Newer concepts in disease management

J.M. Kataria, B.B. Dash, K. Dhama, S. Rahul and Sohini Dey
Division of Avian Diseases,
Indian Veterinary Research Institute,Izatnagar - 243122 (U.P.)
Email: [email protected]

Management of diseases in poultry plays a pivotal rote for the progress of this industry. The changed disease scenario in Indian poultry flocks with complexities in their pathogenesis and resulting economic losses have put a challenge before the Avian Diseases Specialists to address the diseases problems in commercial poultry flocks with modified approaches based on newer concepts of poultry diseases management. With the opening of the markets in this WTO regime the dynamics of the disease management have also been changed to generate poultry products of world class with least disease incidence and higher standard of public health criteria. The newer concepts of poultry disease management covers the newer approaches with favourable results in the area of bio-security, preventive health coverage, diagnosis and immuno-prophylaxis is that can bring about further improvements in the poultry production of our country.

1) Bio-Security & Health Management:
Bio-security is one of the most important criteria for efficient disease management and it refers to all measures taken to secure prevention of all types of pathogens in poultry farms. Bio-security programme can be implemented at all levels of poultry farm management. It envisages proper location and structural criteria of different types of farm (broiler or layer) and facilities needed to respective farm types.

It also ensures standard routine farm operations and specific procedures like disinfections or decontamination of units following flock removal and prevention of contact with exotic avian species from the farm.

Such bio-security measures can be aimed at disease control in a number of areas.

  1. Control of lethal, highly contagious diseases (Avian Influenza, Newcastle Disease, Gumboro Disease).
  2. Reduction of challenge by ubiquitous common organisms known to reduce productivity. (coccidiosis, E. coli)
  3. Reduction or elimination of immunosuppressive diseases (Gumboro disease, chicken anemia virus) and immunosuppressive agents (Mycotoxins, insecticides, heavy metals etc.
  4. Reduction of contamination of poultry and poultry products.

Effective bio-security and implementation of successful hygiene procedures is increasingly dependent on Hazard Analysis Critical Control Point (HACCP) approach to risk assessment and risk management.

Principles of HACCP

1) Hazard analysis
       » To identify hazard, both microbiological, chemical and physical at each step in the process.

2) Critical control points.
       » To identify the important points at which action can be taken to reduce or eliminate hazard.

3) Critical limits
       » To set limits to which the hazard must be reduced.

4) Monitoring
       » To observe and measure control procedures at all points in the process.

5) Correction
       » To set action required if critical limits are not met or are exceeded.

6) Recording
       » To keep accurate records to confirm that a process is in place and is being implemented correctly and continuously.

7) Verification
       » To maintain tests and procedures to ensure the HACCP system is working properly.

Poultry Welfare:

Welfare has been defined as “the state of an individual as regards its attempts to cope with its environment (Brown and Johnson 1993). Good poultry welfare essentially implies good health and freedom from both physical and mental suffering. Establishing that the flock is free from suffering, particularly mental suffering is complex and subjective. Nevertheless, the welfare of farm livestock has become an emotive issue and is protected by law in many countries. The welfare of poultry on the farm, during transport and at slaughter is to be ensured as per specific legislation, which is in place. Systems of intensive poultry husbandry attracted increased public concern on welfare issues, thus leading to the commitment of many a codes and standards.

The five freedoms, widely accepted as an ethical frame work for the determination of animal welfare include

  1. Freedom from hunger and thirst
  2. Freedom from discomfort
  3. Freedom from pain, injury and disease
  4. Freedom to express most normal behaviours, and
  5. Freedom from fear and distress

The scale of commercial poultry production renders examination of individuals birds impossible, assessment of the health and welfare of a flock is judged on the basis of its productivity. Routine necropsy of cells may reveal problems, which would otherwise not be apparent, and some abnormalities may only be evident at slaughter. Feedback of carcass down-grading information from processing plant to farm is essential and is an inexpensive and effective measure of flock welfare, enabling adjustment of resources and husbandry system for subsequent flock cycles.

Various conditions like contact dermatitis including footpad dermatitis hock burn and breast burn, leg weakness in broilers, hunger in broiler breeders, injurious feather pecking and cannibalism are considered indicators of flock welfare. Welfare of birds may be compromised during “depopulation, harvesting and transport to slaughter house. A significant number of birds die in transport due to heat and cold stress.

Scale of production, birds genotype and WTO trade rules render solutions of such welfare issues problematic. But, increased consumer interest in food production and animal welfare renders action by the industry, to address the welfare problems, imperative.

