STREPTOCOCCUS EQUI SUBSP. ZOOEPIDEMICUS

, and Dys (found in S. dysgalactiae) 39  Portable, smartphone-based system to perform end-point fluorescence detection (LAMP) assays on a microfluidic chip, for pen-side detection of respiratory pathogens in nasal swabs from horses ( S. equi , S. zooepidemicus , equine herpesviruses 1 and 4 (EHV1), and equine influenza virus H3N8) 40


IMPORTANCE
Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) is a commensal found in the upper respiratory tract of horses, pigs, and other animals. As an opportunistic pathogen, it causes infections in many species. Until recently, S. zooepidemicus infection in pigs was confined to Asia. However, outbreaks were reported in North America in 2019 -one each in Tennessee and Ohio, 1, 2 and one in Manitoba, Canada. 3 A mortality event linked to S. zooepidemicus also occurred in pigs in Pennsylvania, 4 and epidemiological information suggests similar events may have occurred in other US locations. 5 S. zooepidemicus should be considered an emerging pathogen in swine.

PUBLIC HEALTH
S. zooepidemicus belongs to Lancefield Group C (see Etiology). In humans, Group C streptococci (GCS) cause a variety of clinical syndromes. Most infections are transmitted person-to-person. 6 Zoonotic infection with S. zooepidemicus occurs but is relatively uncommon. 6 Disease is mostly associated with consumption of unpasteurized dairy products or animal contact. 6 Reported illness in humans includes pharyngitis, glomerulonephritis, skin/soft tissue infection, toxic shock syndrome, infectious arthritis, and invasive disease. Clinical syndromes and selected cases are reviewed in Appendix A.
Only one outbreak of S. zooepidemicus in humans has been linked to pork consumption. From 1982 to 1986, 14 cases of GCS septicemia were described in Hong Kong, 11 of which were attributed to S. zooepidemicus. Lyophilized specimens from five of these cases were compared to GCS isolates from septicemic pigs at an abattoir in the same district. All samples were analyzed using biotyping (API 20 Strep) and comparison of colonial morphology, capsular morphology, and chromosomal banding pattern following HindIII and EcoRI restriction endonuclease digestion. The samples from pigs and humans were determined to be identical. No animal contact was noted, nor was consumption of unpasteurized dairy, in any of the human cases. However, the authors note that consumption of raw/undercooked pork is popular in Hong Kong. 7 An overview of the clinical features seen among cases is shown below.

INFECTION IN SWINE
Clinical signs and postmortem lesions associated with known outbreaks of S. zooepidemicus in pigs are summarized below. Diagnostic tests used in each of the described outbreaks are also listed (see Diagnosis). In a recent study by Hu et al., pigs infected with S. zooepidemicus were evaluated over a period of 14 days to understand disease progression. Three isolates were tested: one from the 2019 high mortality swine outbreak in Tennessee, one from a horse with pneumonia and a pleural abscess, and one from a guinea pig lymph node. For each isolate, five sows and six five-month-old pigs were oronasally inoculated and monitored for clinical signs. Clinical signs, postmortem lesions, and morality are described in Table 3.

DISINFECTION
Little information was found on S. zooepidemicus disinfection. S. equi is inactivated by 1% bleach, quaternary ammonium compounds, chlorhexidine, and Virkon ® (potassium peroxymonosulfate). 17 Other disinfectants generally effective against streptococci include 70% ethanol, formaldehyde, glutaraldehyde, and iodine-based products. 18 Efficacy of disinfectants could be affected by biofilm formation, which has been described in S. zooepidemicus ATCC 35246. 19 A 2009 study investigated use of bacteriophage lysin PlyC against clinical isolates of S. equi and S. zooepidemicus.
One μg of enzyme sterilized a 10 8 CFU/ml culture of S. equi in 30 min, and PlyC retained full activity in the presence of hard water, organic matter, and non-ionic detergents. 20

