Eastern Equine Encephalomyelitis in Michigan: Historical Review of Equine, Human, and Wildlife Involvement, Epidemiology, Vector Associations, and Factors Contributing to Endemicity

Eastern equine encephalomyelitis (EEE) is a mosquito-borne viral disease that is an emerging public health concern in the state of Michigan. Although Michigan has one of the highest incidence rates of EEE in the United States, much of the information known about cases in humans, equines, and other animals residing in Michigan is unpublished. This article summarizes such information and explores spatial trends in the historic distribution of EEE in Michigan. Outbreaks in Michigan have occurred over an 80-yr interval, involving only horses in 1942–1943 and 1973–1976, and then episodically from 1980 to 2020, and involving horses, humans, and wild and domestic animals. An estimated 1,036 equine cases (confirmed and suspected) and 36 confirmed human cases have occurred, including 10 in 2019 (6 deaths) and 4 in 2020 (2 deaths). Human cases ranged in age from 1 to 81 yr; 70% were male, and fatality rate of 34.3%. Equine and human cases occurred from July to October, peaked in August, and cluster in space in southwestern and southeastern lower Michigan. Cases occurred in glacial outwash and ice-contact landscapes in glacial interlobate zones. EEE virus (EEEV) was recovered from Culiseta melanura, Coquillettidia perturbans, five species of Aedes, and other mosquito species near horse and human case sites. Virus isolations or presence of neutralizing antibodies in several passerine species of birds suggest broad EEEV–bird associations. White-tailed deer and other wildlife were also affected. Geographic spread to northern areas of the state suggests expansion of this disease system into new and unsuspected foci.

Madariaga Virus

The area affected by the disease caused by Madariaga virus has been expanding recently. Given the geographic location of the endemic regions that overlap with tourist destinations, as well as the confirmed possibility of importing the disease, we cannot rule out the risk of appearance of this disease in Russia. The aim of the study was to analyse the properties of Madariaga virus (Alphavirus genus, eastern equine encephalomyelitis virus complex) and some epidemiological and epizootological characteristics of the virus-associated disease which manifests itself in diffuse inflammation of brain and spinal cord. Based on both ecological and molecular genetic characteristics of Madariaga virus, it is classified as an individual virus which is part of the eastern equine encephalomyelitis virus complex. There is evidence that this pathogen can cause epizootic outbreaks in horses, infect other types of mammals (rats, bats), as well as, potentially, birds and reptiles. The reservoir of infection is the short-tailed cane mouse (Zygodontomys brevicauda). Human cases have also been described, and the potential route of infection was identified as transmission via mosquito bites. Vectors of pathogen transmission are mosquitoes of the Culex, Aedes, and Psorophora genera. Serological studies carried out in Panama revealed the presence of antibodies to Madariaga virus in 2–5% of the examined patients, which indicates the existence of asymptomatic infection along with clinically apparent infection. Phylogenetic analysis of strains isolated from infected people showed that the strains belong to lineage III of the eastern equine encephalomyelitis virus subtype common in Central and South America. The paper summarises the results of assessment of potential risk factors of Madariaga virus infection in endemic regions that was performed using the Akaike information criterion. Risk groups in endemic regions are represented by farm workers and fishermen. The results of the studies show that the area affected by the disease is expanding, and the strains that belong to genetic lineage III of the eastern equine encephalomyelitis virus pose the greatest epidemic risk.

Fatal Eastern Equine Encephalitis in a Patient on Maintenance Rituximab: A Case Report

A 63-year-old woman on rituximab maintenance for follicular lymphoma presented with headaches, vomiting, and fever, and was diagnosed with eastern equine encephalomyelitis by cerebrospinal fluid polymerase chain reaction. Eastern equine encephalomyelitis immunoglobulin (Ig)G/IgM remained negative due to rituximab treatment, and magnetic resonance imaging showed minimal abnormalities, making this a diagnostically challenging case. Despite therapy with intravenous Ig, the patient rapidly declined and died on hospital day 12. Autopsy revealed perivascular and parenchymal chronic inflammation, with an absence of B lymphocytes, and virally infected neurons throughout the central nervous system.