A biologist at Washington University in St. Louis has developed a live vaccine that should greatly decrease the incidences of food poisoning and deaths in humans infected by salmonella bacteria.
Roy Curtiss III, Ph.D., George William and Irene Koechig Freiberg Professor of Biology in Arts and Sciences, has genetically engineered the most common strain of salmonella infecting chickens and developed it as an oral vaccine to be given to poultry. Curtiss deleted two key genes in the strain Salmonella typhimurium UK-1; the deletion weakens the bacteria, allowing it in vaccine form to induce an immune response in a chicken without making the bird sick.
When given a dose of the vaccine, newborn chicks, breeders and laying hens develop a lifelong immunity to salmonella. The immunity is transferred to offspring and eggs that people consume.
The vaccine is in its final stage of testing by scientists at Megan Health Inc. in St. Louis, a company licensed by Washington University to develop and market vaccines based on technology developed by Curtiss.
The U.S. Department of Agriculture (USDA) is expected to license the vaccine by the end of 1997. Megan's vaccine will be the first salmonella vaccine for poultry marketed in the United States.
Curtiss reported the vaccine test results July 29, 1997, at the International Veterinary Vaccines and Diagnostics Conference, held in Madison, Wis.
"Based on years of research and testing, we are confident that our vaccine works very well," says Curtiss, who first developed the strain for his vaccine in 1990, and has spent 17 years researching salmonella bacteria.
When virulent bacteria are introduced into an organism, a fight ensues between the bacteria and the host's immune system.
"By deleting the genes, we make sure that the salmonella has its hands tied behind its back," Curtiss explains. "With those two genes, the salmonella could defend itself against the chicken's natural host defense system, its antibodies, and possibly kill the bird. But by giving the bird the weakened strain contained in the vaccine, the bird always wins."
Bigger winners in this development are American consumers and the poultry industry, which brings 7 billion chickens from processing plants to dinner tables each year. There are higher incidences of salmonella poisoning reported in recent years, and there is increasing evidence that salmonella bacteria are becoming resistant to antibiotics. Salmonella poses the greatest risk for very young children, the elderly and people with compromised immune systems, such as AIDS patients.
A notoriously under-reported disease, salmonella infection is diagnosed about 40,000 times a year in the United States, although experts believe that nearly 4 million people become sickened by salmonella annually. Consuming undercooked eggs and raw eggs used in dressings and condiments are the most common avenues of infection. But the bacteria also are harbored in chickens that elude inspection and make it to the grocery store. Symptoms are diarrhea and nausea, usually lasting a couple of days. In more serious cases, dehydration can occur, and infection can lead to death.
A hallmark of his vaccine is that it is oral and designed to stimulate all three lines of defense: the secretory immune system, which protects the gastrointestinal tract, lungs and genital and urinary organs; the blood, which sends antibodies to fight toxins; and specialized killer cells. Most vaccines are injected and trigger only the second and third lines of defense.
In a related development, Curtiss also announced at the conference the success of a recombinant vaccine that uses the weakened salmonella vaccine as a carrier to immunize chickens against another profit-robbing disease in poultry, Escherichia coli (E. coli). This vaccine has been designed and constructed by Megan Health Inc. scientist Kenneth Roland using some recombinant salmonella clones from the Curtiss laboratory. This vaccine presents a one-two combination that immunizes chickens against both salmonella and E. coli, certain strains of which cause respiratory disease in poultry. The virulent strains are called avian pathogenic E. coli (APEC). They hurt producers in production units, where the sick birds spread the disease and die, and at the processing plants, where E. coli-infected carcasses are condemned by inspectors. Each year, millions of chickens are lost to E.coli.
"We're able to take the genetically engineered salmonella strain and make it express some proteins and other properties from APEC, which imparts immunity to APEC as well," Curtiss says. "The recombinant vaccine is approaching its final round of construction and efficacy testing at Megan Health, and we're anticipating USDA approval on it in a couple of years."
Curtiss has used the recombinant approach with salmonella to research vaccines that would provide immunity to humans for malaria, typhoid fever and leprosy and to livestock for a variety of diseases. Such vaccines would be especially welcome in Third World countries, where these diseases are still common, because they can be taken orally and do not require refrigeration.
"Using needles drives up the cost of vaccines and also raises the chance of contamination," Curtiss says. "But the oral vaccine is 'dirt cheap' in comparison. The biggest costs with the oral vaccine are the glass vials and labels. The vials for the salmonella vaccine will hold between five and ten thousand doses, each dose costing less than a penny per bird."
The vaccine also can be delivered quite effectively as an aerosol spray that deposits the vaccine to the chick's eyes and nares (nostrils)."Sometimes the chicks don't drink anything for a day or two after hatching because they still have lots of fluids from the egg yolk to subsist on," Curtiss says. "Delivering the vaccine as a spray can eliminate the time and cost involved with handling every newly hatched chick."