A human case of swine flu A virus has been detected in Denmark, according to a CDC study


In a recent study published in Emerging infectious diseases Journal, researchers detected a patient with swine influenza virus (IAV) in routine surveillance at the National Influenza Center in Denmark. The detected influenza variant appeared distinct from any previously found in Denmark.

Study: Severe human case of zoonotic infection with swine influenza A virus, Denmark, 2021. Image credit: Liya Graphics/Shutterstock
Stady: Severe human case of zoonotic infection with swine influenza virus, Denmark, 2021. Image credit: Liya Graphics/Shutterstock


According to official records, during the 2021-2022 influenza season, 16,160 cases in Denmark were caused by the influenza A virus, mostly of the H3N2 subtype. During this time, there have been no cases of swine flu virus infection in humans. Since the 2009 influenza A (H1N1) pandemic, there have been no reports of continued human-to-human transmission of IAVs. Also, there have been only isolated reports of human infections with IAVs. However, the zoonotic potential of IAVs is very concerning.

Status report

A young man in his fifties, who works in a pig slaughterhouse in Denmark, was hospitalized after the onset of acute illness on November 24, 2021. He had dizziness at night, followed by chest pain, left arm pain, diarrhea, malaise, but no fever. . The patient suffered frequent convulsions and had to be admitted to the intensive care unit (ICU) and put on ventilation to stop the seizures and manage the low oxygen levels.

He had no impairment of the cardiovascular, renal, or nervous system, or any other abnormality, including pneumonia, which could account for his sudden bout of severe illness. However, the patient’s nasopharyngeal swab sample tested positive for IAV. Notably, no other co-worker in the patient’s workplace reported the flu.

With antiviral drugs (oseltamivir) and supportive treatments, the patient’s clinical condition improved over the next two days, so he was discharged from the hospital. The researchers submitted the sample of the remaining material to the Danish National Influenza Centre, which confirmed that it was positive for the pandemic H1N1 strain.

Further analysis by whole genome sequencing revealed its consensus sequence to be of the H1N1 subtype. Notably, the virus was more similar to porcine IAVs than to human influenza strains. The team uploaded this sequence to the Global Initiative on Sharing the All Influenza Database (GISAID).

The sequence did not match the IAV sequences in GISAID, as indicated by basic local alignment tool (BLAST) searches; However, comparison with endogenous sequences of swine influenza viruses from Denmark showed close similarity to swine 2021 IAVs. This viral strain has many genetic and antigenic differences from other influenza A viruses discovered in Denmark. Also, it had a poor interaction with human seasonal influenza vaccines currently in use. Furthermore, phylogenetic analyzes revealed that most of the genetic segments were similar to the H1N1 subtype. On the contrary, the non-skeletal fragments and neuraminidase belong to the 1C subfamily of avian-like swine influenza A (H1N1) found in Eurasia.


Earlier in Denmark, an elderly patient with comorbidities suffered from an influenza-like illness (ILI). However, the case reported in this study was unique in that a previously healthy adult experienced a sudden, severe illness. Another distinct observation was that this patient had convulsions that are rare in adults and are usually accompanied by fever or encephalitis. Therefore, the viral strains infected in these two cases are likely to be genetically distinct.

The detection of altered IAV via routine monitoring highlighted the importance of continuous monitoring of both human and porcine IAVs of animal potential. In addition, she stressed the need for countermeasures to be urgently put in place for those who come into contact with pigs due to their occupation and suspected influenza experience.


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