Smallpox, once a major scourge on humanity, was eradicated in 1979 following widespread vaccination programs. Vaccination against smallpox stopped then (the vaccine sometimes had severe side effects) and people lost immunity, not only against the smallpox virus, but also against related pox viruses such as monkeypox and cowpox; viruses against whom the smallpox vaccine had also afforded protection. These now pose a new danger to humans and could become a major problem. The full story is recounted by Sonia Shah in Scientific American, March 2013.
A virus is the simplest biological organism, merely consisting of a small number of genes (DNA) sheathed in a protective protein coat. It can only propagate itself by invading the complex cell of a much larger organism (its “host”) and using the specialised machinery of that cell to make copies of itself.
Smallpox is also known as Variola (from the Latin varicus for “spotted”). The disease first emerged in humans about 10,000 years ago. It is caused by a virus that attacks the nose or throat linings and spreads through the body causing a characteristic skin rash dotted with virus-filled blisters. The disease kills one third of those afflicted with it. Smallpox killed about 400,000 people per year in Europe in the 18th century and was responsible for about one third of all cases of blindness. Smallpox killed 300 to 500 million people worldwide throughout the 20th century.
When smallpox reigned as a major disease, the added protection against monkeypox and cowpox afforded by vaccination against smallpox was seen as a small extra bonus – monkeypox and cowpox were seen as relatively minor problem at the time.
Monkeypox is more dangerous to humans than cowpox. It typically lives in African rodents, particularly in squirrels. It is called “smallpox’s little cousin” because the illness it causes is clinically indistinguishable from smallpox. The most virulent monkeypox virus strains kill 10 per cent of those they infect. The incidence of monkeypox cases is sharply on the increase in Africa. A survey of incidence in the Democratic Republic of the Congo showed a 20-fold increase in 2006/2007 compared with 1981-1986.
The increased incidence of monkeypox disease is not surprising since the population is no longer vaccinated against poxvirus. But the data also shows that the monkeypox virus now finds it easier to jump from rodents into humans. The reason for this easier transmission is a matter for speculation. It may be because expanded clearing of land for agriculture and fuel brings humans into much closer contact with infected rodents. Also, Shah reports that because of economic privations resulting from the Congolese civil war, about one third of the rural population eat rodents found dead in the forest.
The fact that monkeypox virus now infects humans much more frequently than heretofore worries virologists. Monkeypox virus can already kill people, but it doesn’t spread from one person to another very efficiently. However, a small change in a current viral trait could transform monkeypox virus into a far more contagious pathogen. Unfortunately, viruses are adept at improving their attack skills against their hosts. They seem to become more virulent by “stealing” genes from their hosts.
The incidence of cowpox is also increasing. In the UK, cowpox poxvirus normally lives harmlessly in rodents such as bank voles and wood mice. The virus infects cats who hunt these rodents, the cats pass the virus on to humans, and vole and wood mice populations are booming because of the milder winters. Rats are picking up the virus and rats can very efficiently transfer disease around the globe.
As the cohort of people globally who have never received a smallpox vaccination grows, the incidence of monkeypox and cowpox will continue to rise. The smallpox virus was beaten because its only natural host is the human and therefore only one chain of transmission had to be broken. A huge vaccination programme against smallpox left nowhere for the virus to hide. On the other hand, monkeypox and cowpox naturally infect terrestrial and arboreal rodents and non-human primates as well as humans. New vaccines are under development to counter the problem should monkeypox and cowpox become a more serious threat.
William Reville is an Emeritus Professor of Biochemistry and Public Awareness of Science Officer at UCC.