Invasive group A streptococcal infection

Group A Streptococcus (GAS, Streptococcus pyogenes, Strep A) is a major cause of morbidity and mortality globally. GAS infections are estimated to account for almost 200,000 deaths globally each year [1]. These deaths are primarily due to invasive infections and serious immunological sequalae such as acute rheumatic fever, rheumatic heart disease and post-streptococcal glomerulonephritis [2].

In Aotearoa New Zealand, GAS infections cause a significant health and economic burden. Each year, GAS infections affect about 1.5% of the population, and results in approximately 34 deaths (31–32 deaths due to invasive infections and 2–3 deaths due to rheumatic heart disease [3]. Rates of invasive GAS (iGAS) infections are highest among Māori and Pacific peoples which have historically been several times higher than in other ethnic groups. Rates of iGAS are also higher in those living in areas of high socioeconomic deprivation [4]. Rates of iGAS infection decreased in Aotearoa during the early years of the COVID-19 pandemic (2021–2022) [5] before rising again in 2023 [6] to mirror the increase of iGAS infections seen globally [7].

A new case definition for iGAS was developed with advice from the iGAS Clinical and Technical Advisory Group for implementation from 1 October 2024, to coincide with iGAS becoming a notifiable disease in Aotearoa. Prior to 1 October 2024, iGAS surveillance in Aotearoa was passive, and consisted of clinical microbiology laboratories around the country voluntarily sending invasive GAS isolates to the national Public Health reference laboratory – the Institute of Environmental Science and Research (ESR) - for recording, analysis, and publication of periodic reports. The transition to iGAS as a notifiable disease with a new case definition changes the way iGAS case information is collected and analysed and will likely improve case detection and iGAS surveillance in Aotearoa.

Detailed information on the epidemiology of iGAS in Aotearoa is available on the ESR website.

Higher rates of iGAS are seen in these demographic groups [8].

• Māori
• Pacific peoples
• People aged 75 years or over (or Māori and Pacific peoples aged 60 or over) [9].
• Children aged under 5 years
• People experiencing socioeconomic deprivation

As of September 2024, we are unable to comment on the risk for disabled people as a priority group, as there is no specific data available.

Medical risk factors include:

• being peripartum, in particular, increased risk between a birthing-person^a and neonate in the first 28 days of life (estimated relative risk is 12 times higher than other close contacts) [9-10]
• immunocompromising conditions (such as those receiving chemotherapy or high-dose corticosteroids)
• chronic wounds or skin breakdown, including chronic ulcers, burns, eczema or a varicella (chickenpox) infection with active lesions [11-14]
• chronic disease (particularly diabetes or heart disease)
• acute viral respiratory infections, particularly influenza [15]
• other Group A Streptococcus (GAS) infections (skin, throat, scarlet fever)

^a ‘Birthing-person’ refers to someone who gives birth, regardless of their gender identity, which may be female, male, nonbinary, or other, and regardless of their relationship with the neonate (e.g. surrogate pregnancy).

Environmental and occupational risk factors include:

• being a household or household-like contact of an iGAS case (estimated relative risk is 19–2,000) [8-9,16] see contact definitions
• being a resident or attendee of institutional settings including childcare centres, aged care or residential care facilities, hospitals, schools and prisons
• experiencing structural or functional household crowding
• experiencing homelessness
• injecting drugs [17-19].

Group A Streptococcus (GAS, Streptococcus pyogenes, Strep A) is a species of bacteria. When grown in the laboratory on blood agar plates (a common bacterial growth medium), GAS breaks down (lyses) the red blood cells in the agar, resulting in clear colourless zones around the area of bacterial growth. This is termed beta-haemolysis (β-haemolysis, complete red cell lysis). Bacteria of the Streptococcus genus that exhibit β-haemolysis are grouped based on the composition of their cell walls using the Lancefield classification system. Streptococcus pyogenes is grouped as ‘A’ in the Lancefield system and is thus commonly called Group A Streptococcus (GAS) [20].

