**3. Vector and reservoir**

In Asia and the Indian Ocean region the main CHIKV vectors are *A. aegypti* and *A. albopictus.*( Jeandel et al. 2004 ; Zeller, 1998). A larger range of *Aedes* species *(A. furcifer, A. vittatus, A. fulgens, A. luteocephalus, A. dalzieli, A. vigilax, A. camptorhynchites)* transmit the virus in Africa, and *Culex annulirostris, Mansonia uniformis*, and anopheles mosquitoes have also occasionally been incriminated (Jupp P.G. et al., 1981; Jupp P.G. & McIntosh, 1990; Lam et al., 2001;). In India, the dominant carrier of Chikungunya virus is *A. aegypti*, which breeds mainly in stored fresh water in urban and semi-urban environments (Yergolkar, 2006). *A. albopictus* has a wide geographical distribution, is particularly resilient, and can survive in both rural and urban environments. The mosquito's eggs are highly resistant and can remain viable throughout the dry season, giving rise to larvae and adults the following rainy season. Originating from Asia, and initially sylvatic, *A. albopictus* has shown a remarkable capacity to adapt to human beings and to urbanisation, allowing it to supersede *A. aegypti* in many places, and to become a secondary but important vector of dengue and other arboviruses (Knudsen, 1995). *A. albopictus* is zoophilic and anthropophilic, is aggressive, silent, active all-day long, and has a lifespan longer than other mosquitoes (up to 8 weeks) and, in the last decades has expanded to several areas previously known to be *Aedes*-free (Charrel et al., 2007). It seems that most new introductions of *A. albopictus* have been caused by vegetative eggs contained in timber and tyres exported from Asia throughout the world. Other emerging events also contributed to the introduction of *A. albopictus* mosquitoes into previously unaffected areas, such as climate change and the increasing use of plastic containers in developing countries. Indeed, climate changes may have several effects on vector biology: increasing temperatures may improve survival at higher latitudes and altitudes, increase the growth rates of vector populations, and alter their seasonality; increased rainfall may have an effect on the larval habitat and population size, and finally an increase in humidity could favourably affect vector survival (Gubler et al., 2001). The use of plastic containers in developing countries, where they are usually not correctly disposed and remain in the environment for years, has also been linked with the spread of the mosquitoes: acting as rain-water receptacles, and being exposed to sunlight, they can become perfect "incubators" for mosquito eggs, where the ideal conditions of temperature and humidity are achieved easily and naturally.

Human beings serve as the Chikungunya virus reservoir during epidemic periods. In Africa some animals (monkeys, rodents, and birds) constitute the virus reservoir during not-epidemic periods, sustaining virus circulation in the environment in the absence of human cases. Outbreaks might occur in monkeys when herd immunity is low; the animals develop viraemia but no pronounced physical manifestations (Inoue et al., 2003; Wolfe et al., 2001). An animal reservoir has not been identified in Asia, where humans appear to be the only host.

### **4. Clinical manifestation**

#### **4.1 General features**

118 Current Topics in Tropical Medicine

CHIKV is an alphavirus belonging to the *Togaviridae* family. Alphaviruses are small and spherical, with a 60-70 nm diameter capsid and a phospholipid envelope. The RNA singlestrand of positive polarity encodes four non structural proteins (nsP1-4) and three structural proteins (C,E1,E2). Viral replication is initiated from the time of attachment of viral envelope to cellular host receptors (Strauss & Strauss, 1994). Endocitosis of the virus occurs, following which, delivery of the viral nucleocapsid into cytoplasm takes place. The replication cycle is considerably fast, taking around 4hours. Alphaviruses are sensitive to disseccation and to temperatures above 58°C (Khan et al., 2002; Strauss J.H. & StraussE.M., 1994). About 30 species of arthropod-borne viruses are included in the alphavirus genus, antigenically classified into 7 complexes. These viruses are widely distributed throughout the world, with the exception of Antarctica, and 7 of them cause a syndrome similar to Chikungunya fever, arthralgia and rash: Barmah Forest and Ross river viruses (Oceania), O'nyong-nyong and Semliki Forest viruses (Africa), Mayaro (South America), Sindbis and Sindbis-like (Africa,

