Diseases Transmitted to Eggs

Early embryonic death, blood ring, dead-in-shell……these terms frustrate and confound aviculturists, novice and professional alike. The reasons that embryos die are many, and diagnosing the specific cause of death can prove elusive in some cases. The serious aviculturist, whether a hobbyist or one who makes her living from bird breeding, should seek to work with an experienced avian veterinarian who can help with all facets of aviculture. Every egg that dies prior to hatching should be examined by an avian veterinarian who can perform an egg necropsy and any other tests that may help to determine the cause of death. Often, histopathology (microscopic examination of tissues) will prove diagnostic, if the embryo has recently died. Bacterial and fungal cultures, stains of egg membranes, viral isolation and DNA PCR probes for specific organisms may help in diagnosing the cause of death.

There are many infectious organisms that can be transferred from the hen to the egg that may cause the egg to die. In some cases, the infectious organism may infect the egg, yet the embryo may continue developing, and may even hatch, carrying the organism at hatch time. If an organism is passed from an infected hen directly into an egg, and then into the developing embryo, this is called vertical transmission. The term vertical transmission is also used to describe transmission of an infectious agent from a parent to an egg during fertilization, during egg development in the oviduct of the hen or immediately after oviposition. Once the egg is laid, some infectious organisms can pass through the eggshell upon contact with contaminated feces, urates or bedding. This is also considered vertical transmission if infection occurs immediately after laying. Some organisms are transmitted from the ovary to the egg, and this is called transovarian transmission. Infectious organisms harbored in the oviduct can also be passed into the egg prior to the shell being formed. Some organisms can infect eggs if contents from the cloaca contaminate the surface of the eggs, and then penetrate the egg. The other method of transmission of infectious organisms is by horizontal transmission. Some ways that horizontal transmission occurs are by preening, inhalation, copulation, insect or animal bites, ingestion, contact with contaminated equipment or fighting.

It seems obvious that prior to the egg membranes and shell being applied to it, the egg would be susceptible to infection by numerous infectious organisms. Even though the eggshell appears solid, it contains microscopic pores that can allow liquids and organisms of small enough size into the egg. The pores allow the transfer of gasses, as well.

Bacterial Diseases

Chlamydia psittaci is a primitive bacteria that can be vertically transmitted from an infected hen through the egg to the embryo. Depending on the pathogenicity of the strain and the number of organisms that are passed into the egg, the embryo may die during incubation, or it may actually hatch as a baby bird with chlamydiosis. It should be noted that transovarian transmission of chlamydiosis has not yet been confirmed by researchers, so it may be that the eggs are contaminated with the organism by some other vertical method.

One avicultural client of mine with over one hundred pairs of large psittacines was having a problem with a pair of blue and gold macaws. They pulled all of this pair's eggs for artificial incubation, and several eggs in the incubator had died about half way through incubation. During the egg necropsies, I tested for chlamydiosis by sending in a swab for DNA PCR testing. I also tested the adult breeder pair for chlamydia, using the University of Georgia tests, which include DNA PCR testing of the blood, DNA PCR testing of a choanal and cloacal swab, plus a latex agglutination titer, which is run at the University of Miami. The eggs were positive for chlamydia, as were the parent birds. I recommended that the breeders remove from the incubator those eggs from the blue and golds, and place them in a small incubator alone. There were five remaining eggs that were in the early stages of incubation. To increase hatchability of the potentially infected eggs, we began a course of egg injections, using injectable doxycycline, which is an excellent drug for chlamydiosis.

The eggs, much to our surprise, all continued to develop, and all five actually hatched on schedule. As soon as the eggs hatched, I instructed the owners to begin medicating the hatchlings with oral doxycycline, which would be continued for 45 days total. Because all baby birds receiving antibiotic therapy should also be prescribed antifungal medication to prevent infection with Candida sp., we also started the babies on a combination of oral nystatin suspension and fluconizole. The babies were also prescribed avian lactobacillus and acidophilus to give them some normal, good bacterial flora.

