Infection of humans by mosquito-borne encephalitis viruses occurs when a mosquito or another arthropod bites first an infected animal and later a human.

The equine encephalitides—Eastern, Western, and Venezuelan—are transmitted by culicine mosquitoes to horses or humans from a mosquito–bird–mosquito cycle (Figure 1). Equines, like humans, are unessential hosts for the maintenance of the virus. Both Eastern and Venezuelan equine encephalitis in horses are severe, with up to 90% of affected animals dying. Epizootic Western equine encephalitis is less frequently fatal for horses. In addition, Eastern equine encephalitis produces severe epizootics in certain domestic game birds. A mosquito–bird–mosquito cycle also occurs in St. Louis encephalitis, West Nile virus, and Japanese B encephalitis. Swine are an important host of Japanese B encephalitis. Mosquitoes remain infected for life (several weeks to months). Only the female feeds on blood and can feed and transmit the virus more than once. The cells of the mosquito’s midgut are the site of primary viral multiplication. This is followed by viremia and invasion of organs—chiefly salivary glands and nerve tissue, where secondary viral multiplication occurs. The arthropod remains healthy.

Fig1. Generalized transmission cycle of mosquito-borne flaviviruses causing encephalitis. Summertime amplification and possible overwintering mechanisms are shown. Humans are dead-end hosts and do not contribute to perpetuation of virus transmission. Wild birds are the most common viremic hosts, but pigs play an important role in the case of Japanese encephalitis virus. The pattern shown applies to many (but not all) flaviviruses. (Adapted from Monath TP, Heinz FX: Flaviviruses. In Fields BN, Knipe DM, Howley PM [editors- in-chief]. Fields Virology, 3rd ed. Lippincott-Raven, 1996.)

Infection of insectivorous bats with arboviruses produces a viremia that lasts from 6 to 12 days without any illness or pathologic changes in the bat. When the viral concentration is high, the infected bat may infect mosquitoes that are then able to transmit the infection to wild birds and domestic fowl as well as to other bats.

There are also tick-borne flavivirus encephalitides. Ticks can become infected at any stage in their metamorphosis, and virus can be transmitted transovarially (Figure 2). Virus is secreted in the milk of infected goats for long periods, and infection may be transmitted to those who drink unpasteurized milk. Powassan encephalitis virus was the first member of the Russian spring–summer complex isolated in North America. The original fatal case was reported from Canada in 1959. Human infection is rare.

Fig2. Generalized transmission cycle of tick-borne flaviviruses, showing hosts for larval, nymphal, and adult ticks. Virus is passed to succeeding tick stages during molting (transstadial transmission), as well as transovarially to progeny of adult ticks. Both male and female ticks are involved in transmission. Tick borne encephalitis virus may be transmitted to uninfected ticks cofeeding on a vertebrate host without the requirement for active viremic infection of the host. (Adapted from Monath TP, Heinz FX: Flaviviruses. In Fields BN, Knipe DM, Howley PM [editors-in-chief]. Fields Virology, 3rd ed. Lippincott-Raven, 1996.)

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اكتشاف هام قد يمهّد لعلاج جديد للسمنة

تغييرات بسيطة في نمط الحياة تقلل خطر أمراض القلب بنسبة كبيرة

ليس اللب فقط!.. قشرة وبذور المانغو تخفي فوائد صحية مذهلة

زيادة استهلاك الكالسيوم ومنتجات الألبان يحمينا من مشكلات صحية خطيرة

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الآخبار التكنلوجية

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لحظة زمنية خاطفة في الدماغ تتحكم في التعلم والحركة

القمر لم يعد مجرد محطة.. خطوة جديدة تغيّر مستقبل البشرية

العلماء يكتشفون "نوعا جديد تماما" من الكواكب

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مواضيع ذات صلة

HIV Infections in Humans: Laboratory Diagnosis

HIV Infections in Humans :Clinical Findings

HIV Infections in Humans : Pathogenesis and Pathology

Animal Lentivirus Systems

Classification of Lentiviruses

Viral DNA Is Integrated into the Host-Cell Genome in Some Nonlytic Viral Growth Cycles

Lytic Viral Growth Cycles Lead to Death of Host Cells

Viral Capsids Are Regular Arrays of One or a Few Types of Protein

Structure and Composition of Lentiviruses

HERPESVIRUSES

POLYOMAVIRUSES

Human T-Lymphotropic Viruses