“You can’t have good human health without good animal health, and you can’t have good animal health without good human health” said the late great Dr. James Harlan Steele, one of three internationally-recognized pillars of Veterinary Public Health. It’s important for legislators to keep Dr. Steele’s sage words in mind as the Massachusetts Legislature considers the potential impact on public health of permitting the use of veal and swine gestation crates and battery cages for hens in Massachusetts.

Maintaining the effectiveness of antimicrobial drugs to treat bacterial diseases in people and animals is a public good as much as is keeping air safe to breathe and water clean to drink. Yet the initiation, growth, and spread of antimicrobial resistance has become a worldwide concern of the World Health Organization, the Food and Agriculture Organization of the United Nations, the World Organization for Animal Health, the federal Centers for Disease Control and Prevention (CDC), and numerous other human and animal health agencies. But what possible connection could this have to crates and cages?

Three factors are necessary for the occurrence and transmission of infectious diseases—the disease agent, the human or animal host, and an environment that favors transmission of the agent among hosts. Veal calves are confined to crates so small that the animals can’t turn around to groom themselves, adopt normal sleeping positions, stand up or lie down without difficulty. Sows kept on industrial farms give birth to five to eight litters over their lifespans and during this time most of them spend 70-80 percent of their lives in gestation crates 6.6 feet by 2.3 feet wide. Temple Grandin, Ph.D., a doctor of animal science and professor at Colorado State University, likened the use of gestation crates to “asking a sow to live in an airline seat.” Battery cage hens spend their lives in spaces the size of a sheet of paper. These are stressful conditions, indeed, and they reduce the ability of the animal’s immune system to fight off disease.

Forcing animals into tiny living spaces enables livestock producers to cram more animals into available space and permits them to produce more animal product at reduced cost. The intense crowding that animals are subjected to in intensive production facilities creates an environment that favors transmission of disease agents to other animals and to people. All that remains to create an infectious disease storm is the disease agent itself.

Physicians and veterinarians treat their sick patients with antimicrobial drugs including penicillin, azithromycin, and tetracycline. The more antimicrobial drugs are used, the greater the likelihood that bacteria will develop resistance to those drugs, whether that occurs within a single bacteria species or by transfer of resistance from other species. The Animal Health Institute, a part of the Coalition for Animal Health that represents America’s veterinarians, farmers and ranchers, food and feed producers, and veterinary medicine manufacturers, estimates that about 30 percent of all antibiotics used in the United States are used in food animals. The Natural Resources Defense Council, a coalition of online environmental activists, lawyers, scientists, and policy experts estimates that about 80 percent of all antibiotics used in the US are used in food animals. Even if one accepts that the true proportion lays exactly half way between these two estimates, more than half of all antimicrobial drugs used in the United States are used in livestock production.

This is not to let physicians off the hook at all. Hospitals are fertile sites for antibiotic resistant bacteria, and the human medical establishment must make even greater efforts to restrict antibiotic use only when necessary to treat disease. But how are antibiotics used in livestock production? There is evidence that administering subtherapeutic doses of antibiotics fosters growth in animals and people. Physicians prescribe antibiotics in therapeutic doses to individual patients to treat disease, and veterinarians who treat companion animals (usually dogs and cats) do the same. But in intensive livestock production, the patient is not a single animal but a group of animals. The presence of a single ill animal often means that other animals in that group are treated, not because they are ill, but to prevent disease that flourishes in stressful and crowded environments.

There is a fine line, if there is one at all, between using antibiotics to prevent and to treat disease. Although current research has been used to justify both the continued use of antibiotics in livestock production and the reduction or outright banning of antibiotics, evidence is mounting that antimicrobial resistance is already one of the biggest challenges to the health of people and animals worldwide, and lacking more forceful action by the medical professions, it will only become more so. The precautionary principle–the precept that an action should not be taken if the consequences are uncertain and potentially dangerous—applies here, and we should err on the side of public health.

The CDC estimates that at least 2 million Americans fall sick every year and about 23,000 die from antibiotic-resistant infections. Concern about animal and human disease and the desire to improve animal welfare led to Canada’s recent proposal to eliminate the confinement of sows in gestation crates. This follows a European Union ban on continuous gestation crate confinement that went into effect in January 2013, legislation banning gestation crates in nine US states including Rhode Island and Maine, and public commitments from more than 50 of North America’s largest pork buyers and producers to eliminate gestation crates from their supply chains and operations. California, Ohio, and Michigan have banned the use or new construction of battery cages.

I urge members of the Massachusetts Legislature to pass H.1456/S.741 for the sake of the health and well-being of public health and animal health in Massachusetts.

Joann Lindenmayer is an associate professor of public health in the Department of Infectious Disease and Global Health at the Tufts Cummings School of Veterinary Medicine.