2.1. Literature search

A literature review was conducted to identify all publications reporting cases of EMCV infection in humans and NHPs. Searches were performed in the PubMed and Google Scholar databases, as well as using the Google search engine. Predefined keywords related to EMCV and NHPs or humans were combined using the Boolean operators “AND” and “OR” to generate multiple search combinations. No restrictions on language or publication date were applied, allowing for the inclusion of both historical and recent studies.

2.2. Study selection

All identified references were initially reviewed based on their titles and abstracts. Articles considered relevant to the objectives of the review were subsequently retrieved and analyzed in full text. Eligible publications included case reports, outbreak investigations, and epidemiological studies documenting EMCV infection in NHPs or humans, as well as relevant data reported in other animal species.

Given the limited amount of available data and the historical nature of many studies, all publications reporting EMCV infection in humans and NHPs since the virus was first identified in 1944 were included. Study selection was based on relevance to the review objectives, rather than strict inclusion/exclusion criteria.

2.3. Data extraction and synthesis

Data extracted from the selected studies included the affected NHPs species, geographic location, year of publication, study type, and key findings related to EMCV infection. A narrative synthesis was then performed to summarize the available evidence of EMCV infection in humans and NHPs.

3.1. Genetic diversity, host range, and geographic distribution

EMCV, a member of the Picornaviridae family, exhibits significant genetic diversity, initially described in the literature in terms of lineages, “serotypes”, and strains [41,42]. Among the serotypes, EMCV-1 is the most extensively characterized and the most widely distributed worldwide. It comprises eight lineages (A–H), which are further structured into four major evolutionary clades (I–IV) [41,42]. These clades encompass diverse strains of EMCV and Mengo virus, reported across a broad range of hosts, including several animal species [2,4,7–13,21], and arthropods [10], as well as humans [11,26]. This group includes historical strains such as EMCV Ruckert, isolated from a chimpanzee in Florida, United States [9], and Mengo M, isolated from a rhesus monkey in Uganda [10].

The recently described EMCV-2 serotype includes the highly divergent strain RD-1338, isolated from a captive wood mouse (Apodemus sylvaticus) in Germany [43], as well as other viral variants frequently detected in wild hazel dormice (Muscardinus avellanarius) in England [44].

In addition, a third putative serotype (EMCV-3), comprising the Sing-M100-02 and Sing-M105-02 strains isolated from captive orangutans in Singapore, has been proposed on the basis of marked phylogenetic divergence. However, this classification remains provisional, as comprehensive serological characterization has not yet been performed [16,40]. Similarly, divergent viruses identified in rats (Bandicota indica) in Vietnam have been suggested to represent a novel serotype (EMCV-4) [41]. Nevertheless, as with EMCV-3, this designation is currently based exclusively on genetic and phylogenetic evidence, lacking serological confirmation [40].

3.2. Genome organization and structure

The EMCV genome is approximately 7.8 kb in length and consists of a single large open reading frame (ORF) flanked by 5’ and 3’ untranslated regions (UTRs) and ending in a poly-A tail. The coding region comprises the leader protein (L), the P1 region encoding the structural proteins VP4, VP2, VP3, and VP1 (genes 1A–1D), and the P2 and P3 regions encoding the nonstructural proteins 2A–2C and 3A–3D, respectively [1]. A poly(C) segment of ~150 nucleotides is present in the 5’ UTR, although its length can vary depending on the viral strain (Fig 2B) [45,46].

4.1. Reported epizooties in NHPs

Since its first identification in 1944–1945 in the USA from a gibbon and a chimpanzee, fatal outbreaks and sporadic cases of EMCV-associated infections have been reported in various captive and semi-captive NHPs species. These cases have occurred in the USA [6,7], Africa [10], Australia [13], Asia [16], Russia [17], and Italy (Fig 3) [18,19].

Download:

• PNG
  larger image
• TIFF
  original image

Fig 3. Global distribution of cumulative encephalomyocarditis virus cases reported in NHPs from 1944 to 2025.

The colored areas (red, blue, orange, and cyan) indicate the geographical regions where EMCV cases have been reported in NHPs and the virus isolation. The most significant cases have been reported in the USA and Russia. Map created with Datawrapper; base map layer (administrative boundaries) provided by Natural Earth (https://www.naturalearthdata.com).

https://doi.org/10.1371/journal.pntd.0014409.g003

In the USA, following the initial report of two sporadic cases involving the deaths of a gibbon on November 1944 and a chimpanze 6 weeks later at the Anthropoid Apes Research Foundation in Florida, the first confirmed epidemic mortality events associated with EMCV in NHPs occurred between 1977 and 1978.

