The multimammate rat Mastomys natalensis is a common feature of savannas and forests in many portions of Africa. These pesky rats often infiltrate people’s homes and make themselves comfortable indoors, feasting on any available food stores. While doing so, they leave behind urine and fecal matter. This can be the start of a local Lassa fever epidemic.
Lassa virus is a single-stranded RNA virus that is member of the Arenaviridae family, similar to the Ebola and Marburg viruses. The virus is vectored by the multimammate rats of Africa. Lassa virus, named after the town in Nigeria where the first case arose, is endemic in Sierra Leone, Liberia, Guinea, and Nigeria. However, cases can also be picked up by travelers and brought back to their home countries. So far this year, Lassa has been reported in Nigeria (273 cases, 149 deaths), Liberia (38 cases, 15 deaths), Germany (2 cases, 1 death), Sweden (1 case), Togo (2 cases, 1 death), and Benin (71 cases, 23 deaths). Lassa is frequently transmitted from the original infected person to healthcare workers, as the disease is not easy to diagnose and is easily spread through contact with infected blood, tissue, or secretions.
The symptoms of Lassa fever are non-specific and almost non-existent in many cases. 80% of those infected will have mild symptoms of fever, general malaise, and/or headache. In 20% of cases, however, much more severe symptoms can occur. Hemorrhaging, respiratory distress, swelling, and vomiting are associated with severe disease. Additionally, Lassa fever can often lead to various degrees of deafness, which can be permanent; as many as 25% of people who survive the disease will suffer from some form of deafness, even if they only present with mild symptoms.
Treatments for Lassa include antiviral drugs, such as Ribavirin, which show the highest efficacy when given early. However, Lassa symptoms do not usually manifest until 1-3 weeks after exposure to the virus, and diagnosis requires the use of an enzyme-linked immunosorbent serological assay (ELISA), which is not cheap and often not available in the clinics. The small Seattle biotech company Kineta recently won a $7.2 million award to develop a novel antiviral specifically for treating Lassa fever. This could help overcome the logistic challenges of treatment. In the current outbreak in Nigeria, for example, health officials have said that logistics support and delayed case reporting by the states is severely dampening their ability to combat the threat.
The typical Lassa virus transmission season is beginning to wind down this year, and WHO believes that the number of cases is on the decline and that the epidemic will end soon. Others, however, are concerned that the WHO and local governments have not taken the outbreak seriously enough. The outbreak was not officially announced until January of 2016, while cases had begun to occur last August. The public in Nigeria has also raised questions as to whether or not the government has been down-playing the significance of the outbreak. This year’s outbreak has been far more deadly and widespread than others in the past. The mortality rate has approached 50% in Nigeria this year, a massive increase from the more typical 1%. Additionally, Lassa has spread to more states in Nigeria than have ever seen the disease before.
Officials have cited increased awareness of disease as a major reason for the uptick in mortality and spread. In the wake of the Ebola outbreak, more cases of fever and hemorrhage have been reported to the health system, allowing for increased diagnosis of Lassa. But beyond the public health aspects at play, some researchers fear the virus itself may be undergoing changes that are allowing the increase in spread and making it more deadly than before. Only time will tell whether it is just increased vigilance or viral mutations that are the driving forces here. For now, all we know for sure is that sales of rat poison are on the rise as the countries continue to fight and manage this most recent epidemic.