When conservation and disease management clash-Bighorns

This is not news in the world of disease management and conservation. Sometimes the best way to control a disease in wildlife is to cull potentially infectious individuals before they come into contact with susceptible individuals in the population (e.g. CWD). However, when we are talking about endangered species, what should we do?

Found this piece reporting clashes between disease managers and conservation groups regarding what to do with endangered Bighorns that have come into contact with domestic sheep in California. People are worried that bighorns might become sick of pneumonia and other diseases. Of course culling an endangered species is a big problem but I don't think its unethical:

"The proposal to kill wild bighorns to `protect' them from domestic sheep is unwise and unethical," said a letter Monday from the Center for Biological Diversity and Friends of the Inyo. "What is necessary is to get domestic sheep away from bighorns now."

Sure, but once they got into contact what else is to do? What is more unethical: to kill a few animals or let a zoonotic event wipe out the entire species? 

On the other hand, the population size of these bighorns is not extremely high:

"In recent years, the population has climbed to an estimated 350 animals, but officials say that has increased the potential for contact with domestic sheep that carry pneumonia and other diseases that can wipe out bighorn herds."

Sure, but for an epidemic to spread population size has to be above a certain threshold. This is basic epidemiology..

N = (b+a+g)/B

where b is the natural death rate of bighorn sheep, a is the number of babies produced per unit time, g is the recovery rate from the disease and B is contact rate between individuals. B might still be very small if a few bighorns contact a lot of sheep, but once these individuals come into contact with other susceptible bighorns the disease can only spread if 

R0 > BN/(b+a+g)

In other words, the small population size of bighorns might buffer them against a possible zoonotic event. With very simple models we can make "educated guesses" as to what action to take. Go get estimates of b,a,g and B and you might have a better answer! Of course we could always vaccinate more sheep (this is how canine distemper is being controlled in East Africa).

Food for thought....

ahumadameister.