These outbreaks resulted in the deaths of a De Brazza monkey (Cercopithecus neglectus) and three chimpanzees at the Lion Country Safari Zoo in West Palm Beach, Florida [6]. The cases were recorded on January 1977 (chimpanzee 1), December 1977 (De Brazza’s monkey), and February 1978 (chimpanzees 1 and 2). Between January and August 1985, another epidemic was recorded, resulting in the death of eight NHPs at the Audubon Park Zoo in New Orleans, Louisiana.

The affected animals included three black and white ruffed lemurs (Varecia variegata), one black-and-white colobus monkey (Colobus guereza), two golden lion tamarins (Leontopithecus rosalia rosalia), and two siamangs (Symphalangus syndactylus) [7].

In subsequent years, sporadic cases and several fatal epizootics were reported in various NHPs species. These included an orangutan (Pongo abelii) in January 1987 [12], baboons (Papio hamadryas) in 1992 [47], and rhesus macaques in October 2008 and December 2009 [15]. The baboon epizootic represents one of the most significant events, resulting in the death of approximately 80 individuals [47].

More recently, between March and April 2019, confirmed EMCV-associated mortality was reported in a zoo in central Florida (USA), affecting three mandrills (Mandrillus sphinx) and one lion-tailed macaque (Macaca silenus) [40].

In Asia, the first reported cases of EMCV infection in NHPs occurred between July 2001 and January 2002, when four orangutans (Pongo pygmaeus and Pongo abelii) died at the Singapore Zoological Gardens [16].

In Australia, fatal EMCV infections were reported at Taronga Zoo between March 1987 and June 1995, affecting a ring-tailed lemur (Lemur catta), a squirrel monkey(Saimiri sciureus), three mandrills, and a chimpanzee [13].

In Russia, the first documented cases date back to the 1970s, involving the deaths of two rhesus macaques, followed by multiple mortality events affecting baboons, green monkeys (Cercopithecus aethiops), and rhesus macaques, across the country [17,41,48]. For instance, between March and April 2007, 37 out of 46 semi-captive hamadryas baboons housed at the Primate Institute in Sochi died due to EMCV infection [41].

In Italy, two confirmed outbreaks of EMCV were recorded at the Natura Viva Zoo in northern Italy between October 2006 and February 2008. The first outbreak led to the deaths of 18 captive NHPs, including black lemur (Eulemur macaco macaco), ring-tailed lemurs, red-ruffed lemurs (Varecia rubra), white-fronted lemurs (Eulemur albifrons), common marmosets (Callithrix jacchus), and barbary macaques (Macaca sylvanus) [18].

The second outbreak, occurring between September and November 2008, resulted in the death of three red-ruffed lemurs [18]. Five years later, another outbreak-associated with a confirmed EMCV infection, occurring between September and November 2012, resulted in the deaths of four barbary macaques at the Rescue Center for Wild and Exotic Animals in central Italy [19].

In Africa, the first reported case of EMCV infection in NHPs was reported in 1946. It involving a paralyzed captive rhesus monkey at the Entebbe Research Institute in the Mengo district of Uganda, [10]. Subsequently, in 2003, two confirmed deaths associated with EMCV infection were recorded in semi-captive bonobos at the Lola ya Bonobo sanctuary in the Democratic Republic of Congo (Table 1) [14].

Download:

• PNG
  larger image
• TIFF
  original image

Table 1. Summary of deaths associated with EMCV in nonhumain primates species in different geographical regions of the world.

https://doi.org/10.1371/journal.pntd.0014409.t001

4.2. Risk factors

EMCV infection in NHPs is strongly associated with environmental risk factors, particularly in captive and semi-captive settings. The primary driver of infection is exposure to rodents, notably those of the genus Rattus, which serve as the main reservoirs of the virus [25,26]. Transmission typically occurs indirectly through the contamination of food, water, or surfaces by rodent excreta. Numerous cases of sporadic infections or reported outbreaks have been associated with the proliferation of infected rodents in sites housing NHPs [6,13,14,18,19]. These observations are consistent with those reported in pigs, where farm-level infections have been frequently traced back to the presence of infected rodents [52,53]. Furthermore, captive conditions characterized by insufficient biosecurity, including inadequate rodent control, inappropriate food storage, and infrastructure facilitating rodent intrusion, significantly increase the risk of exposure [13]. Ecological factors, including seasonal variations that influence rodent population dynamics and the environmental stability of the virus (favored by its non-enveloped structure), may further modulate transmission risk [6]. However, the role of host-specific factors, such as differential susceptibility among NHP species and across age groups, remains poorly characterized due to a lack of comparative studies and the predominance of case reports.

