No 1/3a - 2025

Infectious Diseases 2024

Even though several respiratory infections such as RS virus and whooping cough were on their way back to a pattern corresponding to the period before the pandemic in 2024, Mycoplasma pneumoniae (MP) again reached epidemic levels this year, and never before have so many infected cases been recorded. An outbreak of slapped cheek syndrome/parvovirus also appeared with high intensity in the aftermath of the COVID-19 pandemic.

On August 14, 2024, an ongoing outbreak of mpox in Africa was declared by WHO as an international threat to public health (PHEIC), which requires a coordinated international effort.

Mpox in Africa – an international threat to public health

Since September 2023, an increase in the number of mpox cases has been observed in the Democratic Republic of Congo (DRC) in central Africa, with spread to several surrounding countries. During 2024 (up until December 8th), 20 countries had reported 13,257 confirmed cases, including 60 deaths. The three countries with the most cases in 2024 are DRC (n = 9,247), Burundi (n = 2,523), and Uganda (n = 925). This is due to the mpox virus variants (clades) called Ia and Ib. Clade Ib is new, seems to be driving part of the epidemic, and is a different variant than the one that caused the global mpox outbreak in 2022–2023 (clade IIb). It appears that clade Ib is more contagious and causes more severe disease than clade IIb. There are thus indications that mpox clade Ib to a greater extent also spreads through other forms of close physical contact than sexual contact, and that more target groups are being infected. Furthermore, more infected and sick children have been observed (likely household transmission). In Burundi and Uganda, primarily clade Ib has been detected, while in DRC, both clade Ia and Ib are circulating. There are still several unknowns regarding transmission routes, contagiousness, and severity, but the mortality of clade Ib seems to be lower than initially assumed.

Statens Serum Institut (SSI) has contributed to the international effort by sending an epidemiologist to DRC. There have been imported cases of clade Ib in Sweden, Germany, Belgium, England, Thailand, India, and the USA, but as of yet, no cases of this variant in Denmark. See International outbreak of mpox  (monkeypox) and the Danish Health Authority’s guidelines.

Surveillance of respiratory infections

In the period after the COVID-19 pandemic, we have seen outbreaks of respiratory infections at unusual times of the year and with great intensity – for example, Respiratory Syncytial Virus (RS virus) and a large epidemic of whooping cough in 2023. Both infections are on their way to normal levels in 2024, while other infections, such as pneumonia caused by MP, again reached epidemic levels this year. Although the infection does not frequently lead to hospitalization, it is troublesome and can cause prolonged coughing (reference). With the unusually high disease activity, there have also been significantly more hospitalizations with complications to the infection in the form of encephalitis and Guillain-Barré syndrome.

Infections in pregnant women

Pregnant women are a risk group for severe courses of many different infections, including influenza and COVID-19, and are therefore offered vaccination. 2024 was the year in which the temporary offer of whooping cough vaccination for pregnant women was made permanent. The purpose of whooping cough vaccination during pregnancy is to protect newborns in the first months of life before the child itself can be vaccinated. 2024 saw a severe infection with Zika virus in a pregnant woman after a holiday in Thailand, EPI-NEWS 40-41a/2024, as well as an outbreak of parvovirus B19 (“slapped cheek disease”), EPI-NEWS 22-24/2024. Parvovirus epidemics are normally seen every 3–4 years but had not been observed since 2017 before the outbreak in 2024. It is often a mild childhood disease, and many will not notice that they are infected. Non-immune pregnant women who are infected with parvovirus can transmit the virus to the child. In the period from January to May 2024, 130 cases were detected in pregnant women, and at least one serious complication was seen in 12% (16/130) of the pregnant women, including fetal anemia, fetal hydrops, the need for fetal transfusion, and miscarriage. See also the Danish Health Authority’s guidelines.

Sexually transmitted infections

After some years with significant increases in the number of chlamydia cases, 2023 saw a significant decrease despite increased testing activity, EPI-NEWS 35/2024. This trend appears to have continued in 2024, and there is no immediate obvious explanation for the decline. The increase in gonorrhea cases similarly appears to have stabilized.

