Disease Testing Project
Greg Anderson, North Lander Wildlife Biologist, Wyoming Game & Fish Department
Bighorn Sheep Trapping: Whiskey Mt.: Jan. 12-13, 2012
In total, we captured 47 sheep. The sample included 40 adult sheep and 7 juveniles.
Our main purpose in collecting samples was to estimate the prevalence of various bacteria species in this sheep population. Following is a list of bacteria species of interest to us and a bit of background on each. All of these organisms have been implicated in bighorn sheep die-offs and are currently considered to be potential causes of pneumonia in bighorn sheep.
Bibersteinia trehalosi: Although this bacterium has been blamed for some past bighorn sheep die-offs, more recent research indicates it may not be as detrimental to sheep populations as once thought. It is fairly easy to culture in the lab and may have been identified in past die-offs simply because it was easy to identify and outcompeted other bacteria on culture plates.
Pasturella multocida: Again, this bacterium has been implicated in past bighorn sheep die-offs. Researchers now believe we need to distinguish between two varieties of this bacterium. One type of P. multocida produces a substance known as leucotoxin. This substance essentially poisons white blood cells that are attacking foreign bodies (i.e. bacteria). The white blood cells then disintegrate and release the poison which in turn destroys neighboring cells in an animal's lungs. This chain reaction eventually destroys enough cells and results in pneumonia. Non-leucotoxic varieties of this bacterium are thought to be relatively benign.
Mannheimia haemolytica: More research is beginning to point toward this bacterium as a prime culprit in many pneumonia outbreaks. Like P. multocida, there are leucotoxic and non-leucotoxic varieties of this organism. The leucotoxic variety of this bacterium is believed to be the dangerous one for bighorn sheep. This bacterium is notoriously hard to culture; being easily overgrown by other bacteria on culture plates. Some researchers speculate the leucotoxic variety of M. haemolytica has been responsible for most catastrophic, all-age sheep die-offs throughout the west, but was not implicated because of the difficulties associated with culturing and identifying the species.
Mycoplasma ovipneumoniae: We believe this bacterium may be tied to pneumonia issues a bit differently than the three bacteria mentioned above. Investigators have consistently tried to link the three bacteria above to all-age die-offs in sheep populations. These are catastrophic events where a large percentage of a population may die in a very short period. This is what occurred in the Whiskey Mountain sheep herd twenty years ago. Since that time pneumonia has primarily manifested itself in lambs with adult sheep being largely unaffected in this population. Annually we observe a reasonable number of lambs migrating onto winter range in the fall. Through fall and early winter lambs can be seen coughing violently and lamb numbers drop steadily. Our lab personnel have consistently isolated M. ovipneumoniae from dead lambs. In addition, examination of lungs from these lambs has consistently revealed damage typical of pneumonia caused by Mycoplasma bacteria. Thus, we suspect this bacterium may be at least partly responsible for perennial pneumonia impacting primarily juveniles in the Whiskey Mountain population.
Note: I use the words "may" and "speculate" a lot in the summaries above. This emphasizes the point we are still not sure if we have one deadly bacterial species we need to look for or if the pneumonia problem is a result of some combination of the bacteria listed above. The pneumonia issue in bighorn sheep is complex and we still do not completely understand how all of these pathogens interact.
Sheep Testing Positive:
- B. trehalosi = 46 animals
- P. multocida = 2 animals
- M. haemolytica = 1 animal
- M. ovipneumonia = 14 animals
- M. glucosida = 1 animal
The list above summarizes the bacteria we found in our analysis and how many sheep were carrying a particular species of bacteria. The most noticeable thing on the chart is that 46 of the 47 sampled sheep were carrying B. trehalosi. Obviously this bacterium is extremely common in the population. As I mentioned above, the impacts from this bacteria are not entirely clear. Researchers now more commonly believe this bacterium may not have many health implications for bighorn sheep. Our results tend to support that conclusion. If B. trehalosi were somehow associated with all-age sheep die-offs, we would expect far more significant problems in this sheep population than we currently have, given the prevalence of the bacteria.
One adult ewe and one adult ram (two sheep) tested positive for P. multocida. As mentioned above, we think this bacterium may be problematic, but likely only if it is the leucotoxic positive type. Both the P. multocida samples from our sheep were leucotoxic negative.
Barely noticeable on the chart is one sheep carrying M. haemolytica. Although this finding looks relatively insignificant on the chart, it may be one of the more critical things we found. This is the bacterium several researchers are keying in on as the main culprit in all-age sheep die-offs. In particular, the leucotoxic variety of this bacterium may have devastating consequences in a sheep population if animals are subjected to nutritional or environmental stress. The M. haemolytica sample we collected was leucotoxic positive.
Of the 47 sheep we sampled, 14 were positive for Mycoplasma ovipneumonia. This constitutes 30% of the sheep. Again, we are not sure what the implications are, but we suspect this bacteria may be associated with our perennial lamb pneumonia problems.
Finally, we found one adult ewe carrying Mannheimia glucosida. I did not list this bacteria species in my summary above because it is not typically mentioned in conjunction with bighorn sheep pneumonia. We have never identified this bacterium in any Wyoming bighorn sheep populations and to my knowledge it has never been implicated in die-offs in other states. Its presence does raise a few concerns. Notice it is a species of Mannheimia and thus closely related to Mannheimia haemolytica which we suspect may be particularly deadly. Also, the M. glucosida bacterium we found was leucotoxic positive. Again, we suspect any leucotoxic bacteria may be problematic.
In addition to the bacteria above, several viruses have been linked to bighorn sheep pneumonia. Our results showed no significant exposure to any viruses associated with pneumonia in sheep.