Equine Encephalitis Viruses

Classification

General Characteristics and Epidemiology

General features of equine encephalitis Encephalitis Encephalitis is inflammation of the brain parenchyma caused by an infection, usually viral. Encephalitis may present with mild symptoms such as headache, fever, fatigue, and muscle and joint pain or with severe symptoms such as seizures, altered consciousness, and paralysis. Encephalitis virus Virus Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. Virology (EEV)

• Family: Togaviridae Togaviridae A family of RNA viruses, mainly arboviruses, consisting of two genera: alphavirus (group A arboviruses), and rubivirus. Virions are spherical, 60-70 nm in diameter, with a lipoprotein envelope tightly applied to the icosahedral nucleocapsid. Rubella Virus
• Genus: Alphavirus
• Genome Genome The complete genetic complement contained in the DNA of a set of chromosomes in a human. The length of the human genome is about 3 billion base pairs. Basic Terms of Genetics:
  • Positive-sense, ssRNA
  • 11–12 kb in size
• Properties:
  • Enveloped
  • Lipid bilayer envelope Envelope Bilayer lipid membrane acquired by viral particles during viral morphogenesis. Although the lipids of the viral envelope are host derived, various virus-encoded integral membrane proteins, i.e. Viral envelope proteins are incorporated there. Virology has viral-encoded glycoproteins Glycoproteins Conjugated protein-carbohydrate compounds including mucins, mucoid, and amyloid glycoproteins. Basics of Carbohydrates E1 E1 An aromatized C18 steroid with a 3-hydroxyl group and a 17-ketone, a major mammalian estrogen. It is converted from androstenedione directly, or from testosterone via estradiol. In humans, it is produced primarily by the cyclic ovaries, placenta, and the adipose tissue of men and postmenopausal women. Noncontraceptive Estrogen and Progestins and E2.
  • Small icosahedral capsid Capsid The outer protein protective shell of a virus, which protects the viral nucleic acid. Capsids are composed of repeating units (capsomers or capsomeres) of capsid proteins which when assembled together form either an icosahedral or helical shape. Virology
• Clinically relevant viruses Viruses Minute infectious agents whose genomes are composed of DNA or RNA, but not both. They are characterized by a lack of independent metabolism and the inability to replicate outside living host cells. Virology and geographic distribution:
  • Eastern equine encephalitis Encephalitis Encephalitis is inflammation of the brain parenchyma caused by an infection, usually viral. Encephalitis may present with mild symptoms such as headache, fever, fatigue, and muscle and joint pain or with severe symptoms such as seizures, altered consciousness, and paralysis. Encephalitis virus Virus Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. Virology (EEEV) complex:
    • Eastern equine encephalitis Encephalitis Encephalitis is inflammation of the brain parenchyma caused by an infection, usually viral. Encephalitis may present with mild symptoms such as headache, fever, fatigue, and muscle and joint pain or with severe symptoms such as seizures, altered consciousness, and paralysis. Encephalitis (EEE) virus Virus Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. Virology: North America (Atlantic and Gulf coasts) and the Caribbean
    • Madariaga virus Virus Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. Virology: South and Central America
  • Western equine encephalitis Encephalitis Encephalitis is inflammation of the brain parenchyma caused by an infection, usually viral. Encephalitis may present with mild symptoms such as headache, fever, fatigue, and muscle and joint pain or with severe symptoms such as seizures, altered consciousness, and paralysis. Encephalitis virus Virus Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. Virology (WEEV): North and South America
  • Venezuelan equine encephalitis Encephalitis Encephalitis is inflammation of the brain parenchyma caused by an infection, usually viral. Encephalitis may present with mild symptoms such as headache, fever, fatigue, and muscle and joint pain or with severe symptoms such as seizures, altered consciousness, and paralysis. Encephalitis virus Virus Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. Virology (VEEV): Central and South America

