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.