The clinical spectrum of GAS-related disease is broad, and includes:

• asymptomatic colonisation/carriage of the skin and throat
• invasive and non-invasive infections of varying severity, which may be accompanied by toxin-mediated disease such as streptococcal toxic shock syndrome and scarlet fever
• post-infectious immune sequelae such as acute rheumatic fever, rheumatic heart disease and post-streptococcal glomerulonephritis.

Invasive group A streptococcal (iGAS) infections occur when GAS causes infection in a normally sterile site within the body (i.e. an anatomical location in which bacteria are not normally present in a healthy person).

Invasive infections require microbiological confirmation (growth or detection of GAS in a clinical specimen) and can affect any part of the body, meaning they can present clinically in many ways. Examples of GAS infections and infectious syndromes that are considered invasive when accompanied by appropriate microbiological confirmation are [21]:

• bloodstream infections – bacteraemia, sepsis/septicaemia, septic shock
• skin and soft tissue infections – cellulitis, necrotising fasciitis
• bone and joint infections – osteomyelitis, septic arthritis
• lower respiratory tract infections – pneumonia, empyema
• central nervous system infections – meningitis.

Human reservoir. As a human-adapted pathogen, Group A Streptococcus (GAS) survival requires an unbroken cycle of human-to-human transmission resulting in adherence to the primary infection site (skin or throat), colonisation/carriage (with or without infection), and onwards transmission to a new human host [22].

GAS is generally considered to be spread from person to person by direct contact (i.e. contact with infected/colonised skin, body surfaces, or exudates) or by droplet transmission (i.e. contact with exhaled respiratory droplets from an individual with GAS pharyngitis). Transmission via indirect contact with contaminated surfaces (fomites) may be a possible less common route of transmission [23].

The incubation period for non-invasive GAS generally ranges between 1–3 days (GAS pharyngitis) and between 7–10 days (GAS impetigo).

The incubation period of iGAS is not well defined.

In rare instances, secondary cases of iGAS infections may be identified up to 30 days after the identification of the initial case [24].

For contact tracing purposes, the infectious period for iGAS cases begins 7 days prior to symptom onset.

The infectious period for iGAS ends 24 hours after the commencement of appropriate antimicrobial therapy. The choice of appropriate antimicrobial therapy should be determined by specialist infectious disease or microbiology advice as well as local antimicrobial guidelines [25].

Standard precautions apply. This includes hand hygiene with alcohol-based hand sanitiser or soap and water, usual cleaning, and both mask-wearing and eye protection when examining the mouth, throat or nose of a patient with an undifferentiated illness.

In addition, contact and droplet precautions apply until the end of the infectious period.

For more information, see Appendix 6: Infection, prevention and control guidance

Institutional settings can reduce the risk of iGAS infections by following infection prevention and control (IPC) practices and encouraging basic hygiene practices. IPC guidance can be found on the Health New Zealand website.

Primary prevention should focus on the early detection and treatment of GAS infections (throat and skin infections) in line with clinical practice guidelines.

Improving social determinants of health (including housing and access to healthcare) can reduce the risk of infection in priority populations.

Confirmed case: A person who meets the laboratory definitive criteria.

Probable case: A person who has an illness which meets the clinical criteria AND laboratory suggestive criteria.

Under investigation: A person who has been notified to the medical officer of health, but information is not yet available to classify them further.

Not a case: A person who has been investigated and subsequently found not to meet the case definition.

Clinical suggestive evidence is only required as part of classification of probable cases alongside laboratory suggestive evidence.

Clinical suggestive evidence

Clinical presentation consistent with iGAS infection as a cause of:

• Peripartum infection – Peripartum infection refers to a clinically suspected bacterial infection of the genital tract or its surrounding tissues, occurring at any time between the onset of rupture of membranes or labour, and the 28th day postpartum (e.g. chorioamnionitis, endometritis, maternal/puerperal sepsis). This definition of peripartum infection has been adapted for use in this context from the WHO definition of ‘maternal peripartum infection’ [26].
• Neonatal sepsis – A neonate is a child under 28 days of age (i.e. in the first 28 days of life) as per WHO) [27].

