Web Content Viewer
Insect-Borne Diseases
Culex quinquefasciatus mosquito

The Ohio ADDL has confirmed the presence of four insect borne diseases in horses and white-tailed-deer since August 23, 2019, extending through 9/30/19.


Three horses have been confirmed with eastern equine encephalitis (EEE) virus infection in northeast Ohio, two horses in Ashtabula County – one on the east side of the county and one on the west side of the county – as well as one in Portage County. The Ashtabula County horses, one a 1-year old Morgan crossbred filly and the other a 3-year-old Belgian gelding, were confirmed by a positive IgM titer with a history of no vaccination and clinical signs consistent with EEE prior to dying spontaneously. The horse that died in Portage County, a 3.4-year-old American Standardbred gelding, also died with signs consistent with EEE and histopathology showed a meningoencephalomyelitis in sections of brain and spinal cord. EEE virus nucleic acid was detected in fresh spinal cord tissue by PCR assay. The earliest date of onset was 8/25/19, and clinical signs only lasted 2-3 days in all three cases.

Two horses from Stark and Morrow counties were demonstrated to be infected with west Nile virus (WNV) by IgM titers, a history of no prior vaccination, and compatible clinical signs. Serology results were reported on 9/20/19 for the first equine case this year in Ohio, a 12-year-old Quarterhorse from Stark County. The horse died after showing neurologic signs including ataxia in all 4 limbs, nystagmus with the fast phase to the left, flaccid muzzle, anorexia, acute weight loss, and red mucous membranes. The second infected horse, a 6-year-old Standardbred gelding in Morrow County, first showed clinical signs on 9/16/19 that included muscle fasciculations of the lips and shoulder, depression, ataxia and weakness of the rear limbs. The horse also had a fever of 103.4 F. This horse has responded to supportive treatment and survived as of this date.

Both EEE and WNV viruses are transmitted by mosquitos, and cycle normally between these insects and birds, with infections occurring in both horses and people as dead-end hosts of the virus. Thus, mosquito control is essential to control these virusesNews releases for both virus infections in horses with more information can be found at the ADDL website here and here.


Two horses that died in Hamilton County around Labor Day weekend were both confirmed to be infected with Neorickettsia risticii, the etiologic agent of Potomac Horse Fever, or equine monocytic ehrlichiosis. The diagnosis in each case was made by demonstrating nucleic acid of N. risticii in the feces and colon mucosal scrapings. Each horse suffered an acute course of illness with marked depression, laminitis, fever, and in one case diarrhea. Although the life cycle of N. risticii is complex, it is believed that horses often become naturally infected by ingesting infected aquatic insects such as caddisflies and mayflies that may be found dead and accumulate in pastures near rivers and streams, in feed near light sources and that get trapped on surfaces of drinking water.


There have been seven white-tailed-deer confirmed with epizootic hemorrhagic disease (EHD) virus infection in six counties between 8/23/19 and 9/25/19, six involving wild deer and one case in a captive white-tailed-deer herd. Diagnosis has been confirmed in each case by detection of EHD genetic material in fresh retropharyngeal lymph node by PCR assay. Chilled spleen and lung are also good samples to test suspect cases. Counties in Ohio with positive EHD virus infected deer include Butler (captive and wild deer), Clermont, Knox, Logan, Morrow and Warren. EHD virus is transmitted to susceptible deer by the bites of small Culicoides midges.

Morrow County currently has the distinction of having two confirmed arboviral diseases, WNV in a horse, and EHD in a wild white-tailed-deer.

Dr. Jeff Hayes, MS, DVM, ADDL Section Head of Pathology & Interim Lab Co-Director

Ryan Cleland, senior LMU veterinary student