A select agent is a biological agent or toxin that the U.S. federal government has determined poses a severe threat to public health, animal health, or agriculture. These are pathogens and toxins with the potential for deliberate misuse, and any laboratory that wants to possess, use, or transfer them must meet strict regulatory requirements. The list currently includes some of the most dangerous organisms known, from the viruses that cause smallpox and Ebola to the bacterium behind anthrax.
How the Select Agent Program Works
The Federal Select Agent Program is the regulatory system that tracks and controls these dangerous biological materials across the United States. It’s run jointly by two agencies: the CDC’s Division of Regulatory Science and Compliance handles agents that threaten human health, while the USDA’s Animal and Plant Health Inspection Service (APHIS) oversees agents that threaten livestock, crops, and agricultural systems. Some agents fall under both, since they can infect both humans and animals.
The legal foundation for all of this is the Public Health Security and Bioterrorism Preparedness and Response Act of 2002, passed in the wake of the 2001 anthrax letter attacks. That law gave the federal government authority to regulate who can work with the most dangerous pathogens and toxins, how they must be stored, and how they can be moved between facilities.
What’s on the List
The select agent list covers more than 60 biological agents and toxins, organized into several categories based on what they threaten.
Human-focused (HHS) select agents include some of the most recognizable names in infectious disease: Ebolavirus, variola major virus (the cause of smallpox), Yersinia pestis (plague), Francisella tularensis (tularemia), and Marburg virus. These are pathogens that cause severe illness in people, often with high fatality rates and limited treatment options.
Overlap agents affect both humans and animals. This category includes Bacillus anthracis (anthrax), Nipah virus, Hendra virus, Rift Valley fever virus, and Venezuelan equine encephalitis virus. Because these cross the line between human and animal health, both the CDC and APHIS regulate them.
USDA veterinary select agents target livestock and include African swine fever virus, avian influenza virus, foot-and-mouth disease virus, and rinderpest virus. A separate plant protection category covers pathogens that could devastate crops, such as Ralstonia solanacearum and Xanthomonas oryzae.
Tier 1: The Highest-Risk Agents
Within the broader select agent list, a subset called Tier 1 agents face even tighter controls. These are the agents judged to present the greatest risk of deliberate misuse, with the most significant potential for mass casualties or devastating effects on the economy, critical infrastructure, or public confidence. Tier 1 agents include organisms like Bacillus anthracis, Ebolavirus, smallpox virus, and botulinum neurotoxin-producing species of Clostridium. Facilities working with Tier 1 agents face additional security and personnel requirements beyond what’s already demanded for other select agents.
In a recent update effective January 2025, Nipah virus was elevated to Tier 1 status, reflecting growing concern about its potential for misuse and its high fatality rate in human infections.
Security and Personnel Requirements
Any facility that wants to register with the Federal Select Agent Program must meet detailed physical security standards. These include perimeter fences, locked doors, security windows, and access control systems such as card key readers or biometrics. Select agents held in long-term storage must use tamper-evident devices. Only individuals who have been specifically approved can enter areas where select agents are kept, and anyone without approval must be escorted at all times by someone who does have access.
The personnel screening is equally rigorous. Every person who needs access to select agents must pass a security risk assessment conducted by the FBI’s Criminal Justice Information Services Division. This involves submitting fingerprints, a photo, and a detailed background check form. Access is denied until the assessment clears. The process checks for statutory disqualifiers that would prohibit an individual from working with these materials.
Reporting Theft, Loss, or Accidental Release
If a select agent or toxin is stolen, lost, or accidentally released, the rules are immediate and non-negotiable. The facility must notify the CDC or APHIS right away by phone, fax, or email, along with appropriate law enforcement agencies. The initial report must include the name and identifying information of the agent, an estimate of the quantity involved, the time window during which it happened, and the specific location. A formal written report on an official form must follow within seven calendar days.
For accidental releases, the reporting requirements are similar but also require information about the environment the release occurred in, how many people may have been exposed, and what actions were taken in response. These reports are mandatory even if the missing material is later recovered or the responsible person is identified.
Exemptions for Diagnostic Labs
Clinical and diagnostic laboratories get a limited exemption from the full weight of select agent regulations. If a hospital lab, for instance, identifies a select agent in a patient specimen during routine diagnostic work, it doesn’t need to be registered with the program. But the exemption comes with conditions: the lab must either transfer the agent to a registered facility or destroy it on-site within seven calendar days of identification. For certain toxins like botulinum neurotoxin, that window extends to 30 days. During the time the agent is on-site, the lab must secure it against theft, loss, or release, and it must report the identification to the CDC or APHIS.
A similar exemption applies to proficiency testing specimens. Labs that receive select agents as part of quality assurance testing have 90 calendar days to transfer or destroy them.
The List Changes Over Time
The select agent list isn’t static. Agents can be added, removed, or reclassified as scientific understanding and threat assessments evolve. In December 2024, HHS and USDA published final rules that took effect in January 2025, making several notable changes. Five agents were removed from the list entirely, including three Brucella species (B. abortus, B. melitensis, and B. suis), African horse sickness virus, and a plant pathogen called Peronosclerospora philippinensis. The rules also updated nomenclature, formally changing “Ebola virus” to the genus name “Ebolavirus” and clarifying the naming for SARS-CoV. The exclusion threshold for certain conotoxins was doubled from 100 mg to 200 mg, meaning labs holding small quantities below that amount no longer need to register.
These periodic reviews ensure the regulatory burden stays focused on the agents that genuinely pose the highest risk, rather than growing indefinitely as new pathogens are discovered.