Infection pattern, case fatality rate and spread of Lassa virus in Nigeria

Lassa fever (LF) also called Lassa hemorrhagic fever is a disease caused by infection with a zoonotic virus called Lassa virus (LASV). LASV is a single-stranded RNA virus of the family Arenaviridae [1] belonging to the genera Mammarenavirus [2]. LF was first declared a disease of humans in 1969 within Nigeria [3, 4]. The natural host of LASV is the rodent Mastomys natalensis, a common households’ rat in West Africa. Mastomys erythroleucus (Guinea multimammate mouse) and Hylomyscus pamfi (African wood mouse) are newly reported hosts of the LASV in Nigeria and Guinea Republic [5]. LF is endemic to West Africa especially Nigeria, Liberia and Sierra Leone [1, 6]. Proven cases were also reported in Cote d’Ivoire, Guinea, Central African Republic, Mali, Senegal and Congo [7]. LF is associated with significant morbidity and mortality. The annual incidence of LF in this region is estimated as 100,000 to 300,000 cases with about 5000 deaths and 58 million people at risk [8]. Twenty percent of infected individuals require hospitalization while 80% are asymptomatic infections [9]. The case fatality rate of hospitalized cases ranges from 15 to 20% in Africa [6].

Transmission of the virus to humans occur through direct contact with rat’s excretions such as urine and feces, eating of food and inhalation of contaminated dust containing body secretions of infected rats, as well as eating the rat [6, 7, 10, 11]. Person to person transmission occurs by direct contact with blood or bodily fluids of infected individuals [8, 12]. It can also be transmitted through contact with urine and semen of infected individuals, thereby posing risk for sexual transmission. A recent study reported presence of viral nuclei acid in semen up to 103 days after onset [13]. Infected individuals becomes contagious at the onset of symptom and increases with disease severity [14‐16]. Hospitalized patients with LF may pose a significant risk to healthcare workers (HCWs) and to other patients due to its contagious nature [7, 8, 12]. The virus has an incubation period of usually 7–10 days, with a reported range of 3–21 days [6, 12, 14, 16].

Symptoms include fever and malaise, pharyngitis, gastrointestinal complaints, and cough. In later stages bleeding, facial oedema, convulsions, pericardial effusions and coma are commonly observed [7, 12, 17]. Diagnosis is by blood samples which are examined using LASV specific real time reverse-transcriptase polymerase chain reaction (RT-PCR) [18]. The major control strategy is the control of the rodents around dwellings, avoiding of rats consumption and contact [6]. Currently, there are no vaccines against LASV. Although, off-label treatment consider the use of ribavirin [9], an expensive treatment that is effective when administered for the first six days after the onset of symptoms [19].

In Nigeria, the Federal Ministry of Health (FMoH) through the Nigeria Centre for Disease Control (NCDC) established a number of LF case management centres, often called treatment centres to operate in association with specialist teaching hospitals in endemic states. Despite the dangerous nature of this disease, there is scanty information on the seasonal pattern of infections and distribution of LF in Nigeria. This study was carried out to describe the pattern of infection, distribution, spread and case fatality rate of LF in Nigeria over the last four years. The results of this study will provide information for relevant authorities in the design of appropriate strategies and intervention in the control of this virus.

This study have shown that there is progressive increase in LASV infection in Nigeria, since the outbreak was first reported. The infection exhibited seasonal variability, with the dry season having a significantly higher infection rate than the wet season. The peak infection rate was recorded in February. Similar observation has been reported at Irrua, Edo State [18]. Zhao et al. [24] in their study observed that the major LF epidemic in Nigeria usually occur between November and May during the dry season. A study in Guinea [25] reported that high indoor populations of M. natalensis during the dry season than the wet season might have contributed to the higher outbreaks of LASV during the dry season. M. natalensis plays a significant role in the rodent to human transmission of the LASV, its high abundance in human dwelling has a direct impact on prevalence. Also, the high reproductive ability of M. natalensis is another contributing factor in the spread of the LASV as the rodents can recover its populations within a few months [26]. Human activities such as bush burning which is usually carried out during the dry seasons in the Forest and Guinea Savannah regions of Nigeria where rodents are hunted for meats is another factor favouring the higher prevalence of LASV during the dry season. This activity destroys the rodents’ habitats and thereby encouraging their movement from bushes to human dwellings in search of shelter and food [27‐29].

Recently, the LASV has been reported in other rodent hosts other than M. natalensis. These new LASV reservoirs are the Mastomys erythroleucus (Guinea multimammate mouse) found in Nigeria and Guinea, and Hylomyscus pamfi (African wood mouse) in Nigeria [5]. The current presence of LASV in these hosts provides high chances of transmission and resurgence of the virus from time to time. These hosts are found within the same locality with M. natalensis, thereby aiding the horizontal transmission of LASV i.e. animal to animal transmission [30]. Some studies reported increase in the territorial habitats of M. erythroleucus which is normally a savanna mouse species found in the northern and central part of Nigeria to new localities in southern Nigeria especially in degraded forests [31, 32]. This might explain the current expansion in the geographical spread of LASV across the southern states of the country. In 2017, Olayemi et al. [33] reported the endemicity of LASV in 13 states. In this study, the LASV has reported in 32 states and the FCT as at September, 2020. An evidence that the virus geographical coverage is on the increase. Olayemi et al. [33] further reported that the hosts of LASV i.e. M. natalensis, M. erythroleucus and H. pamfi were distributed across both endemic and non-endemic zones of LF. The presence of these rodents in those communities pose potential risk of spread of LASV.

This study observed that three states in Nigeria; Edo, Ondo and Ebonyi are epicentres for LASV infections. This observation corresponds with earlier reports of NCDC [34] and Usuwa et al. [35]. Series of factors are favouring the increase and spread of LASV infections across Nigeria; the exponential growth of human population, household size, bush burning and urbanisation are some predisposing factors [29]. In Nigeria, the practice of drying agricultural products under the sun especially along roadsides encourages food contamination with urine and faeces of these rodents and hence aiding transmission of LASV [35, 36]. These factors increase human vector contacts thereby putting man at high risk of contracting zoonotic pathogens [37].

This study revealed yearly increase in the number of infected individuals with LASV. The highest monthly infection from LASV was reported in February for the four years period. Edo and Ondo states still remain the epicenter for the virus, accounting for over 60% of annual cases. The LASV is currently endemic in 32 states and FCT with and annual CFR of 18.5%. Therefore, there is urgent need to declare an emergency against this virus by adopting integrated approach in its control which should involve proper surveillance, health education, proper hosts’ identification, vector control, proper diagnosis, morbidity and adequate training of frontline health professionals.