ABSTRACT
West Nile virus (WNV) infection is mosquito-borne zoonoses involving birds, horses, humans and other species of animals. This study was designed to determine the seroprevalence and risk factors of WNV infection in horses and to detect for the viral antigen in mosquitoes in Kaduna State, Nigeria. A total of 368 horses and 31 pools of Culex species of mosquitoes were tested for anti WNV IgG antibodies and antigen using Competitive Enzyme-Linked Immonosorbent Assay and Vector test® respectively in four selected Local Government Areas (LGAs) of Kaduna State. A structured interviewer administered questionnaire was used to determine presence of factors in the stables that are associated with WNV infection in horses. Out of the 368 horses tested, 331(89.9%) were seropositive for WNV infection. Based on the LGA, a statistically significant higher seroprevalence (P=0.003) was found in Kaduna North LGA (98.9%) followed by Zaria, Igabi and Sabon Gari with prevalence of 88.7%, 86.0% and 78.3% respectively. There was no significant association of WNV seropositivity with age and sex of horses. Only 1(3.2%) pool from Zaria LGA out of the 31 mosquito pools, tested positive for WNV antigen. There was low level of awareness on WNV infection and its transmission among horse attendants and presence of wild birds, stagnant water, grasses, trees and waste bins around the stables. This study has identified high seroprevalence of WNV in apparently healthy horses in Kaduna State and Culex species of mosquito in stables at the time of study are carrying WNV. Conditions suitable for transmission of WNV infection are abundant in horse stable environments in the state. There is the need for dedicated surveillance for WNV infection in Nigeria. The inclusion of WNV in the differential diagnosis of fevers of unknown origin in medical hospitals, along-side public health education of the population on the disease is necessary.
CHAPTER ONE
INTRODUCTION
1.1 Background of the study
West Nile virus (WNV) is a re-emerging mosquito-transmitted disease causing WNV disease in humans and animals (Cantile et al., 2000; Durand et al., 2002; Murgue et al., 2001a; Autorino et al., 2002; Charrel et al., 2003 Castillo-Olivares and Wood, 2004). WNV was first isolated in 1937 from the blood of a febrile adult human in the West Nile District of Uganda (Smithburn et al., 1940). The virus (WNV) is a member of the Japanese encephalitis virus complex which includes, Japanese encephalitis virus (JEV), Saint Louis encephalitis virus (SLEV), and Murray Valley encephalitis virus within the genus Flavivirus and family Flaviviridae (Heinz et al., 2000). The genetic material of the WNV is a positive-sense, single stranded RNA that is between 11,000 and 12,000 nucleotides long, which encode seven non-structural proteins and three structural proteins. The RNA strand is held within a nucleocapsid formed from 12-kDa protein blocks, the capsid is contained within a host-derived membrane altered by two viral glycoproteins (Galli et al., 2004). The virus is transmitted in natural cycles mainly between mosquitoes and birds, with humans and horses serving as incidental hosts as well as dead-end host (Burke and Monath, 2001). In humans and equines, WNV infection is usually asymptomatic or characterized by a mild febrile illness to encephalitis with fatal outcome (Petersen and Reohrig, 2001; Bunning et al., 2002; Garcia-Bocanegra et al., 2011). There is no specific treatment for WNV disease and clinical management is supportive. Diagnosis is based on serology, viral isolation and characterization (CDC, 2015a). WNV has since been reported in Africa, the Middle East, Asia, Southern Europe, Australia and North America (Campbell et al., 2002; Roehrig et al., 2002; Zeller and Schuffenecker, 2004; MacKenzie and Williams, 2009). The initial outbreak of the virus in North America was recognized in the fall of 1999 in New York City with reported death in humans, horses, and numerous species of birds, since then, there has been an increase in geographic distribution of WNV in North America (CDC, 2002a; Brien et al,2008). WNV disease is considered a re-emerging pathogen affecting both humans and animals (Morens et al., 2004). Multiple factors contributing to the emergence of the virus include human susceptibility to infections, climate and weather, breakdown of public health measures, economic development and land use, human demographics and behaviour, along with international travel and commerce, contribute to the emergence and re-emergence of the disease (Morse, 1995; Felissa, 2006).
