INTRODUCTION

Muskoxen (Ovibos moschatus) were rare and restricted to certain parts of Alaska, US before they disappeared in the late 19th or early 20th century. Suggested explanations for the disappearance include adverse climatic conditions, fluctuations in food availability, competition for resources, and hunting (Lent 1999). To restore muskoxen to Alaska, 31 muskoxen were brought from Greenland to the University of Alaska, Fairbanks, and subsequently released on Nunivak Island off the western coast of Alaska in 1935 and 1936 (Fig. 1). Descendants of these animals were translocated to different parts of Alaska, including 64 muskoxen released into the eastern North Slope (ENS) in 1969–70 and 36 muskoxen released into Seward Peninsula in 1970, followed by another 35 animals in 1977. Similarly, 36 muskoxen were released into Cape Thompson in 1970, followed by 34 muskoxen in 1977 (Gunn et al. 2013). Annual surveys in ENS indicated that the muskox population increased between 1970 and 1995, stabilized at approximately 650–800 animals during 1995–98, then declined rapidly through 2006 (Reynolds 1998a; Lenart 2011). Calf recruitment evidently declined and predation losses increased during the 1990s (Reynolds 1998a; Reynolds et al. 2002). However, to our knowledge, no observed climatic, anthropogenic, or other environmental influences provided a plausible explanation for these changes or the observed population decline. Conversely, the muskox meta-population on Seward Peninsula in western Alaska (WA) increased rapidly to 2,700 animals by 2007 (Gunn et al. 2013); this population continued to increase through at least 2009 (Gorn 2011). During this period, muskoxen on Nunivak Island were hunted to maintain a stable population, the population on Nelson Island was increasing, and the population on Cape Thompson also grew, although more slowly than other restored populations in Alaska (Gunn et al. 2013).

Figure1. 

Alaska Game Management Units with muskoxen (Ovibos moschatus) populations (number and subunit letter designation) sampled. The eastern North Slope population range is cross-hatched, and the western Alaska, USA population range is shaded in dark gray.

Previous studies have shown that muskox population dynamics are influenced by nutritional conditions, weather, predation, and dispersal (Reynolds 1998b; Reynolds et al. 2002). Nutritional compromise of adult female muskoxen associated with low food abundance and deep snow and evidenced by low serum copper was thought to cause poor calf production and limited herd numbers (Barboza and Reynolds 2004). Disease also influences muskox populations. For example, Yersinia pseudotuberculosis serotype 1B accounted for at least 69% of deaths among 67 carcasses and 53 skeletal remains recovered from Banks Island, Canada, in 1986 (Blakley et al. 2000). Also, between 2004 and 2012, major disease outbreaks occurred in the muskox population on Dovre Mountain plateau, Norway. In 2004, this population experienced a severe contagious ecthyma (CE) outbreak (Vikøren et al. 2008), and in 2006, a multifactorial outbreak of pneumonia occurred during extreme weather events and caused mortality of up to 26% (Ytrehus et al. 2008, 2015). In 2012, Mycoplasma ovipneumoniae was the primary cause of a severe pneumonia outbreak (Handeland et al. 2014). Recently, Erysipelothrix rhusiopathiae was identified as a significant cause of mortality (Kutz et al. 2015) in Canadian muskoxen.

Other conditions of concern that may affect muskox populations include rangiferine brucellosis because it occurs in sympatric arctic caribou (Rangifer tarandus granti) herds and causes reproductive failure and joint lesions (Neiland et al. 1968), although it is rarely reported in muskoxen (Gates et al. 1984; Forbes 1991; Tomaselli et al. 2016). Additionally, Chlamydophila spp. (CLA) causes multiple clinical manifestations in domestic ruminants, such as pneumonia, polyarthritis, abortion, and infertility, that could affect population dynamics (Kaltenboeck et al. 2005). Furthermore, parasitic infestations or comorbidities could exacerbate existent health conditions, resulting in poor disease outcomes. For instance, gastrointestinal parasite infestations occur in arctic ungulates (Kutz et al. 2001, 2004). Notably, Umingmakstrongylus pallikuukensis, a lung nematode, infects muskoxen and may have contributed to a 50% decline in the population in Canada (Kutz et al. 2004), although other organisms are under consideration as causal and contributory. Similarly, Teladorsagia boreoarcticus, a nematode that causes severe abomasal disease, has been observed in muskoxen in Canada (Kutz et al. 2004).

