A report presented at the Annual Meeting of the American Society of Tropical Medicine and Hygiene (ASTMH) on Thursday found the potential for larger, deadlier outbreaks of monkeypox outbreaks in Central Africa and internationally – as the more dangerous Clade 1 of the virus become more transmissible.

“Many people in the US CDC (Centers for Disease Control), the World Health Organization and in health ministries and research institutions across sub-Saharan Africa were calling attention to the rising danger of monkeypox long before we started seeing infections outside of Africa,” said ASTMH President Daniel Bausch. “This study provides important new insights that point to the urgent need to provide additional resources that can help Africans fight this disease.”

There are two “clades” of monkeypox virus. The one that has long been endemic in the Democratic Republic of Congo (DRC) – known as Clade 1 – has a fatality rate of up to 10%, and can include severe complications such as blindness. Clade 2, the source of recent outbreaks outside of the west and central African countries where it is endemic, is fatal less than 3% of the time.

The concern now, the report relates, is that Clade 1 is evolving to become more transmissible, at an increasing rate.

The deadlier variant has increased its rate of transmission 

Data from DRC´s Tshuapa province collected between 2013 and 2017 showed an increase in the transmissibility of the Clade 1 variant. The effective reproduction rate – a closely watched metric for any infectious disease denoting its capacity to infect new hosts – has increased to 0.81 from a baseline of 0.3 to 0.5 seen throughout the 1980s.  When the reproduction rate reaches or exceeds 1, that is a tipping point where the number of new cases increases even faster than the numbers of people recovered or dying, creating the potential for transmission across much larger areas and populations.

“Our data show that monkeypox transmission in the area we studied was notably higher than previous estimates and was getting close to the point where it can cause large and sustained local outbreaks,” said Dr Kelly Charniga, a Prevention Effectiveness Fellow at the CDC and first author of the study. “This research puts the global health community on alert that there may be the opportunity for larger outbreaks in DRC on the horizon.”

The disease’s increased transmissibility has already led to longer outbreaks, increasing the opportunities for the virus to evolve to be able to sustain more person to person spread. Researchers also found evidence of more regular “spillovers” of monkeypox infections from rodents – thought to be the virus’s natural reservoir – to people.

“With today’s interconnected world, outbreaks don’t necessarily stay at their source,” Charniga said. “The best way to prevent monkeypox from causing more outbreaks in DRC and from becoming a bigger global problem is to devote more attention to areas where it is clearly causing the most suffering today.”

Monkeypox burden increasing in endemic countries

Monkeypox cases in endemic African countries have been increasing for several years. The fall in cross-reactive immunity from the smallpox vaccination campaign ended after victory over the smallpox virus in 1982 led to the cessation of smallpox vaccination, which is protective against monkeypox too. That, in turn, has contributed to a rise in infections of both clades across Africa. The current global outbreak of Clade 2 Monkeypox virus began in the United Kingom in May, after a traveler returning  to the UK from Nigeria was reported to have been infected. Since then, some 78,000 monkeypox cases have been reported globally, with the areas of most intensive transmission in Europe and the Americas.  Although the number of new cases reported has declined sharply in the past two months, transmission remains a concern in many countries. 

Improving surveillance in endemic rural areas is the key to stemming the evolution of the disease, but is easier said than done. The regions of the DRC where the strongest monkeypox reservoirs can be found – like Tshuapa province where the study was carried out – are hard to access, researchers noted.

This complicates the distribution of vaccines and antivirals already in limited supply, and many clinics still rely on paper forms to report suspected cases. Despite these difficulties, Charniga emphasized that pro-active health system measures will lead to the most favorable outcomes for everyone.

“The best way to prevent monkeypox from causing more outbreaks in DRC and from becoming a bigger global problem is to devote more attention to areas where it is clearly causing the most suffering today,” she said.

Image Credits: WikiCommons, United Nations.