Coccidiosis: This Poultry
Disease's Impact Is Anything But Paltry
Intestinal infections such as coccidiosis
and salmonellosis inflict heavy economic losses on poultry. The poultry
industry loses more than $700 million annually from coccidiosis alone.
Coccidiosis is caused by protozoans known as
Eimeria that invade the cells in a chicken's or a turkey's intestine.
The bird's ability to absorb nutrients suffers, which results in loss of weight
or death. Coccidia can also damage the immune system and leave poultry more
vulnerable to pathogens like Salmonella.
These protozoan parasites are particularly
difficult to combat because several different species of Eimeria exist
in the field. Chickens may become infected with different species because the
immunity that develops after infection is specific to only one species.
Eimeria have a very complex life cycle which involves many developmental
stages inside host cells. Each Eimeria parasite is able to infect only
one host (for example, chicken or turkey, but not both), and they each attack
different parts of the intestine in their specific host.
Avian coccidiosis has become increasingly
prevalent in the worldwide poultry industry. One factor is the confined-host
rearing conditions, which lead to an increase in the numbers of
oocyststhe stage of coccidia that chickens ingest through litter,
destroying the integrity of the intestine and interfering with nutrient
absorption. Another factor is the increasing incidence of drug resistance to
field strains of coccidia. Confounding this problem is the absence of new drugs
to replace older, ineffective anticoccidials.
Many avian diseases, including coccidiosis,
are currently controlled by drug therapy. Producers add a number of
anticoccidial drugs (coccidiostats) to commercial feed to combat the problem.
Drug-based control measures cost the industry more than $300 million annually.
However, they are increasingly ineffective as drug-resistant coccidia strains
rapidly develop. Also, possible overuse adds to the public's concern over
chemical residues in the food supply.
In Search of a New Approach
Agricultural Research Service (ARS)
scientists at the Henry A. Wallace
Beltsville Agriculture Research Center,
Animal and Natural Resources
Institute in Beltsville, Md., are fighting the disease by seeking
alternative strategies to prophylactic drug usage. Recent findings at the
institute's Parasite Biology, Epidemiology,
and Systematics Laboratory point to cell-mediated immunity as the major
factor conferring resistance to coccidiosis. An increased knowledge of the
interaction between parasites and host could lead to the development of novel
immunological and molecular biological concepts in the control of coccidiosis.
Currently, a coccidiosis vaccine is
available that ARS researchers helped develop. It includes a low dose of the
live parasite as a key ingredient to stimulate protective immunity. It has been
used in millions of chickens. However, the parasite can still cause disease in
vaccinated chickens if their immune systems are already damaged or suppressed
by other infectious agents.
According to Hyun S. Lillehoj, an
immunologist and lead scientist of the coccidiosis research program at the
Beltsville lab, ARS scientists are taking three general approaches to block the
spread of coccidiosis. First, they are developing a better understanding of how
poultry develop immunity in nature. They are also identifying potential vaccine
proteins of coccidia and investigating the most effective ways to deliver
vaccines. And they are developing a better understanding of avian genetics and
identifying host genes controlling birds' response to vaccination, which could
lead to the genetic-based selection strategy for coccidiosis control.
Survival of the Fittest
In the field, once birds have been exposed
to coccidia, they develop immunity after a couple weeks. Although live or
attenuated parasites have been widely used as a commercial vaccine, antigenic
variability between the Eimeria species present in the vaccine and those
in the field restricts the effectiveness of commercial vaccines.
Mark C. Jenkins, laboratory research leader,
and his colleagues identified two proteins on the outside coat of the parasite
that mark it as an intruder and elicit an immune response. The scientists
developed recombinant vaccines to provide protection using only the patented
proteins rather than the whole parasite. These "subunit vaccines" cannot cause
disease, so they are safe to use on poultry when their immune systems are
suppressed. In another study, scientists inserted recombinant DNA into nucleic
acid loops called plasmids and immunized the birds. Chickens that were
vaccinated with the recombinant plasmids showed good levels of protection from
coccidia, but not enough.
Vaccines to Stimulate Natural Defenses
Lillehoj also identified a novel protein
that the parasite uses to enter host cells and patented the antibody to this
protein that could be used to block coccidia invasion of host cells. An antigen
is a protein from the parasite that stimulates a response from the animal's
immune system. Antigens may also be deliberately introduced by vaccination in
order to stimulate production of antibodies. Identification of a target protein
which is recognized by this antibody could lead to a potential vaccine antigen
that can reduce parasite invasion. In collaboration with her colleagues,
Lillehoj generated potentially therapeutic recombinant chicken antibodies that
specifically bind to coccidia parasites.
Another focus at the laboratory is on the
role of certain cells in the gut (B and T lymphocytes) in fighting off
parasites through the production of antibodies and cytokines. Lymphoid tissues
found in the guts of turkeys and chickens contain unique lymphocytes necessary
for protection against coccidiosis. The researchers determined that an
effective vaccine must include parasite proteins which are capable of
stimulating appropriate intestinal T cells.
