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Four poultry workers in Colorado have tested positive for avian influenza, aka bird flu, amid an ongoing outbreak of the virus in the United States. A total of eight people in the U.S. have contracted bird flu since the start of the year.

On Sunday, health officials confirmed four new human cases of highly pathogenic avian influenza A (H5N1) linked to a multi-state outbreak among poultry and dairy cows, according to the U.S. Centers for Disease Control and Prevention. A fifth poultry worker in Colorado has tested presumptive positive and is awaiting confirmation from the CDC.

All of the patients experienced mild illness, including pink eye and respiratory symptoms, and none were hospitalized, according to the Colorado Department of Public Health.

The news comes less than two weeks after officials announced a dairy worker in Colorado had tested positive for bird flu.

Since March, H5N1 avian influenza has spread to over 150 dairy cattle herds in 12 U.S. states, per the CDC. In addition to the five human cases confirmed in Colorado, a dairy worker in Texas and two in Michigan have also tested positive for H5N1.

Last month, the World Health Organization announced a person in Mexico died after becoming infected with avian influenza A (H5N2), which was the first laboratory-confirmed human case of H5N2 reported globally. Additionally, a child in Australia was recently infected with bird flu.

Bird flu is a disease caused by infection with avian influenza A viruses, which occur naturally among wild aquatic birds and circulate among poultry, TODAY.com previously reported.

Occasionally, bird flu viruses spread to mammals and, rarely, to humans. No known human-to-human spread has occurred with the current H5N1 strains spreading in the U.S. or H5N2 in Mexico.

Based on available information, the WHO assesses the current risk to the general public posed by bird flu to be "low." However, the recent human cases and ongoing outbreak has sounded the alarm among officials in the U.S. and abroad, who are monitoring bird flu viruses closely.

8th human cases of bird flu in the U.S.

The latest cases of bird flu in the U.S. were confirmed among workers at a poultry facility experiencing an outbreak of H5N1, according to the CDC. In early July, Colorado confirmed a case of H5N1 among a dairy worker who had exposure to sick cows.

Two farmworkers in Michigan tested positive in May, and one dairy worker in Texas tested positive for H5N1 in April.

The human case in Texas was the first time this strain of H5N1 — referred to as highly pathogenic avian influenza A (HPAI) — had been detected in cows and the first instance of cow-to-human transmission, according to the CDC.

“The current bird flu strain that we’re concerned with, H5N1, has been circulating around the world for quite some time,” Dr. William Schaffner, professor of infectious diseases at Vanderbilt University Medical Center, previously told TODAY.com.

The four new cases in Colorado and the second case in Michigan reported conjunctivitis (pink eye) and respiratory symptoms, officials said. The first Colorado case, the first Michigan case, and the Texas case reported mild pink eye as their only symptom.

The highly pathogenic H5N1 strain causing the U.S. outbreak is severe and often fatal in birds, but appears to be mild in cows.

"Based on the information available at this time, this case does not change CDC’s current A(H5N1) bird flu human health risk assessment for the U.S. general public. The risk to members of the general public who do not have exposure to infected animals remains low," the CDC said.

Fatal case of bird flu in Mexico

In late May, authorities in Mexico reported a confirmed human case of H5N2 to global authorities. The patient was the first human have been infected with an avian H5 virus in Mexico, per the WHO.

The 59-year-old resident died in a hospital in Mexico City in April after developing a fever, shortness of breath, diarrhea, nausea and “general malaise,” per the WHO.

The victim reportedly had multiple underlying health conditions and had already been bedridden for three weeks for other reasons prior to developing symptoms of bird flu.

The source of exposure is unknown, and the victim had no known contact with infected animals, but H5N2 viruses have been detected in poultry in multiple states in Mexico, the WHO said.

Respiratory samples collected from the victim later tested positive for avian influenza A (H5N2), the first human case reported globally. There were no further cases identified, and all potential contacts of the victim are being monitored, the WHO.

What are the symptoms of bird flu?

Bird flu infections in humans can range in severity, experts note. Some people have no symptoms at all or only a mild flu-like illness, while others develop severe disease requiring hospitalization, according to the CDC.

“It can be a serious infection with a high mortality rate,” said Schaffner.

According to the CDC, reported signs and symptoms of bird flu include:

• Fever
• Cough
• Runny nose
• Muscle aches
• Headache
• Fatigue
• Shortness of breath
• Eye redness or inflammation (conjunctivitis)
• Diarrhea
• Nausea
• Vomiting

Bird flu in humans may look similar to seasonal influenza flu or upper respiratory infection. In severe cases, bird flu can lead to pneumonia, respiratory failure and other complications, TODAY.com previously reported.

All eight of the human cases in the U.S. have reported eye redness and conjunctivitis, with five patients reporting additional respiratory symptoms including a cough and a fever. They have since recovered.

The only other human case of H5N1 in the U.S., which was reported in Colorado in 2022, was a mild infection as well.

The case of H5N1 in a child in Australia was a severe infection, but the child fully recovered. Victorian health officials did not release additional details about the patient's symptoms.

An infection with bird flu viruses cannot be diagnosed by signs or symptoms alone, the CDC says. Laboratory testing is required.

How does bird flu transmit to humans?

Transmission of bird flu viruses to humans is very rare. According to the WHO, since 2003, there have been 889 cases and 463 deaths caused by H5N1 in 23 countries. Most human cases worldwide have been caused by H7N9, highly pathogenic H5N1, or H5N6 viruses, per the CDC.

Avian influenza can spread from infected birds to humans in a few ways, per the CDC:

• Directly from an infected bird
• From environments contaminated with bird flu viruses
• Through an intermediate host, such as another animal

Infected birds can shed the virus in their saliva, nasal secretions, mucus and feces, TODAY.com previously reported. People can become infected when the virus particles get into the mouth, nose, eyes or are inhaled, says Schaffner.

Humans typically become infected with bird flu viruses through close, unprotected contact with an infected bird.

When it comes to the dairy workers with conjunctivitis, the CDC noted in a press release that "it's not known exactly how eye infections result from avian influenza exposures."

"It may be from contamination of the eye(s), potentially with a splash of contaminated fluid, or touching the eye(s) with something contaminated with A(H5N1) virus, such as a hand. High levels of A(H5N1) virus have been found in unpasteurized milk from H5N1-infected cows."

Human-to-human transmission of bird flu viruses is extremely rare. The few cases that have been documented have occurred primarily through prolonged, unprotected contact between a symptomatic person and a family member or caregiver, per the CDC.

"The virus doesn’t have the (genetic) capacity to spread easily from person to person,” says Schaffner.

However, in an April press conference, the chief scientist of the World Health Organization called the risk of the bird flu spreading to humans “an enormous concern,” and warned about the potential for the virus to acquire the ability to spread more efficiently between people. Scientists are closely monitoring bird flu viruses for any changes.

Preventing bird flu

There is no evidence that humans can get bird flu from chicken, eggs or beef that's been properly prepared and cooked, and it is safe to drink pasteurized milk, experts say.

The risk of getting bird flu is low, but the CDC recommends people:

• Avoid direct contact with wild birds
• Avoid visiting poultry farms
• When in contact with poultry, wear a mask and eye protection
• Wash hands after any contact with poultry
• Visit a doctor if you become sick after contact with birds

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