Epidemiology

• In the United States:
  • EEEV is the most common of the 3 viruses Viruses Minute infectious agents whose genomes are composed of DNA or RNA, but not both. They are characterized by a lack of independent metabolism and the inability to replicate outside living host cells. Virology.
  • Infections Infections Invasion of the host organism by microorganisms or their toxins or by parasites that can cause pathological conditions or diseases. Chronic Granulomatous Disease are rare and sporadic Sporadic Selective IgA Deficiency.
  • Peak incidence Incidence The number of new cases of a given disease during a given period in a specified population. It also is used for the rate at which new events occur in a defined population. It is differentiated from prevalence, which refers to all cases in the population at a given time. Measures of Disease Frequency: August and September
• Flooding Flooding Psychotherapy increases breeding of mosquitoes → ↑ risk of infection
• Both humans and horses can be infected and develop encephalitis Encephalitis Encephalitis is inflammation of the brain parenchyma caused by an infection, usually viral. Encephalitis may present with mild symptoms such as headache, fever, fatigue, and muscle and joint pain or with severe symptoms such as seizures, altered consciousness, and paralysis. Encephalitis.

Pathophysiology

Vector and transmission

• Mosquitoes: primary arthropod vector
• Virus Virus Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. Virology has different reservoirs or amplifying hosts: domestic and wild birds, mammals
• Transmission:
  • The virus Virus Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. Virology is maintained by birds and mosquitoes (Culiseta melanura with EEEV infection) in a mosquito-bird-mosquito cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation.
  • Viral transmission between vertebrates is facilitated by bridge vectors (other mosquitoes or blood-feeding arthropods).
  • Species of mosquitoes that serve as bridge vectors:
    • Coquillettidia species
    • Aedes species
    • Culex species
  • Humans are infected by the human-biting bridge vectors.
    • While humans can develop illness, they are considered dead-end hosts.
    • Viremia Viremia The presence of viruses in the blood. Erythema Infectiosum in humans is generally insufficient to infect feeding mosquitoes/vectors.
• The virus Virus Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. Virology can also be used as a bioterrorism Bioterrorism The use of biological agents in terrorism. This includes the malevolent use of bacteria; viruses; or other biological toxins against people, animals; or plants. Anthrax agent (via aerosol transmission).