The five baby blue and golds all developed normally and weaned on schedule. Subsequent testing showed that these babies showed no signs of chlamydiosis. It should be noted that testing is not always 100% accurate, and although treatment is often curative, some birds may never clear the organism from their system completely, resulting in asymptomatic carriers. However, these macaws all have thrived and I have followed several to adulthood, and all have remained healthy.

Bacteria of the genus Salmonella can also cause embryos to die in the shell, or if the egg is contaminated by a very small number of bacteria, Salmonella can cause weak hatch babies that may die shortly after breaking out of the egg. The bacteria may cause yolk material to coagulate in the egg, and dead embryos may show hemorrhagic streaks on the liver. The spleen and kidneys may be congested. Pinpoint areas of the liver may be necrotic. Inflammation of the pericardium may also be seen. Salmonella are motile bacteria that can penetrate the eggshell and can be transmitted vertically. Culture of the infected embryo will prove diagnostic.

Some Staphylococcus bacteria can kill embryos. The avian embryo can be resistant to some strains of staphylococci, but can be highly susceptible to other strains. Infected wounds on parent birds can infect eggs, as can staph infections found on the hands of aviculturists, if the egg comes in contact with lesions. Artificial incubators will grow staph readily, and it can spread horizontally in this manner. An embryo can die within 48 hours of exposure to some strains of staph, especially Staph. aureus. The older the embryo is at time of first exposure to staph, the less chance of embryonic mortality. Hemorrhages may be found on various internal organs. A laying hen can develop an ovary infected with Staph. faecalis, which can contaminate the forming egg. Contaminated eggs will have up to 50% mortality. Culturing the egg is important for diagnosis.

E. coli is a common bacteria normally found in the GI tract of mammals, and some birds, as well. It can enter the egg from an infected reproductive tract of a hen. E. coli can also penetrate through the eggshell if the egg is contaminated with fecal material. E. coli commonly causes yolk sac infection, causing the yolk sac contents to appear watery and yellow-green or yellow-brown. Dirty nests and cages can serve as sources of contamination to eggs. The use of water bottles can reduce the amount of E. coli that builds up in the GI tract of birds. In my experience, aviaries that use a watering system and not water bowls will have fewer problems with sub-clinical bacterial infections in their breeder birds and their offspring.

Many embryos infected with E. coli will die late in incubation or shortly after hatching. If an E. coli infection is acquired during incubation, the hatchling may develop an umbilical and yolk sac infection (omphalitis) and they may have poor weight gain. Cracked eggs are more easily infected and may serve as a source of infection for other eggs in the incubator. Cracked eggs should be repaired as soon as the damage is discovered, or they should be discarded.


Mycoplamatales are one order of microscopic organisms that replicate by binary fission. They have no cell wall, but have a three-layer membrane. They are more primitive than bacteria, and must live and grow inside the host. In the environment they live only for a short time. Although we have much to learn about mycoplasmas, they can be involved in problems with cockatiel conjunctivitis and respiratory infections, and also respiratory/eye problems in other species of pet and breeder birds. The organism is spread by the respiratory excretions and by the gonads of both sexes, and infection in the air sacs can lead to contact transmission of the ovary and developing follicle. Transovarian transmission can occur. Mycoplasma can spread to the egg from an infected oviduct or from the semen of infected male birds.

It is possible to treat eggs infected with Mycoplasma infections. Tylosin is injected into the air cell at the start of incubation. A combination of lincomycin and spectinomycin is also effective for egg injection. Dipping the eggs in antibiotic solutions is effective in reducing the incidence of disease, however I have never used this method personally.

A third treatment that has been useful in breaking the transmission cycle of Mycoplasma gallisepticum and M. synoviae involves elevating the temperature in a forced-air incubator to 46 degrees C for 12-14 hours before incubating the eggs normally. This technique inactivates the Mycoplasma organisms, but it will reduce the hatchability by 8 to 12 %.

