Streptococcosis

Last updated August 02, 2024

Streptococcosis is an infectious disease caused by bacteria of the genus Steptococcus . This disease is most common among horses, guinea pigs, dogs, cats, and fish with symptoms varying based on the streptococcal species involved.^[1] In humans, this disease typically involves a throat infection and is called streptococcal pharyngitis or strep throat.

Pathogenesis

Occurring in pairs or chains, streptococci are found to be Gram-positive (although older cultures may lose this characteristic), non-mobile, non-spore forming, and catalase-negative.^[2] Bacteriophages, also known as phages, of streptococcus within different parameters of temperature, pH, and salinity maintain successfully stable and are lytic.^[3] Integrase, transposase, and recombinase coding genes are found to be absent within phages.^[3] Streptococcosis can start occurring due to a weak immune system, or by having bacteria enter wounds.^[1] Spreading of Streptococcus is often sporadic,^[4] and can be done through direct contact (may be done through materials that are likely to carry infection), air transport or (rarely) ingestion.^[1]

Classification

The bacterial species involved in a streptococcosis infection is typically identified through microscopy of the bacteria to observe their morphology, biochemical tests (e.g., hemolysis ability), and tests on which antibodies are produced by the infected organism.^[2] Antibody detection, also known as serologic grouping, categorizes with the labeling of Group A to Group V; it uses differences with cell wall carbohydrates and pili-associated protein.^[2] With the use of hemolysis, species are divided within three different categories: incomplete (α hemolytic), complete (β-hemolytic), and no (γ hemolytic) hemolysis detected.^[2] Two common species seen are S. agalactiae which has been associated with fish and (more significant) S. suis which has been associated with pigs.^[4]

Clinical identification

The clinical manifestations of streptococcus infections differ greatly depending on both the host species and group and strain of the bacteria.^[2]

Alpha-hemolytic streptococci (S. pneumoniae and viridians)

The first group of streptococci is alpha-hemolytic which comprises primarily S.pneumoniae and viridans streptococci. This group is referred to as alpha-hemolysis because the cell membrane of red blood cells is left intact.^[2] When cultured, alpha-hemolysis can be deemed present when the agar gel appears greenish.^[2]

Identifying and diagnosing alpha-hemolytic Streptococcus is done with a sputum gram stain and culture test.^[5] Further identification can be done serologically to test for the presence of capsular antigen, which is the dominant structure on the surface of S. pneumoniae.^[2]^[5] Bile solubility can be used to further distinguish S. pneumoniae from viridans streptococci as S. pneumoniae are bile soluble and viridans streptococci are not.^[6]

S. pneumoniae are the most significant alpha-hemolytic streptococci and are responsible for several infections including:

The identification and diagnosis of these conditions often require a combination of bacteriologic methods with other clinical identification characteristics that are condition-specific.

Beta-hemolytic streptococci (Group A, B, C, D, F, G, and H)

In contrast, the beta-hemolytic group of streptococci includes those capable of complete lysis of red blood cells.^[5] Beta-hemolytic streptococci are further divided into additional subgroups consisting of: Group A, Group B, Group C, Group D, Group F, Group G, and Group H. Beta-hemolysis is identified by its yellow and transparent appearance on the cultured media.^[5]

Clinical identification of beta-hemolytic streptococci relies on culturing the bacteria with agar media that has been supplemented with blood.^[2] This method allows for beta-hemolysis to be easily identified, which is a critical step in further identification tactics.^[2] Identification into subgroups can be done by the Lancefield antigen-determination test which uses antibodies to distinguish B-hemolytic streptococci into different species.^[2]^[7] An additional method used to identify B-hemolytic streptococci is the PYR test, which is primarily used in distinguishing S. pyogenes from other B-hemolytic strains by testing for the presence of pyrrolidonyl aminopeptidase.^[2] Both the Lancefield antigen grouping sera and PYR test are widely available for commercial usage. Each method presents its limitations and studies suggest that a combination of the two protocols be used to achieve the most reliable results.^[2]

