Chikungunya fever, Hong Kong.

To the Editor: Chikungunya virus disease, caused by a mosquitoborne alphavirus, is endemic to Africa and Southeast Asia. It typically causes an acute febrile illness, with joint pain and a skin rash. Chronic arthropathy may develop (,). No treatment or vaccine is available, and relatively little research has been conducted into its pathogenesis, compared with that of other arboviruses, such as dengue. Recent reports have described a massive outbreak of chikungunya disease occurring on islands in the Indian Ocean, off the east coast of Africa (). Reemergence of chikungunya has also been reported from Indonesia ().

During March 2006, a 66-year-old Chinese man from Hong Kong visited Mauritius where he was bitten by mosquitoes 2 days before returning to Hong Kong. On the return trip, he experienced fever (39°C), severe finger joint and muscle pains, mild headache, and a skin rash, and he sought treatment at the Prince of Wales Hospital (PWH) Infectious Diseases Clinic on the second day of his illness. Physical examination showed a generalized erythematous rash over the trunk and limbs and petechiae over the lower limbs. Mild finger joint stiffness was observed, but no joint swelling. No lymphadenopathy or eschar was detected. Level of C-reactive protein was elevated at 10.4 mg/L. Results of screens for malaria and dengue were negative. Results of other routine assessments were unremarkable. His symptoms subsided gradually within a week.

Serum specimens taken on days 2 and 6 were positive for chikungunya virus RNA by in-house reverse transcription (RT)-PCR at the Public Health Laboratory Service (PHLS) (targeting the nonstructural protein-1 [NSP-1] gene) and PWH laboratory (targeting both NSP-1 and the envelope glycoprotein [E1] gene). An additional serum sample taken on day 8 of illness, received by PHLS only, was also positive for chikungunya RNA. Both laboratories confirmed RT-PCR results by sequencing. At PWH, phylogenetic analysis was performed to determine the likely origin of the virus. In-house immunofluorescent slide serologic assays developed at PHLS found chikungunya immunoglobulin G (IgG) titers of <10, 160, and 320 in the serum samples taken on days 2, 6, and 8 of illness, respectively, and detected chikungunya IgM in the day 8 serum. The acute cytokine immunologic response to this virus was also tested (Appendix).

Sequencing and phylogenetic analysis was consistent with an imported infection, almost certainly originating from the current chikungunya outbreaks in the Indian Ocean. Phylogenetic analyses of the NSP-1 and E1 regions, indicated that this virus is most closely related to previous African rather than Southeast Asian chikungunya viruses (Figure A1, Figure A2). The persistence of viremia up to at least day 8 of illness was unusual. Standard texts state that viremia may be present during the first 2–4 days of illness, with neutralizing antibodies appearing by days 5–7 ().

Figure A1

The maximum-likelihood phylogenetic tree for chikungunya envelope glyprotein 1 (E1) sequences (199 bp) constructed by using PAUP* under a Tamura-Nei 1993 model with invariable sites (i.e., TrN+I), as selected by MODELTEST (version 3.7) under the Akaike information criteria. The bracket shows that the PWH patient E1 sequence (DQ489787) clusters most closely with the outbreak La Reunion sequence (LR2006_OPY1_Reunion_2006_E1, DQ443544). Only bootstrap values >70 are shown and considered significant. The tree is rooted against the earliest sequence (AF490259/Ross_Tanzania_1953).

Figure A2

The maximum likelihood phylogenetic tree for chikungunya nonstructural protein 1 (NSP1) sequences (314 bp) constructed using PAUP* under a transitional model with equal base frequencies with a γ-distributed rate of substitution (i.e., TIM+G), as selected by MODELTEST (version 3.7) under the Akaike information criteria. The bracket shows that the PWH patient NSP1 sequence (DQ489788) clusters most closely with the outbreak La Reunion sequence (LR2006_OPY1_Reunion_2006_NSP1, DQ443544). Only bootstrap values >70 are shown and considered significant. The tree is rooted against the earliest sequence (AF490259/Ross_Tanzania_1953).

The most striking finding from the cytokine analysis (Table) is the high level of interferon-γ (IFN-γ)–inducible protein-10 (IP-10/CXCL10), up to 26 and 16 times the upper limit of the normal range at days 2 and 6 after disease onset, respectively. Serum concentrations of interleukin-8 (IL-8), monocyte chemoattractant protein (MCP) 1 (MCP-1) and monokine induced by IFN-γ (MIG/CXCL9) are also elevated in both samples. Notably, serum IFN-γ, tumor necrosis factor-α (TNF-α), and IL-1β, 6, 10, and 12 concentrations remain within normal limits in both samples, although the concentrations at local inflammatory sites (e.g., joints) are unknown. CXCL10 and MCP-1/CCL2 concentrations decreased during clinical recovery. Thus, the cytokine profile demonstrates that the levels of Th1 chemokine CXCL10 was highly elevated and that the levels of chemokines IL-8/CXCL8, CCL2, and CXCL9 were moderately elevated. In contrast, IFN-γ and other inflammatory/Th2 cytokines were not elevated during the illness.

Table

Serum cytokine profiles of patient with chikungunya infection, days 2 and 6 of illness*

Serum cytokine profile (ng/L)	Time after onset of illness
	2 d	6 d
IFN-γ, N<15.6	UD	UD
IL-1β, N<7.2	2.4	1.5
IL-6, N<3.1	UD	1.3
IL-10, N<7.8	1.7	1.7
IL-12 p70, N<7.8	3.6	1.4
TNF-α, N<10.0	UD	UD
CXCL8, N<10.0	45.3	54.3
CXCL10†, N = 232–1,019	26,319	16,156
CCL2†, N = 18–152	445	257
CXCL9†, N = 37–463	1,138	1,605
CCL5, N = 10,349–46,704	24,745	60,671

*N, normal range; IFN, interferon; IL, interleukin; UD, undetected; TNF, tumor necrosis factor.
†Both 2 d and 6 d samples above normal range.

Interpretation of the significance of these cytokine results is necessarily speculative. Some comparison can be made with other viral infections. In severe acute respiratory syndrome–associated coronavirus (SARS-CoV) (,) and H5N1 influenza () infections, very high blood levels of CXCL10 and moderately high CCL2, CXCL9, and CXCL8 concentrations, or their enhanced expressions in vitro, have been reported. In dengue fever, which has similar clinical manifestations as chikungunya fever, only elevated CXCL8, IL-6, IL-10, and TNF-α concentrations have been shown consistently (,), although CXCL10 expression has not been studied.

The function of CXCL10 is to act as a chemoattractant for Th1 cells in the activation of cell-mediated immune response. Its expression can be up-regulated by the Th1 cytokine IFN-γ during acute inflammation. CXCL10 has been implicated in the pathogenesis of SARS-CoV and H5N1 influenza infections, in which persistently high CXCL10 concentrations seem to correlate with disease severity and progression (–). CCL2, CXCL9, and CXCL8, have also been found to have a pathogenic role in H5N1 influenza, SARS-CoV, and dengue infections. Notably, the level of antiviral cytokine IFN-γ was not elevated in our chikungunya case, though admittedly, this is only 1 case. This finding may represent a way that the chikungunya virus evades host defenses and may provide a rationale for the use of IFN as a therapeutic option (). Such IFN therapy has been suggested and tried, experimentally, for SARS-CoV (5) and dengue infections ().