4.3. Interspecies barrier crossing

The epidemiology of EMCV in captive and semi-captive settings highlights its broad host tropism, as evidenced by its ability to infect a wide range of mammalian hosts, including multiple NHP species simultaneously within the same outbreak.

However, rather than indicating frequent interspecies transmission or host-adaptive barrier crossing, these reported cases are more likely explained by a shared-source spillover. This occurs through exposure to a common environmental source, particularly in contexts where different species coexist or share contaminated food and water resources [6,7,13].

Several outbreaks illustrate this pattern of multi-species involvement, without evidence of direct transmission between hosts. At Taronga Zoo in Australia, EMCV infection was associated with the deaths of a diverse array of taxa, including a ring-tailed lemur, a squirrel monkey, mandrills, a chimpanzee, a pygmy hippopotamus, and Goodfellow’s tree kangaroos [13]. Similarly, at Audubon Park Zoo in New Orleans, fatalities affected several NHPs species alongside non-primate mammals such as a Thomson’s gazelle and a dromedary [7]. Comparable multi-species outbreaks have been reported at Lion Country Safari Zoo in Florida and at a wildlife rescue center in Italy, involving both primates and non-primate species [6,19]. The recurrence of such diverse multi-species events underscores the virus’s lack of host specificity. Conceptually, these dynamics further support the hypothesis that transmission is primarily driven by environmental contamination.

Current evidence consistently identifies rodents as the principal reservoir and source of infection, with transmission occurring through the ingestion of food or water contaminated by rodent excreta or secretions, or through contact with infected carcasses [7,12,18,42]. This model provides a parsimonious explanation for the simultaneous infection of multiple species. Furthermore, the frequently reported seasonal clustering during colder months warrants further interpretation [6,13,15,17]. This seasonal pattern may reflect ecological changes in rodent populations, including increased shelter-seeking behavior in human-managed environments, as well as enhanced environmental persistence of the nonenvelopped virus at lower temperatures. Such factors synergistically increase the likelihood of environmental contamination and cross-species exposure among susceptible hosts.

4.4. Natural reservoirs

Rodents, particularly members of the Muridae family, are widely considered key hosts in the ecology of EMCV and are acting as primary sources of environmental contamination through viral shedding in feces and urine [20,21,49].

Epidemiological investigations have consistently reported associations between EMCV outbreaks in NHPs, domestic animals, and the presence of rodents in shared environments such as zoos, sanctuaries, primate centers, and breeding facilities, particularly in proximity to food storage areas and water sources [15,42,47,50,51]. For instance, an epizootic in pigs in central-western New South Wales was linked to a mouse (Mus musculus) infestation, resulting in 47 confirmed outbreaks across 37 farms and over a thousand pig deaths [52].

Molecular and serological evidence further supports this association. EMCV RNA and specific antibodies have been detected in both rodents and affected animal species [10,16,51,54]. In several settings, viral strains isolated from rodents have shown a high degree of genetic similarity to those identified in outbreak-associated species. For example, EMCV strains detected in rodents were nearly identical (98–99.9% nucleotide identity) to those associated with elephant mortalities in zoos in South Africa and France [42,55], and similar findings have been reported in pig farms [51]. In one U.S. zoo, EMCV was detected in multiple rats and mice collected from areas where NHP deaths and seropositive animals had been identified [7].

Together, these findings support a common-source exposure model in which rodents act as potential reservoir hosts and sources of spillover, with transmission to NHPs and other species most likely occurring through indirect exposure to contaminated food, water, or environments, rather than through direct contact. However, the precise origin of infection in individual outbreaks often remains difficult to conclusively determine.

Crucially, a distinction must be made between experimental competence and natural maintenance. Despite strong epidemiological and molecular links, evidence for stable, long-term maintenance of EMCV in wild rodent populations under natural conditions remains limited. Much of the current understanding of rodent competence derives from experimental studies. Under controlled conditions, rats have been shown to develop subclinical infections, shed virus for extended periods, and transmit infection horizontally to conspecifics [20,57]. Viral persistence has also been demonstrated in lymphoid tissues for several weeks post-infection [20], supporting the potential for prolonged carriage.