In response to concerns over the ENS muskox population decline, in 2005, the Alaska Department of Fish and Game (ADFG) initiated a preliminary assessment of the occurrence and prevalence of a variety of diseases and parasites, as well as the nutritional status of muskoxen with regard to important trace minerals. This study included pathologic investigations of muskox carcasses recovered during routine aerial population surveillance. Pathologic examinations were continued until 2008 to establish primary and contributing causes of death. Archived and contemporary sera were tested for selected pathogens, and serologic prevalences were compared during and before the period of decline in both the decreasing ENS muskox population and the increasing WA population. Additionally, hematology, fecal parasites, and trace mineral status were examined to understand factors that may affect the health and population status of Alaskan muskoxen.

MATERIALS AND METHODS

Carcass collection and pathology

Between 2005 and 2008, 13 intact carcasses, and 23 partial remains of muskoxen were recovered by the field staff (helicopter pilot, wildlife biologists, and veterinarian) of ADFG while conducting aerial surveillance of muskoxen. The history, carcass condition, and postmortem examinations of the remains are shown in Table 1. Carcasses were recovered a few to several days after death, except for a lethargic and limping calf that was euthanatized. When carcasses could not be transported intact to the laboratory, gross postmortem examinations were performed in the field, and partial remains such as available limbs and organs (thoracic pluck, liver, kidneys, etc.) were collected for further examination and sampling. Predation, scavenging, autolysis, and carcasses frozen in place limited the ability to perform complete postmortem examinations for definitive diagnoses. Formalin-fixed tissues were processed routinely for histopathologic evaluation by a veterinary pathologist. Samples to be examined by immunohistochemistry (IHC) for CLA and Mycoplasma bovis antigens were submitted to Iowa State University, Veterinary Diagnostic Laboratory (Ames, Iowa, USA). Swabs, tissues, fluids, and aspirates obtained from carcasses for bacterial culture and virology were sent to Washington Animal Disease Diagnostic Laboratory (WADDL; Pullman, Washington, USA) and Wyoming State Veterinary Laboratory (WSVL; Laramie, Wyoming, USA), respectively.

Table 1. 

Factors causing or contributing to morbidity and mortality in 36 muskoxen (Ovibos moschatus) from the eastern North Slope (ENS) and western Alaska (WA), USA, September 2005–May 2008. — = no data; Cu = copper; ppm = parts per million.

Serology and microbiology

Archived sera collected in 1984–92 and 2000 and contemporary sera from 2006 to 2008 were tested for antibodies to bovine viral diarrhea virus; Brucella spp.; CLA; Coxiella burnetii; epizootic hemorrhagic disease virus; infectious bovine rhinotracheitis virus; Leptospira interrogans serovars Canicola, Grippotyphosa, Hardjo, Icterohaemorrhagiae, and Pomona; parainfluenza virus type 3; and respiratory syncytial virus. Archived sera had been evaluated for antibodies to bluetongue virus, CE virus, and ovine-associated malignant catarrhal fever virus. Only contemporary sera were available for screening for antibodies to Neospora caninum and Toxoplasma gondii. Sera were sent to various laboratories and processed using assays presented in Table 2. The number of serum samples evaluated for each pathogen varied depending on availability. Serum samples were grouped into four categories according to location and period of collection: ENS before the population decline (1984–92), ENS during the decline period (2000 and 2006–08), and nonaffected areas (WA) before and during the decline period. Vaginal and conjunctival swabs from adult females were tested by PCR and enzyme-linked immunosorbent assay (ELISA) for CLA at Nebraska Veterinary Diagnostic Laboratory (Lincoln, Nebraska, USA) and at WADDL.

