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In endemic areas of the United States, around a quarter of referrals for equine neurological disease are attributed to EPM.<ref>Reed, S.M, Granstrom, D, Rivas, L.J, Saville, W.A, Moore, B.R, Mitten, L.A (1994) Results of cerebrospinal fluid analysis in 119 horses testing positive to the Western blot test on both serum and CSF to equine protozoal encephalomyelitis.  In ''Proc Am Assoc Equine Pract'', Vancouver BC, AEEP, Lexington, KY, p199.  In: Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  According to the United States Department of Agriculture, the average incidence is 14 cases per 10,000 horses per year.  However, the challenges of obtaining a definitive diagnosis may mean this figure is an underestimate.<ref name="Furr">Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  EPM has been identified in parts of Central and South America, southern Canada and across most of the USA.<ref name="Furr">Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  The disease is noted occasionally in other countries, in horses that have been imported from endemic regions.<ref>Pitel, P.H, Pronost, S, Gargala, G, Anrioud, D, Toquet, M-P, Foucher, N, Collobert-Laugier, C, Fortier, G, Ballet, J-J (2002) Detection of ''Sarcocystis neurona'' antibodies in French horses with neurological signs, ''Int J Parasitol'', 32:481-485.  In: Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref><ref>Goehring, L.S (2001) Sloet van Oldruitenborgh-Oosterbaan MM: Equine protozoal myeloencephalitis in the Netherlands?  An overview, ''Tijdschr Diergeneeskd'', 126:346-351.  In: Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  It is likely that these animals carried a silent but persistent infection during transportation.  There have been reports of EPM in horses that have not travelled to or from endemic regions,<ref name="Furr">Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref> although cross-reacting antigens in immunodiagnostic tests may explain this discrepancy. <ref name="Furr">Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  

In endemic areas of the United States, around a quarter of referrals for equine neurological disease are attributed to EPM.<ref>Reed, S.M, Granstrom, D, Rivas, L.J, Saville, W.A, Moore, B.R, Mitten, L.A (1994) Results of cerebrospinal fluid analysis in 119 horses testing positive to the Western blot test on both serum and CSF to equine protozoal encephalomyelitis.  In ''Proc Am Assoc Equine Pract'', Vancouver BC, AEEP, Lexington, KY, p199.  In: Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  According to the United States Department of Agriculture, the average incidence is 14 cases per 10,000 horses per year.  However, the challenges of obtaining a definitive diagnosis may mean this figure is an underestimate.<ref name="Furr">Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  EPM has been identified in parts of Central and South America, southern Canada and across most of the USA.<ref name="Furr">Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  The disease is noted occasionally in other countries, in horses that have been imported from endemic regions.<ref>Pitel, P.H, Pronost, S, Gargala, G, Anrioud, D, Toquet, M-P, Foucher, N, Collobert-Laugier, C, Fortier, G, Ballet, J-J (2002) Detection of ''Sarcocystis neurona'' antibodies in French horses with neurological signs, ''Int J Parasitol'', 32:481-485.  In: Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref><ref>Goehring, L.S (2001) Sloet van Oldruitenborgh-Oosterbaan MM: Equine protozoal myeloencephalitis in the Netherlands?  An overview, ''Tijdschr Diergeneeskd'', 126:346-351.  In: Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  It is likely that these animals carried a silent but persistent infection during transportation.  There have been reports of EPM in horses that have not travelled to or from endemic regions,<ref name="Furr">Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref> although cross-reacting antigens in immunodiagnostic tests may explain this discrepancy. <ref name="Furr">Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  

The route of infection remains unconfirmed,<ref name="Pasq">Pasquini, C, Pasquini, S, Woods, P (2005) '''Guide to Equine Clinics Volume 1: Equine Medicine''' (Third edition), ''SUDZ Publishing'', 245-250.</ref> but there is an increased risk associated with a young age (1-4 years)<ref>Saville, W.J.A, Reed, S.M, Granstrom, D.E, Morley, P.S (1997) Some epidemiologic aspects of equine protozoal myeloencephalitis.  ''Proceedings of the Annual Convention of the AAEP'', 43:6-7.</ref>and autumn months.<ref name="NAHMS">NAHMS (2000): ''Equine protozoal myeloencephalitis in the US'', Ft Collins, CO, USDA:APHIS:VS, CEAH, National Animal Health Monitoring System.  In: Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  The reported age range for EPM cases is currently 2 months<ref name="EPM8">Gray, L.C, Magdesian, K.G, Sturges, B.K, Madigan, J.E (2001) Suspected protozoal myeloencephalitis in a two-month-old colt.  ''Vet Rec'', 149:269-273.</ref> to 24 years.<ref>MacKay, R.J, Davis, S.W, Dubey, J.P (1992) Equine protozoal myeloencephalitis, ''Compend Contin Educ Pract Vet'', 14:1359-1367.  In: Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  Thoroughbreds, Standardbreds and Quarterhorses are most frequently affected across the US and Canada.<ref>Fayer, R, Mayhew, I.G, Baird, J.D, Dill, S.G, Foreman, J.H, Fox, J.C, Higgins, R.J Higgins, Reed, S.M, Ruoff, W.W, Sweeney, R.W, Tuttle, P (1990) Epidemiology of equine protozoal myeloencephalitis in North America based on histologically confirmed cases, ''J Vet Intern Med'', 4:54-57.  In: Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  This may relate to a breed predispostion or alternatively, managemental factors associated with these breeds.<ref>Boy, M.G, Galligan, D.T, Divers, T.J (1990) Protozoal encephalomyelitis in horses: 82 cases (1972-1986), ''J Am Vet Med Assoc'', 196:632-634.  In: Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  Showing, racing and stress<ref name="Saville">Saville, W.J, Reed, S.M, Morley, P.S, Granstrom, D.E, Kohn, C.W, Hinchcliff, K.W, Wittum, T.E (2000) Analysis of risk factors for the development of equine protozoal myeloencephalitis in horses.  ''J Am Vet Med Assoc'', 217:1174-1180.  In: Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref> have been linked to a greater risk of clinical disease.<ref>Saville, W.J.A, Reed, S.M, Morley, P.S (1999) Examination of risk factors for equine protozoal myeloencephalitis.  ''Proceedings of the Annual Convention of the AAEP'', 45:48-49.</ref>  