No epidemiological criteria are associated with case classification for iGAS.

Laboratory definitive criteria

Isolation of Group A Streptococcus (GAS, Streptococcus pyogenes, Strep A) by culture or detection by nucleic acid testing in a clinical specimen from a normally sterile site.

See the microbiological specimen sterility guidelines for guidance on classification of specimens.

Laboratory suggestive criteria

Isolation of GAS by culture or detection by nucleic acid testing in a clinical specimen from a normally non-sterile site.

See the microbiological specimen sterility guidelines for guidance on classification of specimens.

At the time of presentation invasive group A streptococcal infection (iGAS) is generally indistinguishable from other severe bacterial infections. Laboratory testing of people with severe bacterial infections, such as iGAS, is undertaken to establish the causative organism(s) and direct appropriate antimicrobial treatment.

If the causative organism is found to be group A streptococcus (GAS), then laboratory testing is used to classify the GAS infection as invasive or non-invasive based on the specimen type and anatomical site of the infection (see iGAS case definition). Invasive infection-causing strains of GAS that are grown in the laboratory can be further characterised for surveillance purposes.

People with severe bacterial infections such as iGAS should generally be managed in a hospital setting. Appropriate diagnostic investigations will be directed by the responsible clinical team, with guidance from clinical microbiology and/or infectious diseases as required. Public health does not have a role in initiating testing for iGAS.

Interpretation of laboratory testing results is used to determine whether a microbiologically confirmed GAS infection meets the case definition for iGAS, as per microbiological specimen sterility guidelines (see iGAS case definition).

If after consulting the microbiological specimen sterility guidelines, the specimen classification remains unclear, the result(s) should be discussed with clinical microbiology or infectious diseases.

Bacterial culture

GAS is generally detected in microbiological specimens via bacterial culture methods (growth in the laboratory using specialised incubation conditions and bacterial growth media). GAS generally grows in culture after 18–24 hours of incubation, with final bacterial identification results (+/- antimicrobial susceptibility results) generally available within 48–72 hours of the specimen reaching the laboratory. Antimicrobial susceptibility testing is generally performed and reported for GAS isolates from sterile site specimens.

Nucleic acid testing

Nucleic acid testing, such as polymerase chain reaction (PCR) involves direct detection of GAS deoxyribonucleic acid (DNA) in a clinical specimen. While nucleic acid testing is increasingly being used to detect GAS in throat swabs (a non-sterile site specimen), only a limited number of laboratories routinely use nucleic acid testing to detect GAS in sterile site specimens. GAS that is detected in a specimen using nucleic acid testing but fails to grow in culture cannot undergo antimicrobial susceptibility testing or further genomic characterisation.

Genomic characterisation

GAS genomic characterisation is crucial for surveillance purposes, outbreak investigations and epidemiological studies, and to understand the spread of GAS strains causing invasive infections. Commonly used methods to genomically characterise GAS isolates are sequencing of the emm gene (emm typing) or whole genome sequencing.

Laboratories undertaking testing must notify the local medical officer of health of all confirmed cases of invasive group A streptococcal infection. 

Attending health practitioners must immediately notify the local medical officer of health for both confirmed cases and probable cases of invasive group A streptococcal infection causing peripartum infection or neonatal sepsis as described in the clinical criteria.

Ensure complete case information is entered into EpiSurv.

National escalation for cases of iGAS is only required in the event of an outbreak. If there is an outbreak, the local medical officer of health should first discuss with their regional clinical director (if available) and the on-call protection clinical medical officer of health to make a decision about national escalation. 

See Appendix 5 for further information on escalation pathways. 

There are no routine international reporting requirements.

Follow-up of confirmed and probable cases of iGAS in peripartum people, or neonates should be treated as a high priority. In these instances, public health action should be initiated within 24 hours. 

In all other instances, public health action should be initiated within 72 hours.

