Thank you for this question. As far as I know Marek's Diseases lesions are always based on pleomorphic mononuclear (lymphoid) cell population, and in lymphoid leucosis you can usually find more uniform lymphoid cells. The very typical findings are disseminated focal mononuclear hyperplasia in Marek's Dis and more diffuse lymphoid cell hyperplasia in leucosis as well. So, this ia a short histopathological description. Main gross lesion differentiaton between the Marek's dis and leucosis is the absence of the lesions in e.g. nn. ischiadici in leucosis.

My observations for Marek's Disease differentiation from LL are;

Grossly, We do see opacity of eyes supplemented with paralysis in young as well in adult birds depending upon type of marek's disease (classic form) but not in LL

Secondly, hair folliculitis is a remarkable lesion in case of MD but not in LL.

According to my working Marek pathology, you should see nervus ischiadicus ( nerve enlargement), fitting the typical ballerina,interfollicular tumors in the Bursa of Fabricius, Lymphoid proliferation in skin and feather follicles for Marek disease.

In Marek's disease, tumors are found in the guts of birds, in the central and peripheral nervous system, skin and eye, as in leukemia relevant macroscopic changes are not observed; we observe changes and infiltration of blood lymphoid cells in liver, spleen and other organs.

In addition to peripheral nerves affections (enlargement, loss of striation), eye (grey-eye), skin lesions (nodules at feather follicles), regression of Bursa of fabricius, nodules affecting intestine and mesentery; we have consistently observed emaciation and enlargement of the proventriculus with or without nodular growths.

Hi Dr Kuldip

Grossly, it is quite difficult to differentiate between MD and LL. However, neural involvement is commonly seen in MD and absent in LL. in LL, lesions in bursa composed of focal or nodular tumors whereas in MD bursa generally becomes smaller than normal.  

Dear colleage

It´s impossible to make the differential diagnosis of acute Marek´s disease of Avian Leucosis, specially from avian lymphoid leucosis, both diseases have the same gross appearance, so, you have to do an histological investigation including organs such as brain, liver, heart, brachial and sciatic nerves, lumbosacral spinal cord and red bone marrow of the sternum. Avian Leukosis doesn´t have any change or lession in the brain or nerves, but Marek´s Disease have an interstitiall infiltration of limphocites and plasmacells in nerves and perivascular cuffs in the brain. You should consider that avian reticuloendoteliosis, also a neoplastic disease, can affect nerves such as Marek´s Disease

I agree with Dr Limia, the gross lesions of all three neoplastic viral  diseases of poultry including MD, LL and Reticuloendotheiliosis are overlapping. So it is very difficult to differentiate these diseases on the basis of gross lesions.  One of my student  recent work shows that even the microscopic lesions of these diseases are confusing. The microscopic lesions of reticuloendotheiliosis resembles to the lesions of both MD and LL. In addition, there are mixed infection of avian neoplastic viruses under field conditions. So advanced biomolecular techniques like IHC and PCR are required for differential diagnosis of these oncogenic viral infections.

I must admit that in the last 30 years working as a pathologist, I had never seen the macroscopic expressed hyperplasia or characteristic microscopic findings of mononuclear cells hyperplasia (with or without edema) in the sciatic nerves of poultry suffering from LL.

I agree with you Dr. Marina Tišljar, I have not seen either, however, exceptionally in myeloid leukosis a slight myelocytic infiltrate can be seen in the brachial and sciatic nerves. Here in Cuba we have seen it in layers and broilers, of course, the macroscopic appearance differs from the typical lardaceous appearance of lymphoid leukosis, MD and RE. Although the above is true of what Dr. Kuldip Gupta says, diagnostic techniques such as PCR and ElISA are required to confirm the diagnosis, since diseases are often present simultaneously in the herd.

Tumours on mucosa of bursa are considered virtually pathognomic for LL, enlargement of feather follicles ( folliculitis) only in MD, Nerve enlargement is common in MD not in LL.