What causes egg drop syndrome?

Egg drop syndrome is a viral disease that affects laying hens in various parts of the world. The etiological agent is the duck adenovirus type A which is originated in ducks and was first described in the 1970s. It is also called posture drop syndrome virus or adenovirus 127.

The virus replicates in the nucleus of cells and do not have an envelope, which makes it highly resistant in the environment even at pH conditions of 3 to 10 and at temperatures up to 60°C for 3 hours.

Egg drop syndrome virus has been found to replicate in cells of the infundibulum, oviduct, isthmus, and the mucosa of the respiratory system. This is closely related to the ways of transmission of this disease.

Initially, the disease affected breeding birds, but thanks to management and hygiene strategies, it was controlled in this population. It is currently considered a disease of great importance in the laying hen industry.

Epidemiology of egg drop syndrome

Ducks and geese are the main reservoirs of the egg drop syndrome virus. In addition, serological evidence has been found in other wild aquatic and terrestrial birds. Laying hens, quail and ducks develop the disease with signs that significantly impact posture.

The distribution of the egg drop syndrome in laying hens has been reported in the continents of Europe, Latin America, Asia, and Africa. However, it is not reported in the United States or Canada. It is widely distributed in the world due to the easy transmission.

The transmission of this viral disease in laying hens occurs both vertically and horizontally. In vertical transmission, hens in the production stage excrete the virus in eggs both internally and externally. In addition, chicks hatch with the infection and shed the virus into the environment. On the other hand, the egg drop syndrome is also transmitted horizontally through fomites in poultry farms, water contaminated with feces from wild birds, and iatrogenically with needles contaminated with the virus. On the farm, transmission is by the oral-fecal route, since infected birds eliminate the virus in their feces, contaminating the environment.

Typical signs and lesions of the syndrome in layers

The signs observed in laying hens are associated, as the name of the disease indicates, with a decrease in egg production between 10 and 40%. In addition, the eggs produced have alterations in the shell, which is soft, with color changes and the eggs can even hatch without shells. In the early part of the syndrome, it is difficult to identify these signs because the hens eat these eggs without shells and the eggs are not always altered.

The decrease in egg production is related to the pathological effects that this adenovirus causes on the reproductive system of laying hens.

In birds at the macroscopic level, it is observed that there can be atrophy of the oviducts, ovarian inactivity, and the appearance of edema. At the cellular level, degeneration of the uterine glands may appear, as well as evidence of inflammatory processes in the reproductive apparatus. In the cells of the uterus, isthmus, and glands, intranuclear inclusion bodies can be found, which are evidence of the presence of this type A adenovirus.

Productive impact of duck adenovirus type A

The main impact of the egg drop syndrome in laying hens is a 10-40% decrease in egg production. In addition, egg quality is also drastically reduced due to poor eggshell formation, changes in egg color, and altered laying patterns.

On the other hand, the virus is eliminated through the egg and day-old chicks, which makes it persistent in production. The dissemination of this type A adenovirus implies a greater number of affected birds during their productive stage, although it does not cause mortality. In some cases, secondary infections by bacteria may develop, causing mortality and complicating the course of the disease.

A 2010 study compiled that in the 1980s, about 20,000 tons of eggs were lost in Nigeria due to this egg drop syndrome (Ezema, 2010). In addition, other studies indicated a 30% decline in egg production in Northern Ireland. There are reports in the scientific literature of outbreaks in Bangladesh, Japan (Yamaguchi, 1981), Bolivia (Bishop, 1996), South Africa (Bragg, 1991), and India (Suresh, 2013).

Diagnosis of postural drop in posture syndrome.

Diagnosis for low-laying syndrome in poultry farms starts with the identification of clinical signs related to low egg production, shell disturbance, and some cases of diarrhea. In addition, productive parameters should be monitored to find abnormalities in the flock of laying hens.

The main differential diagnosis of this syndrome is infectious bronchitis since this viral disease also causes a decrease in egg production and abnormalities in egg formation. Other similar diseases are Newcastle disease and avian influenza. The difference between these diseases and the egg drop syndrome lies in the respiratory and nervous signs that they can produce, which are not observed in the egg drop syndrome.

On the other hand, there are laboratory tests available to diagnose this syndrome focused on its detection and isolation. The PCR test is rapid and uses a cloacal swab sample from live birds or uterine glands of slaughtered birds in search of the virus. Samples from the oviduct, spleen, cecal tonsils, and kidneys are also used. Embryonated duck cultures or cell cultures are used for the isolation of FSSV.

Another test available for the diagnosis of this viral disease is the Hemagglutinin Inhibition and ELISA test for the detection of antibodies against the virus. These tests have high sensitivity, but their correct reading requires that the birds have not been vaccinated against this disease. The sample required for these serological tests is blood from birds showing signs.

Prevention strategies for Egg Drop Syndrome

There is currently no treatment available for egg drop syndrome. Prevention measures relate to hygiene strategies and management, These are molecules of botanical origin that have an enhancing effect on mRNA-protein translation at the enterocyte level. Thus, administering pronutrients can increase tissue-specific protein levels that optimize target cell function.

• Introduce replacement birds into the farm that have certificates indicating that they are free of egg drop syndrome.
• Employ on-farm cleaning and disinfection protocols to eliminate viruses from the environment, especially in egg production equipment.
• Implement a sanitary program for the prevention of diseases prevalent in the area, as well as the management of wild birds in the region.
• Carry out periodic vaccinations available in the country with inactivated viruses or use of autogenous vaccines.

Conclusions

Egg drop syndrome is a disease of viral origin that affects laying hens. It was discovered in the 1970s in ducks, so the virus was named duck adenovirus type A. This bird is considered its main reservoir, but it has a great productive and economic impact on laying hens.

This virus is transmitted vertically and horizontally within the poultry farm, making it persistent in the environment. Moreover, it is distributed throughout the world except in North America.

The main signs of this disease are related to a drastic drop in egg production, as well as a decrease in eggshell quality. Laying hens may have difficulty reaching peak production and spread the virus through feces and in eggs.

The clinical signs of the egg drop syndrome develop due to lesions in the reproductive tract of laying hens.

The diagnosis of this disease in poultry farming is made through the productive monitoring of the farm, as well as by laboratory tests such as PCR, Elisa, and Hemagglutinin Inhibition.

Finally, there are prevention strategies related to the management system, biosecurity protocols, hygiene of the facilities.

References.

Bishop, S. C., & Cardozo, P. (1996). Egg drop syndrome’76 in Bolivia. Tropical Animal Health and Production, 28(3), 199-206.

Bragg, R. R., Allwright, D. M., & Coetzee, L. (1991). Isolation and identification of adenovirus 127, the causative agent of egg drop syndrome (EDS), from commercial laying hens in South Africa.

Ezema, W. S., Nwanta, J. A., Aka, L. O., & Ezenduka, E. V. (2010). Egg-Drop Syndrome’76 in different bird species in Nigeria–a review of the epidemiology, economic losses, challenges and prospect for management and control. World’s Poultry Science Journal, 66(1), 115-122.

Suresh, P., Shoba, K., & Rajeswar, J. J. (2013). Incidence of egg drop syndrome–1976 in Namakkal district, Tamil Nadu, India.

Yamaguchi, S., Imada, T., Kawamura, H., Taniguchi, S., Saio, H., & Shimamatsu, K. (1981). Outbreaks of egg-drop syndrome-1976 in Japan and its etiological agent. Avian diseases, 628-641.