Are Domestic Sheep responsible for the Bighorn Sheep Die-off in the Mojave?

Bighorn sheep in the Mojave Desert of California and Nevada have been dying of pneumonia for some months now but, as with many of these outbreaks, the original source of infection hasn’t been conclusively identified. Some reports noted that an angora goat had been shot by a hunter and that it may have been the source of infection, but recently, reports have emerged that sheep being trucked through the Mojave preserve may be the source of the infection. According to an article in the Riverside Press-Enterprise, the carcasses of four domestic sheep were found along with pellets near Halloran Summit on Interstate 15.  Suspected sheep pellets were also found to the south at Foshay Pass and samples were sent to a lab to determine whether they are from domestic sheep.

Sheep are commonly transported from other areas to the Imperial Valley. In an interview with the Riverside Press-Enterprise, Linda Slater, spokeswoman for the Mojave National Preserve, said “the scientists are speculating that perhaps a truckload had some sick sheep on it. Maybe (the driver) had to unload some sheep to get some dead animals off the truck.”

It is clear that contact between domestic sheep and bighorn sheep results in transmission of deadly pneumonia causing agents that are carried by domestic sheep.  In recent years there have been numerous die-off events and, after they have been closely investigated, domestic sheep or goats have been found in close proximity and are presumed to be the source of the infection.  The species must be kept separate in order to avoid such catastrophic die-off events.

Until recently, bighorn sheep populations in the Mojave Desert had been stable due to habitat connectivity and a lack of disease infested domestic sheep.  Even in areas where this is the case, it appears that bighorn sheep are still at risk from irresponsible behavior by livestock interests.



, ,



  1. Michael Lafferty Avatar

    Tossing dead domestic sheep from trucks along this ‘migratory path’ is very clearly a gross violation of the concept of biosecurity. That they are moving at all brings up questions regarding the contamination of wild and domestic herds of animals along the entire route, and at each endpoint.

    It is largely the choice to ignore biosecurity standards in the United Kingdom which has led to the continued transmission of bTB [bovine tuberculosis] in cattle, primarily dairy cattle in the southwestern region of Britain.

    The fatally flawed pilot badger culls underway now in Somerset and Gloustershire are a feeble and desperate attempt by farmers and DEFRA [the Department for Environment, Food and Rural Affairs] to reduce the transmission of bTB. These ‘experiments’ are being conducted against a background of substantial public outcry, and without scientific merit.

    Science suggests that transmission of bTB amongst cattle is primarily a biosecurity issue resulting from the transportation of cattle from a farm where the infection is present, to others where it is not. There is little evidence of substantial spread from badgers to cattle, though even that is an issue controllable through the application of biosecurity measures.

    The movement of domestic herds of any kind, including the sheep involved in this incident, suggest that effective biosecurity measures are not in place and are not adequately enforced to the degree that they do exist, in the United States.

    As is the case regarding the conflicts between apex predators and domestic cattle and sheep, these issues are a reminder of the unintended consequences of the introduction and transportation of non-native species into a wild landscape.

    1. Ralph Maughan Avatar
      Ralph Maughan

      Michael Lafferty,

      Thanks for generalizing the issue by giving this international perspective.

  2. mikepost Avatar

    The big horn sheep was common in California prior to the introduction of domestic sheep to the range (before cattle) in the 1800’s. While market hunting played a roll, in many areas no big horn survived the 19th century.

  3. Ralph Maughan Avatar
    Ralph Maughan

    I am more and more convinced that the demise of bighorn was not market hunting at all. They are too small for meat and hard to get at.

    It was then, as now, domestic sheep.

  4. snaildarter Avatar

    A lot of people are switching from cow dairy to sheep and goat dairy for ecological and health reasons. I suspect sheep and goats are a growth industry again. Not good for BigHorn sheep.

  5. Scott Raine Avatar
    Scott Raine

    That farce of a political slogan couched in language of an article is a truly sick and twisted take on the agenda of eliminating grazing. Are you serious? No, of course you are not. No sane person could ever believe for a second that a wild sheep… could ever get sick from an alleged incident of maybe a dead sheep being illegally dumped. If that dumping really wasn’t a figment of wild imagination then call the cops and charge whomever it was with illegal dumping of animals. Seriously, so you claim some truck driver decided to break the law and just dump a carcass or six and then by some stroke of happenstance a rare Bighorn just happened to wander up and lick or otherwise come into direct contact with that carcass, thereby contracting pneumonia, and thereafter infecting his / her herd. This is really really stretching an already far fetched theory on Bighorn / domestic sheep disease transmission. Did you fail to mention that when tested most wild sheep, even those very very far from domestics have the same bacterial pneumonia (Pasteurella and Mycoplasma) in them but nobody really knows what sets them off multiplying into pneumonia? Yep, failed to mention that and a few hundred other facts that tell us that the conclusions of this ‘article’ are a really bogus political ploy to once again place the blame on the livestock industry. Sure, blame domestics, even when they aren’t present. Great science guys.

    1. Ralph Maughan Avatar
      Ralph Maughan

      Scott Raine,

      In your emotion it seems you didn’t read the post very well. Read this again, “According to an article in the Riverside Press-Enterprise, the carcasses of four domestic sheep were found along with pellets near Halloran Summit on Interstate 15. Suspected sheep pellets were also found to the south at Foshay Pass and samples were sent to a lab to determine whether they are from domestic sheep.” [emphasis mine]

      Sheep pellets mean the sheep were alive. As far transmission of pneumonia from domestic sheep to bighorn, it is now a well established scientific fact, and the two don’t even have to touch each other. Do you know anything about the location of bighorn in the area and where the carcasses were found?

      1. Ralph Maughan Avatar
        Ralph Maughan

        I see the comment got both Ken and I roused for the morning since they were posted at the same time. 😉

    2. Ken Cole Avatar

      I’m not couching anything, I think we should eliminate grazing on public lands. Yes, I’m serious. I think that someone trucking sheep through the Mojave Nation Preserve took sheep off of a truck and bighorn sheep came into contact with them. Bighorn aren’t that rare in that area and they are attracted to domestic sheep. It wouldn’t be the first time that someone from the livestock industry did something illegal but, in this instance, there are probably no laws in place to prosecute someone.

      I did refer back to a previous post and we have written extensively about this very subject of disease transmission from domestic to bighorn sheep. It is a very serious issue that has killed off thousands of bighorn sheep in recent years.

      Yes, many wild sheep have the pathogens you mention but it is likely that the deadliest strains have disappeared from the herd and that the adults have developed enough resistance over time to the specific pathogens they carry to survive. That being said, in many herds that do have these less deadly strains, the lamb recruitment is very low for a decade or more. When another transmission event occurs due to contact with domestic sheep, even if the bighorn already carry the less virulent pathogens, there is a big die-off event that follows.

      You forgot to mention that many of the bighorn sheep that test positive for those pathogens wouldn’t have them had they or their ancestors not contracted them from your disease infested sheep.

      We do know that domestic sheep transmit deadly pathogens to bighorn sheep. There is a ton of science that confirms this and none that refutes it.


Ken Cole is a 5th generation Idahoan, an avid fly fisherman, wildlife enthusiast, and photographer. He is the interim Idaho Director for Western Watersheds Project.

We do not accept unsolicited “guest” authors or advertising.

Subscribe to get new posts right in your Inbox

Ken Cole