This research has explored the (1) historical course of spread of heartworm in California’s coyote populations, the main reservoir for heartworm in California, and (2) its current distribution and abundance as a function of physiographic variables influencing vector populations, along with (3) individual-level effects of the parasite on coyote behavior, body condition, and reproduction, (4) the seasonal timing of transmission relative to vector abundance and coyote pupping activities, (5) multiannual variability in transmission relative to rainfall and vector abundance. Additionally, (6) use of serological tests designed to detect this parasite in dogs were validated for use in wild canids and found to work well in mainland populations of coyotes, gray foxes, and red foxes, but to give rise to frequent false-positives in the critically endangered population of the Channel Island fox. The latter finding helped to channel attention towards actual threats affecting these fox populations. (7) In a focal study in the south San Francisco Bay area involving hundreds of coyotes, gray foxes, and red foxes, we found high prevalence in coyotes and gray foxes but 0% prevalence in >200 red foxes. Because gray foxes use the same habitats as coyotes but red foxes use different (primarily salt marsh) habitat, these finding suggest the possibility that coyotes, which are able to sustain heartworm infections with minimal population impacts, are the primary source of exposure of heartworms (via vectors) to smaller canids.
Variables influencing the distribution and abundance of canine heartworm: temperature (left; from Sacks 2002, PhD dissertation), habitat for western tree hole mosquito vectors (center, from Calif. GAP analysis), and rainfall (right, from Calif. GAP analysis)
Predicted prevalence of canine heartworm in California coyotes
Collaborators
This research involved colloboration and contributions from agency, academic, and private wildlife ecologists, entomologists, veterinary epidemiologists, biotechnologists, and wildlife managers: Gary Roemer (New Mexico State Univ.), Linda Munsen (UC Davis, Vet Sch), Dirk Van Vuren (UC Davis), Kevin Crooks (CSU), Ed Caswell-Chen (UC Davis), C. C. Chang (UC Berkeley), Jerry Theis (UC Davis, Med Sch), Tom Scott (UC Davis), Bruno Chomel (UC Davis, Vet Sch), Rick Kasten (UC Davis, Vet Sch), Mike Johnson (UC Davis), Bernie May (UC Davis), Dairen Simpson (Santa Clara Vector Control District), Noor Tietze (SCVCD), Dave Woodward (Lake County Vector Control District), Art Colwell (LCVCD), Dave Brown (Sacramento-Yolo County Vector Control District) Vickie Kramer (CA Dept. Health Services), Ron Sanders (Synbiotics, Inc), Serge Leterme (Synbiotics, Inc).
Funding
This research was funded in part through grants and fellowships to Ben Sacks from the National Science Foundation IGERT for research on invasive species biology, the UCD Wildlife Health Center (co-investigators BB Chomel, EP Caswell-Chen), Jastro-Shields, and Fletcher Jones foundations.
Publications
Sacks, B. N., B. B. Chomel, and R. W. Kasten. 2004. Modeling the distribution and abundance of the nonnative parasite, canine Heartworm, in California coyotes. Oikos 105:415-425.
Sacks, B. N., D. L. Woodward, and A. E. Colwell. 2003. A long-term study of non-native-heartworm transmission among coyotes in a Mediterranean ecosystem. Oikos 102:478-490..
Sacks, B.N., and E. P. Caswell-Chen. 2003. Reconstructing the spread of Dirofilaria immitis in California coyotes. Journal of Parasitology 89:319-323.
Sacks, B.N., B. B. Chomel, R. W. Kasten, C. C. Chang, R. K. Sanders, and S. D. Leterme. 2002. Validation for use with coyotes (Canis latrans) of a commercially available enzyme-linked immunosorbent assay for Dirofilaria immitis. Veterinary Parasitology 109:45-58.
Sacks, B. N., and K. M. Blejwas. 2000. Effects of canine heartworm (Dirofilaria immitis) on body condition and activity of free-ranging coyotes (Canis latrans). Canadian Journal of Zoology 78:1042-1051.
Sacks, B. N. 1998. Increasing prevalence of heartworm in coyotes from California. Journal of Wildlife Diseases 34:386-389.
Coyote heart with heartworms (left), Fay trap used to survey for western treehole mosquito vectors (center), blood-engorged mosquito (right)