Roundtable Participants

Dr. David Chapman, Department of Poultry Science, University of Arkansas (Chair) Dr. Tim Cherry, Stephen F. Austin University, Texas Dr. Harry Danforth, Technology Transfer Coordinator, USDA Dr. Grant Richards, Eimeria Pty Ltd, Victoria, Australia Dr. Martin Shirley, Institute for Animal Health, Compton, U.K. Dr. Ray Williams, Schering-Plough Animal Health, Harefield, U.K. It was billed as a “controversial roundtable” and was strategically slotted on the last day of the 8th International Coccidiosis Conference in Queensland, Australia. The focus: The role of live vaccines in the sustainable control of coccidiosis in poultry production. Led by some of the heaviest hitters in this branch of poultry science, the roundtable discussion lived up to its billing and attracted a standing-room-only crowd. Following are CocciForum’s notes from this lively session.

Take an Integrated Approach

A decision to use vaccination against coccidiosis isn’t a question of ‘all-or-nothing’. While vaccination isn’t currently used in the same flock with chemical or ionophore treatments, vaccination can become part of an effective rotational program, alternating with existing medication-based regimes.

Harry Danforth gave a graphic example of how this works in practice. He has done trials that confirmed how Coccivac-B repopulated farms with coccidia that were sensitive to salinomycin. This helps enhance the usefulness of the drug if it is used in a later growout.

Watch the Arsenic

Some populations of Eimeria tenella - one of the most significant coccidia species - are becoming very resistant to salinomycin. One way of dealing with this has been to use roxarsone (3-Nitro), an arsenical treatment that’s been used since the 1970s and is currently used by around 60% of the U.S. poultry industry.

Danforth said that with increasing concerns about arsenic in the environment, growers’ access to this tool may be limited in the future. Proven treatments such as vaccination are emerging as viable alternatives.

Keep Tabs on Coccidia

Two presenters at the roundtable explained why it is important to understand what’s happening in a flock as the coccidia respond to treatment. Looking at the numbers of coccidia, as well as what species they are, shows how the treatments are working.

Eimeria populations can be tracked by measuring how many oocysts are excreted by the birds. Dr. Ray Williams said that in medicated flocks, the oocysts per gram (OPG) counts can vary a lot, but usually peak at around week 4 or 5. Williams also noted that, in these flocks, the coccidia can peak again toward the end of the growout after the medication is withdrawn. This can indicate that the birds are still susceptible to the disease, and the prophylaxis has only suppressed the coccidia rather than giving immunity.

In vaccinated flocks, there are typically two peaks in the output of oocysts, the first occurring usually between weeks 2 and 4. This happens as immunity is being established. The second peak represents a wild-type challenge around week 4 or 5.

Once the second peak has passed, the OPG counts drop rapidly — a good indicator that immunity has kicked in successfully. Danforth explained that a new diagnostic tool developed in Europe and based on molecular biology (polymerase chain reaction, or PCR) shows researchers very accurately which of the seven Eimeria species are involved in a flock. This is important to help design good, targeted treatments. It also helps monitor exactly how well different programs are working.

Team Up with Vaccines

Williams said that because vaccination is a biological process using live organisms, growers need to learn to work as a team with the vaccine to make sure it delivers maximum effectiveness.

The treatment depends on the birds successfully excreting a generation of vaccinal coccidia and then “recycling” them by picking them up from the litter. The uptake of coccidia is part of the build-up of immunity. To successfully “sporulate” in the environment, coccidia need the right balance of oxygen, warmth and moisture.

If the litter is too dry, there may not be enough oocysts shed to challenge the birds sufficiently for immunization; if it is too wet, the numbers of coccidia could overwhelm the birds and cause a disease outbreak.

Williams said stocking density and access to feed are two other important factors in the birds’ uptake of coccidia from litter. Good husbandry is essential.

‘Compensatory Gain’

Dr. Tim Cherry said immunity in vaccinated birds is developed by 4 weeks of age. Weight gain and feed efficiency are initially affected as immunity develops, but birds subsequently catch up — a scenario known as “compensatory gain.”

It used to be thought that vaccinated birds compensated for the initial weight gain suppression as immunity was established, usually by 4.8 lbs. But Schering- Plough Animal Health data have shown that weight gain and feed conversion in vaccinated birds are comparable to birds in coccidiostat programs after 35 days.

Drug resistance a problem in China

They don’t do things by halves in China. Professor Mingquan Xie of the Guangdong Academy of Agricultural Sciences gave conference-goers an outline of his country’s massive poultry industry. There are three native types of chicken grown — black, yellow and white — accounting for 80% of poultry output. The remaining 20% is shared between ducks and geese. Production techniques vary from very small-scale and primitive to highly modernized units that would be more familiar to U.S. growers. Xie says anticoccidial drugs have been abused in China, where coccidiosis tends to be worse in the hotter southern and eastern regions, including Taiwan. As a result, vaccination is playing an increasingly important role in management of coccidiosis in that country. An evaluation of anticoccidial drugs showed that about two-thirds of the Eimeria tenella strains were resistant to five or more different products — a worrying sign. In the trials, diclazuril performed very well, Xie said.

The Clostridial Question

Some in the industry are concerned that vaccinated birds may not receive the protection they need from clostridial bacteria. This is because the ionophores normally used to prevent coccidiosis also provide some protection against clostridial diseases like necrotic enteritis. Ionophores cannot be used with live coccidiosis vaccines, as they would compromise their performance.

Nevertheless, Williams said that in trials he’s observed, necrotic enteritis affected both vaccinated birds and control groups of birds treated with a growth promoter and ionophore.

He said there’s a perception in the industry that coccidiosis makes birds more susceptible to clostridial diseases like necrotic enteritis, but more research is urgently required to confirm this idea.

What About Diet?

Factors such as diet could play a part in both coccidiosis and clostridial diseases. Williams said it’s been shown that coccidiosis and clostridial diseases are affected by dietary changes (e.g., more or less wheat or maize content).

How this works isn’t known yet, but it could be a function of vitamins or the “viscosity” of feces. The less viscous, the more easily bacteria are “flushed out” of the bird’s system. More research could show how diet could play a part in coccidiosis management.

Looking Ahead

At the roundtable discussion, Dr. Martin Shirley was very enthusiastic about the future of coccidiosis control through vaccination. He expects to see the growing use of “big” vaccines (i.e., those available currently), complemented by the development of new, specialized vaccines for different markets.

He also expects to see vaccines developed to manage necrotic enteritis. In line with the growth in vaccine use, Shirley said that concerns over the use of in-feed medication were likely to increase, especially within Europe, and the possible wider banning of some or all coccidiostat in-feed medication could now be seen on the horizon.

Shirley also predicted that biotechnology will lead the way to development of more sophisticated and targeted vaccines. This will come about as the parasite genes responsible for inducing immunity are identified, and scientists learn how to manipulate and deliver antigens. He also said biotechnology would help scientists keep up with dealing with any immunologically different (vaccine-resistant) strains should they appear in the field.

Further Information

To read the latest Cocci Forum magazine please click here (PDF format)

Source: Cocci Forum Magazine - Schering-Plough Animal Health - October 2003