In Asia and the Indian Ocean region the main CHIKV vectors are *A. aegypti* and *A. albopictus.*( Jeandel et al. 2004 ; Zeller, 1998). A larger range of *Aedes* species *(A. furcifer, A. vittatus, A. fulgens, A. luteocephalus, A. dalzieli, A. vigilax, A. camptorhynchites)* transmit the virus in Africa, and *Culex annulirostris, Mansonia uniformis*, and anopheles mosquitoes have also occasionally been incriminated (Jupp P.G. et al., 1981; Jupp P.G. & McIntosh, 1990; Lam et al., 2001;). In India, the dominant carrier of Chikungunya virus is *A. aegypti*, which breeds mainly in stored fresh water in urban and semi-urban environments (Yergolkar, 2006). *A. albopictus* has a wide geographical distribution, is particularly resilient, and can survive in both rural and urban environments. The mosquito's eggs are highly resistant and can remain viable throughout the dry season, giving rise to larvae and adults the following rainy season. Originating from Asia, and initially sylvatic, *A. albopictus* has shown a remarkable capacity to adapt to human beings and to urbanisation, allowing it to supersede *A. aegypti* in many places, and to become a secondary but important vector of dengue and other arboviruses (Knudsen, 1995). *A. albopictus* is zoophilic and anthropophilic, is aggressive, silent, active all-day long, and has a lifespan longer than other mosquitoes (up to 8 weeks) and, in the last decades has expanded to several areas previously known to be *Aedes*-free (Charrel et al., 2007). It seems that most new introductions of *A. albopictus* have been caused by vegetative eggs contained in timber and tyres exported from Asia throughout the world. Other emerging events also contributed to the introduction of *A. albopictus* mosquitoes into previously unaffected areas, such as climate change and the increasing use of plastic containers in developing countries. Indeed, climate changes may have several effects on vector biology: increasing temperatures may improve survival at higher latitudes and altitudes, increase the growth rates of vector populations, and alter their seasonality; increased rainfall may have an effect on the larval habitat and population size, and finally an increase in humidity could favourably affect vector survival (Gubler et al., 2001). The use of plastic containers in developing countries, where they are usually not correctly disposed and remain in the environment for years, has also been linked with the spread of the mosquitoes: acting as rain-water receptacles, and being exposed to sunlight,

**2. Microbiology** 

Asia, Scandinavia and Russia) (Taubiz et al., 2007).

**3. Vector and reservoir** 

After infection with Chikungunya virus, there is a silent incubation period lasting 2–4 days on average (range 1–12 days) (Lam et al., 2001). Clinical onset is abrupt, with high fever, headache, back pain, myalgia, and arthralgia; the latter can be intense, affecting mainly the extremities (ankles, wrists, phalanges) but also the large joints (Hochedez et al., 2006; Lam et al., 2001; Quatresous, 2006; Robinson, 1955; Saxena et al., 2006). Skin involvement present in about 40–50% of cases, and consists of (1) a pruriginous maculopapular rash predominating on the thorax, (2) facial oedema, or (3) in children, a bullous rash with pronounced sloughing, and (4) localised petechiae and gingivorrhagia (mainly in children) (Brighton et al., 1983; Fourie & Morrison, 1979). Radiological findings are normal, and biological markers of inflammation (erythrocyte sedimentation rate and C-reactive protein) are normal or moderately elevated (Fourie & Morrison, 1979; Kennedy et al., 1980). Iridocyclitis and retinitis are the most common ocular manifestations associated with Chikungunya fever; less frequent ocular lesions include episcleritis. All ocular manifestations have a benign course with complete resolution and preservation of vision. Retinitis shows gradual resolution over a period of 6 to 8 weeks (Mahendradas et al., 2008). CHIKV infection seems to elicit long-lasting protective immunity, and experiments performed using animal models have shown a partial cross-protection among CHIKV and other alphaviruses (Edelman et al., 2000; Hearn Jr. & Rainey, 1963).

#### **4.2 Arthralgia**

Erratic, relapsing, and incapacitating arthralgia is the hallmark of Chikungunya, although it rarely affects children. These manifestations are normally migratory and involve small joints of hands, wrists, ankles, and feet with pain on movement. Symptoms generally resolve within 7–10 days, except for joint stiffness and pain: up to 12% of patients still have chronic arthralgia three years after onset of the illness. Arthralgia experienced by CHIKF patients closely resembles the symptoms induced by other viruses like Ross River Virus (RRV) and Barmah Forest virus (BFV) (Jacups et al, 2008; Mahalingam et al., 2002). Such alphavirusinduced arthralgia mirrors rheumatoid arthritis, a condition which is characterised by severe joint pains due to inflammation and tissue destruction caused by inflammatory cytokines such as IL-1b, IL-6 and TNF-a (Barksby et al., 2007). It is thus plausible that CHIKV infection induces similar pro-inflammatory cytokines that cause arthralgia, explaining why joint pains are constant ailments of many patients infected with CHIKV even years after recovery from the initial febrile phase (Lakshmi et al., 2008). More recently, global analyses on the specific involvement of cytokines and chemokines have showed that IL-1b, IL-6, and RANTES were associated with disease severity (Ng et al.,

