US CDC: Avian Flu Case in Missouri Raises Concerns, But No Evidence of Human-to-Human Transmission

US CDC: “Genetic sequence and infection route unclear, but no risk of additional transmission”

The first case of avian influenza in the United States without contact with animals such as birds or cows may be a one-time outbreak that will not lead to further transmission, according to an analysis by the U.S. Centers for Disease Control and Prevention (CDC).

As avian flu continues to spread through dairy and poultry herds, the Missouri case raises the possibility of human-to-human transmission of the flu. However, the CDC stressed that there is no evidence that anyone else has been infected and the risk to the general public remains low.

“The evidence so far suggests this is an isolated event,” said Dr. Nirav Shah, CDC’s deputy director. Health officials have released few details about the Missouri case, which was first announced last week, citing patient confidentiality.

The patient presented with chest pain, nausea, vomiting, diarrhea, and weakness and was admitted on August 22 “for reasons related to his underlying medical condition,” Shah said. The patient was tested for the flu and tested positive for influenza A, and subsequent testing revealed that the virus belongs to a category of viruses commonly seen in birds.

The patient had such a low concentration of viral genetic material that it was impossible to identify exactly which virus it was, but the partial genetic sequence was similar to that found in U.S. dairy cows, the CDC said. “Ultimately, it may not be technically feasible to obtain the full sequence,” Shah said.

Influenza viruses are classified based on the types of two proteins on the surface of the virus: hemagglutinin (H) and neuraminidase (N). Highly pathogenic avian influenza is classified as H5N1, meaning it is composed of H5 and N1 types. In this patient’s case, the H type was confirmed to be H5, but the N type has not yet been confirmed.

The patient reportedly had no contact with dairy cows or other animals implicated in the ongoing avian influenza outbreak. He also stated that he had not consumed unpasteurized milk or other unpasteurized dairy products.

“This is the 14th human case of H5 (avian influenza) reported in the United States in 2024, and the first case of H5 without direct exposure to a sick or infected animal,” the CDC said in a statement when the Missouri case first surfaced. All previous cases in the United States had occurred in people who worked around cattle and poultry, so the case raised concerns about disease transmission.

According to the CDC, avian flu has been found in more than 200 dairy herds in 14 states, but none in Missouri. However, there have been cases of avian flu in poultry and wild birds.