Pathogenic features and viral infection

• Infected female mosquito bites human → virus Virus Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. Virology enters the bloodstream → entry into host cells (myeloid or lymphoid cells) via endocytosis Endocytosis Cellular uptake of extracellular materials within membrane-limited vacuoles or microvesicles. Endosomes play a central role in endocytosis. The Cell: Cell Membrane
• Envelope Envelope Bilayer lipid membrane acquired by viral particles during viral morphogenesis. Although the lipids of the viral envelope are host derived, various virus-encoded integral membrane proteins, i.e. Viral envelope proteins are incorporated there. Virology E1 E1 An aromatized C18 steroid with a 3-hydroxyl group and a 17-ketone, a major mammalian estrogen. It is converted from androstenedione directly, or from testosterone via estradiol. In humans, it is produced primarily by the cyclic ovaries, placenta, and the adipose tissue of men and postmenopausal women. Noncontraceptive Estrogen and Progestins and E2 viral proteins Proteins Linear polypeptides that are synthesized on ribosomes and may be further modified, crosslinked, cleaved, or assembled into complex proteins with several subunits. The specific sequence of amino acids determines the shape the polypeptide will take, during protein folding, and the function of the protein. Energy Homeostasis:
  • Aid in virus Virus Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. Virology attachment Attachment The binding of virus particles to virus receptors on the host cell surface, facilitating virus entry into the cell. Virology and penetration Penetration X-rays
  • E1 E1 An aromatized C18 steroid with a 3-hydroxyl group and a 17-ketone, a major mammalian estrogen. It is converted from androstenedione directly, or from testosterone via estradiol. In humans, it is produced primarily by the cyclic ovaries, placenta, and the adipose tissue of men and postmenopausal women. Noncontraceptive Estrogen and Progestins can bring about hemagglutination by fusing to membrane lipids Lipids Lipids are a diverse group of hydrophobic organic molecules, which include fats, oils, sterols, and waxes. Fatty Acids and Lipids
• Viral replication:
  • Occurs in the cytoplasm ( viremic phase Viremic phase Tick-borne Encephalitis Virus)
  • Positive-sense genomic RNA RNA A polynucleotide consisting essentially of chains with a repeating backbone of phosphate and ribose units to which nitrogenous bases are attached. RNA is unique among biological macromolecules in that it can encode genetic information, serve as an abundant structural component of cells, and also possesses catalytic activity. RNA Types and Structure serves as the mRNA mRNA RNA sequences that serve as templates for protein synthesis. Bacterial mRNAs are generally primary transcripts in that they do not require post-transcriptional processing. Eukaryotic mRNA is synthesized in the nucleus and must be exported to the cytoplasm for translation. Most eukaryotic mRNAs have a sequence of polyadenylic acid at the 3′ end, referred to as the poly(a) tail. The function of this tail is not known for certain, but it may play a role in the export of mature mRNA from the nucleus as well as in helping stabilize some mRNA molecules by retarding their degradation in the cytoplasm. RNA Types and Structure.
• Virions mature by budding Budding Mycology from plasma Plasma The residual portion of blood that is left after removal of blood cells by centrifugation without prior blood coagulation. Transfusion Products membranes → infect other cells
• Neuroinvasion:
  • The virus Virus Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. Virology crosses the blood-brain barrier Blood-brain barrier Specialized non-fenestrated tightly-joined endothelial cells with tight junctions that form a transport barrier for certain substances between the cerebral capillaries and the brain tissue. Systemic and Special Circulations and multiplies in the CNS.
  • Leads to neuronal necrosis Necrosis The death of cells in an organ or tissue due to disease, injury or failure of the blood supply. Ischemic Cell Damage/neuronophagia and meningeal irritation Meningeal Irritation Subarachnoid Hemorrhage
  • Invasion depends on:
    • Level of viremia Viremia The presence of viruses in the blood. Erythema Infectiosum
    • Virulence Virulence The degree of pathogenicity within a group or species of microorganisms or viruses as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenic capacity of an organism is determined by its virulence factors. Proteus of strain
    • Host immune response
  • Age-dependent risk for CNS infection, which is increased in:
    • Infants/young children
    • Elderly

Diseases

Minor illness

• 1st phase (from mosquito bite to viral replication in non-neural tissue)
• Incubation Incubation The amount time between exposure to an infectious agent and becoming symptomatic. Rabies Virus period: 4–10 days following a mosquito bite
• 7–10 day prodrome Prodrome Symptoms that appear 24–48 hours prior to migraine onset. Migraine Headache:
  • Fever Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever
  • Headache Headache The symptom of pain in the cranial region. It may be an isolated benign occurrence or manifestation of a wide variety of headache disorders. Brain Abscess
  • Nausea Nausea An unpleasant sensation in the stomach usually accompanied by the urge to vomit. Common causes are early pregnancy, sea and motion sickness, emotional stress, intense pain, food poisoning, and various enteroviruses. Antiemetics/ vomiting Vomiting The forcible expulsion of the contents of the stomach through the mouth. Hypokalemia
• Most cases are subclinical or only produce low-grade fever Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever.
• Patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship with minor viral illness (and no CNS involvement) recover within 1–2 weeks.

