INTRODUCTION
West Nile virus (WNV) is a mosquito-borne RNA virus belonging to the genus Flavivirus in Flaviviridae family1. Though, WNV infection is generally asymptom- atic in human(or causes a flu-like illness) life-threatening neurological complications (e.g. acute flaccid paralysis and meningoencephalitis) have been reported2. WNV is transmitted in nature through a cycle involving birds and ornithophilic mosquitoes3-4. It has been widely reported throughout the world3, 5, including North African region and the Mediterranean basin6-8. Recently, the virus has been intense in North Africa with a large increase in the number of cases in Tunisia, and with the first reported case in Algeria since the 1990s9. However, the distribution of WNV is poorly documented for Libya9-10 and seropreva- lence studies are limited. Serological evidence of human exposure to WNV has been reported only in one screen- ing study meant to define viral and bacterial etiologies causing acute febrile illness in Libya. The study included 985 individuals from all 32 provinces of Libya, selected
randomly from a cohort of >65,000 specimen, wherein the percentage of WNV was 13.1%.
Many of the cases presented with fever in Libya are documented as fever (pyrexia) of unknown origin (PUO), especially if they failed to respond to antimicrobial drugs.
Several arboviruses, such as WNV and yellow fever virus, are frequently considered in the aetiology of acute febrile illness in some African countries11.
Libya is one of the North African countries, extend- ing to the southern coast of the Mediterranean Sea. Its climate is influenced by both, the Sahara Desert in the south and the Mediterranean Sea in the north. It has hot subtropical semi-arid climate with long, hot and dry sum- mer and relatively wet and mild winter and scanty rain- fall12. Libya contains many wet lands along the coast, as well as a number of oases in the desert areas, which are considered as important habitats to numerous species of residential, breeding as well as migratory birds that pass- through Libya (called occasional or accidental visitors).
Some species of birds including migratory ones especial- ly passerines13 may act as potential reservoirs of West Nile
Prevalence of IgG antibodies for the West Nile virus in human population in Tripoli, Libya
Taher Shaibi
1-2, Walid K. Saadawi
2, H. Aghila
2& Badereddin B. Annajar
2-31Zoology Department, Faculty of Science, University of Tripoli; 2National Centre for Disease Control, Ministry of Health; 3Public Health Department, Faculty of Medical Technology, University of Tripoli, Tripoli, Libya
ABSTRACT
Background & objectives: West Nile fever (WNF) is a mosquito-borne viral infection, circulated in natural cycles between birds and mosquitoes, particularly Culex species. It is transmitted to humans through mosquito bites, and causes a variety of clinical outcomes, ranging from asymptomatic or mild febrile illness to severe men in go encepha- litis with some fatalities observed in older or immunocompromised patients. West Nile virus (WNV) transmission is considerably influenced by environmental conditions; and abundance of avifauna and mosquitoes.There are very few reports on WNV exposure in individuals from Tripoli City in Libya. The main objective was to provide basic epidemiological information about the WNV seroprevalence in the human population of Tripoli.
Methods: A total of 400 serum samples were collected from persons (123 females, 277 males; age range: 15–78 yr) approaching the Tripoli Reference Laboratory for the purpose of obtaining health certificate; during the period from August to October 2013. The presence of WNV IgG antibodies was evaluated by a commercial kit based on WNV immunoglobulin G (IgG) enzyme-linked immunosorbent assay (ELISA).
Results: It was observed that 2.75% (11/400) samples were found reactive in the WNV ELISA assay. This result suggests that WNV has a low prevalence in the study area.
Interpretation & conclusion: Seropositivity rates of WNV in Tripoli region of Libya were low. However, continu- ous monitoring of population is important to keep track of the disease prevalence, risk factors, reservoir hosts and vectors for better understaning of the disease epidemiology and designing appropriate control strategies.
Key words IgG antibodies; Libya; prevalence; Tripoli; West Nile virus J Vector Borne Dis 54, June 2017, pp. 183–186
J Vector Borne Dis 54, June 2017 184
virus in the country. The objective of this study was to con- firm the prevalence of West Nile virus IgG antibodies in the population of the capital city, Tripoli in Libya, and its relationship with several demographic variables. The study was primarily intended to provide direct estimate of the general seroprevalence of WNV in the human population.
