Iowa Alumni Magazine | December 2005 | Features

As Sick as a Dog or a Bird or a Pig

By Tina Owen

It was the largest mass vaccination program in United States history. In 1976, more than 40 million Americans received a shot against swine flu, a type of influenza that had crossed from pigs to humans, infecting a number of soldiers at Fort Dix, New Jersey, and causing one death.

Animal Hosts

While bird flu is the concern of late, in the 1970s it was swine flu that worried the nation. And, in a case of strange bedfellows earlier in 2005, a strain of horse flu jumped to dogs.

President Gerald Ford initiated the nationwide campaign after some scientists, doctors, and public health officials warned that a major pandemic could occur. In that instance, the experts were wrong: the virus never spread and the pandemic didn't happen. Today, though, experts know more about how pathogens cross from one species to another (causing what's known as zoonotic illnesses)—and they're more concerned than ever.

"The greatest potential for a pandemic is from reassortment of zoonotic influenza viruses," says Dr. Greg Gray, UI professor of epidemiology and head of the College of Public Health's Center for Emerging Infectious Diseases (CEID).

Recent deadly outbreaks have brought bird flu to the public's attention, but experts worry about other cross-species infections that jeopardize both animal and human health. Earlier this year, a strain of equine flu afflicted thousands of dogs in the U.S. Infection rate was virtually 100 percent, with mortality rates running between one and ten percent.

The H5N1 strain of avian flu that's caused about 130 human deaths so far has also spread to swine, tigers, and cats. That virus, first seen in Hong Kong in 1997, is thought to have spread from migrating shorebirds to ducks via fecal contamination. Ducks then passed the pathogens on to domesticated chickens, which later infected humans at the city's live birds markets. Carriers such as wild birds tend to be immune to the virus, but humans are a much greater risk from the mutated pathogens.

As they migrate from one country to another, infected birds become what the Wall Street Journal called "the epidemiologic equivalent of intercontinental ballistic missiles." Though that threat is real, another zoonotic carrier exists much closer to home, in Iowa's 9,000 farms that raise more than 25 million hogs per year.

Pigs' physical makeup allows them to contract and spread influenza viruses to and from other species. As they're so susceptible to influenza virus infections from other species, pigs may also serve as "mixing vessel hosts" that can produce new strains of the virus. Recent research from the CEID shows that farmers, veterinarians, and, to a lesser degree, meat processing workers have a greater risk of contracting swine influenza.

Public health officials worry that if a pandemic strain of influenza enters the U.S. and infects swine herds or poultry flocks, Iowa's 200,000 swine workers and their poultry counterparts may be some of the first humans infected. Although most people who contract swine flu experience mild symptoms, UI researchers worry that swine workers' exposure to the virus could amplify zoonotic infections among humans.

To help prevent a pandemic, the UI researchers are calling for increasing surveillance for both human and zoonotic forms of influenza among swine workers. They're also advocating that agricultural workers should be included in the government's plans for dealing with a pandemic. "We're really concerned about agricultural workers and their health," says Gray. "We want to make sure that whatever we can do to protect them is done."