Arboviruses Slide Set

Arthropod-Borne Viruses


Arthopod-borne viruses (Arboviruses) are viruses which could be transmitted to man via an insect (arthropod) vector. In most instances, the virus is maintained in nature between the vector and an animal host. Man gets infected incidentally but is a dead end of infection. Sometimes, a human-vector-human cycle may occur e.g. Yellow Fever and Dengue. In this case,  the reservoir may be in either humans or the vector. Transovarial infection may play an important role in the maintenance of infection in the vector. In general arboviruses belong to three families:-

1.  Togaviruses - genera alphaviruses e.g. EEE, WEE, VEE.
2.  Bunyaviruses - e.g. Sicilian Sandfly Fever, Rift Valley Fever, Crimean-Congo Haemorrhagic Fever.
3.  Flaviviruses - e.g. St Louis encephalitis, Japanese Encephalitis, Yellow Fever, Dengue, Zika virus



The Togaviridae comprises of 4 genera: Alphavirus, Rubivirus, Pestivirus, and Arterivirus. The only member of the rubivirus genus is rubella. The Pestivirus genus contains 3 viruses of veterinary importance; bovine diarrhoea virus, hog cholera virus, and border disease virus. The arterivirus genus contains a single member; equine arteritis virus. The alphavirus genus has 25 members of which 11 are recognized to be pathogenic for man. All alphaviruses are mosquitoe-borne and are geographically distributed mainly in the new world.

A. Properties

Belong to the family of Togaviridae
ssRNA enveloped viruses, icosahedral symmetry
virions 60-65nm in diameter
positive stranded RNA genome. A 26S subgenomic mRNA which represents the 3' one third of the genome codes for structural proteins. Translation of the whole genome from the 5' end produces non-structural proteins
alphaviruses are classified into 6 antigenic groups on the basis of neutralization tests and molecular biology

B. The American Equine Encephalitides

The alphavirus genus contains 4 viruses which produce encephalitis; EEE, WEE, VEE and Everglades. EEE, WEE and VEE were first isolated from the brains of dead horses in the 1930s. Focal epidemics of EEE have occurred from time to time in the Eastern USA, WEE is endemic in the Western USA, VEE is endemic in S.America and occasionally causes disease in N. America. The Everglades virus is related to EEE but is restricted to the state of Florida in the U.S.A and only 5 cases of encephalitis associated with it has been reported, despite a high seroprevalence rate in S. Florida (27%). EEE and WEE are maintained in nature between mosquitoes and birds, VEE between mosquitoes and rodents. The strain of VEE pathogenic for man are mainly amplified in horses, producing equine disease prior to the beginning of human disease. This is in contrast to the EEE and WEE viruses, where horses appear to be a dead end of infection.

The clinical manifestations may vary from an inapparent or a mild influenza like illness to the syndrome of encephalitis. The probability of developing encephalitis vary widely between the different viruses; it is highest for the EEE and lowest for the VEE. The mortality rate of encephalitis vary from 5% for WEE, 35% for VEE to 50% for EEE. Laboratory diagnosis may be made by the isolation of the virus from the brain, blood, CSF or throat washings. The viruses can be isolated using suckling mice or a variety of vertebrate cell lines such as Vero, where they produce a CPE, or using mosquito cell lines, where no CPE is seen. Alternatively, a serological diagnosis may be made for which a variety of tests are available.

No specific therapy is available against these viruses. Although inactivated vaccines have been prepared by the U.S. army against these viruses, they are not available for issue to the public at large and their efficacy had yet to be proven. Prevention depends on the surveillance of these viruses in mosquitoes, birds, rodents and horses. Public warnings should be issued when these viruses are prevalent. Vector control is indicated in areas with a high mosquito population.

C. Alphaviruses Producing Fever, Polyarthritis, and Rash

Chikungunya Virus

The best example of a old world alphavirus is Chikungunya virus. It is found in areas of Africa and Asia, especially jungle areas. There are 3 possible modes of transmission, the first mode is sporadic cases arising when man come in contact with the jungle areas where the reservoir is in monkeys. Chikungunya is also prevalent in Thailand, where people to people spread occur by mosquitoes. The third type of Chikungunya exists in India, when during the rainy season, the population of mosquitoes increases greatly and epidemics can occur.

The clinical disease is a flu-like fever of acute onset, followed by a pharyngitis, maculopapular rash and arthritis. Minor haemorrhagic manifestations are occasionally seen. The diagnosis of infection is usually made by serology.