MATERIAL & METHODS Specimen collection
Blood specimens were collected randomly at Tripoli Reference Laboratory, Tripoli from 400 Libyan peoples, attending the laboratory, for the purpose of obtaining health certificate; during the period from August to Octo- ber 2013, regardless of their health situation and whether they had WNF symptoms or not. Samples included 277 males and 123 females. Health certificates are usually compulsory for those applying to join army and police services and also for those working in the field of food industry, hospitals, restaurants, etc. Therefore, most of health certificate applicants were young and middle aged (working age). Blood samples were taken by venipunc- ture; sera were separated by centrifugation and stored at –20°C until tested.
Each patient gave his consent for the study and filled- out a questionnaire in which the following variables were registered—age, gender, occupation, area of residence, blood transfusion, the history of contact with domestic animals, and traveling history. No data regarding race were obtained. Samples were analyzed serologically in the Par- asitology and Vector Borne Disease Research Laboratory at the National Centre for Disease Control, Tripoli, Libya.
Ethical approval: The study was approved by the Libyan National Committee of Biosecurity and Bioeth- ics (Reference 52-13) and signed informed consents were obtained from all the participants.
Serological technique
Serum specimens were tested by immuno-enzymatic test (ELISA) to detect the presence of IgG antibodies against WNV using a commercial enzyme immunoas- say kit, DRG® Flavivirus (West Nile) IgG ELISA (EIA- 4400), DRG International, Inc., USA, according to the manufacturer’s instructions14.
Statistical analysis
Prevalence of antibodies to WNV was calculated as the ratio between confirmed positive sera by ELISA and all tested sera. Chi square test was used to evaluate
if the prevalence by demographic data was statistically significant (p ≤ 0.05). All data were analyzed statistically with SPSS version 20.0 software15.
RESULTS
The study population included 400 samples;
277(69.3%) males and 123(30.7%) females. Demograph- ic characteristics of the blood samples collected is shown in Table 1. Out of the 400 serum samples, 11 (2.75%) were positive for WNV-IgG antibody; the positivity rate was higher among males (3.6%) than females (0.8%), the dif- ferences were not statistically significant (χ2 = 2.492, df = 1, p = 0.115).
The mean age of participants was 31.41±10 (range, 15–67 yr). All age groups showed positive test for WNV-IgG, except the participants older than 59 yr (Table 1).The differences between age groups were not significant (p>0.36). Participants outside Tripoli showed more positivity (10%) than the participants from Tripoli (2.16%), (χ2 = 6.366, df = 1, p = 0.012).
Positivity varied across the settlements, i.e. 7.9% of participants living in farms were positive for WNV-spe- cific antibody, which decreased to 2.33% for the cases living in houses and 1.64% for those living in apartments;
however, the differences were not significant (p >0.06).
Almost half of the participants, who were positive for WNV-specific antibody, had contact with animals. It is worth mentioning that none of the positive tested partici- pants had blood transfusion. Most of participants who were positive for WNV-IgG antibody (72.7 %) had trav- eled out of Libya.
Table 1. Demographic characteristics of participants Characteristics No. investigated No. positive
Total participants 400 11 (2.75)
Gender
Male 277 10 (3.61)
Female 123 1 (0.81)
Age (yr)
>20 49 1 (2.04)
20–29 154 6 (3.90)
30–39 115 2 (1.74)
40–49 53 1 (1.89)
50–59 20 1 (5)
>59 9 0
Blood transfusion
Yes 14 0
No 386 11 (2.85)
Travelling history
Yes 278 8 (2.88)
No 122 3 (2.46)
Figures in parentheses indicate percentages.