DISEASE PREVENTION
New animals should be quarantined for several weeks before introduction to the herd. However, utility of screening is unclear since the prevalence of S. zooepidemicus in healthy North American swine, in different types of production systems, is not known. While S. zooepidemicus has been reported in healthy pigs, in some studies it is much more common in clinically ill animals. 23 Other studies report endemic persistence. 24,25 Pigs that are sick should receive treatment and supportive care to decrease the likelihood of secondary infection with S.
zooepidemicus. Since stress may be a predisposing factor for S. zooepidemicus infection, pork producers should aim to prevent:  Transport of injured or sick pigs (including malnourished pigs)  Transport during extreme temperatures or inclement weather  Overcrowding and loud noise exposure during transport  Commingling of transported swine (which also allows direct contact)  Extended hold times in lairage 5

DISEASE CONTROL
Outbreaks of S. zooepidemicus should be managed carefully to prevent disease spread. Affected premises should be isolated, and any contact premises identified via trace-forward or trace-back should be quarantined. Premises and possible fomites (equipment, vehicles, etc.) should be cleaned and disinfected regularly. Other standard biosecurity practices should be in place.

TRANSMISSION
Modes(s) of transmission are not known for recent swine outbreaks. Transmission in other species, such as horses, is due to direct contact with infected animals, and indirect contact with contaminated equipment, housing, bedding, or clothing. Bacteria can also be ingested via contaminated feed or water. 11 In guinea pigs, oral abrasions are linked to S. zooepidemicus cervical lymphadenitis; illness in dogs has been linked to aerosols and environmental contamination; and infection in camelids has been associated with wound contaminated and ingestion (as cited by Sitthicharoenchai et al.) 1 Humans are most often infected through consumption of unpasteurized dairy or animal contact. 6 Stressors such as overcrowding, poor nutrition, transportation, or weaning are thought to contribute to development of S.
zooepidemicus infection in horses, 26 and stress may have been a factor in recent North American swine outbreaks.

PATHOGENESIS
In horses, S. zooepidemicus enters through the nose or mouth and colonizes the mucosal surface and tonsillar tissue of the nasopharynx. Nasal shedding in horses is usually seen two to three days after onset of fever and persists for several weeks. 11 Pathogenesis is not completely understood but related to virulence determinants including M-like protein (prevents phagocytosis), fibronectin-binding protein (facilitates attachment and invasion), C5a peptidase and SpyCEP (involved in immune system evasion), G-like proteins and IgG degrading enzymes (bind IgG and inactivate proteins), and other enzymes and toxins that lead to an excessive inflammatory response (see Major Virulence Determinants).
In pigs experimentally infected with a swine S. zooepidemicus isolate, clinical signs began within 24 hours post-infection

CULTURE AND IDENTIFICATION
Culture and identification remain common methods to identify S. zooepidemicus infection. In horses, Columbia colistinnalidixic acid (CNA) agar with 5% sheep or horse blood added is used. 11 Biochemical test kits have been tested for identification 27 of GCS in nasal, pharyngeal, or abscessed lymph node samples from horses. Several are commercially available (e.g., API ® , bioMérieux).
Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) has been described to identify S. equi subspecies in various hosts/infection sites. 28

TESTS TO DETECT NUCLEIC ACIDS OR ANTIGEN
Recently a quantitative polymerase chain reaction (qPCR) assay based on a conserved region of szM to detect highly virulent swine isolates was described. The assay was evaluated in porcine clinical samples including tissues, contact swabs, and isolated cultures. It was highly sensitive and specific, with no cross-reactivity to avirulent S. zooepidemicus isolates or S. equi. 30 Some tests described for S. equi/S. zooepidemicus diagnosis in other animals are shown below (in order of development, oldest to newest).
 PCR to detect S. equi in horses (nasal swabs) based on seM, which encodes for the M-like protein SeM 31  Multiplex PCR to detect S. equi in horses based on sodA gene (encodes for manganese-dependent superoxide dismutase, which provides defense against oxidative stress) and seeH and seeI genes (encode for the pyrogenic mitogens SePE-H and SePE-I, which are present in S. equi and S. pyogenes but absent in S. zooepidemicus) 32  Multiplex PCR to identify different strains of S. zooepidemicus in camels and camel milk, based on polymorphisms in the 16S-23S rDNA intergenic space region, and the virulence gene szp 33  Multiplex RT-PCR to detect S. equi in horses based on sodA, seeH, and seeI genes (nasal swabs, tracheal wash fluid) 34  PCR to detect superantigen (SAg) genes (szeF, szeN, szeP) in S. zooepidemicus from necropsy lung washes of dogs 35  RT-PCR assay to detect S. equi in horses (nasal swabs, nasopharyngeal swabs, abscess swabs, guttural pouch washes and tracheal washes) based on the eqbE gene, which forms part of the integrative conjunctive element Se2

TESTS TO DETECT ANTIBODY
There are no antibody tests described for detection of virulent S. zooepidemicus in pigs. Paired titers may be useful for comparing exposure vs. infection status. For S. equi, serum titers peak about five weeks post-exposure, and can remain elevated for more than six months. 11 Enzyme-linked immunosorbent assays (ELISAs) are typically used to screen asymptomatic horses for strangles before they are introduced to a disease-free population, to determine if vaccination is needed, or to assess exposure during an outbreak. Most target SeM, a major virulence factor, and cannot differentiate infected from vaccinated animals. There are several commercially available assays (IDEXX/EBI, Innovative Diagnostics). SeM-based ELISAs remain problematic, however, since SzM (found in S. zooepidemicus) is nearly identical to SeM and cross-reactivity is not well understood. As cited by Robinson et. al, 41 pre-incubating sera with heat-killed S. zooepidemicus can remove cross-reactive antibodies to SzM; however, background reduction has not been implemented in current SeM-based ELISAs leading to the potential for false positive results.
An indirect ELISA (iELISA) that targets two S. equi protein fragments (SEQ2190 and SeM) has been described. 42 It was found to have superior sensitivity and specificity compared to an iELISA that targets only SeM. 41 This assay is available in at least one laboratory. 43 The ability of an S. equi vaccine to interfere with detection of antibodies (via the SEQ2190 and SeM duplex iELISA) was found to be clinically insignificant. 44

SAMPLES
Collection of fixed and fresh tissues is recommended including lymph node, lung, liver, kidney, spleen, and tonsil. 5 Oral fluids have not been evaluated for detection of S. zooepidemicus. However, S. suis has been found in swine saliva, 45, 46 and detected in oral fluid samples, 47 which is unsurprising since many streptococci are upper respiratory tract commensals.

SPECIES AFFECTED
S. zooepidemicus is best known as a commensal found in the upper respiratory tract of horses. 48 It has been described in other healthy animals, including pigs. 18 Yet, in a culture study of 395 swine tonsils collected at slaughter, S. zooepidemicus was found much less frequently than other streptococci including S. equisimilis, S. porcinus, and S. suis (1.3% vs. 29.4%, 19.5%, and 53.7% respectively). 49 Additional studies have identified streptococci, but not S. zooepidemicus, in tonsils of healthy swine. 50,51 S. zooepidemicus causes infection in many species. In horses, respiratory disease (rhinitis, bronchitis, pneumonia) and endometritis are most commonly seen. 11,48,52 Infections also occur in cows, 53 sheep, 54 goats, 55 dogs, 56 cats, 57 monkeys, 9 llamas and alpacas, 58 guinea pigs, 59 chinchillas, 60 poultry, 61 pigs 2, 3, 8, 9 , and humans. Although zoonotic infections are rare, they can be very serious (see Public Health). 48

GEOGRAPHIC DISTRIBUTION
Historically, S. zooepidemicus has been the most common cause of swine streptococcal disease in China. A large outbreak occurred in 1975 8 and epidemics occurred regularly after that. S. zooepidemicus was also isolated from diseased pigs and monkeys in Indonesia in 1994. 9 In the early 2000s, S. suis became the predominant streptococcal species isolated in China 62 and it remains so today. 63 Until recently, S. zooepidemicus was not reported in pigs outside of Asia. In 2019, S. zooepidemicus was linked to sudden death and abortion in Canada 3 and sudden death in the United States 1, 2 (see History in Swine).

MORBIDITY AND MORTALITY
The prevalence of S. zooepidemicus in US swine is unknown. The Iowa State University Veterinary Diagnostic Lab (ISU VDL) has reportedly identified only six clinical cases of S. zooepidemicus in pigs since 2010. 1 In a study of pigs condemned due to tuberculosis-like lesions at a slaughterhouse in Spain, streptococci were cultured from nearly one third of tissue samples, including submandibular lymph nodes, lungs, liver, and spleen. Of 65 streptococci isolates, S. suis was isolated most frequently, followed by S. porcinus, S. dysgalactiae subsp. equisimilis, S. zooepidemicus, S. agalactiae, and S. alactolyticus. 64 A subsequent study of condemned free-range pigs with tuberculosislike lesions showed similar results. 65 The 1975 outbreak of S. zooepidemicus in China reported high mortality, with the death of 300,000 pigs. 8 Authors of the 1994 report on S. zooepidemicus in pigs and monkeys in Indonesia stated that "most of the [diseased] animals died within a few days." 9 The number of cases is not stated.
In the second quarter of 2019, S. zooepidemicus was linked to several swine mortality events in Canada. The S. zooepidemicus outbreak at four sow farms in Manitoba, Canada was characterized by sudden death, with an excess of 1000 sow deaths occurring over a 12-week period, and abortion, at a rate 11 times greater than normal. 3 A packing plant in Manitoba reported an increase in sows "dead on arrival" at the end of July. A US processing plant also flagged an increase in sow deaths linked to S. zooepidemicus. Around the same time, an assembly yard in western Canada reported shipments with an increase in pigs "dead on arrival." 66 High mortality was seen in the US outbreaks that occurred in 2019. In Ohio, feeder pigs and cull sows at a buying station experienced mortality of 30% to 50% over a period of eight to 10 days, and a Tennessee abattoir reported the deaths of 30% to 40% of sows in lairage. 2 In Pennsylvania, a livestock dealer's holding facility (linked to a case of S. zooepidemicus) had a case fatality rate of 81%. 4

CHARACTERISTICS OF STREPTOCOCCI
Streptococci are Gram positive bacteria in the family Streptococcaceae. They are non-motile, non-sporeforming cocci that often occur in pairs or chains. 18 The genus Streptococcus is divided into more than 70 species that are classified by habitat, pathogenicity, physiological characteristics, macromolecule characteristics, and Lancefield group. 67 Three types of hemolysis are seen: α, β, and γ. β-hemolytic strains cause most acute streptococcal disease and are divided into two groups based on colony size (large-forming vs. small-forming). 18 First described in 1933, Lancefield grouping is based on the identification of carbohydrate antigens associated with β-hemolytic streptococci. 68 There are 20 recognized serotypes, identified as A to H and K to V. 18 Some streptococci belong to more than one Lancefield group, based on more recent evidence of intergroup phage reactions and intergroup transduction between strains. 67

CHARACTERISTICS OF S. ZOOEPIDEMICUS
Phenotype and Lancefield group have traditionally been used to identify streptococci. 68 S. zooepidemicus is β-hemolytic.
Most strains produce hyaluronic acid, giving a mucoid appearance. In horses, colonies from the tonsil are non-mucoid due to the production of hyaluronidase. 11,69,70 However, primary cultures from lung or tracheal washes may be briefly mucoid, 69 making them grossly indistinguishable from S. equi. 11 S. zooepidemicus colonies recently isolated from US pigs were large, clear, and mucoid in appearance. 1 Streptococci are catalase negative. Most strains of S. zooepidemicus hydrolyze esculin and starch, and ferment ribose, sorbitol, and lactose. Some, but not all, ferment trehalose. 68 S. zooepidemicus belongs to Lancefield Group C, carrying N-acetylgalactosamine on the oligosaccharide side chains of its cell wall carbohydrate antigens. 67

CLASSIFICATION OF GROUP C STREPTOCOCCI
Group G streptococci (GGS) are closely related to GCS; both belong to the pyrogenic streptococci group. Group A streptococci (GAS), an important cause of illness humans, also belong to the pyrogenic group. Both GCS and GGS are related to GAS (80% homology), and contain genes acquired through horizontal transfer. 67 There are four GCS and GGS species groups, containing 12 individual species based on 16S rRNA as shown in Table 4. 67 S. zooepidemicus and S. equi are very closely related (98% DNA homology). 48 S. equi is thought to be a descent of S. zooepidemicus. 71,72  Major virulence determinants of S. zooepidemicus are described below and summarized in Table 5.

Fibronectin Binding Proteins 7, 16
Most GCS and GGS bind fibronectin, which mediates adhesion to host cells and cell internalization. S. zooepidemicus produces three fibronectin binding proteins-FNZ, FNZ2, and SFS. In some isolates the fnz gene has undergone a onenucleotide deletion, giving rise to fne gene which encodes FNE in S. equi. FNE is functional but has reduced fibronectinbinding capability compared to FNZ. SFS is produced by both S. equi and S. zooepidemicus, and a similar counterpart is found in GAS.

Complement Fragment C5a Peptidase 7, 16
Complement fragment C5a is a chemotaxin that helps recruit phagocytes to infection sites. The C5a peptidase gene scpA has been detected in human GGS, but not in animal GGS. In GCS, S. zooepidemicus produces ScpZ (isolated from a pig with septicemia), and S. equi produces ScpE, both homologues of ScpA. The scp genes of S. equi and S. zooepidemicus are associated with a transposon, rather than being within the mga regulon encoding the emm gene as seen in S. pyogenes.

IgG-Binding and Protein Inactivation 7, 16
Protein G contributes to adherence, inhibition of phagocytosis, and host mimicry in human GCS and GGS. The role of G-like proteins in pathogenesis is not fully known, but likely related to inhibition of phagocytosis via proteinase-complexed α2M. There are other IgG-binding proteins that are functionally different from protein G, such as MAG and MIG, described

Other Factors 22
The Fic domain-containing protein BifA was recently shown to be required for development of meningitis in mice (i.e., allowing S. zooepidemicus to cross the blood-brain-barrier), and for penetration of a human brain endothelial monolayer in a culture model. In 1975, a S. zooepidemicus outbreak in Sichuan Province led to the death of 300,000 pigs. Respiratory disease and sudden death were observed. The genome of strain ATC 35246, which caused the outbreak was described in 2011.
Bali, Indonesia (1994) 25 In early 1994, an outbreak of arthritis, diarrhea, and respiratory disease occurred in pigs and monkeys in Bali, Indonesia.
Most animals died within a few days of becoming ill. Cases were first reported in pigs in a small village on Bali. Over the next weeks to months, the outbreak spread to surrounding swine and a monkey population. Within 3 months, cases were seen in pigs on two additional islands, Sumatra (to the northwest) and Sulawesi (to the northeast). Postmortem lesions included polyarthritis, bronchopneumonia, pleuritis, epicarditis, endocarditis, and meningitis.
S. zooepidemicus was isolated from 30 pigs and four monkeys from April-July. Despite some phenotypic variation, all isolates were biochemically identical. Digestion with SmaI revealed identical DNA patterns for 32 isolates. In the remaining isolates, from two different pigs on Bali, there were differences seen in two fragments. Control strains had DNA profiles unrelated to outbreak strains. In 2004, endemicity of S. zooepidemicus was confirmed in Indonesian monkeys and swine. 25 Manitoba, Canada (April 2019) 3 n April 2019, a S. zooepidemicus outbreak occurred in four loose-housed, commercial swine farms in Manitoba, Canada.
Farms were part of a large, vertically integrated swine system with more than 9000 sows. The outbreak was characterized by sudden death, with 1000 excess sow deaths occurring over a 12-week period, and abortion, at a rate 11 times greater than normal. Clinical signs developed rapidly; sows were reportedly healthy during morning checks, but became unwilling to stand, febrile, and lethargic, with death occurring in hours. Others aborted first, then went on to develop similar signs.
Outbreaks seemed to be exacerbated by comingling/stress. No signs of illness were seen in pigs from affected sows.
Postmortem lesions included rhinitis, pulmonary edema, gall bladder edema, and hemorrhagic lymphadenopathy of the submandibular, cervical, and bronchial lymph nodes.
Gram-positive cocci were found in heart and lymph node imprints, and S. zooepidemicus was cultured from liver, kidney, heart, brain, lung, spleen, and submandibular lymph nodes. Sequencing showed that all isolates were similar to ATCC 35246 (previously associated with a high mortality outbreak of septicemia in China). 8 All isolates were identified as ST194.
Around the same time, additional S. zooepidemicus-linked swine deaths were described. An assembly yard in western Canada experienced an increase in pigs "dead on arrival" in the second quarter of 2019, and a Manitoba packing plant noted an increase in sows "dead on arrival" in July 2019. 66 Tennessee and Ohio (September-October 2019) 1,2,5 In 2019, the first swine outbreaks of S. zooepidemicus septicemia occurred in the United States. In late September, mortality of 30% to 40% was seen in cull sows in lairage over a five-to seven-day period at a Tennessee slaughter plant.
Deaths were also detected in an epidemiologically linked buying station in Ohio, where mortality of 30% to 50% was seen in feeder pigs and cull sows over eight to 10 days. Clinical signs in pigs included weakness, lethargy, and high fever.
Splenomegaly and hemorrhagic lymph nodes were noted at necropsy.
Three cases were submitted to the ISU VDL. Microscopic lesions were consistent with acute bacterial septicemia (vasculitis, fibrin thrombi, fibrinosuppurative polyserositis, intralesional bacteria). S. zooepidemicus was isolated from spleen, lung, and kidney. Next-generation sequencing identified S. zooepidemicus and porcine partetravirus (a parvovirus previously known as hokovirus).
Whole genome sequencing was performed on 24 S. zooepidemicus isolates, including eight from the outbreak, an outbreak-unrelated swine isolate from Arizona, and 15 S. zooepidemicus isolates from other animal species (six equine, three feline, three guinea pig, one canine, one caprine, one chinchilla). An additional 24 isolates from GenBank, from different countries and years, were included in the phylogenetic analysis.
Results showed that the eight isolates from Tennessee and Ohio were indistinguishable, suggesting a common source.
These isolates were identified as ST194 and clustered together with the Chinese strain ATCC 35246, which has previously caused high mortality outbreaks in China. 8 There were also three guinea pig-associated human cases from Virginia (unassigned ST) that were closely related to the outbreak isolates. The swine isolate from Arizona belonged to ST340 and was distant to the outbreak.
Comparative genomic analysis showed that the outbreak isolates and two Chinese isolates (ATCC 35246 and CY) had similar genomic islands and virulence genes, including the M-like protein genes szp and szm, the Fic domain-containing protein BifA, the fimbrial subunit protein encoding gene fszF, and the protective antigen-like protein coding gene spaZ.

Pennsylvania (December 2019) 4, 29, 30
S. zooepidemicus was detected in a cull sow at a livestock market in Pennsylvania in December 2019. Routine African swine fever/classical swine fever (ASF/CSF) surveillance on-site identified splenomegaly, with fibrin and pus present, as well as a large amount of cloudy, orange fluid in the abdomen. Culture showed heavy growth of S. zooepidemicus. The sow was malnourished and consigned with several others by a small livestock dealer. Traceback showed that additional pigs at the dealer's holding facility had died. Additionally, surviving sows were sent to auction, and three were known to be sent to slaughter. The overall case fatality rate was 81% (17/21). Sequencing showed that isolates from this outbreak were genetically similar to S. zooepidemicus ATCC 35246. Virulence determinants identified included the scI1 gene and M-protein.

POST-EXPOSURE
Immunity to S. equi persists for five years or more in most horses following recovery. Surface-exposed proteins elicit IgG response, including SeM, Se44.2, Se46.8, Se45.5, and Se42.0. 11 No information was found on post-exposure immunity to S. zooepidemicus in animals.

VACCINES
There are both killed and modified live vaccines available for S. equi, which are administered to horses based on risk. 11 Vaccination of horses against S. equi does not confer immunity to S. zooepidemicus. According to the USDA, there are currently no commercial vaccines available for S. zooepidemicus in any species. 5 The diversity of S. zooepidemicus has complicated vaccine development efforts; however, autogenous vaccines may be an option for pigs in the future.
Experimental vaccines that have been described for S. zooepidemicus include the following (shown below in order of development, oldest to newest).

CROSS-PROTECTION
The S. zooepidemicus population is highly diverse. Prolonged, repeated upper airway infections are common in young foals, suggesting that immunity is not acquired until individuals have been infected with many different S. zooepidemicus strains. 11 In horses, no cross-protection occurs between S. zooepidemicus and its closest relative, S. equi. 84

GAPS IN PREPAREDNESS
The role of S. zooepidemicus in recent swine mortality events remains poorly understood. Stress seems to be a predisposing factor in some cases, but S. zooepidemicus has been confirmed as a potential primary pathogen. Notably, few cases of S. zooepidemicus have been reported in US swine. Whether testing reflects the actual prevalence is unclear. In the clinical laboratory, GCS isolates may not be speciated, leading to an underestimate of the disease burden of S. zooepidemicus. In addition, misidentification of clincal swine specimens could be a concern. In a study of S. suis identification (biochemical testing vs. PCR), very high false-positive (71%) and false-negative (60%) rates were seen. 100 To develop preventive measures, the epidemiology of S. zooepidemicus in pigs must be further investigated. from community controls. 106 Antibodies to the M-like protein Szp5058 were found in convalescent sera from 33/44 cases tested. 107 The genome of the causative strain, S. zooepidemicus MGCS10565, was sequenced in 2008.
While the genome was similar to GAS in many ways, it lacked prophages that encode the majority of strain-specific gene content, and a gene related to streptococcal pyrogenic exotoxin B (SpeB), a suspected causative antigen for post-streptococcal glomerulonephritis. 108 In addition, proteins such as streptokinase (which are implicated in post-streptococcal glomerulonephritis due to GAS) were highly divergent in strain MGCS10565. 108 Skin and soft tissue infections due to S. zooepidemicus are less common than other GCS, and usually involve animal exposure. 6 Reported cases include the following.
 In 2017, sepsis, multi-organ failure, and necrotizing myositis were described in a 73-year-old Norwegian farmer. He reported having sores and abrasions on his fingers and having direct contact with two Shetland ponies at his stable.
The ponies did not show signs of illness, and no nasopharyngeal swabs were collected. 109  A 1986 case report from California described a 56-year-old man with pain and swelling on the right side of his face.
He had developed fever, night sweats, and a dry cough prior. The man was a horse caretaker with 20 years of experience, and he lived close to a stable. His case was described as "human strangles." 110  A 1990 case report from Australia described S. zooepidemicus cellulitis in a renal transplant patient who had contact with horses at a show. 111 GCS and GGS have been occasionally identified in cases of streptococcal toxic shock syndrome (STSS). 6 Only in a few instances has S. zooepidemicus been the cause.
 Necrotizing fasciitis, and underlying cirrhosis, lead to the death of a 64-year-old man in Japan in 2004. S. zooepidemicus was identified as the cause of STSS. Animal exposures were unknown. 112  A report from 2004 describes a 63-year-old Australian man who developed left thigh pain and swelling while on an airplane flight. Shortly after he experienced fever, rigor, a rapidly progressing skin rash on his trunk and limbs, and death. He had received an IM injection of prochlorperazine in the left thigh for presumed acute labyrinthitis two days prior. He also had frequent contact with horses. 113  In 2013, a case of STSS was identified in a 57-year-old man with multiple underlying conditions that worked at a horse farm. He initially developed influenza-like illness but was later admitted to the hospital with respiratory failure.
He developed septic shock but responded to treatment and recovered in 6 weeks. 114 GCS are a known cause of infectious arthritis in humans, but S. zooepidemicus has been isolated infrequently. 6 Reported cases include the following.