Group A

Group A streptococcal infections are predominantly caused by S. pyogenes. Human pathologies are mostly associated with Group A streptococci and arise most often as respiratory or skin infections.^[8]

Group A streptococcal infections include:

The identification and diagnosis of these conditions often require a combination of bacteriologic methods with other clinical identification characteristics that are condition-specific.^[8]

Group B

Group B streptococcal infections, most commonly associated with S. agalactiae , are extremely prevalent among pregnant women, newborns, and the elderly. Cattle have also been shown to be important reservoir hosts for S. agalactiae. Reports of S. agalactiae have also been identified in several other mammals, fish, and reptiles.^[4]

Economic impacts and considerations

Streptococcosis has been shown to have serious consequences on Aquaculture industries around the world as a result of various streptococcal-based infections in marine and freshwater organisms.^[9] Streptococosis in fish specifically has proven to be a public health concern due to the zoonotic capabilities of streptococcal infections and diseases.^[9] Mitigating streptococcosis in marine and freshwater organisms, has the potential to improve the economics of the aquaculture sector and decrease the risks of human illness.

Traditionally, antibiotics and other chemotherapeutic drugs have been used to combat streptococcosis infections in aquaculture settings.^[9] However, re-infection rates, drugs accumulating in aquatic ecosystems, demand for chemical-free aquaculture products, and the diversity of species and strains within the Streptococcus genus has proven to be a major challenge.^[9] Since re-infection rates among fish populations are high, multiple treatments are often needed which introduces an additional problem of increased antibiotic resistance.^[9] In search of alternative solutions, current research is investigating the possibility of using dietary supplements or medicinal herbs and other plants as alternatives to antibiotics, and recent findings have generated promising results.^[9]

The existing literature has placed a strong emphasis on the economic impacts of streptococcosis in tilapia cultures.^[10] Tilapia have rapid growth rates, exhibit tolerance to numerous environmental conditions, and are available globally which causes the species to be of major importance in the global aquaculture sector.^[10] Tilapia production is often conducted by large-scale producers in intensive systems, which increases their susceptibility to disease and infection due to the density of cultures and subsequent water quality issues.^[10] Streptococcosis has been identified as the most important pathogen affecting these systems and has caused considerable economic losses to the industry.^[11] In general, preventing disease and infection should be a priority compared to simply controlling and mitigating outbreaks.^[10] Research acknowledges that disease prevention may be possible by utilizing effective biosecurity measures at both global and local levels.^[10] In addition, recent studies have found several benefits of using medicinal herbs to treat streptococcosis in aquaculture. Studies suggest that a combination of vaccines, antibiotics, and phytotherapy may be the most viable solution to improve both the economics of the industry and mitigate public health concerns.^[9] Considerations and adjustments will have to made depending on national regulations, the countries economic status, and the farms production capacity.^[10]

Epidemiology

Host range

Streptococcosis encompasses a spectrum of diseases caused by bacteria from the genera Streptococcus and Lactococcus .^[12] Various species within these genera can cause infections in both wild and cultured animals, including fish and terrestrial species.

Commonly affected organisms include:

Fish species: Streptococcus iniae , Streptococcus agalactiae , Streptococcus dysgalactiae , Lactococcus garvieae , Lactococccus piscium, and Streptococcus parauberis have a significant impact in aquaculture, impacting freshwater, marine, and brackish water species.^[13] Among these L. garvieae, S. iniae, and S. parauberis are considered the primary causative agents responsible for diseases in marine aquaculture among the streptococcal bacteria affecting fish.^[13]^[12]

Terrestrial animals: Streptococcus agalactiae, commonly found in cattle and dromedary camels, has been detected in numerous species, including small ruminants, llamas, horses, and marine mammals, often associated with human sources.^[14]Streptococcus dysgalactiae primarily infects cattle but also affects small ruminants, pigs, dogs, horses, and vampire bats. Streptococcus equi subsp. zooepidemicus, prevalent in horses, is also present in guinea pigs, pigs, monkeys, and various other animals, including dogs, cats, ferrets, and birds.^[15] Additionally, Streptococcus suis mainly affects suids but can be found in other animals like cattle, sheep, goats, and chickens, with different genotypes found in rabbits and chickens compared to pigs.^[14]^[15]

Humans: Streptococcal infections in humans are primarily caused by Streptococcus pyogenes, the most common beta-hemolytic group A streptococcus, often referred to simply as group A streptococcus.^[14] Similarly, group B streptococcus typically denotes Streptococcus agalactiae, although minor beta-hemolytic group B streptococci like S. troglodytidis exist.^[15] While most streptococcal illnesses in humans originate from species adapted to humans, such as S. pneumoniae or S. pyogenes, there are zoonotic species capable of causing infections.^[15] These include S. canis, S. dysgalactiae subsp. dysgalactiae, S. equi subsp. zooepidemicus, S. halichoeri, S. iniae, and S. suis, along with some animal-associated genotypes of S. agalactiae.^[16]^[17] Notably, some streptococci found in animals may infect humans under certain circumstances. Fish-associated S. agalactiae, primarily affecting farmed freshwater and marine fish, have also been implicated in human illnesses, particularly the ST283 genotype.^[17] The prevalence of specific S. suis serotypes varies by region, impacting disease incidence in both pigs and humans.

Transmission routes

Members of the Streptococcus genus are frequently found as part of the normal microbial community in both animals and humans, commonly inhabiting sites such as the upper respiratory tract, urogenital tract, mucous membranes, mammary glands, or skin.^[18] While these organisms can occasionally cause infections as primary pathogens, they more commonly act as opportunistic pathogens, particularly in carriers.^[19] However, their transmission between hosts does not always lead to disease manifestation.^[19] Streptococci are typically transmitted through close contact, though aerosols may sometimes play a role. Some species, such as S. suis, S. equi subsp. zooepidemicus, and S. agalactiae ST283, can be acquired through the consumption of undercooked pork, horsemeat, or fish, respectively, or via unpasteurized dairy products. S. iniae infections in humans often occur through skin abrasions during fish cleaning. The mode of transmission among fish is not fully elucidated but can occur orally or through exposure to contaminated water baths, particularly in laboratory settings. Streptococci can also be transmitted through fomites and can persist in the environment for varying durations, especially in organic material under moist, cool conditions. For instance, S. suis can remain viable for approximately a week in pig feces at 25 °C (77 °F) and up to six weeks in carcasses at 4 °C (39 °F).^[19]

Geographic distribution

The strains of Streptococcus, including S. canis, S. dysgalactiae subsp. dysgalactiae, S. equi subsp. zooepidemicus, S. suis, and mammalian S. agalactiae, maintained in domestic animals are widely distributed and their presence follows the hosts that they reside in.^[20] Regional variations in the predominant serotypes of S. suis may impact disease prevalence in both pigs and humans. S. iniae infections have predominantly been documented in regions such as North America, the Caribbean, parts of Asia (such as Japan, China, Singapore, and Taiwan), Australia, and the Middle East. Meanwhile, occurrences of S. halichoeri have been reported in certain parts of Europe and South Korea, with potential wider distribution.^[20]^[21] Notably, S. agalactiae ST283 appears to be primarily found in Asia but has recently been identified in farmed fish in South America.^[20]^[21]

Related Research Articles

Streptococcus is a genus of gram-positive coccus or spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales, in the phylum Bacillota. Cell division in streptococci occurs along a single axis, thus when growing they tend to form pairs or chains, which may appear bent or twisted. This differs from staphylococci, which divide along multiple axes, thereby generating irregular, grape-like clusters of cells. Most streptococci are oxidase-negative and catalase-negative, and many are facultative anaerobes.

Group A streptococcal infections are a number of infections with Streptococcus pyogenes, a group A streptococcus (GAS). S. pyogenes is a species of beta-hemolytic Gram-positive bacteria that is responsible for a wide range of infections that are mostly common and fairly mild. If the bacteria enter the bloodstream an infection can become severe and life-threatening, and is called an invasive GAS (iGAS).

Streptococcus pyogenes is a species of Gram-positive, aerotolerant bacteria in the genus Streptococcus. These bacteria are extracellular, and made up of non-motile and non-sporing cocci that tend to link in chains. They are clinically important for humans, as they are an infrequent, but usually pathogenic, part of the skin microbiota that can cause Group A streptococcal infection. S. pyogenes is the predominant species harboring the Lancefield group A antigen, and is often called group A Streptococcus (GAS). However, both Streptococcus dysgalactiae and the Streptococcus anginosus group can possess group A antigen as well. Group A streptococci, when grown on blood agar, typically produce small (2–3 mm) zones of beta-hemolysis, a complete destruction of red blood cells. The name group A (beta-hemolytic) Streptococcus is thus also used.

The viridans streptococci are a large group of commensal streptococcal Gram-positive bacteria species that are α-hemolytic, producing a green coloration on blood agar plates, although some species in this group are actually γ-hemolytic, meaning they produce no change on blood agar. The pseudo-taxonomic term "Streptococcus viridans" is often used to refer to this group of species, but writers who do not like to use the pseudotaxonomic term prefer the terms viridans streptococci, viridans group streptococci (VGS), or viridans streptococcal species.

Streptococcus suis is a peanut-shaped, Gram-positive bacterium, and an important pathogen of pigs. Endemic in nearly all countries with an extensive pig industry, S. suis is also a zoonotic disease, capable of transmission to humans from pigs.

Streptococcus agalactiae is a gram-positive coccus with a tendency to form chains. It is a beta-hemolytic, catalase-negative, and facultative anaerobe.

Hemolysis is the breakdown of red blood cells. The ability of bacterial colonies to induce hemolysis when grown on blood agar is used to classify certain microorganisms. This is particularly useful in classifying streptococcal species. A substance that causes hemolysis is a hemolysin.

Group B streptococcal infection, also known as Group B streptococcal disease or just Group B strep infection, is the infectious disease caused by the bacterium Streptococcus agalactiae. Streptococcus agalactiae is the most common human pathogen belonging to group B of the Lancefield classification of streptococci—hence the name of group B stretococcal (GBS). Infection with GBS can cause serious illness and sometimes death, especially in newborns, the elderly, and people with compromised immune systems. The most severe form of group B streptococcal disease is neonatal meningitis in infants, which is frequently lethal and can cause permanent neuro-cognitive impairment.

Anti-streptolysin O is the antibody made against streptolysin O, an immunogenic, oxygen-labile streptococcal hemolytic exotoxin produced by most strains of group A and many strains of groups C and G Streptococcus bacteria. The "O" in the name stands for oxygen-labile; the other related toxin being oxygen-stable streptolysin-S. The main function of streptolysin O is to cause hemolysis —in particular, beta-hemolysis.

The CAMP test (Christie–Atkins–Munch-Peterson) is a test to identify group B β-hemolytic streptococci based on their formation of a substance that enlarges the area of hemolysis formed by the β-hemolysin elaborated from Staphylococcus aureus.

The Streptococcus anginosus group (SAG), also known as the anginosus group streptococci (AGS) or the milleri group streptococci (MGS), are a group of several species of streptococci with clinical similarities. The group is named after a principal member species, Streptococcus anginosus. The older name Streptococcus milleri is now pseudotaxonomic, as the idea that these streptococci constituted a single species was incorrect. The anginosus group streptococci are members of the viridans streptococci group. They have been implicated as etiologic agents in a variety of serious purulent infections, but because of their heterogeneous characteristics, these organisms may be unrecognized or misidentified by clinical laboratorians. The unique characteristic of them from other pathogenic streptococci, such as S. pyogenes and S. agalactiae, is their ability to cause abscesses.