Viral Diseases

Several important viral diseases are vertically transmitted in birds. Psittacine Beak and Feather Disease, (PBFD), has been demonstrated to be vertically transmitted, since the virus is found in the blood of infected birds. It has been shown that artificially incubated baby birds from PBFD-infected hens will consistently develop PBFD. So, attempting to control PBFD by pulling eggs for artificial incubation is futile.

Avian paramyxovirus 1, (Newcastle's Disease or PMV 1), is one of a group of nine distinct serovars (with several more yet to be characterized) of the virus that are dangerous to birds. Although paramyxovirus is theoretically vertically transmissible, this mode of transmission is considered unlikely because infected hens will generally stop laying eggs when they are viremic. Eggs contaminated by virus-laden feces immediately after laying could contaminate an incubator, and can serve as a source of virus for recently hatched neonates.

Herpesviruses, most of which are quite species-specific, include Pacheco's Disease Virus, Amazon tracheitis virus, respiratory disease in Neophema sp. and Psittacula sp., wart-like or flat plaque-like lesions on the skin of psittacine birds, budgerigar herpesvirus, pigeon herpesvirus (infectious to budgies and cockatiels), falcon herpesvirus (infectious to budgies and Amazon parrots), and Marek's disease (suggestive lesions in budgies). It has been theorized that some hens latently infected Pacheco's Disease virus can pass the virus (and antibodies to the virus) to their eggs. The resulting neonates would be latently infected carriers that might not develop detectable levels of antibodies. Herpesvirus of European budgerigars causes feather abnormalities (referred to as "feather dusters") and is thought to be egg transmitted, and has been demonstrated in dead-in-shell embryos and is considered a major cause of early embryonic death in affected flocks, resulting in decreased egg hatchability.

Proventricular dilatation disease, PDD, is an enigmatic disease that is being diagnosed with increased frequency. Although we have much to learn about this disease, my personal experience indicates that PDD may be vertically transmitted. I am working with an aviary that has a pair of severe macaws whose eggs were taken for artificial incubation because the parents often damaged the eggs after being laid. The eggs were placed in a brand new incubator, and the babies were the only ones in the nursery while they were being hand-fed. The owner had problems with the babies from day one, as the crops were slow to empty, and they did not gain weight properly. The babies had to be given antibiotics, antifungals and motility enhancers (cisapride) to get them to digest their food at all. One baby died when it was six weeks of age, and histopathology showed all the classic PDD lesions. The second baby died shortly after weaning and once again, histopath confirmed PDD. Histopathological examination of tissues from a dead bird (especially the proventriculus, ventriculus, crop, small intestines, and brain) is the only way to confirm PDD in a dead bird, as grossly, many diseases can look like PDD. When the PDD tests that Dr. Branson Ritchie at the University of Georgia is developing are available, we will be very interested in testing the parent birds of these two babies. At this time, barium radiographs may render a presumptive diagnosis, and biopsy of areas of the gastrointestinal tract may prove diagnostic if positive, but once the new testing becomes available, it will be much easier to screen for this terrible disease.

Some adenoviruses, REO viruses, and reticuloendotheliosis viruses can be vertically transmitted. Influenza A may be vertically transmitted, as well.


Oddly enough, some parasites have been documented to occur within eggs. Adult ascarids (roundworms) have been found within eggs. These worms get into the egg by moving from the cloaca up into the oviduct, where the eggshell is then placed around the aberrant parasite. The fluke, Prosthogonimus ovatus can be found in the oviduct of Galliformes and Anseriformes, and may also be trapped within an egg, but the flukes are more likely to result in abnormal eggshell formation.


As our knowledge of avian medicine and theriogenology grows, we may discover other organisms that can be vertically transmitted. But with the information that we have today, it may be possible to save some eggs that have acquired an infectious agent. Egg injections are routinely performed in my practice, and this has greatly increased the hatchability of infected eggs. And by testing for the infectious organisms that we can, it is possible to cull, as in the case of PBFD persistently infected birds, or treat the parents, as in the case of many bacterial or chlamydial infections, resulting in healthier baby birds.

Copyright © 2006 Margaret A. Wissman, D.V.M., D.A.B.V.P.
All Rights Reserved

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