The route of infection remains unconfirmed,<ref name="Pasq">Pasquini, C, Pasquini, S, Woods, P (2005) '''Guide to Equine Clinics Volume 1: Equine Medicine''' (Third edition), ''SUDZ Publishing'', 245-250.</ref> but there is an increased risk associated with a young age (1-4 years)<ref>Saville, W.J.A, Reed, S.M, Granstrom, D.E, Morley, P.S (1997) Some epidemiologic aspects of equine protozoal myeloencephalitis.  ''Proceedings of the Annual Convention of the AAEP'', 43:6-7.</ref>and autumn months.<ref name="NAHMS">NAHMS (2000): ''Equine protozoal myeloencephalitis in the US'', Ft Collins, CO, USDA:APHIS:VS, CEAH, National Animal Health Monitoring System.  In: Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  The reported age range for EPM cases is currently 2 months<ref name="EPM8">Gray, L.C, Magdesian, K.G, Sturges, B.K, Madigan, J.E (2001) Suspected protozoal myeloencephalitis in a two-month-old colt.  ''Vet Rec'', 149:269-273.</ref> to 24 years.<ref>MacKay, R.J, Davis, S.W, Dubey, J.P (1992) Equine protozoal myeloencephalitis, ''Compend Contin Educ Pract Vet'', 14:1359-1367.  In: Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  Thoroughbreds, Standardbreds and Quarterhorses are most frequently affected across the US and Canada.<ref>Fayer, R, Mayhew, I.G, Baird, J.D, Dill, S.G, Foreman, J.H, Fox, J.C, Higgins, R.J Higgins, Reed, S.M, Ruoff, W.W, Sweeney, R.W, Tuttle, P (1990) Epidemiology of equine protozoal myeloencephalitis in North America based on histologically confirmed cases, ''J Vet Intern Med'', 4:54-57.  In: Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  This may relate to a breed predispostion or alternatively, managemental factors associated with these breeds.<ref>Boy, M.G, Galligan, D.T, Divers, T.J (1990) Protozoal encephalomyelitis in horses: 82 cases (1972-1986), ''J Am Vet Med Assoc'', 196:632-634.  In: Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  Showing and racing have been linked to a greater risk of clinical disease.<ref>Saville, W.J.A, Reed, S.M, Morley, P.S (1999) Examination of risk factors for equine protozoal myeloencephalitis.  ''Proceedings of the Annual Convention of the AAEP'', 45:48-49.</ref>  

Increasing age and environmental temperature have been associated with an increased seroprevalence of ''S. neurona''.<ref>Tillotson, K, McCue, P.M, Granstrom, D.E, Dargatz, D.A, Smith, M.O, Traub-Dargatz, J.L (1999) Seroprevalence of antibodies to ''Sarcocystis neurona'' in horses residing in northern Colorado, ''J Equine Vet Sci'', 19:122-126.  In: Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  Seroprevalence for this species is typically higher than for ''N. hughesi''.<ref name="Furr">Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>Other risk factors for EPM include the presence of opossums, rats, mice and woodland, increased population density of humans and horses, bedding horses on shavings or wood chips and the use of purchased grain.<ref name="Furr">Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>Case clustering may operate where all the risk factors occur, but the majority of cases appear in isolation.<ref name="Furr">Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>

Increasing age and environmental temperature have been associated with an increased seroprevalence of ''S. neurona''.<ref>Tillotson, K, McCue, P.M, Granstrom, D.E, Dargatz, D.A, Smith, M.O, Traub-Dargatz, J.L (1999) Seroprevalence of antibodies to ''Sarcocystis neurona'' in horses residing in northern Colorado, ''J Equine Vet Sci'', 19:122-126.  In: Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>  Seroprevalence for this species is typically higher than for ''N. hughesi''.<ref name="Furr">Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>Other risk factors for EPM include the presence of opossums, rats, mice and woodland, increased population density of humans and horses, bedding horses on shavings or wood chips and the use of purchased grain.<ref name="Furr">Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>Case clustering may operate where all the risk factors occur, but the majority of cases appear in isolation.<ref name="Furr">Furr, M (2010) ''Equine protozoal myeloencephalitis'' in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref>