Issue 10, February 2002
About this Newsletter

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 oocysts–the 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 cytokines–natural, 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 stocks.

For more information on ARS poultry disease research, contact:

Hyun S. Lillehoj, (301) 504-8771

Research Briefs

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 species.
Kenneth E. Overturf
(208) 837-9096

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
(405) 262-5291

An ARS scientist helped Kentucky researchers characterize the 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
(309) 681-6397

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 body.
Neil Talbot
(301) 504-8216
Tom Caperna
(301) 504-8506

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 disease.
Stacey Schultz-Cherry
(706) 546-3464

ARS scientists are examining an experimental way to reduce foodborne pathogens and treat various poultry diseases. They used 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
(501) 575-2104

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 tracks.
David L. Straus
(870) 673-4483, ext. 275

A new commerical blood test kit, based on ARS technology, detects 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.
Lucy Lee
(517) 337-6836

Awards

These ARS researchers have been honored recently for their achievements:

L. 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.

ARS 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 States.

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 swine.

Healthy Animals archive

Top

ARS Animal Health Research Laboratories

For More Information