There are several other alphaviruses which produces a similar picture to that of Chikungunya virus;-

  1. O'nyong-nyong virus - found mainly in East Africa.

  2. Sindbis virus - clinical cases of sindbis disease have been reported from Africa, Middle East, Asia and Africa. Sindbis virus is maintained in nature by a cycle involving mosquito-bird- mosquito.

  3. Mayaro virus - found in S. America
  4. Ross River virus - causes epidemics in Australia and S.Pacific islands.
  5. Ockelbo virus - Sweden and USSR.



The Bunyaviridae family comprises more than 200 named viruses. Membership is usually based on antigenic interrelatedness or morphological similarity. The family is divided into 5 genera;

  1. Bunyavirus Bunyamwera, La Cross, Tahyna virus, transmitted mainly by mosquitoes
  2. Phlebovirus Sandfly fever, Rift valley fevers, transmitted by sandflies
  3. Uukuvirus transmitted by ticks, not associated with human disease
  4. Nairovirus Crimean-Congo Haemorrhagic Fever, transmitted by ticks
  5. Hantavirus Hantaan Virus, transmission does not require insects

Bunyaviruses, with the exception of Hantaviruses, are thought to be transmitted in nature by arthropods; most frequently mosquitoes but occasionally phlebotomine sandflies, midges and ticks. Vertebrate reservoirs have been demonstrated for some viruses. In others, tranovarial transmission is thought to play a dominant role in virus maintenance. Man is not known to be a natural or reservoir for any of these viruses, with the probable exception of sandfly fever.

A. Properties

Enveloped ssRNA viruses, virions 100 nm in diameter
5-10 nm projections visible on the surface
genome consists of 3 pieces of negative stranded RNA (the small RNA of phlebovirus is ambisense)
virion has 2 surface glycoproteins G1 and G2, with HA and virus neutralization epitopes
uncertain whether reassortment takes place is an uncommon event


B. Bunyavirus Genus

  1. Bunyamera Group - These mosquito-borne viruses are found on every continent except Australia. Bunyamera virus is the prototype of the group, which is an important human pathogen in sub-saharan Africa. Infection may be subclinical or may result in a fever. A severe encephalitis may develop in a few cases.

  2. Bwamba Group - Bwamba virus is widespread in Central and Eastern Africa where it causes a mild febrile illness with a maculopapular rash.

  3. Group C - the 11 members of this group is found in Central or South America. Clinical disease from these agents is usually a benign self-limited febrile illness.

  4. California Group - these viruses are found in tropic, temperate and Arctic regions where they are closely associated with their mosquito vectors. Transovarian transmission where overwintering takes place in the egg stage is thought to be responsible for the maintenance of the viruses, particularly in the Arctics. La Cross virus was first isolated from the brain of a child who died of encephalitis. It is now recognized to be a significant cause of encephalitis. It is probably the most prevalent mosquito-borne disease after St Louis encephalitis in the USA. Cases occur in the Summer months. Infection is usually subclinical or result in a mild febrile illness. Cases of encephalitis usually occur in children. The prognosis is good; complete recovery is the rule and residua are rare. The diagnosis is made by serology as virus isolation had usually been unsuccessful. Treatment is symptomatic and supportive and no vaccine is available. The Tahyna virus is found in Central and Western Europe where is causes a mild febrile disease to aseptic meningitis. Snowshoe hare virus is found in the Arctics and is associated with febrile CNS disease.

  5. Guama Group - found exclusively in the New World tropics, they cause mild febrile illness.

  6. Simbu Group - these viruses are found in temperate and tropical zones of the world, with midges being an important vector although they are also found in mosquitoes. The Orepuche virus had caused 8 major epidemics in Northern Brazil within the last 20 years. It causes a febrile illness of acute onset, often accompanied by a rash and meningitis. The virus probably occurs in nature in a sylvatic cycle (primates, sloths and birds), and an urban cycle.

C. Phlebovirus Genus

There are at least 45 members which are mostly associated with sandflies. These viruses do not usually replicate in mosquitoes. There is evidence for transovarial transmission of many phleboviruses.

  1. Naples and Sicilian Sandfly Fever Viruses - They are the best known and the most widely distributed phleboviruses and are thought to be the cause of most clinically described sandfly fever in the Mediterranean, SW Asia, India and perhaps China. It causes a febrile illness of acute onset, with severe headache, general malaise and arthralgia. The disease is self-limiting with complete recovery. In areas where sandflies are endemic, most of the population is thought to be infected during childhood.