185 Shaibi et al: Prevalence of West Nile virus in Tripoli
DISCUSSION
West Nile virus is a re-emerging Flavivirus in coun- tries like North Africa, Middle East and the Mediterra- nean Region. These countries have been dealing with recurrence of seasonal outbreaks of WNV in the last few years, especially countries neighbouring to Libya such as Tunisia and Algeria where number of cases have been re- ported during the year 2012–13. An increase of up to 86 human cases and one death was reported from Tunisia, while Algeria reported one case and Morocco reported 11 confirmed cases16. A considerable number of cases have been reported from the Middle East region. A study on seroprevalence of WNV from Jordan found that 8% of the study population have exposure to WNV infection17. An- other study in Iran showed 3% seroprevalence for WNV among school children18. In Sudan, the prevalence of IgG antibodies against WNV among humans was 54.9%19. In the present study, only 2.8% of the samples were positive for IgG antibody.
The presence of IgG antibodies may infer a previous exposure to WNV or perhaps another Flavivirus, as the ELISA techniques lack specificity due to cross reaction with antibodies directed to other Flaviviruses, especially those of the Japanese encephalitis serogroup. The IgG positive results must be reconfirmed with other tests16, 20.
WNV could be transmitted by blood transfusions through receipt of blood products and transplantation21-27. The results obtained in this study suggest that WNV is circulating in the population of Tripoli. Since, none of the positive cases experienced/underwent blood transfusion,and only 2.88% of the cases having travel- ling history were positive for WNV, it can be said that transmission was less likely due to traveling or blood transfusion. Therefore, it can be assumed that the virus was transmitted by mosquito vectors. Several species of Culex and Aedes have been recorded in Libya; however, there are no data/studies reporting them as potential vec- tors for WNV transmission from Libya23-24. Hence, further entomological studies need to be conducted to answer this critical question.
It is widely acknowledged that climate has a signifi- cant impact on the distribution of infectious diseases25. Tripoli is characterized by semi-arid climate with hot and dry summers and relatively wet and mild winters,and a Mediterranean rainfall pattern,which differ from year to year; with some years receiving higher rainfall, causing increase in mosquitoes’ abundance, in contrary to drought years. The transmission and outbreaks of WNV depends on the abundance of mosquito species,feeding patterns of infected mosquitoes, factors influencing human expo-
sure to mosquitoes, and the appropriate climatic condi- tions26-29. Tripoli as a city, is characterized by urban cycle of WNV (domestic birds, and mosquitoes feeding on both birds and humans); and the poor infrastructure offers suit- able grounds for mosquito breeding and resting. On other hand, the cycle in rural areas depends on wetlands and irrigation systems.
To predict future outbreaks of infection due to emerg- ing zoonotic pathogens, it would be useful to have a better understanding of avian migration patterns as Libya is situ- ated on bird migration routes between Africa and Europe and can contribute to introduction of infectious diseases to the country.
Based on the results of the study, high seroprevalence rate among male participants could be attributed to their social economic activities (working outside and leisure) that increase the risk of mosquito bites. Moreover, men clothing style covers lesser body parts and exposes more skin, (which is less protective against mosquito bites) than traditional clothing worn by women in Libya. How- ever, both male and female could be at risk if they visit places where there is high population/density of birds and mosquitoes.
Participants with WNV seropositivity had no knowl- edge of WNV disease, as WNF is generally asymptom- atic which is due to different strains of WNV that vary in their behavioural and biological attributes, that may affect the degree of virulence7. Therefore, future research needs to focus on the identification of the WNV strain(s) circulating in Libya.
CONCLUSION
The study result suggests that WNV is circulating in the population of Tripoli, although its prevalence is low. However, continuous monitoring of popula- tion is important to keep track of the WNV prevalence, risk factors, reservoir hosts and vectors at different provinces, especially in the west of Libya to understand the epidemiology and for designing appropriate control strategies.
Conflict of interest
The authors declare that they have no conflict of interest.
ACKNOWLEDGEMENTS
The authors thank the team of Tripoli Reference Laboratory, Tripoli for their help during specimen’s collection.
J Vector Borne Dis 54, June 2017 186
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Correspondence to: Dr Shaibi Taher, Assistant Professor, Zoology Department, Faculty of Science, University of Tripoli, Tripoli, P.O. Box–13793, Libya.
E-mail: [email protected]
Received: 15 September 2016 Accepted in revised form: 10 February 2017