WNV is globally distributed and the first case was reported in Western Hemisphere in 1999 in New York City (Nash et al., 2001). The virus has also spread to Europe, beyond the Mediterranean basin, and a new strain of the virus was reported in Italy in 2012 (Barzon et al., 2012). WNV is considered to be an endemic pathogen in Africa, Asia, Australia, Middle East, Europe and United States, of which there was an experience of one of its worst epidemics in the year 2012 resulting in death of 286 people in the United States of which Texas being mostly affected with the disease outbreak (Murray et al., 2013), making the year (2012) the worst record for WNV disease outbreak in the United States (Fox, 2013). WNV can occur in horses as West Nile encephalitis virus, resulting from mild febrile illness to encephalitis, the incubation period for equine West Nile encephalitis following
mosquito transmission is estimated to be 3–15 days (OIE, 2013). A transient viraemia of low virus load anticipate clinical onset of WNV infection (Schmidt and El Mansoury, 1963; Bunning et al., 2002). In humans and equine, most WNV infections are asymptomatic; approximately 20 to 30% of infected individuals develop flu-like clinical manifestation characterized as West Nile fever (Perelygin et al., 2002; Wang et al, 2004a). In a subpopulation of individuals, at least 1 out of 150 develops a neuroinvasive or encephalitis disease caused by WNV. (Petersen et al., 2001; Perelygin et al., 2002; Pestka et al., 2004). The clinical features of severe WNV infection varies and it includes, severe headache, ocular manifestations, muscle weakness, vomiting, anorexia, cognitive impairment, tremors, and a poliomyelitis-like flaccid paralysis (Ceausu et al., 1997; Perelygin et al., 2002; Bakri and Kaiser, 2004; Sejvar, 2006). The mortality rate following neuroinvasive infection is approximately 3-15% (Nathanson and Cole, 1970; Petersen and Marfin, 2002) and long-term neurological sequelae are common (King and Kesson, 1988; Scholle and Mason, 2005). Neuronal damage is most prevalent in the brain stem and anterior-horn, neurons of the spinal cord; although in immunosuppressed individual’s infection can spread throughout the central nervous system (CNS) (Guarner et al., 2004; Klee et al., 2004). The most important risk factor for WNV transmission is through the bite of infected mosquitoes (Hayes et al., 2005a; Winters et al., 2008). Also, inappropriate personal protection measures could establish risk for WNV infection (Mandalakas et al., 2005). The existence of mosquitoes breeding site, such as stagnant water body, old tires and bushy environment favours the spread of the virus (Han et al., 1999). Risk factors that independently potentiate the development of neuroinvasive diseases rather than West Nile fever in California in 2005 include old age, male sex, hypertension and diabetes mellitus (Jean et al., 2007).
Successful isolation of the virus from dead birds has allowed the subsequent identification of WNV as the etiologic agent of both human and animal disease, (CDC, 1999; WHO, 2011). Seropositivity of the virus was reported in patients in Western part of Nigeria, in 1951 and 1955 (Macnamara et al., 1959). Recently, other researchers reported seroprevalence of WNV infection in camels, horses, domestic birds and febrile humans in some parts of Nigeria (Mishelia, 2014). High prevalence of WNV antibodies was reported in some State in South-western part of Nigeria (Olaleye et al., 1990; Sule et al., 2015). Evidence of WNV infection has also been reported in febrile individuals in South-eastern and North-eastern part of Nigeria (Ezeifeka et al., 1986; Baba et al., 2006). Seroprevalence to WNV infection was reported in domestic birds in Kaduna State, Nigeria (Mishelia, 2014).
1.2 Statement of the Research Problem
Most sicknesses in Africa, especially in Nigeria that present with high temperature are reported as fever of unknown origin, if they do not respond to anti-microbial treatment (Olaleye et al., 1990). The knowledge and experience of medical practitioners and other medical specialists in Nigeria regarding the management of Arboviral diseases seem inadequate therefore; ignorance of doctors on Arboviral diseases could lead to misdiagnosis and thereby adopt inappropriate treatment (Baba et al., 2006). This speculation is supported by a report that, due to the non-specific nature of WNV infection it often escapes medical attention (Monath et al., 2001). Since WNV outbreaks in animals precede human WNV cases, the establishment of an active animal health surveillance system to detect new cases in birds and horses is essential in providing early warning for veterinary and human public health authorities (WHO, 2011). The disease outbreak experienced in U.S. some years ago was evidence that the geographical distribution of WNV has expanded and the number of States in the U.S. where infected mosquitoes, birds and animals have been found increased from 4 in 1999 to 26 in 2001 (Senay, 2002). During the recent outbreak of the viral infection, horses were the mammalian species mostly affected in North America with high morbidity and mortality, (Ostlund et al., 2001). Despite the availability of two commercial vaccines to prevent WNV infection and immerse public notice on the devastating impact of WNV infection in horses, it still causes widespread morbidity and mortality in California horses, (Minke et al., 2004). There has been an expanding host range of WNV infection. Infection of the virus has been occasionally reported in reptiles and amphibians including Lake Frog (Jose et al., 2006).