For Mycoplasma genitalium (MG), national figures could be presented for the first time, as MG was included in the laboratory monitoring of infectious diseases. There are very large differences in testing activity between different parts of the country, but generally, the positivity rate is almost as high as for chlamydia. With far more than half of the infections being untreatable with azithromycin, and with the increased testing activity, treatment failures with moxifloxacin, which is recommended for confirmed macrolide resistance, are now more frequently observed. These infections require specialist treatment with antibiotics under individual dispensation. It is therefore important to only test for MG in patients with symptoms.

Foodborne outbreaks in 2024

In the spring of 2024, two Salmonella outbreaks caused by minced beef (from England and Denmark, respectively) were observed, followed by a prolonged outbreak of Listeria in ready-to-eat smoked fish products. After the summer, we had an international outbreak – with more than 150 cases, 22 of which were in Denmark – caused by Salmonella Umbilo in arugula and baby spinach from Italy.

In November 2024, there was also an outbreak of Listeria in fish cakes. In 2022, there was an outbreak with the same type of Listeria, which was also caused by fish cakes from the same producer. Additionally, an outbreak of Salmonella Strathcona from Italian tomatoes, which SSI investigated back in 2011, is worth mentioning because it has again caused a larger international outbreak and pan-European investigation in 2024.

Luise Müller and Steen Ethelberg, both from SSI, received the Fritz Kauffmann award this year for their significant efforts over the years in identifying and investigating disease outbreaks.

Vector-borne diseases in 2024

2024 was the year when Usutu virus was first detected in Denmark in blackbirds. Furthermore, there were outbreaks in sheep and cattle of the disease Bluetongue, which is spread by midges. The mosquito-borne viral disease West Nile fever has been detected in animals just south of the Danish border with Germany (Hamburg), and Aedes mosquitoes (tiger mosquitoes), which can spread dengue and chikungunya viruses, have been found in southern Sweden. Additionally, there has been geographic spread of the tick-borne disease TBE in 2024, with presumed transmission to humans with one new case in Egedal, Randers, and Odsherred municipalities, which are thus classified as possible new risk areas .

Zoonotic influenza

Influenza from birds and other animals, such as pigs, can in rare cases infect humans, also referred to as zoonotic influenza.

Highly pathogenic avian influenza (HPAI A(H5N1) virus of clade 2.3.4.4b) has spread extensively among wild birds worldwide, leading to outbreaks among domestic birds and infections in several land and marine mammals. In 2024, HPAI A(H5N1) was detected for the first time in dairy cattle in the USA, resulting in an ongoing outbreak in which dairy farms in several American states are affected.

This spread can lead to an increased risk that humans are exposed to these viruses, and there may be an increase in sporadic infections in humans as a result of exposure to birds and animals, although the risk of these viruses spreading from animals to humans has not increased.

Sporadic cases of infection with zoonotic influenza viruses have been reported in humans globally. Most human infections with avian influenza viruses have been associated with unprotected exposure to poultry, live poultry markets, or contaminated environments. Additionally, there have been recent human cases of A(H5N1) after exposure to dairy cattle infected with HPAI A(H5N1)-virus. Most cases have had mild disease, such as conjunctivitis. No human-to-human transmission of A(H5N1) has been observed. There have also been a few cases of zoonotic influenza infections without known exposure to infected animals or their environments, detected through routine surveillance systems for seasonal influenza.

No infections with avian influenza have been reported among humans in Denmark (or the EU/EEA). In Denmark, there have been two documented cases of zoonotic influenza virus transmitted from pigs, both in 2021. The two cases were unrelated and did not result in further transmission to other people.

Detections of A(H5N1)-influenza virus of clade 2.3.4.4b in humans, which have caused symptomatic infections, have shown varying symptoms from predominantly mild to a few severe or even fatal courses of illness. See also risk assessments from DK-VET and SSI.