Table 2. 

Serologic and microbiologic assays used to test Alaskan muskox (Ovibos moschatus) samples and laboratories where samples were processed.^a — = not applicable.

RESULTS

Postmortem findings

Major pathologic findings and suspected or confirmed causes of death are presented in Table 1. Mortality was classified as probable predation when a brown bear (Ursus arctos) was present at the scene or there was evidence of massive antemortem hemorrhage and bite wounds consistent with bear predation. Predation contributed to 42% of deaths in the carcasses examined; however, some cases classified as predation may have been judged to be so as the result of scavenging, and not all observed predation deaths were sampled for this analysis. Preexisting conditions were identified in five carcasses classified as predation events as evidenced by hoof lesions (Fig. 2a, b), polyarthritis, bronchopneumonia, hepatocellular vacuolar degeneration, and Sarcocystis spp. in cardiac or skeletal muscles.

Figure2. 

Dorsal (a) and plantar (b) views of postmortem hoof lesions in a muskox (Ovibos moschatus) with deficient liver copper stores. Arrows indicate severe hoof wall separation from the laminae. Ruler in centimeters.

Gross and histopathologic lesions of polyarthritis without evidence of predation was the primary lesion in a female calf found abandoned in ENS and euthanatized in 2005. Carpal and tarsal joints were externally grossly distended with a thick fibrinohemorrhagic exudate. Articular cartilage erosions and thickening of the joint capsules with red, edematous, synovial membranes were evident (Fig. 3). Chlamydophila spp. antigen was detected by IHC of synovial cells of one of the six affected joints. This pathogen was also suspected to have caused necrotizing suppurative arthritis of the elbow joint in an adult female, but IHC of the joint cartilage and capsule for CLA was inconclusive, possibly because of a lack of suitable synovium samples. Other cases of polyarthritis were documented (Table 1), but culture of joint fluid and IHC of the articular surfaces, capsules, and synovia of affected joints were negative for CLA and M. bovis.

Figure3. 

Synovitis positive for Chlamydophila sp. on immunohistochemistry staining, in a swollen hock joint of a copper-deficient muskox (Ovibos moschatus) calf that displayed lameness before euthanasia. Ruler in centimeters.

Bronchopneumonia was an important finding, was often found with other conditions, and likely contributed significantly to mortality in only one case (2007-081, Table 1) where aspiration of plant material into the lungs could have predisposed to infection with Trueperella pyogenes and Pasteurella trehalosi, causing acute severe fibrinosuppurative bronchopneumonia and consolidation of the cranioventral right lung. In addition to causing pneumonia, T. pyogenes caused septicemia with peritonitis, omphalitis, hepatitis, and enteritis in a calf. Notable pathologic findings in calves included maternal abandonment resulting in dehydration and starvation, premature birth, and jejunal intussusception. Noteworthy findings in pregnant females were hepatocellular vacuolar degeneration and dystocia.

Serology

We detected evidence of exposure to all pathogens of interest except for bluetongue virus, infectious bovine rhinotracheitis virus, and T. gondii (Table 3). Prevalence of antibodies to CLA was relatively high in WA during both periods; in contrast, prevalence was low in ENS and was only detected during the decline period. Antibodies against Brucella suis serovar 4 and bovine viral diarrhea virus were detected in ENS only during the decline period, but not from WA during either period. Coxiella burnetii was detected at low prevalence from both areas during the decline period. Because of specimen availability and funding sources, only archived sera were tested for CE, and antibodies were detected in ENS but not from WA.

Table 3. 

Serologic data for selected pathogens in muskoxen (Ovibos moschatus) from the eastern North Slope (ENS) and western Alaska (WA), USA, during the ENS predecline period (1984–92) and ENS decline period (2000, 2006–08). — = no data; CI = confidence interval.