Several species of wild mammals have been reported to be commonly exposed to WNV, occasionally with high seroprevalence rates (Root, 2013). Dead and sick tree squirrels have been reported to be ubiquitous signs of WNV activity in some parts of the United State (Heinz-Thaheny et al., 2004; Kiupel et al., 2003; Padgett et al., 2007).
Culex mosquitoes have been incriminated as the major primary transmission vector; the main mosquito vector of WNV in the western United States that feed on a variety of avian and mammalian species is Culex tarsalis (Kilpatrick et al., 2005; Reisen et al., 2005). Other Culex mosquitoes vectors have shown to be competent for both infection and transmission of WNV includes; C. quinquefasciatus, C. stigmatosoma, C. thriambus, C. pipiens, and C. Nigripalpus (Reisen et al., 2006a; Hamer et al., 2008; Vitek et al., 2008). Vector competency studies have indicated that, Aedes aegypti is capable of transmitting WNV infection (Ecdc, 2014). The virus has also been isolated from this mosquito species in the field (Turell et al., 2005). It has also been reportedly isolated from hard (ixodid) and soft (argasid) tick species in some parts of Europe, Africa, and Asia (Hoogstraal et al., 1976; Platonov, 2001). However the vector (Ticks) has not been adequately investigated (Lawrie et al., 2004). Non-vector transmission has also been reported among humans by blood transfusion, organ transplantation, transplacental transmission, and through breast milk (Pealer et al., 2003).
Due to poor environmental conditions it is common to keep horses in shaded stables and under trees which serve as favourable locations for mosquitoes to breed. Fumigation of these places is usually rarely carried out in Kaduna State. Moreover, most of the environments have poor drainage system which favours mosquitoes breeding site in the State. Not much is known about WNV in Nigeria especially in Kaduna State. Horse owners and horse groomers in polo farms infested with mosquitoes are at risk of WNV infection and there is no outbreak investigations and surveillance of WNV in Kaduna State. Therefore, there is need to investigate the presence of the virus in horses in Nigeria. It is important to carry out study on the occurrence of the disease which will help to support the differential diagnosis of other mosquito transmitted diseases in Nigeria.
1.3 Justification of the Study
Many febrile illnesses in Nigeria are misdiagnosed as malaria and typhoid, because of absence of surveillance for arboviral infections. The study will further highlight the importance of surveillance in order to bring out the extent of the circulation of the virus in Kaduna State. This will help fill the gap in surveillance for arboviruses. Polo and race tournaments are common events in the State, which bring horses and humans together providing conducive environment for possible disease outbreak like, influenza and WNV infections. This study will provide an insight for public health and fill the gaps in biosecurity measures and zoonotic disease spread associated with horses and human congregations, including the need for more intensified environmental mosquito control. WNV provides a good platform for activating and implementing one health approach in disease control where humans and animals environmental health overlap strongly towards providing both animal and humans epidemics control. Detecting the virus in mosquitoes is the major evidence needed to establish possibility of animal and human outbreaks or occurrence of undetected outbreaks. This will help convince medical authorities to include WNV infections as part of routine differential diagnosis of febrile illnesses in Nigeria, which will reduce misdiagnosis of the infection and the risk of further transmission of the disease through blood transfusion, organ transplantation in hospitals. Similarly providing evidence of WNV infection in horses and that of contemporary virus circulation will help justify vaccination of priced horses such as, thorough bred race and polo horses against the disease in Nigeria.
1.4 Aim of the Study
To determine the seroprevalence of West Nile virus infection in horses and to detect the virus antigen in mosquitoes in horse stables in Kaduna State, Nigeria.
1.5 Objectives of the Study
- To determine the seroprevalence of West Nile virus infection in horses in selected LGAs in Kaduna State.
- To detect West Nile virus antigen in mosquitoes from horse stables in selected LGAs in Kaduna State.
- To determine demographic management and environmental factors associated with the occurrence of West Nile virus infection in horses in Kaduna State, such as, presence of mosquitoes, trees, grasses, stagnant water, waste bins wild birds.
1.6 Research Questions
- What proportion of horses has West Nile virus antibodies in selected LGAs in Kaduna State?
- Are the mosquitoes found in the stables in selected LGAs in Kaduna State infected with West Nile virus?
- Are there demographic and environmental factors associated with the occurrence of West Nile Virus infection in horses in Kaduna State?