Due to the severity of iGAS, cases are likely to be in hospital being supported by their treating team and other hospital services. Follow-up action by public health services primarily relates to contact management. See Contact management: Manaaki and wellbeing for further detail.

Confirmed or probable iGAS cases do not require source investigation.

Public health services should ask whether the case:

• is resident at any institution to support outbreak identification
• attends a school or early childcare setting and questions to identify the high-risk settings described in exposure event management.

Public health action is focused on contact management only. For further information, see contact management.

iGAS cases should be isolated until 24 hours following the start of appropriate antimicrobial treatment. Precautions are as outlined in infection prevention and control.

Treatment for a confirmed or probable iGAS case will be determined by the treating team with specialist infectious disease and/or clinical microbiology advice where possible.

Cases should be provided with appropriate information by their treating team.

An early childcare, te kōhanga reo, kura, or school setting with one case of iGAS AND evidence of ongoing GAS infections, chickenpox or influenza transmission.

In this situation, the risk of further iGAS cases is higher than in other contexts.

After conducting a risk assessment, public health services should consider the following:

• Whether any control measures including those outlined in the outbreaks section are appropriate.
• Chemoprophylaxis can be considered in certain circumstances, based on a risk assessment. Factors to considered include:
  • evidence of co-circulation chickenpox or respiratory viral infections alongside GAS infections (e.g. scarlet fever, impetigo, strep throat).
• Have a low threshold for seeking advice/support from the national team.
• In settings with co-circulating chickenpox, consider post-exposure prophylaxis with varicella vaccination for children under 5 years:
  • within 3 days of exposure
  • children from 9 months of age and staff with no clear history of chickenpox could be offered 2 doses of varicella vaccine, 4 to 8 weeks apart.

Flight contact tracing

Flight contact tracing of infectious iGAS cases, even on long-haul flights, is not required.

There are no actions for exposures in low-risk settings.

For the purposes of public health follow-up, the following groups are considered close contacts of an iGAS case. 

Birthing-person – neonate pairs

Definition: Where either the birthing-person or neonate develops iGAS disease during the peripartum period (any time between the onset of rupture of membranes or labour, and the 28th day postpartum).

A‘birthing-person’ is someone who gives birth, regardless of their gender identity, which may be female, male, nonbinary, or other, and regardless of their relationship with the neonate (e.g. surrogate pregnancy).

A neonate is defined as a child under 28 days of age (i.e. in the first 28 days of life) as per WHO).

Key actions:

1. Prophylaxis: Routine chemoprophylaxis recommended for the close contact within the birthing-person—neonate pair. Administer antibiotics as soon as possible, ideally within 48 hours of exposure or notification.
2. Provide iGAS information sheet.

Household or household-like contacts

Definition: Any person who spent at least 24 hours (cumulative) in the same household or household-like setting with the case during the case’s infectious period (7 days prior to symptom onset) or had sexual relations, or other intimate contact with a case during the case’s infectious period.

Chemoprophylaxis is generally not recommended for household contacts. However, individuals with additional risk factors may be considered for chemoprophylaxis on a case-by-case basis.

Key actions:

1. Provide iGAS information sheet.

Airway exposed healthcare workers

Definition: Healthcare workers who have had unprotected (i.e. without wearing a procedural mask or other suitable PPE) close exposure of their airway to respiratory droplets of an iGAS case with a respiratory manifestation (i.e. iGAS infection of the upper or lower respiratory tract infection including the lungs, airway, pharynx, nose or mouth) during airway management (e.g. suctioning, intubation), or mouth-to-mouth resuscitation within the infectious period.

Chemoprophylaxis is generally not recommended for airway exposed healthcare workers. However, individuals with additional risk factors may be considered for chemoprophylaxis on a case-by-case basis.

Key actions:

1. Public health services should inform hospital and/or ambulance infection prevention and control (IPC) and occupational health services of iGAS cases with respiratory manifestations (i.e. iGAS infection of the upper or lower respiratory tract infection including the lungs, airway, pharynx, nose or mouth) wherever possible to enable appropriate awareness and follow-up by these services.
2. IPC and occupational health should provide the iGAS information sheet or equivalent contact health information to identified contact meeting this definition.
3. For airway exposures in non-respiratory manifestations of iGAS, no routine follow-up is recommended, although IPC/occupational health may choose to provide contact health information to healthcare workers in this setting.