  2. Rift Valley Fever Virus - This virus is able to cause severe disease in domestic animals and man. The disease is found in all parts of Africa, where epidemics have occurred from time to time with significant morbidity and mortality. It was originally isolated in Kenya during an epizootic of fatal hepatic necrosis and abortion in sheep. Since that time, it has repeatedly infected herds of sheep, cattle, and goats in which it produced 10 to 30% mortality. The impact of the virus on domestic animal raisers in the Rift valley and southern Africa is substantial. Rift valley fever virus extended into Egypt in 1977 causing a widespread epidemic with at least 600 deaths. The virus subsequently disappeared from Egypt. The virus is thought to transmitted mainly by mosquitoes although it can be transmitted by sandflies. Aerosol transmission had also been documented where man had become infected after coming into contact with animal carcasses. Most infections are symptomatic and usually present as a mild non-specific febrile illness, a small proportion (~1%) develop haemorrhagic fever, retinal vasculitis and encephalitis. Treatment is supportive, although ribavirin, interferon and passive immunization have been shown to be useful in animal models. Certain ribavirin should be considered as part of the management. A formalin-inactivated cell culture vaccine is available and is thought to be effective and safe, although it is very expensive and thus its use should be confined to susceptible laboratory and veterinary workers.

D. Uukuvirus_Genus

Members of this genus have been found in ticks but they have yet to be associated with any human disease, even though antibodies against these viruses have been demonstrated from Czechoslovakia, Hungry and Norway.

E. Nairovirus Genus

All known members of the Nairovirus genus are thought to be transmitted by ticks and have often shown to be vertically transmitted. Their ecology is not completely understood.

Crimean-Congo Haemorrhagic Fever - During the Second World War, 200 cases of severe haemorrhagic fever was described from the Crimea. Similar diseases have been recognized from the USSR and parts of Eastern Europe. A similar disease was subsequently described in the Congo. It is now clear that a single antigenic strain of the virus is distributed from the sub-Saharan Africa, Eastern Europe, Middle East and possibly India. The virus is carried by ticks where transovarian transmission have been demonstrated. Certain vertebrate hosts are also infected, such as hedgehogs, hares and domestic animals, where amplification is known to occur. Medical personnel are at some risk of secondary infection and accidental parenteral inoculation is dangerous. Following an incubation period of 3 to 21 days, a non-specific febrile illness of abrupt onset develops. The second phase of the illness sees the development of haemorrhagic manifestations which lasts several days. Petechiae, GI haemorrhage, haematuria may be seen as are marked neurological manifestations. Hepatosplenomegaly may be present as well as bradycardia, hypotension and in some sever cases, shock. A recovery phase then follows. Mortality rates of 10-40% had been reported. The virus may be readily isolated from the blood by the intracranial inoculation of suckling mice, which rapidly die of paralytic disease. A CPE is also seen in vero cells and other cell cultures can be used. Treatment is mainly supportive. The drug ribavirin has encouraging results in vitro and systemic therapy with this agent should be considered in severe cases. Hyperimmune convalescent serum may be useful and those with a high titre of virus neutralizing antibodies should be selected. An inactivated vaccine had been prepared. Although it has few side effects, it does not appear to elicit a high titre of virus neutralizing antibodies.

F. Laboratory Diagnosis Bunyavirus Infections

  1. Virus Isolation - intracranial inoculation of suckling mice is thought to be the most sensitive system available for virus isolation. However, several sensitive cell culture systems are available such as vero. LLC-MC2 and mosquito cells. Once isolated the virus can be types by neutralizing tests.
  2. Rapid Diagnosis - antigen detection systems and the detection of specific IgM antibodies are becoming available as means of rapid diagnosis. RT-PCR is also being increasingly used.
  3. Serology - a wide variety of serological techniques are available such as HI, CFT, IFA, neutralization tests and ELISAs.



The viruses of the family Flaviviridae are important arthropod-borne viruses in both human and veterinary medicine. They are transmitted by mosquito and ticks and usually are maintained in a transmission cycle in nature. They are widely distributed throughout the world with the exception of the polar region, although a specific flavivirus may be geographically restricted to a continent or a particular part. They produce a broad spectrum of clinical responses in humans ranging from asymptomatic infection to fulminant encephalitis or haemorrhagic fever. Nearly 60 flaviviruses are known to exist but many are yet to be shown to cause disease in humans.