Health coverage:

The concept in routine health coverage has been changed from indiscriminate use of antibiotics to the use of probiotics, immunomodulators and performance promoters. The term probiotics denotes a microorganism or a combination of microorganisms which would quickly establish in birds digestive tract to inhibit colorization and growth of pathogenic organisms. The mechanism of action of probiotics:

  1. Establishment and maintenance of normal gut micro flora.
  2. Rapid colonization prevents the pathogens getting established in the gut.
  3. Efficient digestion and utilization of nutrients
  4. Stimulate the immune system and making the birds less vulnerable to diseases
  5. Recolonising the gut with beneficial bacteria, which are depleted by the use of antibiotics and in certain disease conditions.
  6. Liberation of antibacterial substances (Ex. lactocidin, organic acids)
  7. Provides digestible proteins, vitamins, enzymes and other important co-factors.
  8. Reduction in gut pH by production of lactic acid.
  9. Absorption of toxins.

The naturally occurring organisms mostly used as probiotic feed additives are Lactobacillus acidophilus, Streptococcus faecium, Saccharomyces cervisiae, Bacillus coagluans, Bacillus subtlis etc.

Saccaromyces cerivisiae, an yeast, releases mannon oligosaccharides which binds to lactins of the pathogenic bacteria, preventing its binding to gut epithelia and colonization.

The probiotics can protect chicken from pathogenic bacteria such as Compylobactor, Salmonella, Staphylococcus, E. coli etc. These pathogenic bacteria are also responsible for food born diseases and even death to humans. It will help farmers to shun the use of antibiotics and to produce eggs or meat with least antibiotic and chemical residues, so that their products find a place in international market.

2) Vaccination:

Vaccination is practiced in all the farms to protect the birds against diseases. When the birds are grouped by ten of thousands in one big pen, segregation and treatment of individuals is impractical and uneconomic. So mass vaccination is usually followed.

Mass vaccination in medium using feed is of little value compare to water, as birds showing in appetence, are reluctant to eat but not to drink. Care should be taken for quality of water to be supplied as chlorinated or treated water may destroy the vaccine, however, addition of protein in water can neutralize chlorine. The water tank should also be of non-reactive substances.

Other methods of mass vaccination proved to be economic are aerosol delivery and in-ovo immunization, used to immunise 18 days old embryo against Marek’s disease, by an instrument called in-ovo jet. In 1997, over 80% of birds in USA were vaccinated by this method. In rural economics with village chicken, methods have been developed for the delivery of Newcastle disease vaccine virus in feed pellets. Vaccines were introduced by Edward Jenner in 1798 and with the advent of cell culture techniques in 1950s, a second era of vaccination was introduced and many attenuated virus and inactivated virus vaccine were developed. Today, the field of vaccinology has entered in third Era through the application in many different ways of recombinant DNA technologies. Third generation vaccines are now complementing and in some cases, replacing them.

NEW GENERATION VACCINE

Vaccine produced by expression of viral proteins in eukaryote:

It involves the identification of critical viral protein conferring protection and gene coding this protein is cloned into one of a wide choice of expression plasmids. The gene is expressed in any of several cell systems. If the desired protein is glycosylated then eukaryotic expression system is used. Useful eukaryotic expression system includes Yeast cells, Insect cells and various mammalian cells. Mammalian cells offer advantage over cells from lower eukaryotes is that, they are more likely to possess the machinery for correct post transnational processing.

VLPs (Virus like particles):

Genes coding for the production and cleanse of capsid proteins of viruses are cloned into plasmids and expressed, the resulting individual capsid proteins assemble into virus like particles (VLPs). These VLPs may be used as a vaccine. So far VLPs have been made and shown to be immunogenic include, picornavirus, calcivirus, rotavirus and orbivirus. VLPs are called as empty virus particle and may be equated to an inactivated whole virus vaccine but without the potentially epitope damaging step of chemical inactivation.

Vaccine utilizing bacteria as vectors:

Recombinant DNA technology allows the expression of viral epitope on the surface of bacteria that directly infects the host. This approach includes insertion of DNA encoding a protective viral antigen or epitope into a region of the genome of a bacterium or one of its plasmids that encodes a prominent surface protein. The bacterium can multiply and present the viral epitope to the immune system of the host.

Vaccine utilizing virus as a vector:

Recombinant DNA technology allows any foreign gene to be introduced into viral genome and the product of the foreign gene is then carried into and expressed in the cell. The method is in use in veterinary medicine and involves the use of viruses as a vector to carry the genes for protective antigen of other viruses.

Eg fowl poxvirus is a choice as a vector for avian vaccine due to its large genome size, which can accommodate at least a dozen foreign genes and still be packaged satisfactorily within the virion. Fowl poxvirus has also been shown to be a useful vector in mammals.