A manaaki-centred approach to follow-up with close contacts is crucial to establishing relationships and trust between public health services, cases, and their contacts. For further details on this approach, see Equity chapter.

As iGAS is a severe illness, close contacts are likely to be distressed. Cases are likely to be cared for in high pressure intensive care settings, and some illnesses will result in death. The following approaches should be considered when following up close contacts of an iGAS case.

• Understanding the needs and priorities of whānau and friends of the case first will help with finding the most effective approach to follow up.
• Allow plenty of time to connect and communicate with close contacts. It may be more difficult for them to process information and make decisions at this time.

When following up with people who have recently given birth, it is recommended to link with their lead maternity carer to provide additional support.

The priorities for public health follow-up of close contacts and recommended timeframes, if resources are available identify close contacts that require:

• chemoprophylaxis and provide information within 48 hours
• require information within 3 days.

On notification of a probable or confirmed case of iGAS, all people meeting the close contact definition should be identified and contacted for provision of health information where possible and for birthing-person—neonate pairs, ensuring provision of chemoprophylaxis.

Symptomatic close contacts should be provided the information sheet for guidance on seeking medical attention for symptoms.

Contacts of a confirmed or probable iGAS case are not recommended to quarantine under typical circumstances.

Chemoprophylaxis is only recommended routinely for birthing-person—neonate pairs, where either individual has been diagnosed with an iGAS infection from the peripartum period up to 28 days after the birth.

Routine provision of antibiotics for chemoprophylaxis to all close contacts of a single case is generally not recommended, as evidence of the efficacy of this strategy in preventing secondary cases is limited [28].

Close contacts with additional risk factors may be considered on a case-by-case basis for chemoprophylaxis by the responsible medical officer of health. See Additional information.

Antibiotics for chemoprophylaxis should be given to eligible close contacts of a single case of iGAS as soon as possible after the close contact is identified, preferably within 48 hours of exposure to the index case, or, at least, within 48 hours of the case being notified. There is limited evidence to support giving chemoprophylaxis beyond 10 days of diagnosis of the iGAS index case.

Table 1 describes appropriate antibiotic prophylaxis regimens. 

Table 1 Antibiotic prophylaxis regimens

All close contacts should be provided with an information sheet about iGAS as soon as possible. This information may be disseminated by the case, next of kin or the treating team.

Information in the information sheet should be readily adapted for the cultural and literacy needs of recipients, but all information sheets provided to contacts should include the following information and advice.

• Close contacts may be at higher risk of developing an iGAS infection for up to 30-days following their last contact with a case.
• Monitor for symptoms of GAS and iGAS.
• If symptoms of non-invasive GAS infection develop (e.g. impetigo, pharyngitis):
  • contacts should seek non-urgent medical review
  • information provided should also include specific messaging to groups at high risk of acute rheumatic fever as a sequela of streptococcal pharyngitis.
• If symptoms of iGAS develop, contacts should seek immediate medical attention and inform the medical practitioner that they are a contact of an iGAS case.

Household cluster

Where two or more cases of iGAS occur in the same household or household-like setting within 30 days of symptom onset in the initial case.

Institutional settings

• Confirmed cluster: two or more cases within an institution that occur within a 3-month period, where cases are both epidemiologically and genomically linked and are not household or household-like contacts of each other.
• Possible/suspected cluster: two or more cases within an institution that occur within a 3-month period that are epidemiologically linked, where cases are not household or household-like contacts of each other.

Epidemiological link: Where cases occur in a physical or geographical context and a plausible mode of transmission accounts for infection spreading between people.