Attention to pandemic risk of zoonotic influenza

The European Centre for Disease Prevention and Control recommends that all countries increase their attention to zoonotic influenza. The concern regarding zoonotic influenza is that the virus mutates and may subsequently cause human-to-human transmission, thus potentially leading to a pandemic. It is therefore important to monitor the extent of possible transmission between animals and humans so that authorities can gain more knowledge about the risk of transmission for risk assessment and monitoring and react quickly with preventive measures. SSI, in collaboration with the University of Copenhagen, has initiated a project on active surveillance of zoonotic influenza among people with occupational exposure in Denmark, EPI-NEWS 44/2024. Furthermore, a coordination group (KOZO) with the participation of all relevant authorities has been established.

SSI hosted the first EuroMOMO meeting after the pandemic and the Nordic vaccine meeting

Representatives from 22 countries in Europe as well as ECDC and WHO met in September for a three-day workshop to exchange experiences and discuss new methods and opportunities for monitoring excess mortality during epidemics such as COVID-19 and influenza. Read more about the EuroMOMO project here.

The Nordic vaccine meeting is an important forum for vaccine specialists from the Nordic countries to exchange knowledge and experiences about vaccines and vaccination programs. The meeting, which has been held every other year for the past 30 years, was organized together with the Danish Health Authority, held in September, and had more than 200 participants.

SSI focuses on antibiotic resistance

The Central Unit for Infection Hygiene has led the large EU project JAMRAI-2, which brings together efforts across several departments at SSI and national health authorities and, together with several European countries, works to implement action plans to combat antibiotic resistance.

As in the summer of 2023, there were again cases of impetigo caused by a methicillin-resistant Staphylococcus aureus (MRSA), which was also resistant to fusidic acid and gentamycin, in the summer of 2024. In total, the Reference Laboratory for Antibiotic Resistance at SSI received 83 isolates, both from children with infections and positive screening tests from family members. The same clone has been detected in several other European countries, and a comparison of their genomes with the Danish ones shows a close relation and several possible introductions into Denmark, followed by local spread.

The SSI Reference Laboratory for Antibiotic Resistance in 2024 became the leader of a newly established European reference laboratory for antibiotic resistance (EURL-PH-AMR) in a consortium with the Technical University of Denmark (DTU Food) and the EUCAST Development Laboratory in Sweden. The new EURL-PH-AMR will act as a link between ECDC and the national reference laboratories in EU/EEA countries. It will support progress toward common European laboratory methods for detecting and reporting antibiotic-resistant bacteria, focusing on carbapenem- and colistin-resistant Enterobacterales, Pseudomonas aeruginosa, and Acinetobacter baumannii.

The appointment as EURL-AMR follows the conclusion of two four-year capacity-building projects (EURGen-RefLabCap and FWD AMR-RefLabCap), which SSI and DTU Food managed for the EU. Both projects aimed to broadly improve the functions of national reference laboratories as well as specifically in the use of whole genome sequencing of bacteria for outbreak detection and surveillance.

SSI has also entered into a collaboration with Ukrainian authorities to support improving their surveillance of antibiotic resistance. The problems with antibiotic-resistant bacteria in Ukraine are significant and have also become visible in other European countries, including Denmark, which has received patients transferred from Ukraine.

Electronic reminder letters about vaccination of children born to women with chronic hepatitis B

Since last summer, SSI has replaced paper letters to general practitioners and parents reminding them about vaccination of children born to women with chronic hepatitis B with letters sent via e-Boks. The new dispatch procedure ensures that both parties are still reminded about the important vaccination and contributes to the ongoing digitization process.

Comments

The observed spread of mosquito species that can introduce new infections, such as West Nile fever, and the spread of TBE to possible new risk areas depends on many factors but may be a sign of emerging health consequences of climate change, even in Denmark. The increased spread of zoonotic influenza HPAI A(H5N1) means that there may be a rise in sporadic infections in humans due to exposure to birds and animals, although the risk of these viruses spreading from animals to humans has not increased. It is therefore important to monitor the extent of possible transmission between animals and humans, and in line with this, a project on active surveillance of zoonotic influenza among people with occupational exposure has been initiated in Denmark.

With this summary of 2024, we wish all readers of EPI-NEWS a happy new year!

(P. Valentiner-Branth, Department of Infectious Disease Epidemiology and Prevention)