FLAVIVIRUSES AT THE TERRITORY OF SERBIA – PRESENT SITUATION AND CHALLENGES

The aim of this study is to summarize the data on the occurrence, presence and prevalence of some zoonotic flaviviruses, which have been actively circulating in the territory of the Republic of Serbia during past decade. The genus Flavivirus, family Flaviviridae,encompasses vector-transmitted viruses with zoonotic potential. The genus is comprised of more than 70 viruses transmitted to humans by mosquitoes and ticks. Some of those are associated only with human diseases (such as yellow fever and dengue fever), whereas others can cause diseases in both humans and animals. Animals are major reservoirs and primary hosts for the latter group. The virus transmission to other animal species or humans occurs via diverse vectors – mostly mosquitoes and ticks. A range of these virus species is widely distributed worldwide, especially in tropical and sub-tropical climatic zones. Some representatives of these species have only recently been identified at the territory of Republic of Serbia. This paper gives an evidence on the presence and 1* Corresponding author tomy@niv.ns.ac.rs Arhiv veterinarske medicine, Vol. 11, No. 2, 53 70, 2018 Petrović T. ... et al.: Flaviviruses at the territory of Serbia...


PRESENCE AND PREVALENCE OF WEST NILE VIRUS (WNV)
West Nile virus (WNV) is a neurovirulent mosquito-borne flavivirus with zoonotic potential. The virus is maintained in nature in an enzootic transmission cycle between avian hosts and mosquito vectors, mainly from the genus Culex but also some other mosquito species. The virus occasionally infects other vertebrates, including humans and horses, in which it may cause sporadic disease outbreaks that could result in fatal outcomes (Martin-Acebes et al., 2012). WNV was first isolated in 1937 (Smithburn et al., 1940) Petrović et al., 2013).
In the aspect of human and animal health, wild birds are particularly important due to migration across national and intercontinental borders they contribute to a long-range virus spread (Linke et al., 2007). Many bird species, though not all that are susceptible to infection, produce levels of viremia that are sufficient for transmitting the virus to mosquitoes (Komar et al., 2003). Humans and mammals, especially horses, are incidental "dead-end" hosts and play limited roles in the natural cycle because viremia after infection is generally too low to allow further virus transmission to the mosquitoes at the moment of bite (Hrnjaković- Cvjetković et al., 2009;Valiakos et al., 2010). However, in both humans and horses, severe neuroinvasive disease with even fatal outcomes can occur. Infections in humans predominantly take a subclinical course (80-90% of cases). Some infected individuals (up to 20%) can manifest mild clinical symptoms resembling influenza associated with sudden fever, headache, sour throat and pain in the back, muscles and joints, fatigue, mild transient rash and lymphadenopathy. However, in only a small subset (one per 150 cases) of mostly elderly patients and patients with comorbidities the infection can progress to severe neuroinvasive disease with encephalitis or meningoencephalitis, occasionally with fatal outcomes (Blitvich, 2008;Hrnjaković-Cvjetković et al., 2009). Similarly to humans, WNV infection in horses is usually not accompanied with apparent clinical symptoms. Nevertheless, neurological symptoms can occur in some 10% of cases with a mortality rate that can reach even 50% (Blitvich, 2008;Calistri et al., 2010). The virus has been present in Europe for many years, but dramatic increase of the incidence and severity of epidemic outbreaks with neurological complications in birds, humans and horses has been recorded in central and southern Europe in recent years, which is considered an emerging veterinary and public health problem (Martin-Acebes and Saiz, 2012; Petrović et al., 2013).Apart from some historical data, first reports on the presence of West Nile virus (WNV) in Serbia became available upon testing of horse blood sera for the presence of WNV-specific antibodies in the region of Vojvodina Province. The first study was performed using blood sera collected during 2009 and 2010. The presence of WNV-specific antibodies was detected applying immunoenzyme as-say (ELISA) and subsequent plaque reduction neutralization test (PRNT) in 12% (46/349) of examined samples (Lupulović et al., 2011). Surprisingly, high percentage of seropositive horses initiated further research, thus, serological testing has continued during following three years. WNV-antibody testing applying ELISA conducted on horse blood sera in 2011 revealed 28

PRESENCE AND DISTRIBUTION OF USUTU VIRUS (USUV)
Usutu virus (USUV) was first isolated in 1959 from Culex neaevii mosquitoes in the region along the river Usutu (after which the virus is named) in Swaziland (since recently, Eswatini -South Africa region) (Williams et al., 1964). First human isolates were detected in 1981 and 2004 in ill persons from the regions of Central African Republic and Burkina Faso, respectively (Nikolay et al., 2011). The presence of USUV in Europe was first established when the first recognized outbreak of USUV occurred among blackbirds Turdus merula in Austria in 2001 (Weissenböck et al., 2002), and later also in other European countries. The recently published research confirmed the presence of the virus in archived tissue samples from Italian wild birds found dead in 1996 strongly suggesting that USUV was circulating in the region of

INSTEAD OF A CONCLUSION
The objective of this article was to summarize the research in the field of epidemiology and diagnostics of flavivirus infections conducted in the region of Serbia during past decade and to offer an overview of current epidemiological and epizootic situation as well as of measures of the surveillance of WNV currently in force in Republic of Serbia. Research results obtained through annual surveillance programmes have already offered answers to some questions on the presence and distribution of some flavivirus infections such as WNV. However, they launched other emerging issues related to the incidence and prevalence of TBEV and USUV. Also, there are issues of interaction between these three viral infections in terms of the problem of differential diagnosis due to similar clinical manifestations, as well as the hazards to human and animal health. Understanding of public health importance of the presence and prevalence of different flaviviruses requires further comprehensive seroepidemiological, clinical and virological research. Apparent and pronounced climatic changes entail intensive spread of specific vectors towards North; thus, occurrence of certain flavivirus infections so far unknown in the region of Serbia should be expected. In that respect, further research should involve permanent monitoring of vector distribution and their infection status in view of the presence of pathogenic agents of human and animal diseases, as well as the presence of pathogens among the population of natural hosts and reservoirs. Upon that point, establishing of a comprehensive surveillance program is compulsory to obtain all necessary information and to get the answers related to risk analysis and adequate preventive and control measures.

AKNOWLEDGMENTS
This work was produced within the framework of Project TR31084 funded by the Ministry of Education, Science and Technological Development of Republic of Serbia.