Institutions can include, but are not limited to:

• childcare centres
• schools
• aged care or residential care facilities
• prisons
• hostels
• military barracks
• hospitals (hospitals take the lead in managing outbreaks within their facilities with support from the public health service. This includes conducting source investigation and following up with staff, patients, and visitors to prevent further spread.)

Household clusters

The entire household should be offered antibiotics for chemoprophylaxis as outlined in Contact management: Prophylaxis and should be provided with a contact information sheet. People ordinarily resident in the affected household should be considered part of this group regardless of whether they were identified as a close contact of a case.

The initial household case does not require additional chemoprophylaxis if a secondary case is identified following their initial course of antibiotics.

The administration of antibiotics for chemoprophylaxis to the household should not be delayed while waiting for genomic characterisation.

Institutional settings

It is preferable that a cluster be confirmed via genomic characterisation prior to initiating a public health management response.

A public health response may be initiated prior to receiving results for a suspected cluster or outbreak if:

• there is strong epidemiological evidence of a cluster
• staff members, residents or other attendees of an institution belong to a group at increased risk of infection (see At risk and priority populations).

Key considerations for iGAS outbreak management in institutional settings are described below.

• Consider whether regional or national escalation is appropriate. See Appendix 5: Escalation pathways
• Consider convening an Incident Management Team for coordinating outbreak management.
• Based on a risk assessment, medical officers of health should consider supporting provision of antibiotics for chemoprophylaxis to all asymptomatic residents and staff when: [29-30]
  • a cluster or outbreak is confirmed via genomic characterisation
  • there is clear and strong epidemiological evidence of transmission within the facility or a subdivision of the facility (with little to no crossover of staff and residents).
• Screening asymptomatic people for GAS carriage through testing is not routinely recommended.
• Public health services should work with institutional management to:
  • distribute health information
  • review cleaning, hygiene, and infection prevention and control practices
  • implement appropriate transmission-based precautions if caring for cases onsite
  • establish enhanced surveillance of the facility for further linked cases for at least 3 months after the most recent case was diagnosed, by ensuring that management are aware of iGAS symptoms and have a process by which they can notify the public health service of further cases.

Other community clusters

Clustering of iGAS cases in community settings outside institutions and healthcare settings present distinct challenges for public health response. Community clusters can occur in a range of settings and contexts, including universities, sports clubs (where the sport involves close contact), among people who inject drugs and among people experiencing homelessness.

Key considerations for community clusters of iGAS in people who inject drugs or people experiencing homelessness are described below.

• Stigma, marginalisation, and criminalisation of injecting drug use and homelessness are a challenge to effective engagement, and management approaches should take this into account.
• Public health services should work with community organisations who have established trusted relationships with individuals who may be contacts of the cluster.
• Priority actions when following up these clusters are:
  • identify and follow-up contacts where possible
  • identify whether a case has been linked to sheltered accommodation, a drug service or specific injecting network, prison setting, healthcare setting or other institution in the 7 days prior to onset of symptoms
  • consider providing chemoprophylaxis to close contacts in people who inject drugs or people experiencing homelessness populations who are high-risk (e.g. shared needles with the case or with current open wounds). See Additional information for advice on individual risk assessment.

Decisions to provide antibiotics for chemoprophylaxis to any close contacts of a single case should consider the benefits and risks, including: 

• An individual’s risk factors for developing iGAS, see at risk and priority populations section
• An individual’s risk of other sequelae of GAS infection (e.g. risk of rheumatic fever recurrence in someone with known severe rheumatic heart disease and previous heart valve surgery)
• the specific context of a case (including case severity) and their close contacts
• the risk of side effects as a result of antibiotic treatment, and
• other potential consequences of providing antibiotics to asymptomatic persons (i.e. elimination of protective flora, or the potential to develop antibiotic resistance).

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[24] CDNA National Guidelines for Public Health Units pp.5

[25] BCCDC Guidelines for iGAS pp.2

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[27] WHO Newborn Health Guidelines

[28] Household transmission of Invasive Group A Streptococcus infections in England

[29] CDNA National Guidelines for Public Health Units

[30] UK Health Security Agency Guidelines