Perspectives for the control of coccidiosis in poultry by chemotherapy and vaccination
H. D. Chapman, Department of Poultry Science, University of Arkansas,
Fayetteville, AR, 72701, USA. dchapman@uark.edu
Introduction
Despite the introduction of live vaccines, in most countries prophylactic
chemotherapy is still the preferred method for the control of coccidiosis (Chapman,
2000). Significant improvements in the performance of commercially reared
poultry have been made during the last half of the twentieth century and a
recent study has shown that in the USA these improvements have continued, and
are measurable even for the relatively short time period of 1995-2001 (Chapman
et al., 2003). It has often been assumed that these improvements would not have
been possible without the introduction of a succession of ever more effective
anticoccidial agents to control coccidiosis. In recent years, however, few new
drugs have been introduced. The most widely used compounds, ionophorous
antibiotics, have been utilized with apparent success for more than thirty years;
these drugs have been used extensively in commercial broilers and are also
employed to some degree during the rearing of broiler breeder and replacement
layer stock (Chapman, 1993; 2001). Synthetic drugs (chemicals) have been used to
a lesser extent; in the USA nicarbazin was introduced in 1955 and is still used
in broiler production (Chapman, 1994a)! In the 1970s many other highly
efficacious synthetic drugs were introduced only to be withdrawn because of the
development of drug resistance.
At first it was thought that resistance would not develop to the ionophores but
many recent studies have shown this not to be the case. For example, Mathis
(1999) examined the sensitivity of recent isolates of E. acervulina, E. maxima,
and E. tenella to monensin, salinomycin, narasin and lasalocid and found that
these drugs were only marginally or poorly effective; similar conclusions have
been made by others (e.g. Chapman and Hacker, 1994). Anecdotal information
provided to the author by those involved in rearing poultry indicates that
coccidiosis is not generally considered to be a major problem. How can this be
true if the principal drugs used are no longer as effective as in the past?
Possible reasons are discussed in this article.
Acquired drug
resistance
Conventional methods for evaluating drug efficacy involve challenging birds with
isolates of Eimeria obtained from the field using a sufficiently large dose of
oocysts to induce a depression of weight gain and/or lesions in susceptible
birds (Chapman, 1998; Holdsworth et al., 2004). Such studies can indicate
whether a drug is able to control an infection capable of causing clinical
disease and if not the isolates may be classified as showing various degrees of
“resistance” to the drug. Some authors have questioned whether current
experimental methods for determining resistance are appropriate (Watkins, 1997).
Experiments with isolates considered “resistant” to ionophores showed lack
of control by these drugs whether a large or small dose of oocysts was
administered, and weight gain or oocyst production in the feces respectively
used to assess efficacy (Chapman and Shirley, 1989). An explanation for the
efficacy of ionophores under commercial conditions where ionophore resistant
strains are present must be sought elsewhere. A possibility is that these drugs
do not prevent the acquisition of immunity and that this may develop before
birds are exposed to potentially pathogenic numbers of oocysts (Chapman, 1999a).
Indeed, reliance upon immunity development in flocks medicated with ionophores
is a major strategy used to control coccidiosis by poultry producers in the USA
(see below). Whether resistance will translate into drug failure in the field
will depend upon the extent of exposure to infective oocysts in the commercial
poultry house. Such exposure will in turn depend upon many environmental and
management factors.
Our knowledge of the extent to which resistance is present in the field would be
increased if methods were available for identifying resistant strains; such
information may be forthcoming in the future (Sangster et al., 2002).
Management and
Environment
The pioneering studies of Johnson and Tyzzer in the 1920s laid the groundwork
for our understanding of coccidiosis and the important management and
environmental factors that affect the incidence and epizootiology of this
disease (Chapman, 2003). Of the many improvements in husbandry and housing that
have occurred over the years, one of the most significant may have been the
comparatively recent introduction of nipple type drinking systems. These systems
require careful management but have the potential to reduce litter wetness that
is known to be a major factor in the occurrence of clinical coccidiosis. The
introduction of efficient ventilation systems and enclosed housing has
facilitated better environmental control and may also have helped reduce the
likelihood of coccidiosis. An example of the effect of management upon the
effectiveness of coccidiosis control programs was given by Williams; higher
parasite numbers were produced by vaccinated birds where “clean-out” between
flocks was demonstrably inadequate (Chapman et al., 2002).
Other diseases
and pathogens
There is some published evidence that coccidiosis is more prevalent in flocks
where other diseases, bacterial and viral, are present. The introduction of
modern vaccination programs for a variety of such diseases may therefore have
contributed to the decreased occurrence of clinical coccidiosis. Many
antibiotics have been introduced to improve the performance of commercial
poultry and are generally thought to work by suppressing the growth of
pathogenic bacteria; these are often used along with anticoccidial drugs. A
recent study has clearly demonstrated improved performance of flocks where
growth promoters are included in the feed in combination with anticoccidial
drugs (Chapman and Johnson, 2002).