A. Properties

ssRNA enveloped viruses, 40-50 nm in diameter
positive RNA genome of around 10,000 nucleotides
3 structural proteins (an envelope glycoprotein, a nucleoprotein, and a small membrane protein), and a number of non-structural proteins.
one single reading frame, no subgenomic mRNA
classified in terms of cross-reactivity and the host.

The flavivirus family is divided into 7 subgroup of viruses. The antigenic determinants are carried by the envelope glycoprotein which is recognized by both HI and neutralization tests. 3 types of antigenic determinants are found for flaviviruses;

  1. Type-specific determinants

  2. Complex-reactive determinants shared by closely related viruses

  3. Group-reactive determinants shared by all serologically related flaviviruses.

The envelope glycoprotein contain mainly type-specific determinants and to a lesser extent complex-reactive and group-reactive determinants. The nucleocapsid proteins had only been shown to contain group-reactive determinants.

B. Flaviruses Producing Encephalitis

The flavivirus family contains many viral agents which produces encephalitis. Flavivirus encephalitides are either mosquito- borne, tick-borne, or have an unknown vector.



St Louis encephalitis      birds                     Americas, Caribbean

Japanese encephalitis      birds, pigs               N and SE Asia, India

Murray Valley              birds                     Australia

West Nile                  birds                     Africa, Asia, Europe

Ilheus                     birds (humans?)           Central and South America


Russian spring-rodents,    birds                     Russia
summer encephalitis

Central European           rodents, birds,           Western and Central Europe
encephalitis               goats

Louping ill                rodents, sheep            Great Britain

Powassan                   Squirrels, chipmunks      N. America, USSR

No known vector

Rio bravo                  bats                      Americas

Rocio                      birds (humans?)           Brazil

Negishi                    unknown                   Japan

C. Mosquito-Borne Flaviviruses

2. St Louis Encephalitis

St. Louis encephalitis occurs in endemic and epidemic form throughout the Americas and is the most important arboviral disease of North America. It is closely related to Japanese encephalitis and the Murray Valley encephalitis viruses. From 1955 to 1988, over 5000 cases of SLE have been reported to the Centers for Disease Control. The reported cases are only a fraction of those that actually occur. The largest epidemic occurred in 1975 when 1815 cases were reported. The virus is maintained in nature by a bird-mosquito-bird cycle.

The incubation period is 21 days. The ratio of inapparent to apparent infection ranges from 16:1 to 425:1. Children are much more likely to have inapparent infection than adults. The morbidity and mortality rate increases with age. Patients who are symptomatic will usually present with or progress to one of three syndromes (1) febrile headache (2) aseptic meningitis (3) encephalitis. The laboratory diagnosis is usually made by serology. Treatment is supportive and no vaccine is available.

2. Japanese Encephalitis

Japanese encephalitis is a major public health problem in Asia, SE Asia, and the Indian subcontinent. Prior to 1967, thousands of cases with several hundred deaths were reported each year. In endemic areas where vector control and vaccination had been undertaken, the incidence had dropped dramatically. Epidemics had been reported from Japan, China, Korea, Taiwan, USSR, Vietnam, Philippines, ASEAN countries, India and Bangladesh. The transmission cycle in nature involve the Culex and Aedes mosquitoes and domestic animals, birds, bats, and reptiles. Man is not a preferred host for Culex mosquitoes and transmission of JE virus does not usually occur until mosquito populations are large.


Japanese encephalitis produces a high inapparent to apparent infection ratio, ranging from 25:1 to 500:1 case of encephalitis. However, when encephalitis occur, the mortality rate is in the range of 20 to 50%. Some patients will only show an undifferentiated febrile illness or have mild respiratory tract complaints. The diagnosis is usually made serologically as virus isolation is not usually successful. No specific treatment is available. An inactivated suckling mouse brain vaccine had been available since the early 1960s which had been extensively used throughout Asia. The efficacy rate ranges from 60 to 90%. Despite the vaccine being a mouse brain preparation, no postvaccination demyelinating allergic encephalitis had been reported. Mild symptoms occur in 1% of vaccinees and thus the vaccine is generally to be considered as safe.

3. Murray Valley Encephalitis

This virus is closely related to Japanese encephalitis and resembles JE clinically. It is confined to the Australia and New Guinea, where it is an important cause of epidemic encephalitis periodically. In the 8 epidemics that took place between 1917 to 1988, 330 cases were reported in Australia. The diagnosis is made by serology and no specific treatment or vaccine is available.

