Introduction

What are the symptoms of immunosuppression?

Problems caused by mycotoxins

Immunosuppression caused by aflatoxins

HI titre against new castle

Salient points

Conclusion

Poultry performance and efficient disease prevention are obvious goals for the poultry industry worldwide.  Most disease problems in poultry today are usually caused by interaction of many factors where immunosuppression plays a key role (for example, CRD & respiratory complexes, Wingrot).  There are varieties of agents that can impair the normal resistance mechanism of birds.  The presence of such agents in modern poultry operation is of great importance as they can directly affect the performance and incidence of disease. 

 The economic impact of immunosuppression can be quite large, therefore, it is important to identify the cause and take a corrective action to put the immune status in place.  The best-known causative agents are  probably immunosuppressive viruses and mycotoxins.    

The immune system is obviously the defense mechanism of the body. The key organs involved in the immune system are thymus, bursa, liver and spleen.  In poultry, the bursa, an organ located in the lower back of the chicken, along with the thymus in the neck, are key sites for immune response.  If the feed contains mycotoxins, there will be regression of these organs and hence the ability to produce immunoglobulins decreases.  This immune suppression can have chronic effects in times of stress or challenge Very often, immune suppression can be seen as the accumulation of fluid in the heart, or a balloon-like enlargement in the proventricular.  To illustrate the physical impact of mycotoxins, let us take the example of the broiler breeder.

Breeders are considered as the center piece of poultry production to supply the necessary flock for egg and meat production. The quality of Day Old Chick (DOC) depends directly on the status of the breeder flock. First, the aflatoxin has a structure that interrupts the vitamin’s role in calcium metabolism.  The net effect is a reduction in leg strength and reduced physical ability of the cock to mate; hence, fertility is reduced.  By impacting directly on the liver, crucial transfer of vitamins (fat-soluble) to the egg and hence the chick is further impaired. One of the most important aspects to consider in a DOC is the level & quality of maternal antibodies, which should be high & uniform to provide good protection against disease outbreak during first week. With a reduced size in antibody-producing organs such as the spleen and bone marrow, fewer antibodies are transferred to the egg.  Whenever a breeder flock is subjected to immunosuppression, the level of circulating antibodies will be affected and thus transfer of immunity to DOC.  This decreased level of immunity combined with other agents such as stress, can have devastating effect as  the chick must depend on transferred antibodies for the first ten to fifteen days.  The vaccination against diseases such as gumboro of chicks at ten days typically takes five days to show response; but if sufficient antibody has not been produced, the chick may actually have no immune defense mechanism for a few days.

It is now universally recognized that the mycotoxins contamination result in increased susceptibility to infectious agents that would not normally infect the animal so easily.  The impact of mycotoxins on resistance to infectious diseases revolves around their interference in one or many immune mechanisms.  It was demonstrated that aflatoxins drastically impair both number and activity of phagocytic cells. ie,  lower than the normal number and lower than the normal movement in response to pathogens.  This is generally termed as ‘Lazy Leucocyte Syndrome’.  Increased susceptibility of chickens to IBD and more severe lesions are associated with aflatoxin contamination.  Similarly, impaired antibody response following vaccination to NCD & IB are noticed.  On other hand, Trichothecene toxins impair the protein synthesis and thus interfere with antibody production and membrane integrity. 

In a nut shell, chronic ingestion of mycotoxins result in:

• Increased susceptibility of birds to various infectious agents including bacteria and viruses.
• Impaired disease resistance through an alteration of various defense mechanism.
• Interfere with normal T & B cell immunity including suppression of antibody production.

 

  Salient Points: 

• Mycotoxin contamination appears to be a growing economic problem for live stock and feed industries.
• The presence of mycotoxins in feed stuff reduces the feed quality in terms of both energy and protein value 25% reduction in ME & 5% protein).
• High moisture content (>12%) and grain damage favors mold growth.  Mycotoxins may get concentrated from 30-500 times in broken grain as compared to whole grain.
• Risk from mycotoxin not only depends on the degree of contamination, but also on duration of exposure.
• There is no safe level for mycotoxin as the damage depends on too many factors including health and nutritional status of the bird, stress and other disease conditions and presence of other mycotoxins. 
• Testing the feed for one particular toxin may not give a clear picture as there could be number of other mycotoxins.  In practical feeding situations, it is rare to find a single mycotoxin. 
• Mycotoxins generally have synergistic effects.  So the damage caused by the combination is more devastating.   Multiple mycotoxins at low to moderate levels may produce symptoms often distinctly different from those associated with individual mycotoxins.
• Immunosuppression  is one of the serious outcome of Mycotoxin contamination, often unnoticed, making bird susceptible to infection and complex disease problems.
• Control and prevention of mycotoxicoses ideally should begin with from the point of harvesting,  then storage and transportation.  However, many times, it goes beyond control. 
• Purchasing a clean feed stuff is an important practice wherein, a proper sample drawn from the lot is analyzed for presence of toxins.  But this is not always possible in practical feeding situations.
• Simple thing is to avoid feed stuff which appears to be moldy or dilute such feed stuff with fresh feed. 
• Despite of all efforts, mycotoxins invariably creep in to the feed stuff. It is practically unavoidable. 
• Use of mould inhibitors and toxin binders provides practical solution.  However, make sure, the selected product is effective at the field condition.

 

Problems caused by mycotoxins
Feed refusal	Immunosuppression
Poor growth and weight gain	Toxic residues
Low egg production	Organs damage
Poor shell quality	Leg weakness
Infertility	Mortality
Bottom Line:  Economic Loss

 

 

Immunosuppression caused by aflatoxins

Groups	NCD-HI titres
0.38 ppm	6.00 + 1.00
0.75 ppm	4.66 + 0.57
1.5 ppm	3.33 + 0.57
3.0 ppm	2.33 + 0.56
Control	6.61 + 1.52
	Ref: C.S. Bakshi & others, Ind. J. Comp. Microbiol, Immunol. & Infect. Dis.

 

HI titre against New Castle Disease
Aflatoxin B1 level (ppm)	5th day	27th day	42nd day	56th day	8th week bursal weight (per cent live weight)
0.00	1.618a + 0.05	1.975a + 0.04	2.389a + 0.04	2.021a + 0.04	0.127a + 0.00
0.10	0.505ab + 0.03	1.731b +0.04	2.051b + 0.04	1.806b + 0.06	0.108b + 0.00
0.20	1.392bc + 0.05	0.693b+ 0.04	1.919bc + 0.04	1.662bc+ 0.04	0.105b + 0.00
0.30	1.317c + 0.05	1.524c + 0.05	1.825cd + 0.03	1.448cd+ 0.08	0.098cd + 0.00
0.40	1.242c + 0.05	1.317d + 0.05	1.731de + 0.04	1.214de + 0.07	0.094cd + 0.00
0.50	1.129c + 0.05	1.185d + 0.05	1.637e + 0.065	1.968e + 0.07	0.090d + 0.00

 

 Conclusion:  

This situation of immunosuppression most certainly occurs more frequently than is currently recognized.  Therefore, the poultry industry must exercise to extreme caution to manage mycotoxicoses with specific regard to maintenance of health status and immune status.  In a field condition, situation may arise which often confuse.  For eg., following vaccination, development of immunity is expected.  However, due to destruction of cells involved in immune process by mycotoxins, expected immunity may not occur.  In such situations, vaccine failure is attributed to poor vaccine, improper administration and host of other factors.   Regretfully, the failure of bird to develop immunity is seldom linked to  mycotoxins.  From the practical point of view,  disease control means improved immunity which, obviously calls for control of mycotoxicoses.