 In 2001, 2 cases of acute arthritis due to Streptococcus dysgalactiae subsp. equisimilis were reported in Spain. A review of the literature identified 22 additional cases of GCS arthropathy. In 2 of these, S. zooepidemicus was identified as the cause. In one case the patient reported collecting cervical cultures from infertile mares 4 days before the onset of symptoms, one of which was positive for GCS. 115  In 2021, a case report was published for a recently retired veterinarian with severe GCS infection. The patient, a 65-year-old male who was previously healthy, developed septic polyarthritis, native mitral valve endocarditis, and lumbar discitis/osteomyelitis. An animal source was presumed but not identified. 116 Invasive disease caused by GCS and GGS appears to be increasing, especially in people with underlying disease. Primary bacteremia due to S. zooepidemicus is most often associated with animals or animal products. 6 Reported outbreaks include the following.
 An outbreak of 11 cases of bacteremia was described in West Yorkshire, England, in 1988. Endocarditis and meningitis were also identified in many cases. Seven patients died; all were greater than 70-years-old. All cases had consumed unpasteurized milk from a dairy herd with mild, intermittent mastitis. 117  A 1999 literature review identified 88 cases of bacteremia caused by GCS. Of these, 21 reported exposure to ani-mals or animal products (unpasteurized milk, agricultural work, work as a butcher, other animal contact) and were diagnosed with S. zooepidemicus. Five cases of S. zooepidemicus-endocarditis, with animal exposure, were also described. 118  An outbreak of S. zooepidemicus septicemia, linked to unpasteurized goat cheese, occurred in Finland in 2003.
Seven cases were identified, one of which had purulent arthritis. All were previously healthy, and no deaths occurred. 119  A report from 2009 examined a case of bacteremia, with complications involving meningitis, mitral endocarditis, and blindness (due to bilateral endophthalmitis) in a 59-year-old Canadian woman. She had multiple underlying conditions. Sporadic contact with horses was noted, and she recovered with some permanent sequelae. 120  A 2010 case report described wound infection and subsequent meningitis, caused by S. zooepidemicus, in a 79-year-old man who was trampled by his horses. A further literature review identified 20 cases of S. zooepidemicus meningitis. Animal contact was noted in all but one, with suspected transmission due to inhalation, ingestion of unpasteurized dairy, and wound inoculation. 121  Another case report from 2010 detailed a case of meningitis in an 83-year-old horse trainer from Florida. He had a history of hypertension, pacemaker placement, and penicillin allergy. He survived following a 4-week-course of ceftriaxone. No horses with GCS were identified on the farm following an epidemiological investigation by the state.
A literature review identified 36 cases of GCS meningitis, about 1/3 reported horse exposure and about 1/5 reported ingestion of dairy products. 122  A 2015 case report documented endogenous endophthalmitis in a 73-year-old man that had contact with a sick horse in Germany. 123  In 2016, 2 cases of S. zooepidemicus were described in King County, Washington. A 37-year-old woman (patient A), who operated a horse riding and boarding facility, developed mild pharyngitis and a cough. Around the same time, a horse at the facility (horse A) developed upper respiratory disease. A 71-year-old woman (patient B), who was patient A's mother, also developed an upper respiratory infection. She had lived in the same household as patient A, and had contact with horse A. Several weeks later, she developed vomiting and diarrhea, was found unconscious, and was transported to the hospital where she died. Both women were previously healthy. 124  A nationwide outbreak of S. zooepidemicus-associated respiratory disease occurred in Iceland in 2010, spreading through 77,000 horses. All equine activities were stopped, and a self-imposed ban on equine exports was instated. 257 isolates were analyzed, and ST209 was thought to be responsible for the epidemic. Concurrent with this epidemic ST209 was isolated from a human case of septicemia. 125  A 2019 case report documented meningitis possibly caused by S. zooepidemicus in a 6-month-old girl. She presented with fever, vomiting, fussiness, decreased energy, and imbalance during crawling. She also had a respiratory infection two weeks prior. Her only animal contact was with two pet dogs that recently had respiratory infections. 126  Additional cases of invasive disease linked to S. zooepidemicus were found in the literature. 127