Microbiology

Both populations had evidence of CLA infection by PCR—however, at different times of the year. In ENS, CLA was detected in 7 of 11 vaginal swabs collected in March and July 2007 (during the calving season) and none of 10 swabs collected in October 2007 (noncalving season). In contrast, in WA, CLA was detected in vaginal swabs from female muskoxen captured during October 2008 (Table 4). Vaginal and conjunctival swabs were also analyzed by ELISA; however, all results were negative, even in situations for which PCR results were positive.

Table 4. 

Prevalence of Chlamydophila spp. in conjunctival and vaginal swabs of adult female muskoxen (Ovibos moschatus) captured from the eastern North Slope and western Alaska, USA, 2006–08. — = no data.

Trace minerals

Trace mineral concentrations in serum and blood from live-captured adult females and in livers of dead muskoxen are given in Tables 5 and 6, respectively. Muskox liver range (Puls 1994) and published muskox values (Salisbury et al. 1992; Puls 1994; Blakley et al. 2000; Barboza et al. 2003) are included for comparison. Generally, copper, iron, molybdenum, and zinc levels in serum of live adult female muskoxen in both areas were similar. Blood selenium levels in ENS were low compared with WA, and manganese levels in animals from both areas were below adequate when compared with sheep (Ovis aries; Puls 1994).

Table 5. 

Trace mineral levels (parts per million wet weight) in serum and blood (selenium) of adult female muskoxen (Ovibos moschatus) captured from the eastern North Slope and western Alaska, USA, in 2006–08. — = no data.

Table 6. 

Trace mineral levels in liver (parts per million [ppm] wet weight) of muskoxen (Ovibos moschatus) found dead in the eastern North Slope and western Alaska, USA, 2005–08. All values have been adjusted to ppm for comparison. — = no data.

Table 6. 

Extended.

Although mean liver copper levels were marginal to adequate in both areas compared with Canadian muskoxen (Salisbury et al. 1992), the median liver copper was low from the ENS, and some animals were low enough to be deemed deficient. Liver molybdenum was low when compared with muskoxen from Victoria Island, Canada (Blakley et al. 2000). However, liver zinc was high compared with muskoxen from Victoria and Banks Islands (Salisbury et al. 1992; Barboza et al. 2003) and the range provided by Puls (1994). The mean and median liver selenium values were higher in WA than ENS samples, but both were higher than Canadian muskoxen.

Hemogram

Adult female muskoxen from ENS had higher total leukocyte counts (P=0.013, t=2.69, df=24), monocyte counts (P=0.06 t=1.95, df=24), and total eosinophil (P<0.0001, Mann-Whitney) and lower hematocrit (P=0.013, t=2.68, df=24) compared with those in WA (Table 7).

Table 7. 

Hemogram for adult female muskoxen (Ovibos moschatus) from the eastern North Slope and western Alaska, USA, 2007–08.

Fecal parasites

The prevalence of parasite stages and their likely identities based on Kutz et al. (2012) are detailed in Table 8. Infestation with more than one parasite was more common in ENS than WA. Parasites that occurred at high prevalence in ENS include Protostrongylus spp., small trichostrongylids, and Nematodirinae, whereas dorsal-spined larvae were detected in all feces from muskoxen in WA.

Table 8. 

Prevalence (%) of parasite stages in feces from adult female muskoxen (Ovibos moschatus) in eastern North Slope and western Alaska, USA, 2007–08.

DISCUSSION

Wildlife population declines are often the result of complex combinations of interrelated mortality factors, including nutritional imbalances, disease agents, and adverse climatic and environmental conditions. Predation and human take can also be significant contributors to demographic changes. We identified several comorbidity conditions that could have contributed to mortality and limited ENS muskox population numbers.