Major illness

• 2nd phase: viral multiplication in the brain Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification
• Encephalitis Encephalitis Encephalitis is inflammation of the brain parenchyma caused by an infection, usually viral. Encephalitis may present with mild symptoms such as headache, fever, fatigue, and muscle and joint pain or with severe symptoms such as seizures, altered consciousness, and paralysis. Encephalitis or inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body’s defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation/ swelling Swelling Inflammation of the brain Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification parenchyma affects 2% and 6% of infected adults and children, respectively.
  • Signs and symptoms include: 
    • Headache Headache The symptom of pain in the cranial region. It may be an isolated benign occurrence or manifestation of a wide variety of headache disorders. Brain Abscess
    • High fever Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever
    • Muscle pain Muscle Pain Ion Channel Myopathy
    • Photophobia Photophobia Abnormal sensitivity to light. This may occur as a manifestation of eye diseases; migraine; subarachnoid hemorrhage; meningitis; and other disorders. Photophobia may also occur in association with depression and other mental disorders. Migraine Headache
    • Altered mental status Altered Mental Status Sepsis in Children
    • Seizures Seizures A seizure is abnormal electrical activity of the neurons in the cerebral cortex that can manifest in numerous ways depending on the region of the brain affected. Seizures consist of a sudden imbalance that occurs between the excitatory and inhibitory signals in cortical neurons, creating a net excitation. The 2 major classes of seizures are focal and generalized. Seizures and cranial nerve palsies Cranial Nerve Palsies Cranial nerve palsy is a congenital or acquired dysfunction of 1 or more cranial nerves that will, in turn, lead to focal neurologic abnormalities in movement or autonomic dysfunction of its territory. Head/neck trauma, mass effect, infectious processes, and ischemia/infarction are among the many etiologies for these dysfunctions. Diagnosis is initially clinical and supported by diagnostic aids. Management includes both symptomatic measures and interventions aimed at correcting the underlying cause. Cranial Nerve Palsies in 50% of patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship
    • May manifest as bulging fontanelle Fontanelle Any of six membrane-covered openings between the cranial sutures in the incompletely ossified skull of the fetus or newborn infant. The fontanelles normally close sometime after birth. Skull: Anatomy in infants
  • Mortality Mortality All deaths reported in a given population. Measures of Health Status rate can reach 20%–30% depending on the EEV type and host factors.
  • Survivors have severe neurologic sequelae that can include: 
    • Intellectual impairment
    • Epilepsy Epilepsy Epilepsy is a chronic brain disorder marked by recurrent and unprovoked seizures. These seizures can be classified as focal or generalized and idiopathic or secondary to another condition. Clinical presentation correlates to the classification of the epileptic disorder. Epilepsy
    • Paralysis
    • Deafness
    • Blindness Blindness The inability to see or the loss or absence of perception of visual stimuli. This condition may be the result of eye diseases; optic nerve diseases; optic chiasm diseases; or brain diseases affecting the visual pathways or occipital lobe. Retinopathy of Prematurity

Diagnosis and Management

• Diagnostic approach:
  • Clinical findings consistent with encephalitis
  • Laboratory findings:
    • Leukocytosis
    • Hyponatremia
    • CSF analysis: pleocytosis, ↑ protein concentration
  • Determination of virus:
    • Virus-specific IgM: < 4-fold rise in specific antibodies Antibodies Immunoglobulins (Igs), also known as antibodies, are glycoprotein molecules produced by plasma cells that act in immune responses by recognizing and binding particular antigens. The various Ig classes are IgG (the most abundant), IgM, IgE, IgD, and IgA, which differ in their biologic features, structure, target specificity, and distribution. Immunoglobulins: Types and Functions 
    • Viral antigen Antigen Substances that are recognized by the immune system and induce an immune reaction. Vaccination or genomic sequences (using PCR PCR Polymerase chain reaction (PCR) is a technique that amplifies DNA fragments exponentially for analysis. The process is highly specific, allowing for the targeting of specific genomic sequences, even with minuscule sample amounts. The PCR cycles multiple times through 3 phases: denaturation of the template DNA, annealing of a specific primer to the individual DNA strands, and synthesis/elongation of new DNA molecules. Polymerase Chain Reaction (PCR) assays) in CSF, blood, or tissue
  • Imaging: MRI is more sensitive in showing abnormalities (focal lesions in basal ganglia Basal Ganglia Basal ganglia are a group of subcortical nuclear agglomerations involved in movement, and are located deep to the cerebral hemispheres. Basal ganglia include the striatum (caudate nucleus and putamen), globus pallidus, substantia nigra, and subthalamic nucleus. Basal Ganglia: Anatomy, brainstem, and thalami).
• Treatment: supportive (no specific treatment)
• Prevention: 
  • Mosquito control
  • Avoidance of mosquito bites (protective clothing, mosquito repellants)