Unusual Swine Flu Case in the Netherlands: Patient Had No Contact with Pigs

In a perplexing development, the World Health Organization (WHO) has reported an isolated case of swine flu in the Netherlands. This case, reported on September 2, 2023, has raised concerns as the infected individual had no direct contact with pigs, diverging from the typical transmission pattern of swine-origin H1N1 influenza.

An Atypical Case Emerges

The Netherlands Ministry of Health, Welfare and Sports detected a laboratory-confirmed case of influenza virus variant (v) infection, specifically swine-origin H1N1 influenza. Notably, the patient in question had no documented interactions with pigs. This anomaly was uncovered during routine surveillance for respiratory diseases.

As of September 7, 2023, there have been no symptomatic contacts linked to this case, and no further instances have been identified through daily monitoring. Five close contacts underwent monitoring for a maximum incubation period of 10 days, with none developing symptoms. Consequently, there is no evidence of human-to-human transmission, classifying this case as a sporadic human occurrence of influenza A (H1N1). The risk of widespread community transmission or international propagation is assessed as low.

Understanding Swine Influenza Variants

Swine influenza A viruses, including H1N1, H1N2, and H3N2, are prevalent in swine populations worldwide. Occasionally, these viruses cross into humans, typically via direct or indirect exposure to pigs or contaminated environments. Infections with swine influenza variants tend to result in mild illness, though severe cases can occur.

Historically, sporadic human infections caused by influenza A (H1N1v) and influenza A (H1N2v) have been documented in the Netherlands, but sustained human-to-human transmission has not been observed. This marks the first instance of human infection with the influenza A H1N1 virus in the Netherlands in 2023 and the third occurrence in the past five years. Nine such cases have been recorded in the country since 1986.

Case Description and Public Health Response

The infected individual, an adult from the province of Noord-Brabant, exhibited no underlying health conditions or a history of occupational exposure to animals. Symptoms, including fatigue, respiratory issues, chills, sneezing, coughing, headache, and fever, manifested on August 20, 2023. Participatory surveillance for acute respiratory infection initiated on August 21, leading to the collection of a nasopharyngeal swab sample. Subsequent testing confirmed influenza A virus presence but ruled out influenza A H1N1 )pdm09.

Further genetic analysis conducted via whole-genome sequencing identified the virus as A/Netherlands/10534/2023, a Eurasian avian swine influenza A (H1N1) v clade 1C.2.2 virus. Notably, this strain is sensitive to neuraminidase inhibitors such as oseltamivir and zanamivir.

The patient has since recovered as of September 13. Investigations by national authorities, the WHO, and veterinarians have not pinpointed a definitive source of infection. No animal monitoring or testing has been conducted, as other influenza viruses detected in the Netherlands do not belong to the swine influenza A virus variant.

This unique case continues to be monitored, with ongoing antigen identification efforts. The situation remains under close observation by health authorities and international organizations, given its unusual circumstances.