Synthetic vaccine:

After locating and defining epitopes on viral proteins, the corresponding antigenic determinants or epitopes are synthesized chemically. Such synthetic peptides have shown to elicit neutralizing antibodies against various animal pathogens. One limitation to this technique is that the most epitopes that elicits humoral response are conformational and short synthetic peptides lack any tertiary or quaternary conformation. Most antibodies raised against them are incapable of binding to virions, hence neutralizing antibody titre may be of a lower magnitude than that evidenced following vaccination with inactivated whole virus or purified intact virus.

Anti-idiotypic vaccine:

The antigen binding site of an antibody produced by each B-cell contains a unique amino acid sequence known as its idiotype or idiotypic determinant. Because anti-idiotypic antibody is capable of binding to the same idiotype as binds the combining epitope on the original antigen, the anti-idiotypic antibody mimics the conformation of that epitope so that anti-idiotypic antibody mimics the conformation of that epitope, so that anti-idiotypic antibody raised against neutralizing monoclonal antibody to a particular virus can conceivably be used as a vaccine.

Immune complex vaccines:

Immune complexes are formed routinely during the immune response to an introduced antigen by binding of newly formed specific antibodies to that antigen. A small portion of the immune complex is trapped in B-cell follicles on the processes of follicular dendritic cells and can be retained there for a long period via binding to Fc receptors and complement C3 binding to Fc receptors and complement C3 receptors. Antigen preserved in this way is believed to play a crucial role in the generation of memory B cells. And the maintenance of long-term human/immune responses, immune complexes bound to follicles dendritic cells are involved in the selection of B lymphocytes that have undergone affinity maturation. In vitro preformed immune complexes have been shown to be 100 times more efficient in inducing humoral responses in vivo than the native protein antigen. This specific potential of immune complexes has suggested their uses as vaccine antigens. Recently, a number of vaccines have been developed consisting of immune complexes formed by mixing a certain amount of specific antibodies obtained from the serum of hyper immunized chickens with live infectious bursal disease vaccine virus.

Thermostable vaccine:

Thermo stable live vaccines are the alternatives for the conventional vaccines requiring cold-chain as the mandatory. These thermostable vaccines will be useful in India with tropical climatic conditions and in rural areas with lack of uninterrupted electricity. A thermostable live mutant vaccine strain IBD virus has been developed from an Indian virulent field strain with similarity to intermediate type of IBD vaccine strain. The thermostable vaccine has been found to provide protective immunity on challenge against virulent IBD virus. This vaccine is now ready for field use.

Vaccination with immunomodulators:

Immunomodulators are generally used for the pharmacological manipulation of the state of activity of the immune system. If the pathogenesis of the disease is known, then an immuno modulator which can generate a protective immune response can be selected for vaccine production.

Immuno modulation may result in general regulation of the entire immune system, but most commonly results in up regulation of certain cytokines and concomitant down regulation of others. In addition to that, it also determines the isotype of IgG, with other immunoglobulins and how much cell mediated immunity is generated. The immune response never swings totally in one direction or the other. The most notable swings are produced by aluminum salts resulting in >90% Th2 (humoral response) and bacterial endotoxins or derivatives which induce predominantly Th1 (CMI) response.

This knowledge of different immunomodulators is used to enhance different effects on immune system as desired for particular antigen. Most adjuvants modify the cytokine network to exert their effects as immunomodulators. Great advances have been made in the field of vaccines especially in the development of component vaccine and recombinant antigens by the application of biochemical and genetic engineering technologies. Chemical synthesis of antigenic determinants of vaccines have been attempted and the synthetic peptides have been used for vaccination in experimental models. For effective application of these recombinant antigens and component vaccines to poultry, the development and combination of active immuno-modulator is essential.

3) Diagnosis of diseases:

Recently the DNA based techniques have been used for the detection, differentiation and characterization of various avian pathogens. The DNA based techniques involve PCR, RE analysis, hybridization and nucleotide sequencing for detail investigation of important emerging and reemerging diseases.

DNA finger printing:

In case of DNA viruses the finger printing can be obtained with the help of enzymes restriction endonucleases (RE), which recognize the specific base sequences to cleave DNA into segments of different sizes. These fragments of DNA can be separated by agarose gel electrophoresis, stained with ethidium bromide to be viewed under ultraviolet light. It helps in molecular epidemiology of a particular disease and to determine the origin of infection and involvement of different types of etiological agents in the country. So accordingly strategic measures can be adopted for control and eradication of diseases.

Nucleic acid probes and hybridization:

The fragments of nucleic acids can be made diagnostically useful by isolating specific nucleic acid sequences which are usually cloned DNA probes used to identify specific virus types having complementary base sequences in their genomes from clinical specimens. In this method the native nucleic acid of the virus present in the tissues is separated out and if double stranded it is allowed to denature to single strand by physical or chemical means and allowed to bind with the proves on the basis of complementary base sequences with high affinity which can be detected by auto-radiography, if the probes are tagged with radio isotopes or enzymatically.

