After a relatively quiet decline, bird flu cases are surging again in Michigan.

When state officials announced the presence of avian influenza at another poultry farm in Ottawa County on Dec. 16, 2024, it was the first time in more than six months that the H5N1 virus had been detected in Michigan poultry. The USDA has since confirmed seven more outbreaks in the state — five in commercial flocks and two in backyard flocks — most recently in Oakland County.

Meanwhile, on January 6, 2025, the U.S. Centers for Disease Control and Prevention reported the first death from avian influenza in Louisiana.

Avian influenza is a complex and evolving disease that has a significant impact on the poultry and dairy industries. Since the outbreak in poultry and cattle, nearly 135 million poultry and more than 900 cattle herds have been affected across the country. A wide range of wildlife including raccoons, skunks, foxes and bobcats have also succumbed to the virus, as well as marine mammals and domestic cats.

Marine mammals and domestic cats have also died from the virus.

Kimberly Dodd, dean of Michigan State University's College of Veterinary Medicine, is an expert in responding to emerging infectious disease outbreaks. Michigan was one of the first states to experience an H5N1 outbreak in commercial poultry and dairy facilities. State and federal officials are working closely with the Michigan State University Veterinary Diagnostic Laboratory to identify newly infected cattle while continuing to monitor the virus in poultry and wild birds.

Dodd spoke with The Conversation about the risks H5N1 poses to families, pets and livestock, and how researchers are working together to find ways to limit its spread.

What do Michigan researchers know about the outbreak?

In the case of cows infected with the H5N1 virus, scientists and animal health experts in Michigan and across the country are working to understand a common disease in a new species.

Transmission of the avian influenza virus H5N1 to dairy cows was first confirmed in Texas in March 2024 and has since spread to 15 other U.S. states, most recently California.

The virus affects milk production and poses a significant risk to other birds, particularly poultry and some mammals. The CDC also confirmed mild human cases and fatal cases in Louisiana in January 2025.

Diagnostics staff at Michigan State University and the USDA National Animal Health Laboratory Network provide rapid diagnostics to identify infected cattle and monitor the virus in wild birds and mammals to help control the spread.

Based on sequencing of circulating viruses, we know that a specific strain or H5N1 genotype B3.13 is circulating in cattle and poultry. Subtle changes in the virus's genetic makeup over time suggest a single spillover event. That means the virus could have spread from a wild bird to a cow in Texas, and then from one cow to another. We also know that there is a separate H5N1 genotype, D1.1, currently circulating in wild birds and poultry.

Researchers in our College of Veterinary Medicine are working with federal and state officials and farmers to determine when cows produce infectious milk. The goal is to understand how cows become infected within and between herds so we can better mitigate the spread.

Why is it called bird flu if it also makes other animals sick?

Influenza viruses affect many animals, including humans, pigs, dogs, and horses.

There are four types of influenza viruses: A, B, C, and D, which are loosely defined based on the species they can infect. Avian influenza viruses are considered influenza A viruses. Interestingly, influenza D virus is the virus that primarily infects cattle. But the H5N1 influenza virus currently circulating in cows is the same influenza A virus currently causing outbreaks in birds.

This is of particular concern because only influenza A viruses have been associated with human pandemics.

Avian influenza viruses circulate among wild birds but usually do not cause severe illness. Although many avian influenza viruses can infect poultry, their behavior in these hosts classifies viruses as either highly pathogenic or low. It is important to note that this classification does not necessarily indicate how easily the virus spreads or how likely it is to spread to other species.

The currently circulating strain is classified as highly pathogenic avian influenza (HPAI) because it causes high mortality in poultry. One of the many unusual features of the current outbreak is the high mortality rate among wild birds. As a result, we see many young carnivores and scavengers becoming infected with the virus.

Infection in other species often requires exposure to large amounts of the virus, or a compromised or underdeveloped immune system. These are often "dead end hosts". They become infected but do not play a role in the spread of the virus because the animals either die or become infected but are not contagious.

The current HPAI H5N1 outbreak has been ongoing since 2021. This outbreak is notable for its long duration, wide geographic spread, and unusual impact on non-poultry species. It has caused severe illness and death in wild birds such as ducks and geese, as well as mammals exposed to the carcasses of infected birds such as cats and skunks.

However, when the USDA unexpectedly identified H5N1 as the cause of significant disease in dairy cows in early 2024, it marked the first time the virus had been identified in U.S. dairy cows.

What effect does this virus have on cattle?

Most cows infected with H5N1 recover spontaneously without treatment. Symptoms include fever and sluggishness. Since the virus is concentrated in the mammary glands and milk, the appetite of lactating cows will also decrease and milk production will decrease significantly.

There are three reasons for concern about ongoing outbreaks of H5N1 influenza in dairy cows:

First, reduced milk production and the contagiousness of the virus pose challenges to farmers in controlling the infection and lead to economic losses. In addition to production losses, there are additional labor and supply costs required to control outbreaks in cattle herds, including isolation and supportive care of sick cattle. Staff are also required to use personal protective equipment to prevent the spread of the virus to healthy animals and to protect themselves from potential infection.

Secondly, cows produce a large amount of viruses in their milk, which are highly contagious. While pasteurization kills viruses, raw milk still poses a major infection risk. Large numbers of wild mammals and domestic cats on dairy farms have died from the virus after consuming raw milk. It also poses a risk to dairy workers.

The virus has also spread from dairy farms to poultry farms, causing high bird mortality. Experts are exploring whether clothing, shoes, trucks, equipment and other items contaminated with raw milk containing the virus could lead to inadvertent fatal exposure of poultry.

Third, prolonged transmission in cattle increases the risk of virus adaptation to mammals, including humans.

To monitor this risk, all H5N1-positive samples are sent to the USDA for genetic sequencing to identify mutations that may increase the virus's ability to infect mammals and provide important information about how the virus spreads within and between populations.

How do we respond to the H5N1 outbreak?

Biosecurity in poultry and dairy facilities is more important than ever. Biosecurity measures include restricting visitor access to farms and facilities; disinfecting tools, footwear and equipment; avoiding contact between infected and uninfected species; and quarantining sick animals.

These measures play an important role in containing the virus and keeping it away from other animals, property and humans.