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Symptoms/clinical signs[edit]
Brainworm affects neurological and behavioral responses. Deer rarely show any external symptoms of P. tenius infection due to their high acquired resistance. Moose, however, have low resistance, and may show a number of symptoms. Though infrequent, cases of moose recovering from brainworm infection have been reported. In both deer and moose, symptom severity does not necessarily vary with severity of infection.


 * Infected individuals may not have any external symptoms.
 * Mild symptoms may include slower movements and response time, frequent stumbling, unusually tilted head, and emaciation.
 * Severe symptoms include extreme weakness, lameness, walking in circles, partial or whole blindness, loss of fear for humans, ataxia, and mortality. [24]

Several other ungulates are susceptible to brainworm infection, including elk, caribou, mule deer, sheep, goats, llamas,[4][18] alpacas[19], rarely cattle[20], and rarely horses. [20] Severe neurological damage similar to that of infected moose is shown to occur in these species.[18]

Diagnosis[edit source]
Presently, a commercial antibody test that can detect P. tenuis antemortem dies not exist[20][22]. Diagnosis in deer can be conducted by analyzing fecal pellets for larval P. tenuis, or post mortem necropsy to detect presence of adult P. tenuis in the brain cavity or second- and third-stage worms along the spinal cord.[1][9][10] However, brainworm larvae are difficult to distinguish from other parasitic worm species that can also be found in fecal pellets,[11] so detection of adult worms through necropsy is recommended. Diagnosis in moose is conducted with necropsy to detect worms in the brain or spinal cord.[6][11] Diagnosis in horses can be conducted with postmortem samples used for polymerase chain reaction (PCR) testing[20][21]. Positive PCR results show evidence of P. tenuis infecting equines[20][21].

Epidemiology
The geographic ranges of moose and white-tailed deer were historically separate prior to the 20th century.[1][9] Moose are well adapted to winter survival, whereas deer are not. They could not withstand the harsh winters in these regions of the northeastern United States and the southeastern provinces of Canada.[8] Deer populations began to move into the southern portions of moose range in the early 1900s following changes in climate,[10] logging, mining, forest fires, and increasing human development.[1][9] High-quality deer habitat associated with these changes has led to dramatic increases in deer abundance in these regions.[1][9]

White-tailed deer are the normal host of the P. tenuis parasite and are immunologically adapted to its presence. Deer and P. tenuis have coadapted in an evolutionary arms race over time. Deer remain largely unaffected by the presence of P. tenuis because of the immunity they have built as a result of coadaptation.[4][18] The prevalence and infection rate of P. tenuis in deer is density dependent; increased rates of infection by the parasite are the result of higher deer densities.[1][9]

Moose populations on the southern fringes of their range have recently experienced dramatic declines. As deer encroached upon the southern fringes of moose range, they introduced the parasite to a naïve host, the moose.[11] Upon transmission of the pathogen into moose, the worm causes cerebrospinal nematodiasis, a disease of the nervous system that often results in death.[4] P. tenuis may be one factor that has contributed to moose declines in southern portions of their range. This disruption of host-pathogen dynamics due to habitat alterations has promoted the spread of this parasite to a naïve host and has potentially contributed to declines in moose abundance and productivity in regions with high deer densities.[8] Continued increase in deer abundance and density in the southern fringes of moose range are predicted facilitate sustained transmission of P. tenuis to moose. Continued transmission of deer-related parasites, coupled with low productivity, habitat degradation, and a northward shift in the moose thermonuetral zone, leads to a troubling prognosis for southern moose populations.[11] Murray et al. (2006) predict, if current trends continue, moose populations in these regions will not be viable and population declines will persist.

Treatment
Currently, there is no definitive treatment for P. tenuis in mammals, though research is still being conducted. The use of anthelmintics (ivermectin and fenbendazole) have been attempted in white-tailed deer. The results indicate, however, that ivermectin was ineffective against larvae that had already reached the spinal cord. [23]

Current and past research[edit]
The majority of moose brainworm research in North America has been conducted in northern Minnesota, where a historical moose population of 4,000 to 5,000 moose had declined to an estimated 1,200 individuals by 1997. In this region, wildlife managers are challenged to predict and attempt to mitigate moose declines resulting from deer-related pathogens. Because no effective methods to prevent the transmission or infection of brainworm in moose have been found, managers have focused their efforts on decreasing deer densities in these regions. The Minnesota DNR identified that population densities greater than 12 deer/square mile result in increased moose mortality as a result of brainworm. Managers in these areas are responsible for evaluating suitable habitat for moose and deer, as well as setting management priorities and population goals to decrease the transmission of Parelaphostrongylus tenuis in moose. Though the main focus of research has been on white-tailed deer and moose, research has found that the guinea pig can be used as an experimental model of P. tenuis infection. [24]

References[edit]

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 * 16) ^ D.A. Benson and G. D. Dodds. (1997) The deer of Nova Scotia. Department of Lands and Forests, Province of Nova Scotia. Cited in: Edmund S. Telfer. (2004) "Continuing Environmental Change – An Example from Nova Scotia", Canadian Field-Naturalist 118(1): 39-44
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 * 20) ^ Jump up to: a b Mittelman, N.S.; Divers, T.J.; Engiles, J.B.; Gerhold, R.; Ness, S.; Scrivani, P.V.; Southard, T.; Johnson, A.L. (2017-05-01). "Parelaphostrongylus tenuis Cerebrospinal Nematodiasis in a Horse with Cervical Scoliosis and Meningomyelitis". Journal of Veterinary Internal Medicine. 31 (3): 890–893. doi:10.1111/jvim.14691. ISSN 1939-1676. PMC 5435076. PMID 28317172.
 * 21) Tanabe, M., Gerhold, R. W., Beckstead, R. B., de Lahunta, A., & Wade, S. E. (2010). Molecular confirmation of Parelaphostrongylus tenuis infection in a horse with verminous encephalitis. Veterinary pathology, 47 (4), 759-759. doi:10.1177/0300985810363488
 * 22) Dobey, C. L., Grunenwald, C., Newman, S. J., Muller, L., & Gerhold, R. W. (2014). Retrospective study of central nervous system lesions and association with Parelaphostrongylus species by histology and specific nested polymerase chain reaction in domestic camelids and wild ungulates. Journal of Veterinary Diagnostic Investigation, 26(6), 748-754.DOI: 10.1177/1040638714553427
 * 23) Kocan, A. A. (1985). The Use of Ivermectin in the Treatment and Prevention of Infection with Parelaphostrongylus tenuis (Dougherty) (Nematoda: Metastrongyloidea) in White-tailed Deer (Odocoileus virginianus Zimmermann). Journal of Wildlife Diseases, 21(4), 454-455. doi:10.7589/0090-3558-21.4.454
 * 24) Haigh, J., Mackintosh, C., & Griffin, F. (2002). Viral, parasitic and prion diseases of farmed deer and bison. Revue Scientifique Et Technique De LOIE, 21(2), 219-248. doi:10.20506/rst.21.2.1331