Pathologic investigations were challenged by examination of incomplete carcasses, as well as carcasses that had undergone various degrees of autolysis, producing poor-quality samples. Nonetheless, these examinations revealed important findings. Preexisting conditions in animals apparently succumbing to predation included hoof abnormalities, polyarthritis, pneumonia, and pregnancy. When good-quality samples were available (e.g., euthanasia of the sick calf), a definitive diagnosis could be made, such as that of polyarthritis caused by CLA. Some cases of polyarthritis were suspected to have been caused by CLA or Mycoplasma; unfortunately, tests were inconclusive. Chlamydophila spp. can cause stiff gait, lameness, weakness, abortion, retained placenta, infertility, and prenatal death in domestic ruminants (Kaltenboeck et al. 2005). This pathogen has also been implicated in polyarthritis, keratoconjunctivitis, and death in bighorn sheep (Ovis canadensis) in Yellowstone National Park, Wyoming, and may have predisposed individuals to predation (Meagher et al. 1992).

The high prevalence of CLA antibody in samples from WA during both periods suggests it has established host-pathogen balance in that population. On the other hand, detection of CLA from ENS only during the decline period is consistent with that of a naïve population with recent pathogen introduction. Furthermore, the confirmation of CLA in the synovium of a polyarthritis case and detection in vaginal tissue of adult females suggests active disease and transmission in ENS. Also, detection of CLA in vaginal tissue of adult female muskoxen without demonstrable antibodies in their serum suggests antibodies are not always produced rapidly at detectable levels. It could also be that those antibody levels wane quickly—a situation similar to Alaskan caribou which, when sampled repeatedly over a 2-yr period, had transient antibody titers (K.B.B. unpubl. data). The detection of low antibody titers to CLA in serum of adult female muskoxen from WA without detection of CLA in vaginal tissue from the same animals suggests these animals could have been infected but were no longer shedding CLA, or the bacteria might not have localized in the reproductive tract. Both PCR and ELISA were used to evaluate CLA in vaginal and conjunctival tissue. The ELISA failed to detect CLA in any of the samples, including those positive by PCR. The ELISA used here may have limited utility for screening muskox tissues for CLA, although neither assay has been validated for use in muskoxen.

Another condition that appeared to contribute to morbidity and mortality was verminous, bacterial, or aspiration pneumonia. Multifactorial pneumonia, including contributing pasteurellosis has caused mortality in Norwegian muskoxen during extreme weather events (Ytrehus et al. 2008), and recently, M. ovipneumoniae has been suggested as the primary cause of a severe pneumonia outbreak in Norwegian muskoxen (Handeland et al. 2014). In our study, culture and IHC of lung tissues with pneumonia that were negative for M. bovis. T. pyogenes, P. trehalosi, and lungworms (Parelaphostrongylus odocoilei) have also been identified as causes of pneumonia-associated mortalities in Dall's sheep (Ovis dalli dalli) (Jenkins et al. 2007), and one lamb mortality was identified in a sheep population sympatric with ENS muskoxen (K.B.B. unpubl. data).

We record the first case of dystocia in free-ranging muskoxen. In wild ruminants, dystocias are rare and difficult to detect. The muskox fetus was in breech presentation, and the dystocia was most likely caused by mal-presentation. However, concurrent findings of bacterial bronchopneumonia and low hepatic copper in the fetus (which should be at its highest at birth) and verminous pneumonia, septicemia, and low hepatic copper in the dam points toward a complex diagnosis, likely involving comorbidities. Also, finding hepatocellular vacuolar degeneration in pregnant females suggests these animals were in negative energy balance and could have had other toxicities (Cal et al. 2009).

Other indicators of complex interactions between nutritional status and diseases, leading to impaired health in this study, include hoof lesions that could be attributed to copper deficiency—a condition that can cause abnormal hoof keratinization and population declines (Flynn et al. 1977). Other trace mineral aberrations (elevated hepatic zinc and iron) could have exacerbated copper deficiency, since these minerals can be copper antagonists (Puls 1994). Average liver zinc levels in both areas were high when compared with muskoxen from Victoria Islands and Banks Island (Salisbury et al. 1992), and some individual levels were in the range considered toxic for sheep (Puls 1994). Hepatic storage of iron is reported to increase in sheep during copper deficiency, so detection of high liver iron in both areas could be further evidence of copper deficiency (Puls 1994).

