Making Coccidia Less Cocky
In tests at the Beltsville laboratory, a vaccine injection is used to immunize
chicks against coccidia with recombinant DNA containing two promising proteins
from the outside coat of oocysts, the protozoa's infectious stage.
Each year, U.S. poultry producers raise about 7 billion broilers. That's a
lot of beaks to feed. And feed is the costliest item on the producer's balance
sheet--topping even labor costs.
That's why chickens are routinely given drugs to prevent infection by tiny,
single-celled protozoa known as coccidia. These organisms invade cells in a
chicken's gut, where they reproduce and make it harder for the bird to absorb
feed and gain weight quickly.
Coccidiosis is among the top-five chicken diseases that prevent weight gain,
says Agricultural Research Service microbiologist Harry D. Danforth. He is with
ARS' Parasite Biology and Epidemiology
Laboratory at Beltsville, Maryland. Each year, the protozoa cost producers
worldwide an estimated $600 million in treatment and low carcass weights.
And it could become worse, because the protozoa are developing resistance to
standard drugs. That has Hyun S. Lillehoj and Mark C. Jenkins, who are based at
the ARS Immunology and Disease Resistance Laboratory at Beltsville, working on
ways to use the birds' own immunity against coccidia. "We have short- and
long-term goals," says Danforth, who is the agency's scientific liaison
with the poultry industry. The short-term goal is a gamma-irradiated vaccine
Danforth tested this year at Perdue Farms, Inc., in Salisbury, Maryland.
First, however, Jenkins had to determine the radiation dose needed to weaken
the live oocysts--the infectious stage of coccidia. This prevents them from
developing or reproducing. Next, he figured the dose of weakened oocysts needed
to produce immunity in the chicks. Then, Danforth put the oocysts in a gel
delivery system he and researchers in the vaccine industry had developed
earlier to get live vaccine into chicks at the hatchery. The gel was added to
the feed of chicks destined to become Cornish hens.
Danforth says it takes about 1.6 pounds of feed for each pound of bird. With
the gamma-irradiated oocysts, "feed conversion was 3 points better than
with the anticoccidial drugs," he says. "That means the treatment
reduced, by three hundredths of a pound, the feed needed to raise a 2-pound
Cornish hen. That may seem small, but it's nothing to sneeze at when multiplied
by thousands or millions of birds. The company would realize an extra $1
million a year for its Cornish hens alone, says Danforth.
Donna Hill, Perdue's director of health services, says the vaccine had other
benefits. The birds were more uniform in size and had good color. The results
were so promising that "if it were commercially available, we would
probably start using an attenuated vaccine right away for Cornish hens and
begin testing it on broilers," she says.
Trouble is, it takes 10,000 of the killed oocysts to immunize each bird, and
the oocysts have to be grown in live chickens. About 40 billion
gamma-irradiated oocysts per week would be needed just to inoculate all the
broilers raised by Perdue alone, says Danforth. "It's a numbers game.
We're trying to give the producer some relief now, until researchers streamline
ways to enhance chickens' immune response to coccidia."
And that's no easy task.
Not Too Little--Not Too Much
The chicken's immune system is more complicated than a Chinese puzzle. And
launching an immune response is walking a fine line. It can protect a bird--or
destroy it, if it goes too far. So researchers first have to unravel the
complex inner workings in the bird's gut before they can get the optimum immune
response without an overreaction.
Jenkins approached this problem by identifying proteins in the oocysts that
mark it as an intruder and elicit an immune response. He, Lillehoj, Danforth,
and Michael D. Ruff have a patent on the recombinant DNA for two promising
proteins from the oocysts' outside coat.
Jenkins inserted the recombinant DNA for those two coat proteins and another
promising protein separately into DNA loops, called plasmids, taken from E.
coli bacteria. In small-scale tests at the Beltsville lab, he shot the
plasmids straight into chicken legs using a jet gun, like the ones dentists use
to numb their patients' teeth and gums.
"We got the best protection with mixtures of all three plasmids,"
Jenkins says. Weight gain in the immunized chicks was significantly better than
in the unimmunized birds. But it was not as good as in the birds that never got
close to a coccidia oocyst. "We have a little way to go because we want to
have complete protection," he says. And a more efficient system for
delivering such an inoculum in an industry setting is needed.
Lillehoj's lab identified another promising protein. It enables the stage
that emerges from the oocyst, called a sporozoite, to invade the bird's T
cells. Lillehoj patented a monoclonal antibody to the protein that she
says "consistently blocks this invasion in culture dishes." She is
now collaborating with scientists in Japan and Korea to find the DNA that
directs production of that protein.
The All-Natural Boost
Lillehoj's main focus is on the substances immune cells generate to
communicate with one another. Animals produce these natural, hormonelike
chemicals, called cytokines, during an infection. They are potent and function
at low levels.
Some cytokines enhance the immune response. But others can cause disease
symptoms, "so you have to know which ones are protective," says
Lillehoj. "Understanding how this works may be a way to control infection
without introducing anything unnatural."
She is looking for umbrella protection against the six or seven chicken
coccidia species because, once cytokines are produced, they aren't picky about
the species. They may also be given along with vaccines to increase
their effectiveness.The challenge is that there are more than 20
different cytokines that regulate immune response, Lillehoj says. "We're
just beginning to understand how they work."
One all-purpose cytokine that has proved effective is interferon gamma. (See
"Two Strategies for Protecting Poultry From Coccidia,"
Agricultural Research, Oct. 1996, pp. 12-13.) Interferon gamma activates
macrophages--cells that behave like the Pac Men of the immune system, gobbling
up invaders. Interferon gamma inhibits coccidia multiplication, so the birds
lose less weight. Lillehoj says the birds' immunity level correlates with their
interferon gamma level.
Another cytokine that's showing promise in laboratory tests is
interleukin-15. IL-15 prompts the all-important, infection-fighting T cells to
multiply. Last year, Lillehoj's lab cloned the gene for IL-15, and they are
testing its use under three protocols.
In one, they inject the IL-15 protein the gene produces directly into the
chick's muscle. In another, they use a gene gun to inject the chick with the
naked IL-15 DNA. And in the third, they insert the DNA into a vector, such as a
weakened version of the fowl pox virus that is now used to immunize poultry
against fowl pox.
"All three methods enhance the chickens' innate immune response against
coccidia," Lillehoj says. "The animals have more T cells, which are
critical for defense, and they lose less body weight."
Lillehoj is collaborating with three companies on chicken cytokine research.
But progress has been slow, she says, because knowledge about human cytokine
DNA isn't much help. Poultry DNA is only about 30 percent similar to human DNA,
compared to a 70-percent similarity in large meat animals.
Lillehoj is also collaborating with Perdue on a different tactic for
controlling coccidiosis. Since some chickens are genetically more resistant to
the disease than others, her lab is searching for the genes that confer this
resistance. Once they are identified, poultry producers can breed for the
hardiest birds.--By Judy
McBride, Agricultural Research Service Information Staff.
Harry D. Danforth is at the
Biology and Epidemiology Laboratory, 10300 Baltimore Ave., Beltsville, MD
20705-2350; phone (301) 504-8300, fax (301) 504-5306.
Mark C. Jenkins and
Hyun S. Lillehoj are at the
and Disease Resistance Laboratory, 10300 Baltimore Ave., Beltsville, MD
20705-2350; phone (301) 504-8201, fax (301) 504-5306.
"Making Coccidia Less Cocky" was published in the
January 1999 issue of
Agricultural Research magazine.