A field study of clinical immunosuppression in a layer flock

Introduction

Immunosuppression clinical pictures in commercial poultry are observed with greater intensity in current breeding systems. These can be due to environmental, nutritional, health, and toxic causes, without being able to determine with certainty which of these causes was the primary agent that triggered the clinical picture. They usually occur in chronic form, with not very high mortality (Rosales, 1999). This type of cases has been attributed to the action of certain viruses, such as the Chicken Anemia Virus and Gumboro disease, mycoplasmosis, mycotoxicosis, etc. (Dohms & Saif, 1984)

This paper presents a case that occurred on a replacement breeding farm, located in the Luján area, province of Buenos Aires, Argentina, during the month of May, 2010, in batch of layer flock for egg production, 8 weeks of age, housed in cages. There was a high mortality rate, which lasted for three weeks, leaving as a sequel to a marked lack of uniformity and a poor immune response to vaccines applied later.

Materials & Methods

The batch of 24,000 birds was housed in cages, with a density of approximately 200 cm² per bird. An increase in mortality occurred, which increased during the next few days and persisted up to 13 weeks. Mortality rates ranged from 3.13% to 11.20% from week 8 to week 13 of age.

Birds were vaccinated at the incubation plant using recombinant vaccines against Marek disease, Gumboro disease, Infectious Laryngotracheitis and avian smallpox. Two doses of Newcastle and three of bronchitis were applied via drinking water; the last of them 12 days before the start of symptoms.

The balanced food used was self-produced. Periodic physiochemical and mycotoxicology controls were performed on the latter and its raw materials.

The general condition of the batch was good at the beginning, only manifesting mild respiratory symptoms. Weights were inferior to the standards of the line. At that time, medication was applied with tylosin phosphate associated to oxytetracycline for 7 days in food, at therapeutic doses. Food was reinforced with choline chloride and vitamin-mineral supplement in doses recommended by the nutritionist.

The following determinations were made:

Necropsy of birds with clinical symptoms, and recently dead.
Histopathology of organs: liver, spleen, intestine.
Detection of antibodies in 15 sera against IBD, IBV, CAV, MS and MG using the ELISA technique proposed by Idexx Laboratories (Kleven, 2001).
Investigation of Enterobacteria from liver and air sac content in 10 birds (isolation in Mac Conkey agar).
Investigation of mycotoxins in balanced food through thin-layer chromatography (TLC), according to methodology proposed by the A.O.A.C. (1990).

Results and Discussion

The autopsy showed observed marked congestion and increase in size of the liver, liver friability, generalized bleeding in intestine with bleeding content inside, presence of hemorrhagic contents in abdominal cavity, with marked alteration of clotting, marked atrophy of the thymus, fibrinose aerosaculitis, opacity of air sacs.

Histopathological studies described a splenitis with amyloidosis, and venous congestion, necrosis, hyperemia and heterophile infiltrations. In liver, marked venous congestion, hyperplasia of bile duct, vacuolar hepatosis and necrosis, corresponding to lesions of toxic origin.

The serological studies performed at 11 weeks of age indicated seroconversion for CAV, testing positive to all the examined sera. 55% of the analyzed sera tested positive to MS and all the sera tested negative for MG. The IBD titles were high, according to expectations for a recombinant vaccine, indicating a challenge posed by the disease. The IBV title average was high, indicating a possible challenge posed by the disease. Bacterial colonies compatible with Escherichia coli were isolated in air sac content. No other enterobacteria were detected.

Studies for the determination of mycotoxin led to T₂ values between 500 and 1000 ppb in soy extrusion used in the manufacture of balanced food, in the period between one month before and one month after this case happened.

The mortality was gradually falling (Figure 1), showing an approximate weight uniformity of 50%. In the month of February of the year 2011, the birds were laying eggs, without apparent clinical symptoms, with a percentage of egg laying of 5 points below the standards of the genetic line and antibody titles against low egg-laying, non-protective, so the batch was revaccinated and it has shown no clinical symptoms up to the present.

This clinical picture of immunosuppression was caused by factors which acted synergistically: Anemia virus, mycoplasmosis, colibacillosis, mycotoxicosis, high population density and failures in biosecurity systems, as health deficiency vacuum and poor hygiene and disinfection procedures.

Figure 1. Mortality curve of birds from week 1 through week 18 of age.

Conclusions

The high productive requirements to which birds of the current genetic lines are subjected are leading to diminished resistance to certain stress factors. The permanent health challenge in rebreeding farms of multiple ages, coupled with shortcomings in biosecurity levels, require that these birds have a competent immune system.

It is important to implement a routine procedure for determining mycotoxins, in order to foresee, as far as possible, the diversion or rejection of contaminated supplies, and a routine serological monitoring to assess the levels of antibodies developed in response to the applied vaccines.

Bibliography

AOAC International. 1990. Method no. 986.17 and 986.18. In: Official Methods of Analysis of the Association of Official Analytical Chemists, Helrich K. (ed.), Arlington.

Dohms JE & Saif YM. 1984. Criteria for evaluating immunosuppression. Avian Dis. 28:305-310.

Kleven SH. 2001. Some perspectives on micoplasma diagnosis and control. En XVII Congreso latinoamericano de avicultura. Guatemala.

Rosales AG. 1999. Inmunosupresión causada por enfermedades virales, estrés, micoplasmas, manejo y nutrición. En III Encontro Internacional de Ciencias Agrarias. MG. Brasil.

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A field study of clinical immunosuppression in a layer flock

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