Although moose populations are expected to fluctuate from year to year, scientists have been concerned about an overall downward trend in North American moose populations in the last few decades. In Wyoming, the estimated statewide moose population was 12,000–13,000 individuals in 1990, but only 3,500–4,000 individuals by the late 2010s. The Jackson-area moose population in northwest Wyoming is experiencing a particularly steep decline, from an estimated population of 3,500 in 1992 to just 400–450 in 2019.
But what is causing these declines? Is a single disease responsible, or are several factors contributing? Do these causes vary by region? The answers to these questions are vital to conservation efforts. Based on past studies of various North American moose populations, there seems to be no single answer, and additional studies are needed to determine what factors are most important among different subpopulations. Possible causes include habitat loss, shifts in local ecology, parasitism, malnutrition, and disturbance by human activity, including roadway collisions.
The Wyoming Game & Fish Department currently is conducting a field study to investigate causes of mortality in the declining Jackson-area moose population. As a veterinary pathologist at the Wyoming State Veterinary Laboratory, I specialize in the diagnosis of animal diseases, and I am collaborating with Wyoming Game & Fish on this project.
Field studies, which investigate conditions as they exist in the natural world, rather than using laboratory-based experiments, are a vital component of wildlife research. Surveys of causes of death (mortality) among wildlife populations are an important step in solving the mystery of population declines. The results of these studies help scientists identify diseases and other factors affecting the health of individual animals in a population. Reviewing this data can help us to identify the most significant factors contributing to a population’s decline, thus providing a basis for informed conservation management decisions.
How do we determine causes of mortality?
To determine why an individual animal has died requires a thorough investigation. First, a field scientist (usually a biologist or veterinarian) observes the situation in which the deceased animal was found. Relevant observations include evidence of visible hazards, such as nearby roads or toxic plants, to which the animal could potentially have been exposed, as well as eyewitness accounts from people who may have seen the animal’s behavior prior to or at the time of death.
Next, we conduct an autopsy, which starts by determining the age and sex of the moose and looking for external abnormalities such as skin parasites, injuries or emaciation. Then we perform a thorough dissection to examine the internal organs for evidence of diseases that may not be visible externally. We take samples of all major internal organs, as well as feces, serum, gastrointestinal contents and parasites. The background information we gather and any abnormalities we discover during the autopsy are documented and relayed, along with the tissue samples, to the WSVL. My role is to examine the collected moose tissue samples under the microscope—because some diseases can only be detected microscopically—and decide if any other tests are needed. At the conclusion of all testing, I integrate all the findings for each moose to identify the factors that likely contributed to its death, as well as the presence of any concurrent but non-fatal diseases.
An unexpected finding
Between March 2020 and December 2021, we investigated the deaths of 33 moose and discovered that a variety of factors contributed to their deaths. Causes of death included infectious diseases, such as bacterial pneumonia; trauma, such as from vehicular collisions; exposure to environmental toxicants; and, among some calves, severe tick parasitism.
In many of the adult moose, we also found microscopic evidence of kidney disease. No moose died primarily of kidney disease, but this is still an important finding because kidney disease can cause weight loss and a decline in overall health. The kidney disease among these moose varied from mild to severe, in some cases to the extent that the body’s ability to excrete cellular waste products was compromised. Kidney disease therefore appears to be affecting the health of some of these moose, and in this way may potentially be contributing to population decline.
Causes of kidney disease
What is causing kidney disease in these moose? To begin answering this question, we need a basic understanding of kidney anatomy.
Each kidney is made up of millions of microscopic structures called nephrons, which are responsible for filtering cellular waste products out of the bloodstream into the urine. One component of the nephron is the glomerulus, which is a spherical structure made up of a meshwork of small blood capillaries surrounded by cells forming a membrane, a selective barrier that allows waste products and fluid, but not larger blood components such as proteins and red blood cells, to be released from the bloodstream into the urine. When the glomerulus is damaged, this selective barrier becomes leaky, and excess proteins and fluids are lost with the urine.
A common cause of selective damage to the glomerulus is the deposition of antibody complexes in the glomerular membrane. Antibodies normally are formed by white blood cells in response to infectious diseases, helping to bind and inactivate pathogens, and are present in circulation. Most short-lived antibody responses do not cause glomerular damage, but antibodies produced over an extended period can deposit in the glomerular membrane, leading to glomerular membrane thickening and dysfunction. Chronic infections can result in glomerular damage.
Since these moose have selective glomerular damage, might a chronic infection be to blame? We hypothesize this to be the case. Although some of the moose with kidney disease died of infectious disease, others did not. Might there, then, be a chronic infection present in many of these moose that is not causing death, but is leading to damage of the glomerular membranes through persistent antibody deposition?
One possible culprit is the parasitic arterial worm, Elaeophora schneideri. The adult stage of this small worm resides in the carotid arteries of the moose’s neck, in many cases causing little or no apparent disease. However, its persistent presence in the moose’s body may result in continuous low-level antibody production, potentially leading to glomerular damage. We have found E. schneideri worms in a majority of the adult moose in this study, and we currently are working on testing methods to improve our ability to detect the parasite. Once these testing methods are optimized, we will begin looking for correlation between the kidney disease and this parasite’s presence.
An ongoing study
With only 33 moose, it is difficult to draw any definitive conclusions about mortality trends in the Jackson moose population, but multiple factors do appear to be involved. Kidney disease, though not a primary cause of death, may compromise the health of individual moose, and we are continuing to investigate its implications for Wyoming’s moose population. As this study continues over the next few years and more deaths are investigated, we expect trends to become apparent that will help us identify the most important factors contributing to this specific population’s decline.
By Jacqueline P. Kurz, Clinical Assistant Professor, Department of Veterinary Science, Wyoming State Vet Lab. To contact: (307) 766-9953, jlarose@uwyo.edu.
Reprinted from the 2022 issue of Reflections, the College of Agriculture, Life Sciences and Natural Resources annual research magazine.