4. West Nile Fever

West Nile fever is a dengue-like illness that occurs in both epidemic and endemic forms in Africa, Asia, and the Mediterranean countries. Areas of high endemicity include Egypt and Iran where most of the adult population will have antibodies. West Nile virus is a member of the St Louis encephalitis complex and is transmitted by Culex mosquitoes. The virus is maintained in nature through a transmission cycle involving mosquitoes and birds. Children will usually experience an inapparent or a mild febrile illness. Adults may experience a dengue-like illness whilst the elderly may develop an encephalitis which is sometimes fatal. The diagnosis is usually made by serology although the virus can be isolated from the blood in tissue culture. No vaccine fro the virus is available and there is no specific therapy. Among the arboviruses, it is difficult to distinguish clinically between West Nile, dengue and Chikungunya. In the absence of a rash, a number of toga and bunyaviruses should also be considered in the differential diagnosis.

5. Ilheus Virus

This virus is found in Latin America where it causes a febrile illness with arthralgia. Occasionally a mild encephalitis is seen. The virus can often be confused with dengue, St Louis encephalitis, yellow fever and influenza viruses.

II. Tick-Borne Encephalitis Viruses

Tick-borne encephalitis viruses occur in temperate climates of Western and Eastern Europe and the USSR. These viruses are so closely related antigenically that it is uncertain whether to group them as separate viruses or as variants of the same virus. TBE viruses can be transmitted to a wide range of animals by ticks and is probably maintained in nature by small rodents. Humans can be infected via tick bites or by drinking milk of infected animals such as goat, cows and sheep. The clinical presentation vary from asymptomatic infection to fulminant encephalitis and death. The diagnosis is made serologically. By the time overt clinical manifestations are seen, the viraemia had already subsided so that the virus cannot be isolated from the blood or CSF. The treatment of TBE is supportive. A formalin-inactivated vaccine is available for use in the USSR which is recommended for persons living in endemic areas and for laboratory workers who may be exposed to the virus.


Louping ill is primarily a disease of sheep in England, Ireland and Scotland. Cattle, pigs, deer and some small mammals and ground-dwelling birds are also infected. It is relatively rare disease of humans caused by contact with infected tissue of sheep (butchers and vet), laboratory accidents and through tick bites. The disease caused resembles that of a mild form of tick-borne encephalitis. The disease starts of with a mild influenza-like illness which may proceed to a mild meningoencephalitis. A vaccine is available for sheep which should reduce human disease.

C. Flaviruses_Producing_Haemorrhagic_Fever


1. Yellow Fever

a. Epidemiology

Yellow fever, once a scourge of the port cities of North America and Europe, remains an important endemic and epidemic disease of Africa and South America. Yellow fever occurs in 2 major forms: urban and jungle (sylvatic) yellow fever. Jungle YF is the natural reservoir of the disease in a cycle involving nonhuman primates and forest mosquitoes. Man may become incidentally infected on venturing into jungle areas. The S American monkeys are more prone to mortality once infected with YF than the old world monkeys, suggesting that American YF probably originated from the old world as a result of sailing ships.

The urban form is transmitted between humans by the Aedes aegypti mosquito and thus the potential distribution of urban YF is in any areas where infestation with Aedes aegypti occurs, including Africa, S and N America and Asia. Although the urban vector is present in Asia, yellow fever has never been established there. The majority of reported human YF cases come from Africa (Angola, Cameroon, Gambia, Ghana, Nigeria, Sudan, and Zaire) and S America (Brazil, Bolivia, Columbia, Peru, Ecuador and Venezuela). Both of these continents have jungle yellow fever transmitted in a monkey-mosquito-monkey cycle. In these areas, YF is reintroduced into urban populations from time to time as a result of contact with jungle areas. YF cases occur more frequently at times of the year when there are high temperatures ad high rainfall, conditions which are most conducive to mosquito reproduction.