Polymerase chain reaction:

This is a quick, reliable and a new method of gene amplification under in- vitro condition. This reaction involves denaturing DNA to be investigated followed by hybridization with short oligo-nucleotides called as primers to “certain regions of the both strands of the starting nucleic acid. The hybridization regions of the oligo-nucleotides are separated from each other by certain distance. The hybridized oligo nucleotides are then elongated by a DNA polymerase by the addition of nucleotides to produce a new strand of DNA which is complementary to the initial sequences, which can be separated in agarose gel for detection of complementarity of the specific primers of particular viral genome, to the wild genome present in the tissues resulting in the amplification of the genome is of diagnostic importance. The PCR products corresponding to the important genes of the full genome are then subjected to RE analysis or sequencing to know the diversity in the specific segment of the genome of different isolates of a specific organism, to identify different types, subtypes variants, or pathotypes of the aetiological agent prevailing in the country.

These molecular techniques have been used successfully in the diagnosis of the following important avian diseases.

Newcastle disease(ND):

ND is caused by Newcastle disease virus, classified as avian paramyxovirus-1, which is a major problem in the country. Pathotyping of the NDV is important to establish the involvement of lentogenic or mesogenic or velogenic strain of the virus in an outbreak. Since it is an RNA virus, the RT PCR was used to amply the F or HA gene of NDV isolated from several species of poultry to group them under different pathotypes of the virus. The sequencing of the connecting peptide region revealed differences among the pathotypes of the isolates.

Infectious bursal disease (IBD):

The gene of interest in IBD virus is VP2 which has genetic markers to identify virulent, mild, very virulent and variant strains of IBDV, which are located in the variable region of the VP2 gene. RT-PCR of the variable region of VP2 using primers designed from flanking conserved region was followed by sequencing was found successful in detecting the virus types, in the field cases.

Avian adenoviruses:

Avian adenoviruses are involved in causing two major economically important diseases of poultry in our country. Hydropericardium syndrome or litchi heart disease and Egg Drop Syndrome-76 (EDS-76). The gene encoding the hexon protein has been the focus of attention for diagnosis of adenoviruses. Hexon based PCR followed by RE-analysis has been used to confirm the involvement of FAV-4 serotype in HPS in majority of cases. But the incidence of HPS by FAV-12 serotype is an interesting observation, which warrants a modified strategy in diagnosis and control of this disease. In case of EDS-76, identification of the virus by isolation is a time consuming and cumbersome process but with PCR from the blood samples of the affected birds can be helpful in diagnosis of this disease within no-time.

Infectious bronchitis (IB):

IB has been considered as a re-emerging disease in the poultry flocks in our country at present. RT-PCR of 3 untranslated region (UTR), N gene and S1 and S2 parts of Spike protein gene has been widely used in characterization of this virus. It has been observed that 3 UTR and N gene are highly conserved sequences which used to design primers for the general PCR helpful in diagnosis of this disease, where as the amino terminal of S1 region of spike protein gene carries variable region of the IB virus. So RT-PCR followed by RE analysis and sequencing of this variable region helps in differentiating the isolates of IB virus.

Marek’s disease:

MD is a lymphoproliferative disease of chicken and quail. Herpes virus of turkey (HVT) is widely used as a vaccine strain for the control of this disease with a diverse level of protection, so serotype 1 attenuated MD virus vaccines are being in use. Moreover, there are reports of the involvement of very virulent MD virus in outbreak therefore, type specific PCR coupled with RE analysis and sequencing would help in differentiating virus types, on the basis of their host origin, virulence and serotypes.

Avian leucosis:

Avian leucosis virus (ALV) of retroviridae family have been classified into six subgroups, A, B, C, D, E, and J. The subgroup J was identified in 1959 causing myelocytic myeloid leucosis in broiler chickens. Primers designed for amplification of env gene and LTR region of different subgroups of ALV helps in differentiation of the virus isolates based on RT- PCR.

Avian Reo virus infection:

A number of disease manifestations of chicken viz. viral arthritis mal absorption syndrome and proventriculitis are associated with Reo virus infection. RT-PCR based amplification of the S1 gene was commonly used for the detection and characterization of Reo viruses.

Conclusion

The intensive poultry farming with increased threshold of commercialization has caused the emergence of new disease conditions with changed symptoms and lesions resulting into severe economic impact has attracted newer concepts with modified and advanced technologies to address the disease problems with efficacy. Therefore, new generation vaccines with newer approaches to enhance their immunogenicity and recently developed molecular approaches in diagnosis of poultry diseases with promising results in accuracy, specificity and sensitivity in least possible time frame can revolutionize the poultry ventures in our country with rapid progress. These newer concepts in poultry disease management can put Indian poultry enterprise at the prominent position of the world poultry map.

Source : IPSACON-2005

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