

JE virus IgM antibodies are usually detectable 3 to 8 days after onset of illness and persist for 30 to 90 days, but longer persistence has been documented. Therefore, positive IgM antibodies occasionally may reflect a past infection or vaccination. Serum collected within 10 days of illness onset may not have detectable IgM, and the test should be repeated on a convalescent sample. For patients with JE virus IgM antibodies, confirmatory neutralizing antibody testing should be performed.

Confirmatory testing in the US is only available at CDC and a few specialized reference laboratories. In fatal cases, nucleic acid amplification, and virus culture of autopsy tissues can be useful. Viral antigen can be shown in tissues by indirect fluorescent antibody staining.

Equine viral arteritis (EVA) is a disease of horses caused by equine arteritis virus, an RNA virus of the genus "Arterivirus". The virus which causes EVA was first isolated in 1953, but the disease has afflicted equine animals worldwide for centuries. It has been more common in some breeds of horses in the United States, but there is no breed "immunity". In the UK, it is a notifiable disease. There is no known human hazard.

Most individuals with HSE show a decrease in their level of consciousness and an altered mental state presenting as confusion, and changes in personality. Increased numbers of white blood cells can be found in patient's cerebrospinal fluid, without the presence of pathogenic bacteria and fungi. Patients typically have a fever and may have seizures. The electrical activity of the brain changes as the disease progresses, first showing abnormalities in one temporal lobe of the brain, which spread to the other temporal lobe 7–10 days later. Imaging by CT or MRI shows characteristic changes in the temporal lobes (see Figure). Definite diagnosis requires testing of the cerebrospinal fluid (CSF) by a lumbar puncture (spinal tap) for presence of the virus. The testing takes several days to perform, and patients with suspected Herpes encephalitis should be treated with acyclovir immediately while waiting for test results.

The course of encephalitis lethargica can vary significantly between individuals, particularly when accompanied by preexisting or simultaneous diseases and disorders. It is characterized by high fever, sore throat, headache, lethargy, double vision, delayed physical and mental response, sleep inversion and catatonia. In severe cases, patients may enter a coma-like state (akinetic mutism). Patients may also experience abnormal eye movements ("oculogyric crises"), parkinsonism, upper body weakness, muscular pains, tremors, neck rigidity, and behavioral changes including psychosis. Klazomania (a vocal tic) is sometimes present.

Characteristics of a viral infection can include pain, swelling, redness, impaired function, fever, drowsiness, confusion and convulsions.