In contrast, field-based longitudinal data documenting sustained viral circulation within natural rodent populations are scarce. This distinction is important, as experimental findings may not fully reflect the complex ecological dynamics in the wild. Additionally, susceptibility appears to vary among rodent species: while rats (Rattus spp.) often exhibit subclinical carriage with prolonged shedding, the high mortality seen in some mice species might limit their role as long-term reservoirs [17].

EMCV has been successfully isolated from a range of wild rodent species, including mice (Mus spp.), rats, Mastomys spp., dormice (Myoxus glis), cotton rats (Sigmodon hispidus), water rats (Hydromys chrysogaster), and spiny rats (Proechimys guyanensis) [56]. However, whether these species serve as true reservoirs or are merely incidental hosts during outbreaks remains unclear. It is possible that other rodent species, or even other as yet unidentified hosts in nature, could serve as natural reservoirs for the virus (Fig 4).

Download:

• PNG
  larger image
• TIFF
  original image

Fig 4. Reservoir hosts and transmission cycle of encephalomyocarditis virus.

This diagram illustrates the multi-host ecology of EMCV, where rodents serve as the primary natural reservoirs maintaining the virus within the natural ecosystem through persistent infections, with bats also potentially involved in this maintenance. Transmission to other species occurs via two main reported pathways: the ingestion of food or water contaminated with rodent excreta or secretions, and the consumption of infected rodent carcasses. The potential role of arthropod vectors (mosquitoes and Acarina) in transovarian or transstadial transmission remains a hypothesis requiring further investigation. A wide range of susceptible hosts can be infected, including domestic pigs, various wild and captive animals (such as primates and elephants), and highly sensitive rodents like mice, while humans are considered accidental hosts. In the diagram, solid lines represent reported and verified transmission routes, while dashed lines indicate plausible hypotheses. Created in BioRender. FENOLLAR, F. (2026) https://BioRender.com/cxhowt9.

https://doi.org/10.1371/journal.pntd.0014409.g004

Overall, current evidence indicates that rodents play a central role in the ecology and dissemination of EMCV, particularly as sources of environmental contamination that facilitate spillover into susceptible hosts. However, their role as stable reservoirs in natural ecosystems, as well as the mechanisms underlying viral persistence between outbreaks, remains incompletely understood and warrant further investigation.

4.5. Other potential reservoirs

In addition to rodents, certain bat species, such as Miniopterus fuliginosus, have also been suspected of being potential reservoir hosts or carriers of EMCV in the wild (Fig 4) [58].

However, unlike the relatively robust body of research on rodents, available data on EMCV in bats remain limited and inconclusive.

EMCV has also been found in arthropods, particularly mites and mosquitoes [10,42], including various wild animal species such as wild boar (Sus scrofa) [50,59]. Nevertheless, experimental investigations in mosquitoes have provided no evidence of viral transmission. The presence of the virus in these arthropods is most plausibly explained by the ingestion of blood meals from infected hosts [10].

7.1. Molecular testing

EMCV RNA is routinely detected and characterized by RT-PCR and sequencing [4,16,42,51]. However, genomic data from human and NHPs remain scarce, with most sequences originating from older studies and only a few strains fully characterized. Partial and complete genomes, as well as specific gene segments, have been obtained from clinical samples in lemurs, bonobos, orangutans, macaques, baboons, and chimpanzees [4,14,16]. The majority of nucleotide sequence data currently available comes from studies in swine and rodents (Table 2) [4,62–65].

Download:

• PNG
  larger image
• TIFF
  original image

Table 2. Nucleotide sequence references of genomes and genes from EMCV strains isolated from NHPs, wild and domestic animals, humans, and arthropods.

