Epidemiological outbreak investigation of influenza A virus in cattle
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EmailApproaching an outbreak from an epidemiological perspective.
In March 2024, the U.S. experienced an unusual outbreak of highly pathogenic avian influenza A (HPAI) in cattle, spreading across several states. This event marked the 1st occurrence of this disease, which is usually associated with wild birds. While each disease outbreak is unique, the investigation follows similar steps. Epidemiologists, policymakers, and stakeholders work together to take action to understand the disease to prevent and control its spread. As veterinary epidemiologists, the researchers describe the steps of the epidemiological outbreak investigation of influenza A virus in cattle that is underway.
Step One
Confirm the outbreak and the causative agent. In early March, veterinarians in Texas noticed unusual disease symptoms in dairy cows. Among the symptoms observed, one distinctive and abnormal finding was milk with a thicker consistency and a yellow to brown color, resembling colostrum. This was a very specific and unlikely symptom, as no known mastitis pathogen causes such changes without additional udder inflammation symptoms.
Furthermore, mastitis pathogens were not identified in these cases. As the disease occurred in more cows and herds, concerns grew. After extensive testing, the USDA National Veterinary Services Laboratory identified HPAI in the samples and, therefore, a possible cause. Additional cases and confirmatory testing supported this suspicion. Soon after, the CDC confirmed the first and human H5N1 infection associated with this outbreak. Identifying the microorganism helped develop hypotheses about how infection is occurring and define control measures.
Step Two
Form an investigation team. Michigan is a very strong dairy-producing state, and the well-integrated network of stakeholders has greatly facilitated communication and collaboration. Within MSU, the College of Veterinary Medicine, Extension, Veterinary Diagnostic Laboratory, and AgBioResearch have been working closely with MDARD and milk producers. This collaborative effort is focused on collecting the data needed to provide information and guidance to dairy producers and advisors on implementing preventive and control measures.
Step Three
Introduce preliminary control measures. As outbreaks evolve and new information about the disease emerges, federal and state agencies typically issue orders to control the spread of the disease. In the current HPAI outbreak, this includes the USDA's order to test lactating cattle moving interstate and MDARD's order to implement biosecurity measures such as record keeping and designated visitor areas on dairy farms. Additionally, the USDA announced financial assistance for producers with affected herds to improve on-site biosecurity.
Step Four
Perform epidemiologic studies. Understanding the dynamics of the disease is crucial for making informed decisions. During outbreak investigations, one of the most common approaches is case-control studies. In these studies, animals that test positive for HPAI are considered cases and are compared with healthy animals with similar characteristics, known as matched controls. Analyzing the differences between these two groups can provide valuable insights into the disease.
We aim to understand how the disease is transmitted within and across dairy herds and how long the virus circulates in cows and the herd. Additionally, we seek to determine the best strategies to prevent herds from becoming positive and control the disease once it is on the farm. To do that, we visit farms for a few weeks and take samples from animals and the environment. Answering these questions is essential to inform farmers and policymakers about effective management strategies to prevent and control the spread and impact of the virus. In addition, other universities are conducting additional studies to better understand the disease, complementing our efforts.
Step Five
Develop new control measures and/or update existing ones. As more science-based information is generated and the disease evolves, control measures are adjusted accordingly until they can be safely removed. For instance, if there is reasonable certainty that the virus spreads in a specific way, resources and biosecurity efforts may focus on that mode of transmission. This targeted approach ensures that the most effective strategies are employed to control the outbreak.
We are now entering the fourth step of the outbreak investigation, which means there is still more to learn and accomplish. Once all these steps have been achieved, the disease will enter a stage of surveillance and evaluation of control measures. While much remains unknown, implementing biosecurity measures and conducting frequent testing is the logical approach to eventually reaching a stage where we understand this virus better and provide specific advice tailored to the epidemiology of the virus. We are heading in the right direction, but not there just yet.
Producers who are willing to support the Michigan State University research project are encouraged to contact the authors to learn more. All information collected will be confidential and anonymized.
