Group A Streptococcal Infection in Northern Ireland

1 Background

Group A Streptococcus (GAS) are common bacteria. GAS can be present on or in the human body, without causing infection (colonisation), but may cause skin, soft tissue and respiratory infections, including scarlet fever. GAS infections range from mild to very severe. Invasive GAS (IGAS) occurs when the bacteria proliferate in a normally sterile body site, such as the blood, soft tissues or joints. Streptococcal toxic shock syndrome (STSS) can be a severe complication of IGAS, caused by toxins that are produced by GAS, which act on host cells or tissues.

GAS is spread by close contact between individuals, through respiratory droplets and direct skin contact. GAS can also be transmitted indirectly through contact with inanimate objects, such as towels or bedding. GAS is usually diagnosed by microbiological culture of a specimen from the affected site or by a characteristic illness, such as scarlet fever.

Public and clinician awareness of GAS infections has changed over time, especially during December 2022, which may contribute to changes in trends. IGAS, being more severe, is likely more consistent over time.

Detail of case definitions and methods are at the end of this document. Data were correct at 9am on 23 November 2023.

2 Cumulative number of scarlet fever and IGAS cases, 2022-2023.

The number of cases from the start of 2022 until the report date are shown (Table 2.1).

Table 2.1: Counts of cases of IGAS and scarlet fever, 2022-2023.

	IGAS	Scarlet Fever
	(N=172)	(N=2593)
Infection
Confirmed	157 (91.3%)	214 (8.25%)
Probable	15 (8.72%)	2379 (91.7%)

3 Scarlet Fever

3.1 Trends

Scarlet fever notifications in Northern Ireland have remained relatively low in recent weeks. (Figure 3.1; incident cases are shown by date of recording by PHA). December had the highest number of notifications by month in 2022. Notifications recorded by PHA in January 2023 exceeded those in December 2022, however as cases are reported by date recorded by PHA, not date of onset of symptoms, this is likely to reflect delays in statutory notification process over the Christmas and New Year holiday period. (Figure 3.2). Notifications from February 2023 onwards were much lower than in December or January and a steadily decreasing trend has been observed in the past few months, with the number of notifications in October 2023 similar to that reported in October 2022. The increases noted in December 2022 and January 2023 are likely due to an increase in reporting due to increased awareness as well as a true increase. The reduced number of cases in 2020 and 2021 during the COVID-19 pandemic is likely in part due to reduced social contact and increased use of preventive measures, and therefore lower transmission during that period. The reduction might also partly reflect changes in healthcare seeking or access during that period. Supplementary Table 7.1 shows counts of probable and confirmed scarlet fever notifications recorded by week.

Figure 3.1: Incidence of scarlet fever notifications, by week of notification, all ages, 2022 - 2023.

Please note: in the figure above the most recent week is incomplete

Figure 3.2: Incidence of scarlet fever notifications, by month of notification, all ages, 2016 - 2023.

Please note: in the figure above the most recent month is incomplete

4 Invasive Group A Streptococcal Infection

4.1 Trends

IGAS reports typically follow a seasonal pattern, with cases typically highest in spring. NI experienced unusually high levels in December 2022 and January 2023. (Figure 4.1). The number of monthly cases reported recently (September and October 2023) is comparable to previous years.