The serum copper levels in adult females in both areas were above mean levels reported in copper-deficient free-ranging Alaskan muskoxen (Barboza and Reynolds 2004). Although the serum copper levels we observed in adult females may appear adequate because of homeostatic mechanisms, the liver stores may be inadequate for optimal reproduction and fetal growth. We also found evidence of manganese and zinc deficiencies in both areas and low selenium in ENS. Manganese deficiency is associated with silent estrus, reduced reproductive performance, abortions, and weak calves. Selenium deficiency can cause reduced calf survival and reproductive performance (Flueck 1994). Copper deficiency and other trace mineral perturbations could be indicators of poor nutritional status in both ENS and WA muskox populations and could be affecting population health and reproduction.

Although Zarnke (1983) did not detect antibodies to B. suis serovar 4 in Alaskan muskoxen, recent exposure of ENS muskoxen to B. suis was an important finding of our study and that of Nymo et al. (2016). In contrast, WA muskoxen were negative despite exposure to the caribou herd with the highest B. suis serovar 4 antibody prevalence in Alaska and proximity to herds of infected reindeer (Dieterich et al. 1990). Rangiferine brucellosis has not been recognized as an important disease in muskoxen until recently, when a case was diagnosed concurrently with CE and lungworm in a hunter-harvested male (Tomaselli et al. 2016). In Alaskan caribou, it causes chronic infections that manifest as reproductive failure, abortions, lameness, metritis, bursitis, epididymitis, orchitis, and infertility (Neiland et al. 1968), warranting further evaluation in muskox populations.

We also report CE-like lesions in dead animals and low CE antibody prevalence in ENS muskoxen. Subsequently, we identified parapoxvirus by PCR and amplicon sequencing from interdigital or mucocutaneous lesions of one ENS and two WA muskoxen from the ENS population (M. Tryland pers. comm.), and a case was reported in Canada (Tomaselli et al. 2016). Considered a disease of domestic sheep and goats (Capra aegagrus hircus), CE occurred in an outbreak in muskoxen in Norway, where infected cows with udder and teat lesions refused to let calves suckle. Also, lesions on the lips and interdigits of calves might have contributed to decreased calf survival (Vikøren et al. 2008).

Blood cell counts and fecal parasite examinations are often used to evaluate the general health status of animal populations. The ENS group had significantly higher total leukocyte, monocyte, and eosinophil counts than the WA group, strengthening concern about this population's overall health status and susceptibility to infectious diseases. Monocytosis occurs with chronic inflammation and is often caused by chronic bacterial infections, such as brucellosis and chlamydiosis (Weiss and Wardrop 2010). The eosinophilia was most likely due to high parasite prevalence and multiple parasitic infestations. High parasite loads, especially during stressful periods, could exacerbate other health conditions (Kutz et al. 2004).

Unlike muskox population declines documented elsewhere, the ENS population did not exhibit a sudden, large-scale die-off that could be attributed to adverse weather or a particular disease or nutritional limitation (e.g., Ytrehus et al. 2008; Kutz et al. 2015). Rather, considering all of the evidence collected during this study, we conclude that the ENS muskox population was adversely affected by complex nutritional and infectious disease dynamics resulting in comorbidity that also likely increased susceptibility to predation. We documented pathogens that have been newly detected in Alaskan muskoxen and are worthy of continued monitoring, especially considering the effects of global climate variability in the arctic region (Kutz et al. 2009). We most strongly recommend continued serologic surveillance and pathologic investigation of CLA, brucellosis, and CE. Furthermore, nutritional and trace mineral investigations should be expanded because of the evidence of copper deficiency and the possible relationship between poor nutrition and disease. It is likely that, for the ENS muskox population and other wildlife populations experiencing the complex stressors of our modern times, more significant health insights and well-informed management recommendations could be best realized through holistic investigations of disease agents, trace minerals status, and nutritional factors in conjunction with intensive demographic and environmental analyses.