https://doi.org/10.1371/journal.pntd.0014409.t002

7.2. Cell culture and isolation

The first strain of EMCV, designated EMCV Ruckert (EMCV-R), was isolated in 1944 in Florida from a chimpanzee that died suddenly from acute myocarditis [9]. In 1946, a second strain referred to as Mengo virus, classified under the EMCV-1 serotype, was isolated from the cerebrospinal fluid of a paralyzed rhesus macaque at the Mengo Research Institute in Entebbe, Uganda [10]. Since these initial discoveries, EMCV strains have been progressively isolated from a wide variety of species worldwide, notably from both domestic [2,4,25,66] and wild animals [7,8,13,54,55], including small rodents [21,56] and even in humans [11,26]. In NHPs, virus isolation has been documented not only from chimpanzees [9], and rhesus macaques [10,15,17,48], but also from bonobos [14], orangutans [16], grivet [17], lemurs [18], barbary macaques [18,19], and baboons [17,47]. Virus isolation is typically performed using baby hamster kidney fibroblast (BHK-21) cells, African green monkey kidney (Vero) cell lines, and laboratory mice (Table 2). A range of clinical specimens has been used, including pleural fluid and spleen [9], cerebrospinal fluid [10], serum, whole blood, pericardial fluid, cardiac tissue [14], and tissue samples from the heart, lungs, brain, and liver [13,16,19,41].

In humans, EMCV has been successfully isolated from serum and blood samples of patients presenting with febrile illnes and neurological symptoms [11,26]. Most EMCV strains have been isolated from rats and pigs [4,21,62–64,67,68].

7.3. Immunohistochemical and serological tests

Serological and immunohistochemical methods have been employed to detect EMCV-specific antibodies or antigens in humans and NHPs species. The serological assays include immunofluorescence [13,16], hemagglutination inhibition [12,17], and virus neutralization [13,16,17].

In a study by Yeo et al [16], neutralizing antibody titers against the EMCV Sing-M105-02 isolate, originally isolated from orangutans, were detected in 22 of 122 zoo animals tested, including capybaras, chimpanzees, and orangutans. Similarly, neutralizing antibodies were observed in eight of 23 rhesus macaques, one captive grivet, and 19 baboons among 630 animals tested in Uganda and the USA [10,47]. In addition to serology, immunohistochemical testing has revealed the presence of EMCV antigens in tissue samples from infected NHPs [14].

7.4. Microscopic testing

Histopathological analyses have consistently demonstrated cardiac lesions, most notably myocardial involvement characterized by extensive, focal, or diffuse myocarditis [14–19].

These inflammatory changes in the myocardium are among the most consistent and indicative findings associated with EMCV infections. Furthermore, electron microscopy has enabled the visualization of virus particles within affected tissues of NHPs [14].

8.1. Treatment

Due to the rapid progression of EMCV infection, therapeutic intervention is often challenging, if not futile. In addition, the disease remains poorly recognized, and appropriate diagnostic tools are typically implemented only at advanced stages of infection or not at all in live animals resulting in significant delays in clinical management [56]. In the absence of specific antiviral therapy, treatment is primarily supportive and may be attempted in animals presenting with mild to moderate clinical signs. However, in most cases, euthanasia should be seriously considered, given the poor prognosis, particularly in individuals with cardiac involvement or heart failure [15,40,56]. Various therapeutic approaches have been described in the literature, although with limited success. For instance, intravenous administration of ceftriaxone (50 mg/kg) was attempted in two bonobos at the Lola ya Bonobo Sanctuary (Democratic Republic of the Congo) during the advanced stages of an acute respiratory syndrome characterized by tachypnea and dyspnea; both animals died within 24 hours of the onset of clinical signs [14]. In other reports, captive elephants and a capybara exhibiting compatible clinical signs succumbed to the disease within a few days despite receiving supportive treatment, including antibiotics, analgesics, and antispasmodics [6,73]. Overall, these observations highlight the limited efficacy of currently available therapeutic interventions and underscore the critical importance of early detection and preventive measures.

8.2. Vaccination Strategies Against EMCV

Efforts to develop vaccines against EMCV have been ongoing for several decades, with overall heterogeneous and often inconsistent outcomes. Various vaccine strategies have been explored, including autogenous inactivated vaccines derived from outbreak-specific viral isolates, as well as live attenuated formulations [7,69–72].

The first vaccination trials were initiated following an outbreak in a zoological setting in New Orleans, USA, involving multiple species, including NHPs, elephants, and lions [7]. Since then, several vaccine candidates have demonstrated the ability to induce humoral immune responses across a range of species, including NHPs.

However, despite these encouraging findings, evidence supporting long-term immunity and cross-protection against genetically diverse EMCV strains remains limited and inconsistent [13,56]. These limitations highlight the ongoing challenges in developing broadly effective and durable vaccination strategies against EMCV.