The Re-Emergence of an Old Disease: Chikungunya Fever 121

Other rare complications described after CHIKV infection are mild hemorrhage,

Diagnosis of infection with CHIKV is based on molecular biology (RT-PCR) and serology methods. The first one is useful during the initial viraemic phase, at the onset of symptoms and normally for the following 5-10 days, when CHIKV RNA reaches very high levels (viral loads of 3.3 x 109 copies/ml) and can be detected (Carletti et al., 2007; Parola et al., 2006). Afterwards, the diagnosis is based on serological methods (ELISA, immunofluorescence,

IgM specific against CHIKV are detectable 2-3 days after the onset of symptoms by ELISA immunofluorescent assay and persists for several weeks, up to 3 months (Litzba et al, 2008; Sam & AbuBakar, 2006); rarely, IgM can be detected for longer periods, up to 1 year. IgG specific against CHIKV appear soon after IgM antibodies (2-3 days) and persists for years. Testing of a couple of sera collected in the acute and the convalescent phases of the disease is mandatory for the identification of recent infection using serology methods that cannot distinguish IgG Ab from IgM Ab (i.e. HI and Nt). It is also very useful to confirm results obtained with other methods, especially taking into account the although rare persistence of IgM antibodies. Viral isolation can be performed from serum of infected patient on insect or mammalian cell lines (i.e. C6/36 or Vero E6) during the early phase of the disease, when the viral load is very high and the immune response is still not detectable; however it is useful only for epidemiology or pathogenesis studies or for thorough molecular characterization (Fusco et al., 2010). The sensitivity and specificity of rapid bedside tests commercially available are poorly established, and the possibility of false-positive reactions resulting from cross-reactivity with dengue or other arboviruses such as o'nyong-nyong virus has to be considered (Blackburn et al., 1995). Serologically, chikungunya virus is most closely related to o'nyong-nyong virus and is a member of the Semliki Forest antigenic complex. Individual serological testing is not particularly useful, except when faced with atypical or severe

hemoagglutination inhibition (HI) and infectivity neutralization (Nt)).

forms, or in travellers returning from an epidemic zone (Pile et al., 1999).

Currently, there are no available specific therapeutics against CHIKV. Treatment is purely symptomatic and can include rest, fluids, and medicines to relieve symptoms of fever and aching, such as ibuprofen, naproxen, acetaminophen, or paracetamol. Non-steroidal antiinflammatory drugs (NSAIDs) are primarily used to treat inflammation but high doses, administrated to control arthralgia, could cause thrombocytopenia, gastrointestinal bleeding, nausea, vomiting and gastritis (Jain et al, 2008; Pialoux et al., 2007). Steroids have been occasionally used but their efficacy was not significant (Taubitz et al., 2007). Some time ago chloroquine, a drug useful for prophylaxis and treatment of malaria, showed promising results for treating chronic Chikungunya arthritis (Brighton, 1984), while a recent trial conducted on French Reunion Island proved that there is currently no justification for the use of chloroquine to treat acute chikungunya diseases (De Lamballerie et al., 2008). However, the usefulness of chloroquine in the treatment of Chikungunya infection deserves further investigation that could take advantage on the availability of a non-human primate animal model (Labadie et al., 2010). Ribavirin (200 mg twice a day for seven days) given to

myocarditis, hepatitis (Lemant et al., 2008) .

**5. Diagnosis** 

**6. Treatment** 

2009). Moreover, since high concentrations of these pro-inflammatory factors were found in the joints of humans afflicted with RRV-induced polyarthritis, they probably have a causative role in chronic joint and muscle pains that plague patients (Lidbury et al., 2008). The finding that aberrant Type I interferon signalling in mice led to severe forms of CHIKF (Couderc et al., 2008) further highlighted the important role cytokines play in the pathology of CHIKV infection.
