The cancer that kills chicken slowly

Some diseases are baffling even to us scientists, who find understanding their occurrence difficult.

You see, some of such diseases appear in the population and then just disappear. This reminds us that there is still a lot we do not understand about our environment.

Last week, I had a situation where I had to ask myself and my colleagues why we have not encountered a disease we were all seeing for the first time. We excused Dr Veronica because she is still less than a decade in practice.

But Dr Mwikali and I, with our combined practice of 68 years, had not come across the chicken disease.

The call came from Mercy a poultry farmer. She said her three lots of chickens in their prime production age kept dying at a slow rate of about one to six birds per day. They also were producing below 70 percent and continuously dropping. The oldest lot was almost reaching the uneconomical production limit of 65 percent.

None of the lots had reached 70 percent and above. Under normal circumstances, her birds do above 85 percent. She wondered what the problem could be. On further producing, the farmer said some of the birds would also lose weight before dying. An animal health service provider had told her the birds had a viral infection called Avian leucosis complex.

Other farmers had told Mercy they had a similar problem but their service providers had given varying reasons. Some had said it was a problem with the feed while others had diagnosed aflatoxin poisoning. She needed a professional second opinion.

Mercy further told me her dead birds had white spots in the liver. The livers also broke easily when pressed. She wondered whether the current problem in availability of feeds could be the issue.

We agreed it would be better for her to bring some fresh dead birds to the clinic for me to examine since I may need to run samples in our laboratory.

Mercy brought three dead layers that my colleagues and I promptly examined. External review showed nothing abnormal with the birds.

Upon opening, the intestines were very clean. The answer, however, was in four internal organs. The liver, spleen and mesentery had multiple white spots. The mesentery is the sheet-like organ that covers and wraps the abdominal organs. Some of the spots, especially in the liver and the spleen, fused to form a whitish-grey layer. The liver and spleen were easily breakable on application of slight pressure.

The other organ affected was the bursa of Fabricius. This organ is responsible for development of immunity in birds but by the age of sexual maturity, it regresses and disappears. In all the three birds, which were already laying eggs, the bursa was still present and enlarged. It also contained whitish nodules.

I cut the surfaces of all the affected organs in the first bird and took an impression smear for microscopic examination. I did not take samples from the other two birds because they presented the same disease picture.

By this time, I was convinced the birds had an attack of Avian leucosis. Examination of the smears confirmed the diagnosis. There were numerous immature cells called lymphoblasts, some which showed signs of rapid division that we call mitotic figures.

Mitotic figures are a cardinal sign of diagnosing cancerous growths. The type of cancer is known by the type of cells that are involved and the organ they originated from. In this case, the first organ affected was the bursa of Fabricius because it was enlarged and still present when it should have disappeared in chicken of egg-laying age.

I explained to Mercy our findings and advised her accordingly. Avian leucosis complex is a set of cancerous diseases caused by the Avian leucosis virus. It affects all chicken breeds and some wild birds including pheasants, partridges and quails. Avian leucosis virus is not known to affect humans and other mammals.

The virus has many different types. The strain in Mercy’s birds infects the bursa of Fabricius and causes the organ to swell and produce cancerous lymphocyte cells of the B type. The cells are seen as immature stage called lymphoblasts. The cancerous cells migrate from the bursa through the blood to other body organs especially the liver, spleen and mesentery. They may also be found in the kidneys and the heart. The cancerous cells replace the normal cells of the affected organs, cause progressive weight loss, organ failure and death.

Affected flocks have low but sustained mortality because the spread of the cancerous cells in the body and replacement of cells in organs takes time.

The disease has no treatment or vaccination. Farmers should prevent it through good hygiene and purchasing bird stocks from hatcheries confirmed to be free of the disease; through regular testing of the parent stock by the hatcheries.

The virus is transmitted through infected eggs or chicks from contaminated environments. Meat and eggs from infected flocks is fit for human consumption except where the disease has advanced to make organs unsightly with whitish coloring and nodules. I advised Mercy to clear all her affected flocks, clean and disinfect her buildings properly before restocking. She would also need to discuss with her chicks supplier so that they could confirm freedom from the disease in their flocks.

I still could not understand why I had never come across the disease.

The writer is a veterinary doctor


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