Sores of boreal moose reveal a previously unknown genetic lineage of parasitic nematode within the genus Onchocerca

Abstract

Long-standing reports of open sores on the hind legs of moose (Alces alces) have been recorded in Alaska (as well as Canada, Europe, and Michigan), eliciting concerns about causes and infection. We used histological and genomic methods to investigate the sores from 20 adult moose on the Kenai Peninsula, Alaska. We paired this with thermal imagery and molt scoring of adult moose to further describe sore formation and understand its timing. Severe, ulcerative and eosinophilic dermatitis was found in all moose with sores present, and microfilariae within intraepidermal pustules were additionally found in four samples. Genetic analysis of sores from moose revealed a previously unknown genetic lineage of Onchocerca. Adult moose molt and lose their barrier of protection against flies in June and July during peak fly activity, leaving them vulnerable and allowing the development of sores. In summary, our results indicate that the cause for the sores on the hindleg of moose is a previously unknown genetic lineage of Onchocerca, probably transmitted by black flies, in timing with the molt cycle of adult moose. These sores leave moose exposed to pathogens, making them vulnerable, and challenging their health and fitness.

Fig 1. Thermal analysis.

Surface temperature of the same moose on May 28, 2021 (A) and July 30, 2021 (B). Surface temperatures were measured inside of the ellipses (black circles) and polygons (black irregular quadrilaterals) drawn over the images, representing perianal and leg temperatures respectively.

Fig 2. Molt and sores.

A. The progression from winter coat to molted, summer coat in adult female moose (2015: n = 11; 2016: n = 12; 2021: n = 12). B. Molt of the hind leg area above the hocks and the appearance of sores. Points represent individual observations.

Fig 3. Sores.

Variation in sores on the hind leg of adult female moose. A. Two sores (July 21, 2021). B. 25 sores (July 22, 2021).

Fig 4. Observed skin temperature (open symbols) in regions of interest on the caudal surface of adult female moose (n = 12) over summer.

Lines are predicted relationships between skin temperature and day of year for the temperature of the perianal (solid line and open triangles), hind leg maximum (dotted line and open circles), and hind leg minimum (dashed line and open diamonds).

Fig 5. Representative histologic photomicrographs of moose skin collected between May and July 2021.

A. Skin biopsy taken before appearance of sores in May, 2021. There are small numbers of perivascular eosinophils and lymphocytes in the dermis. No pustules or microfilariae are observed. Hematoxylin and eosin, 200x magnification; insert: higher magnification of small numbers of eosinophils (black arrows) and lymphocytes. Hematoxylin and eosin, 600x magnification. B. Biopsy from a leg sore of a live adult female moose. There is a diffuse, severe infiltration with eosinophils throughout the dermis. Note the diffuse ulceration and a serocellular crust on the left. Hematoxylin and eosin, 40x magnification; insert: higher magnification of the eosinophilic infiltration. Hematoxylin and eosin, 600x magnification. C. Skin sample from a dead wild moose sampled in July, 2021. There is a diffuse, severe infiltration with eosinophils throughout the dermis. Note multiple intraepidermal pustules (dashed circle). D. Higher magnification of the pustule highlighted in C. The pustule is filled with eosinophils with a microfilaria in the center surrounded by degenerated eosinophils forming an eosinophilic sleeve. Hematoxylin and eosin, 600x magnification.

Fig 6. Phylogenetic tree.

Phylogenetic tree created using a maximum likelihood method and a General Time Reversible best fit model with gamma distribution (2,000 bootstrap replicates) showing the relationship of all known Onchocerca cervipedis species complex species in conjunction with other Onchocercidae species. At this time, all Onchocerca species comprised within the O. cervipedis species complex (n = 4, denoted with *) form distinct clades including the positive samples (n = -17) from this study (denoted in black with **). These samples were accessioned in GenBank (OP265723-39).