Killing rats at the first signs of a human outbreak of bubonic plague will raise he death toll, not lower it.
This is the surprising conclusion of a population dynamics model created by Matt Keeling of Cambridge University and his colleagues. 鈥淭he timing of a cull is very important,鈥� he says.
Animals harbour bubonic plague in every continent except Australia. In humans, the disease has become more common over the past 15 years.
Advertisement
The fleas that transmit the plague bacteria would much rather live on rats than on humans, says Keeling. They only switch to humans when there aren鈥檛 enough rats around 鈥� usually, when most have succumbed to the plague.
So killing more rats will only force more infected fleas to move to humans. Rats should only be culled well after any human outbreak, or at least after measures to kill the fleas, Keeling says.
鈥淭his makes sense,鈥� says David Dennis of the US National Center for Infectious Diseases in Fort Collins, Colorado. 鈥淚n the case of an outbreak, first you want to protect people from the fleas 鈥� by spraying insecticide around homes, for example. Then you want to start reducing the rodent population.鈥�
Cities at risk
Keeling鈥檚 team also found that as few as 3000 rats per square kilometre are, in theory, enough for a plague epidemic among the rats to spread to humans.
鈥淭here are thought to be as many rats as people in even modern developed cities,鈥� says Keeling. This means that if the plague takes hold in rats, there鈥檚 the potential for it to cause a human epidemic, he says.
Most cases of plague occur in East Africa. 鈥淏ut if there is poor sanitation and poor rat control, there is always the potential for an epidemic to spread into previously unaffected countries,鈥� says Dennis.
Epidemics don鈥檛 occur in areas such as the western US mainly because humans and infected animals don鈥檛 come into close contact there, he believes.
Death cycle
Rats nearly always carry low levels of plague, says Keeling. Every seven or eight generations the disease peaks, killing about 97 per cent of the animals exposed to the bacteria.
The few that recover are immune to the disease. Immunity passes to their offspring, but it gradually disappears leaving the population susceptible again.
鈥淭he disease spreads very slowly among largely immune populations,鈥� says Keeling. 鈥淏ut when it hits a lot of susceptible rats all in one place, you can have a very big epidemic.鈥� This is when contact with humans can lead to a human epidemic.
Around 3000 people die every year from bubonic plague, according to World Health Organization figures. But many experts think the true number may be much higher.
More at: Nature (vol 407, p 903)
