This week, Nigeria continues to battle a Lassa fever outbreak that began more than a year ago. Nigerian health officials began reporting cases to the WHO in December 2016, and as of the most recent WHO African Region weekly bulletin (data provided through January 25), they have reported 1,319 total cases (385 confirmed), including 149 deaths (overall case fatality of 11.3%). Current data from the Nigerian CDC shows a total of 615 suspected cases across 17 states from the start of 2018 through February 11. Among these cases are 14 healthcare workers. Laboratory tests have confirmed 193 of these cases in 2018, which account for 43 of the 47 reported deaths so far this year.

Dr. Wondimagegnehu Alemu, WHO Representative to Nigeria, noted that Nigeria is “observing an unusually high number of cases for this time of year.” At this same time last year, there were only 138 suspected cases (37 confirmed) across 10 states and 21 deaths. According to the WHO Emergency Response Framework, the current outbreak has risen to a grade 2 emergency, calling for increased international assistance to prevent further disease spread. The Nigerian government and the WHO coordinated to establish Lassa fever Emergency Operation Centers and enhance surveillance activities early in the outbreak. The WHO also recently donated much-needed personal protective equipment to support local responders, particularly in the hospital setting, as well as laboratory reagents to support timely diagnosis.

This week, Outbreak Observatory takes a look at the increased geo-temporal spread of Lassa fever in Nigeria and identifies several particular areas of concern.

What is Lassa Fever?

Lassa fever is an acute viral hemorrhagic illness that can be transmitted via contact with the urine or feces of infected rodents or the bodily fluids of an infected individual. Due to the long viral incubation period, symptoms may not manifest until 1-to-3 weeks after the initial infection. It is estimated that 80% of cases develop only mild symptoms—such as slight fever, malaise, or headaches—and go undiagnosed. Because many of these individuals do not seek care or otherwise go undiagnosed due to their mild symptoms, outbreak response activities like contact tracing are particularly challenging. The remaining 20% of cases experience severe symptoms, including facial swelling; bleeding from the gums, eyes, or nose; respiratory distress; and neurological issues such as encephalitis. Currently, there is no vaccine to protect against Lassa fever. The WHO and CDC both note that the antiviral drug ribavirin has been shown to be an effective treatment option if administered early in the course of the illness, but is not effective as post-exposure prophylaxis.

Concerning Trends

Geographic Spread

Lassa fever is endemic to Nigeria, but sizable outbreaks are becoming more widespread across the country. From its emergence in 1969 through 2008, Lassa fever cases were reported in only seven of Nigeria’s 36 states. This number increased to 14 states between 2009 and 2015 and rose further to at least 26 in 2016 alone.

The European Civil Protection and Humanitarian Aid Operations (ECHO) recently reported that the increase in cases is overwhelming local healthcare systems in the most affected areas—presumably the southern states of Edo and Ondo, where 75% of the total confirmed cases have been reported. States with little or no prior experience identifying or treating Lassa fever patients may not be properly equipped to handle the influx of patients they are receiving, potentially also contributing to the infection of healthcare workers. Additionally, these same states may not have the necessary surveillance systems in place to facilitate rapid case identification and contact tracing efforts.

The ECHO report also notes delays in transporting clinical specimens to diagnostic laboratories. It is unclear whether this is a result of the patient surge itself or inadequate access to properly equipped laboratories. If laboratories capable of performing the necessary diagnostic tests are located in areas historically affected by Lassa fever, they may not be readily accessible for areas where the disease has recently spread, which could further hinder local outbreak response activities.

Temporal Trends

The seasonality of Nigeria’s rainy and dry seasons provides ideal conditions for sustaining Lassa fever year to year. The moist environment of the rainy season (approximately May-October) provides favorable conditions for the survival of the virus in the environment, outside of vertebrate hosts, promoting infection in rodent vectors. Additionally, while the rodents that carry the virus mate year round, there is an uptick in reproductive rates that coincides with the rainy season—possibly linked to crop maturation and increased food availability—resulting in an increased probability of human-rodent contact as they mature. During the dry season (approximately November-April), the decreased humidity results in high viral aerosol stability, facilitating infection in humans. Additionally, dry season agricultural activities such as clearing and burning land in preparation for planting may place humans in close proximity to rodent populations by driving rodents indoors to seek shelter.

In contrast to the historical incidence trends, data from the Nigerian CDC shows that a substantial number of Lassa fever cases were reported from June through September 2017. While the current surge in cases coincides with the rainy season, Nigeria could continue to see a high burden of Lassa fever throughout the rainy season if the 2017 trend repeats itself. If the 2018 rainy season incidence is comparable to the current surge—like was seen in 2017—and the outbreak enters the 2018-19 dry season already exhibiting high levels of sustained human-to-human transmission, Nigeria could potentially see even higher case counts than this season.

Bottom Line

In light of these trends, it will be important to effectively implement Lassa fever preventive measures, not only for Nigeria, but also its neighboring countries. The WHO recommendations include the promotion of good “community hygiene” to mitigate zoonotic transmission of the virus. Necessary rodent control measures such as proper food storage and garbage disposal can reduce human contact with rodents. In an attempt to further reduce human-rodent contact in homes, one local Nigerian official has asked residents to avoid using fire or chemicals to clear grassy areas, which can increase the number of rodents attempting to enter homes in search of shelter. As mentioned above, burning land in preparation for planting crops is common during the dry season, so this may continue to pose a challenge for health officials.

As mentioned above, 14 healthcare workers have been infected in 2018, highlighting the importance of maintaining high vigilance and infection control standards in healthcare settings. The WHO guidance addresses the risk to healthcare workers and laboratorians and outlines specific measures to ensure their health and safety. The WHO also notes that the disease can be exported to other countries via ill individuals. This concern was highlighted recently when a Lassa fever victim in Guinea traveled to Liberia, where he died, reminiscent of the initial spread of Ebola in West Africa in 2013-14. The abundance of rodent vectors across Africa makes it impossible to completely eliminate the disease, but adoption of these simple measures can ultimately help prevent and control future outbreaks and reduce the overall burden of Lassa fever in areas where it is endemic.


Photo: Stained photomicrograph of liver tissue from a Lassa patient.

Photo courtesy of CDC/Dr. W. Winn

Outbreak Observatory aims to collect information on challenges and solutions associated with outbreak response and share it broadly to allow others to learn from these experiences in order to improve global outbreak response capabilities.