<i>Streptococcus canis</i> Species of bacterium

Streptococcus canis is a group G beta-hemolytic species of Streptococcus. It was first isolated in dogs, giving the bacterium its name. These bacteria are characteristically different from Streptococcus dysgalactiae, which is a human-specific group G species that has a different phenotypic chemical composition. S. canis is important to the skin and mucosal health of cats and dogs, but under certain circumstances, these bacteria can cause opportunistic infections. These infections were known to afflict dogs and cats prior to the formal description of the species in Devriese et al., 1986. However, additional studies revealed cases of infection in other mammal species, including cattle and even humans. Instances of mortality from S. canis in humans are very low with only a few reported cases, while actual instances of infection may be underreported due to mischaracterizations of the bacteria as S. dysgalactiae. This species, in general, is highly susceptible to antibiotics, and plans to develop a vaccine to prevent human infections are currently being considered.

Streptococcus dysgalactiae is a gram positive, beta-haemolytic, coccal bacterium belonging to the family Streptococcaceae. It is capable of infecting both humans and animals, but is most frequently encountered as a commensal of the alimentary tract, genital tract, or less commonly, as a part of the skin flora. The clinical manifestations in human disease range from superficial skin-infections and tonsillitis, to severe necrotising fasciitis and bacteraemia. The incidence of invasive disease has been reported to be rising. Several different animal species are susceptible to infection by S. dysgalactiae, but bovine mastitis and infectious arthritis in lambs have been most frequently reported.

Streptococcus zooepidemicus is a Lancefield group C streptococcus that was first isolated in 1934 by P. R. Edwards, and named Animal pyogens A. It is a mucosal commensal and opportunistic pathogen that infects several animals and humans, but most commonly isolated from the uterus of mares. It is a subspecies of Streptococcus equi, a contagious upper respiratory tract infection of horses, and shares greater than 98% DNA homology, as well as many of the same virulence factors.

Streptococcus dysgalactiae subsp. equisimilis is a species of Streptococcus, initially described by Frost in 1936. As a result of several DNA hybridization studies in 1983, the species was merged into Streptococcus dysgalactiae. Subsequently, S. dysgalactiae was divided into the subspecies Streptococcus dysgalactiae subsp. equisimilis and Streptococcus dysgalactiae subsp. dysgalactiae. Although the name Streptococcus equisimilis is no longer valid, it is still encountered both in clinical practice, and in scientific journals.

Streptococcus iniae is a species of Gram-positive, sphere-shaped bacterium belonging to the genus Streptococcus. Since its isolation from an Amazon freshwater dolphin in the 1970s, S. iniae has emerged as a leading fish pathogen in aquaculture operations worldwide, resulting in over US$100M in annual losses. Since its discovery, S. iniae infections have been reported in at least 27 species of cultured or wild fish from around the world. Freshwater and saltwater fish including tilapia, red drum, hybrid striped bass, and rainbow trout are among those susceptible to infection by S. iniae. Infections in fish manifest as meningoencephalitis, skin lesions, and septicemia.

Perianal cellulitis, also known as perianitis or perianal streptococcal dermatitis, is a bacterial infection affecting the lower layers of the skin (cellulitis) around the anus. It presents as bright redness in the skin and can be accompanied by pain, difficulty defecating, itching, and bleeding. This disease is considered a complicated skin and soft tissue infection (cSSTI) because of the involvement of the deeper soft tissues.

Lancefield grouping is a system of classification that classifies catalase-negative Gram-positive cocci based on the carbohydrate composition of bacterial antigens found on their cell walls. The system, created by Rebecca Lancefield, was historically used to organize the various members of the family Streptococcaceae, which includes the genera Lactococcus and Streptococcus, but now is largely superfluous due to explosive growth in the number of streptococcal species identified since the 1970s. However, it has retained some clinical usefulness even after the taxonomic changes, and as of 2018, Lancefield designations are still often used to communicate medical microbiological test results.

Granada medium is a selective and differential culture medium designed to selectively isolate Streptococcus agalactiae and differentiate it from other microorganisms. Granada Medium was developed by Manuel Rosa-Fraile et al. at the Service of Microbiology in the Hospital Virgen de las Nieves in Granada (Spain).

Granadaene is the trivial name of a polyene of non-isoprenoid biosynthetic origin, that constitutes the red pigment characteristic of Streptococcus agalactiae.

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