Once infected, the mosquito vector remains infectious for life. transovarial transmission of Aedes aegypti had been demonstrated and may provide a mechanism for the continuation of the jungle or urban cycle. Once the virus is inoculated into human skin, local replication occurs with eventual spread to the local lymph nodes and viraemia occurs. The target organs are the lymph nodes, liver, spleen, heart, kidney and foregut.

b. Clinical Features

The incubation period varies from 3 to 6 days, following which there is an abrupt onset of chills, fever, and headache. Generalized myalgias and GI complaints (N+V) follows and signs may include facial flushing, red tongue and conjunctival injection. Some patients may experience an asymptomatic infection or a mild undifferentiated febrile illness. After a period of 3 to 4 days, improvement should be seen in most patients. The moderately ill should begin to recover, however, the more severely ill patients with a classical YF course will see a return of fever, bradycardia (Faget's sign), jaundice, and haemorrhagic manifestations. The haemorrhagic manifestations may vary from petechial lesions to epitaxis, bleeding gums, GI haemorrhage (black vomit of YF). 50% of patients with frank YF will develop fatal disease characterized by severe haemorrhagic manifestations, oliguria and hypotension. Frank renal failure is rare. Rarely, other clinical findings such as meningoencephalitis in the absence of other findings have been described.

c. Laboratory Diagnosis

The differential diagnosis of YF include typhoid, leptospirosis, tick-borne relapsing fever, typhus, Q fever, malaria, severe viral hepatitis, Rift valley fever, Crimean-Congo haemorrhagic fever, Lassa, Marburg and Ebola fever. Yellow fever can be diagnosed serologically or by virus isolation. The serological diagnosis can be made by HI, CF and PRN tests. Virus isolation can be attempted from the blood which should be obtained within the first 4 days of illness. A variety of techniques are available for virus isolation, such as intracerebral inoculation of newborn Swiss mice or inoculation into Vero, LLC MK-2, BHK, or arthropod cell lines. Molecular methods such as real-time RT-PCR are being increasingly used for the rapid diagnosis of Yellow Fever infection.

d. Treatment and Prevention

No specific antiviral therapy is available and treatment is supportive. Intensive medical treatment may be required but this is difficult to provide as many epidemics occur in remote areas. Yellow fever is regarded as a quarantinable disease of international public health significance and public health officials should be notified as soon as possible so that vector eradication and mass immunization can be carried out as soon as possible to prevent an epidemic. A live attenuated vaccine known as the 17-D had been available since 1937. The vaccine is regarded as highly effective and generally safe, with mild reactions such as headache, myalgia and low grade fever occurring in 5 to 10% of vaccinees. Vaccination is recommended for residents of endemic areas and should be included in routine vaccination programs. Travelers to endemic areas should also be vaccinated. It is officially recommended that a booster dose should be given every 10 years although this may change in view of recent data on the long persistence of YF antibodies. The contraindications to the use of 17-D vaccine are pregnancy, altered immune states, and hypersensitivity to eggs.

2. Kyasanur Forest Disease

This is a tick borne disease closely related to the tick-borne encephalitis complex and is geographically restricted to Karnataka State in India. Haemorrhagic fever and meningoencephalitis may be seen. The case-fatality rate is 5%.

3. Dengue

Hundreds of thousands of cases of dengue occur every year in endemic and epidemic forms in tropical and subtropical areas of the world. The attack rates during epidemics can reach as high as 50%. Dengue is a prevalent public health problem in SE Asia, the Caribbean, Central America, Northern South America and Africa. (See red coloured areas in map below). In hyperendemic areas, most cases occur in young children as the majority of the population had already been infected with multiple serotypes. In other areas, older children and adults are more likely to be affected. Maximum number of cases occur during the months of the year with the highest rainfall and temperatures, when Aedes aegypti populations are at their highest. A. aegypti mosquitoes deposit their eggs in waterfilled containers and thus reproduction is highest during periods of high rainfall.


4 serologically distinguishable types of dengue are recognized (DEN 1-4). The vector mosquito becomes infected by feeding on a viraemic host. The virus becomes established in the salivary glands of the mosquito from where it can be transmitted to susceptible individuals. Following an incubation period of 2 to 7 days, the virus is disseminated (route unknown), to the organs of the RE system (liver, spleen. bone marrow and lymph nodes). other organs may be involved such as the heart, lungs and GI tract.

a. Clinical Manifestations

The clinical presentation of dengue in children is varied. The disease may be manifested as an undifferentiated febrile illness, an acute respiratory illness, or as a GI illness: atypical presentations which may not be recognized by clinicians as dengue. Older children and adults infected with dengue the first time will display more classical symptoms: sudden onset of fever, severe muscle aches, bone and joint pains, chills, frontal headache and retroorbital pain, altered taste sensation, lymphadenopathy, and a skin rash which appears 3 days after the onset of fever. The rash may be maculopapular, petechial or purpuric and is often preceded by flushing of the skin. Other haemorrhagic manifestations may be seen such as epitaxis, gingival bleeding, ecchymoses, GI bleeding, vaginal bleeding and haematuria. Severe cases of bleeding should not be diagnosed as Dengue haemorrhagic fever (DHF) or Dengue shock syndrome (DSS) unless they meet the criteria below.