Host genetics
It is known that different breeds of chicken may vary in their susceptibility to
infection with Eimeria species. Although breeding programs to select for
resistance to coccidiosis have not been undertaken by the commercial poultry
industry, selection for “fitness” will include resistance to common
environmental pathogens of which Eimeria is one component. It would be of
interest to establish whether the modern broiler is less susceptible to
coccidiosis than its antecedents.
Ionophores and
immunity
Recent studies have shown that a factor in the efficacy of ionophores has been
their lack of interference with the development of immunity (Chapman, 1999b).
The poultry industry has taken advantage of this by increasing the withdrawal
period of medication prior to slaughter with considerable savings in the costs
of medication. This can, however, result in poorer performance, particularly in
situations where birds may be exposed to heavy infections late in life (Chapman
et al., 2004).
Vaccines
It seems logical that if immunity is desired then this should be achieved by use
of live vaccines. Guidelines to assist those interested in designing studies to
evaluate the efficacy of new vaccines have recently been published (Chapman, et
al., 2005b). Vaccination has become more practical with the development of new
methods for vaccinating birds in the hatchery. It has recently been shown
however that even with highly immunogenic species of Eimeria, such as E. maxima,
reinfection is necessary for the establishment of solid immunity at 4 weeks of
age when birds are given a small dose of oocysts following hatch (Chapman et
al., 2005a). Exposure to large numbers of oocysts prior to 4 weeks could result
in clinical coccidiosis before birds acquire protective immunity. Thus with
vaccines as well as drugs success of vaccination is likely to depend upon
environmental and management factors that affect numbers of infective oocysts in
commercial poultry houses.
In recent years several new methods of vaccinating chickens have been introduced
but in few cases has the route of infection been clearly demonstrated and the
proportion of birds infected established; the latter is relatively easily
determined (Chapman and Cherry, 1997).
Restoration of drug sensitivity
Use of live vaccines comprising drug sensitive strains results in the
restoration of sensitivity to ionophores such as monensin (Chapman, 1994b);
similar conclusions were reached for salinomycin and diclazuril (e.g. Mathis,
2003). It is difficult to demonstrate that restoration of sensitivity has
resulted in long term improvements in flock performance, nevertheless programs
involving the rotation of vaccines with traditional chemotherapy have been
advocated (Chapman et al., 2002) and are used by the poultry industry.
Current
coccidiosis control in the USA
In the USA accurate information on the use of feed additives, including
anticoccidial drugs, in the feed of poultry is available from a commercial
database that covers almost the entire broiler industry (Agri. Stats Inc.). Data
from 1995-1999 indicate drug usage had a characteristic cyclical annual pattern
(Chapman, 2001). Programs comprising a single or two different ionophores were
used extensively (80-90% of broiler complexes) in the summer and fall
(June-November) followed by a decrease (70-50% of complexes) in winter and
spring (December-May). During the latter period there was an increase in the use
of shuttle programs (20-50% of complexes) in which a synthetic drug and
ionophore were employed. Overall, more than 95% of broilers were given an
anticoccidial drug in the feed for some part of their life.
Data from 2004/2005 indicate a different pattern of drug usage. Shuttle programs
in which a synthetic drug was employed during winter and spring were still
widespread (20-60% of complexes). However, during the summer and fall the use of
a single or two ionophores had declined to approximately 50-60%. The remaining
complexes (10-30%) used no anticoccidial medication during this period and it is
speculated that the reason was the adoption of coccidiosis vaccines. In the USA,
vaccines are primarily used in the summer months; it will be interesting to see
if this pattern persists in the future.
The Future
Crystal ball gazing is not without risk as the premature anticipation of
recombinant DNA vaccines illustrates. The dearth of new drugs to control
coccidiosis makes it tempting to speculate that the “age of chemotherapy”,
having been extremely successful and lasting almost sixty years, (since the
demonstration in 1947 that sulphonamides can be used prophylactically) may be
nearing its end. As already indicated, in the USA where accurate data are
available, the use of drugs is still extensive; in Brazil, a country with a
broiler industry similar in size to that of the USA, approximately 92-95 % of
broilers receive an anticoccidial drug in the feed (Viana, personal
communication). Although coccidiosis vaccines have been available since the
1950s their use in the USA is limited although as indicated above this situation
may be changing (attenuated vaccines that are apparently widely used in Europe
and elsewhere are not presently available). It may be premature therefore to
consider that the ‘age of vaccination” is upon us. An unknown factor is the
extent to which national authorities, in response to perceived consumer
pressure, may impose restrictions on the use of anticoccidial drugs in poultry
feeds. Vaccination is now considered a more realistic proposition than in the
past but this does not mean that it is without problems and many questions
remain to be answered. In the case of attenuated vaccines, not the least will be
the logistics of providing sufficient oocysts to vaccinate billions of broilers
worldwide. A likely scenario in the coming decades is a combination of
approaches in which chemotherapy and vaccination are integrated in programs
designed to achieve sustainable coccidiosis control for the foreseeable future.
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