They are also examining the possibility of
harnessing the chickens' own
hormone-like chemicals produced by lymphocytes to fight infections and used by
immune cells to communicate with each other to impair parasites' replication.
The ARS researchers showed for the first time that Eimeria parasites
activate intestinal T lymphocytes to produce immunologically active factors
such as interferon-gamma (IFN-gamma). IFN-gamma is a cytokine that is produced
by activated T lymphocytes and inhibits coccidia multiplication by activating
macrophages that attack invaders. Lillehoj found that birds' immunity levels
correlate with their IFN-gamma levels. She and her colleagues also demonstrated
for the first time that certain cytokines enhance the efficiency of protection
induced by recombinant DNA vaccination.
For example, IFN-gamma and interleukin 2
(IL-2) enhanced the effect of recombinant DNA vaccination against coccidiosis.
Chicken IL-15, another cytokine that Lillehoj and a colleague identified in
1998, prompts infection-fighting T cells to multiply. The lab has plans to
study several different ways to deliver recombinant chicken cytokines in
conjunction with antigens to enhance their vaccinal immunity.
These potential future generations of
recombinant DNA and subunit protein vaccines have shown promise in experimental
infections but have yet to be commercially developed.
Taking Stock in Poultry Breeds
The lab is interested in helping poultry
breeders select better stocks with enhanced natural immunity. Researchers want
to know why some chickens are more resistant to diseases than others. A lack of
understanding of the mechanisms of protective immunity against most avian
diseases makes practical genetic selection difficult. The scientists want to
compare many different breeds with different disease-susceptibility patterns to
find the genes with positive traits for the commercial market.
Recently, Lillehoj identified a DNA marker
on a chromosome associated with coccidiosis-resistant traits in commercial
broiler chickens. DNA marker technology avoids many of the problems in
selecting poultry stocks with superior disease resistance. This finding could
assist poultry producers in identifying valuable economic traits, known as
quantitative trait loci, to improve the genetic quality of commercial broiler
For more information on ARS poultry disease
Hyun S. Lillehoj, (301)
An ARS researcher has
pinpointed a family of
rainbow trout that thrive on grain-rich feed. Feeds are the fish farmer's
biggest expense and are usually made with fishmeal from saltwater species.
Using more grain in fish feed would help prevent overfishing of these saltwater
Kenneth E. Overturf
ARS researchers found that kenaf, a crop usually
grown to make paper, could replace alfalfa
pellets as a crude protein supplement for lambs fed bermudagrass or fescue
hay without affecting feed intake or weight gain. They then used the lamb
feeding as an experimental model for cattle.
William A. Phillips
An ARS scientist helped Kentucky researchers
bacterial cause of a reproductive disease of thoroughbred racehorses that's
caused hundreds of cases of weakened or stillborn foals on farms in the state's
Bluegrass region. The accomplishment helps veterinary scientists diagnose
horses and research ways to prevent the disease.
David P. Labeda
ARS scientists have cultured a pig liver cell line
that performs some of
the organ's functions in a petri dish. With PICM-19, animal researchers can
design in vitro models of the liver to study gene expression, nutrient
metabolism, drug toxicity and blue duct formation outside the animal's
A never-before-described virus that infects young
turkeys, has been identified and its
sequence deciphered by ARS scientists. The virus, associated with Poult
Enteritis Mortality Syndrome (PEMS), has caused problems for southeastern
poultry producers since the early 1990s and is now circulating throughout the
United States. This information may lead the way to vaccines for the emerging
ARS scientists are examining an experimental way
to reduce foodborne pathogens and treat various poultry diseases.
bacteriophages to treat an air sac infection, called air saculitis, in broiler
chickens. Early success in the laboratory with bacteriophages could open
the door to another way of preventing and treating animal disease.
William E. Huff
Fish farmers may soon have a better way to treat
a serious parasitic disease of catfish, known as ich or whitespot.
ARS Scientists found
that potassium permanganate quickly stops the parasite in its
David L. Straus
673-4483, ext. 275
A new commerical blood test kit, based on ARS
chicks infected by an extremely virulent strain of avian leukosis virus,
ALV-J. A 1996 epidemic produced shortages of breeding stock that threatened
the poultry industry's ability to meet the burgeoning demand for chicken on the
dinner table. Breeders can use the kits to reduce ALV-J infection.
These ARS researchers have been honored recently
for their achievements:
Dale Van Vleck, Roman L. Hruska U.S. Meat Animal
Research Center, received a T. W. Edminster Research Associate Award for
his proposal to develop
procedures identifying valuable economic traits (quantitative trait loci)
associated with genomic markers in livestock.
Science Hall of Fame inducts class of 2001:
William L. Mengeling,
National Animal Disease Center, for
developing a diagnostic test instrumental in eradicating hog cholera from U.S.
swine herds and for isolating porcine parvovirus from swine in the United
Lawrence Johnson, formerly of the
Animal and Natural Resources
Institute, for developing the USDA-Beltsville Sperm Sexing Technology and
Beltsville Thawing Solution, as well as co-developing the first successful
method of deep-freezing swine semen for use in artificial insemination of