DHF or DSS are usually seen in children and usually occurs in 2 stages. The first milder stage resembles that of classical dengue and consists of a fever of acute onset, general malaise, headache, anorexia and vomiting. A cough is frequently present. After 2 to 5 days, the patient's condition rapidly worsens as shock begins to appear. Haemorrhagic manifestations ranging from petechie and bleeding form the gums to GI bleeding may be seen. An enlarged nontender liver is seen in 90% of cases. The WHO recommended the following criteria for the diagnosis of DHF and DSS:

1. Fever
2. Haemorrhagic manifestations including at least a positive tourniquet test.
3. Enlarged liver
4. Shock
5. Thrombocytopenia (<=100,000 ul)
6. Haemoconcentration (HcT increased by =20%)

A diagnosis of DSS is made when frank circulatory failure is seen, and occurs in one third of cases of DHF. DHF has been graded by the WHO on the basis of its severity.

Grade I 

Grade II 

Grade III 

Grade IV

Fever accompanied by non-specific constitutional symptoms, the only haemorrhagic manifestation is a positive tourniquet test 

Spontaneous bleeding in addition to the manifestations of Grade I patients, usually in the form of skin and/or other haemorrhages 

Circulatory failure manifested by rapid and weak pulse, narrowing of pulse pressure (20 mmHg or less) or hypotension, with the presence of cold clammy skin and restlessness 

Profound shock with undetectable blood pressure and pulse

Grade III and Grade IV DHF are also considered as Dengue Shock Syndrome.


b. Immunology

The immunological response to dengue infection depends on the individual's past exposure to flavivirus. The flavivirus group shares cross-reacting antigen(s). Primary infection results in the production of antibodies against the infecting serotype predominantly. Reinfection with another dengue serotype (or other flaviviruses) usually produces a secondary (heterotypic) response characterized by very high titres to all 4 dengue virus serotypes and other flaviviruses, so that serological identification of the infecting agent is quite difficult if not impossible. After a first infection with one dengue serotype, cross-immunity to other serotypes may persists for a few months, but after 6 months, reinfection with another serotype may occur.

c. Pathology

There are 2 theories proposed for the pathogenesis of DHF and DSS: virus virulence and immunopathological mechanisms. The weight of the available evidence supports the immunopathological theory. DHF and DSS occurred most often in patients with a secondary (reinfection) serological response. However, the observation that DHF and DSS occurred in infants with a primary response cast some doubt on this theory until it was demonstrated that preexisting maternal antibody had a similar effect to acquired antibody. The antibody-dependent theory proposes that in the presence of non-neutralizing heterotypic antibody (whether maternally derived or not) to dengue, virus-antibody complexes are formed which are more capable of infecting permissive mononuclear phagocytes than uncomplexed dengue virus.

d. Laboratory Diagnosis

1. Serology - HI, CF and PRN tests are commonly used. The high degree of cross-reactivity between flaviviruses can make the interpretation of serological results very difficult.

2. Virus isolation - this can be accomplished by the intracerebral inoculation of sera from patients into suckling mice. Sera can also be inoculated intrathoracically into Aedes mosquitoes. Head squash preparations are examined for the presence of antigen by the FA technique. Cell cultures such as LLC MK-2 and several mosquito-derived cell lines can be used.

3. Direct detection - RT-PCR may detect virus in serum and can reliably identify the infecting dengue type.

e. Treatment and Prevention

There is no specific antiviral treatment available. Management is supportive and intensive medical management is required for cases of severe DHF and DSS. No vaccine for dengue is available but a tetravalent live-attenuated vaccine has been evaluated in Thailand with favorable results. Scale-up preparation for commercial production of the vaccine is underway and it is anticipated that the vaccine will become available soon and evaluated in large scale clinical trials. To avoid dengue, travelers to endemic areas should avoid mosquito bites. Prevention of dengue in endemic areas depends on mosquito eradication. The population should remove all containers from their premises that may serve as vessels for egg deposition. Vector surveillance is an integral part in control measures to prevent the spread of dengue outbreaks. Regular inspections as part of law enforcement may be used in the control of mosquito vectors. The object of source reduction is to eliminate the breeding grounds in and around the home environment, construction sites, public parks, schools and cemeteries. Illegal dumping of household refuse provide favorable breeding sites for mosquitoes. Long-term control should be based on health education and community participation, supported by legislation and law enforcement. Domestic water supplies should be improved in order to reduce the use of containers for the storage of water. No dengue vaccine is available although candidate attenuated vaccines have been developed. against all serotypes. It would be important to immunize against all 4 dengue serotypes at the same time because of concerns that DHF and DHS may be triggered if the patient is infected in future by a dengue type that he had not been immunized against.


4. Zika Virus

Zika virus is a flavivirus that is primarily transmitted by Aedes mosquitoes, mainly Aedes aegypti. It can also be sexually transmitted and possibly via blood transfusions. Most infections are asymptomatic. When symptoms are present, they are usually mild and resemble dengue.  They include a mild fever, skin rash, conjunctivitis, muscle and joint pain, malaise or headache. These symptoms generally last for 2-7 days. Interest in this virus exploded with increasing scientific evidence that it is associated with microcephaly and Guillain-Barré syndrome. The distribution of zika virus is similar to Dengue, being found mainly in tropical areas of Asia, Africa and America.

Zika Virus Infections Worldwide.

Zika virus was first isolated in Uganda in 1947 and the first documented outbreak in Micronesia in 2007. A massive outbreak occurred from 2015-16 that originated in Brazil which led the WHO to declare a Public Health Emergency. That outbreak spread to other American, Pacific, Asian and African countries. The outbreak virus was thought to be Asian in origin and was possibly brought to Brazil by a traveler from French Polynesia.

Zika virus was first reported to be associated with microcephaly and Guillain-Barré syndrome by Brazil in 2015. Since then, there is scientific consensus that Zika virus is a cause of congenital abnormalities including microcephaly and a trigger of Guillain-Barré syndrome. There is ongoing research to investigate whether there is any linkage between Zika virus and other neurological disorders.

A diagnosis of Zika virus infection is generally accomplished by the detected of virus RNA from blood, urine or saliva by RT-PCR. The virus can also be isolated in Vero cells. IgM antibodies may be detected early in the infection. There is no specific treatment for Zika virus which is normally a mild self-limiting disease. Pregnant women who were infected are advised to have their fetus followed up closely by ultrasound. Protection against mosquito bites and control of mosquito population are the main measures used to prevent Zika virus infection. The fact that Zika virus can be sexually transmitted is of concern due to to the association of Zika virus and congenital disorders. All people with Zika virus infection and their partners should be counseled as to the risks of Zika virus infection in pregnancy. Pregnant women in infected areas should practice safer sex or abstained altogether.



Orbiviruses are insect-borne viruses primarily of veterinary importance. Orbiviruses contain dsRNA arranged in 10 segments except for the members of the Colorado tick fever serogroup which have 12 segments of dsRNA. They vary in size from 50 to 90nm with 92 capsomers. The capsid is a double-layered protein. The outer coat is diffuse and unstructured while the inner layer is organized in pentameric-hexameric units. Colorado tick fever is caused by a virus belonging to the family of Reoviridae. It is a zoonotic disease of rodents and is transmitted to man via tick bites. It is prevalent in the Rocky Mountains and more western regions of the USA. It is a dengue-like illness albeit with a relatively low incidence. Colorado annually reports 100 to 300 cases but the disease is underreported there and from other western states as well. There is a strong seasonal trend with the majority of cases occurring between February and July. Chipmunks and squirrels serve as amplifying hosts.

Following an incubation period of 3 to 6 days, a high fever of acute onset is seen, along with chills, joint and muscle pains, headache, N+V. A maculopapular rash may be seen in a minority of patients. A more severe clinical picture may be seen in children, who may develop haemorrhagic manifestations including severe GI bleeding and DIC. Aseptic meningitis or encephalitis may be seen. CTF may be diagnosed by virus isolation whereby the patient's blood is inoculated into suckling mice or cell culture lines such as Vero, followed by identification by IF, N or CF tests. More rapid diagnosis can be made by performing IF directly on the blood clots. A IgM tests as well as other serological techniques are available for serological diagnosis. No licensed vaccine for CTF is available nor is it practicable because of the rarity and the benign nature of the disease. Public health education remains the most preventative measure.

Arboviruses Slide Set