There has for quite some time been considerable controversy regarding the ability of Campylobacter to pass from one generation of poultry to the next through fertile eggs. However, there are scenarios in which the fertile egg can serve as a vehicle for passing Campylobacter from one generation of broilers to the next that do not involve transovarian, vertical transmission.
Why the lack of consensus?
The main reason for this lack of consensus is that egg passage, which can involve contamination of the reproductive tract and/or the follicles of the hen with naturally occurring Campylobacter, has been extremely difficult to consistently demonstrate with laboratory cultural methods.
An ideal cultural procedure that recovers and isolates Campylobacter from a variety of biological and food specimens on a regular basis is not available. Most of the cultural methods were developed for the recovery of Campylobacter from fecal samples in which there are large populations of Campylobacter. These methods can be inadequate for detecting small numbers, sublethally injured or stressed cells, or viable nonculturable cells of Campylobacter in foods or biological samples (Cox et al. 2001).
Because of the inability to fully validate this phenomenon culturally, there has been a strong bias against the concept that fertile eggs can be a source of introduction of Campylobacter into breeder and broiler flocks. As a result of this bias, many scientists have overlooked the fact that egg passage can involve much more than vertical, transovarian transmission (Cox et al. 2012).
Semen's role in colonization of fertile eggs
To begin with, the intestinal tract of the male breeder can be colonized by Campylobacter which can, in turn, heavily contaminate a rooster's semen. This semen can then contaminate the hen's reproductive system from the vagina to the infundibulum and thereby contaminate the internal components of the fertile egg as it descends and ultimately exits the hen's body. Cloacal/fecal contamination of the semen and not the male reproductive organs is thought to be the primary source of the organism.
F rom a total of 275 semen samples collected from commercial broiler breeder roosters, about 10 percent contained Campylobacter levels as high as 1,000 cells per ml. By using scanning electron microscopy, C. jejuni was shown to be in close association with chicken spermatozoa.
Contamination of eggshell surface
Another scenario is that feces can easily contaminate the shell surface of a fertile egg because the egg and feces both pass through the cloaca. Then bacterial contamination (including Campylobacter) can be drawn through the shell as the egg cools, aided by the presence of moisture (condensation of the egg).
Even though Campylobacter would have a difficult time surviving on the dry shell or in the hostile albumen, the organism can survive in the moist eggshell membranes. Then, as the chick pips and emerges from the egg, it may ingest the Campylobacter entrapped in the shell membranes and become colonized, subsequently spreading this contamination to flock mates through ingestion of cecal droppings during brooding. Since this is likely to occur only in a small number of birds placed in a grow house, it is not surprising that it will take an insensitive sampling method such as a drag swab approximately four weeks to detect Campylobacter from the litter surface.
Hence, there is more than one scenario in which the fertile egg can serve as a vehicle for passing Campylobacter from one generation of broilers to the next that does not involve transovarian, vertical transmission.
Detection of Campylobacter in circulating blood
Vertical transmission of an infectious agent, specifically Salmonella Pullorum by chickens, was a difficult concept to accept when it was first reported in 1914 and was aggressively disputed contrary to what was known about transmission of congenital infections such as syphilis (Rettger, 1914). However, following the work of Rettger and later the emergence of Salmonella Enteritidis in the 1980s, it is now accepted that Salmonella Enteritidis and certain other Salmonella serovars and Escherichia coli can be transmitted vertically. In the early 1990s, Salmonella Enteritidis was cultured from the ovaries and oviducts of spent hens, but the prevalence was low and difficult to consistently demonstrate (Poppe et al. 1992).
The situation regarding acceptance of the concept of Campylobacter is similar. Campylobacter has been found to reside naturally in the thymus, spleen and liver/gallbladder in commercial broilers and broiler breeders. It was also cultured from all the various segments of the reproductive tracts of actively laying commercial breeder hens.
In addition, Campylobacter has been cultured from the mature and immature follicles of hens and from their bloodstreams. Detection of Campylobacter in the circulating blood provides insight into a possible means by which they are able to disseminate rapidly to an assortment of tissues and organs within the bird. These findings provide evidence that Campylobacter is not strictly limited to the digestive and reproductive tracts.
Evidence that egg transmission of Campylobacter occurs
It is generally agreed that horizontal transmission of Campylobacter is an important route of the contamination of commercial broilers and breeder flocks. However, there is disagreement among scientists regarding the concept of egg passage of Campylobacter through eggs from one generation of poultry to the next even though the published literature provides evidence of egg transmission of Campylobacter. This includes many publications that document the presence of Campylobacter in breeder flocks, fertile eggs and newly hatched chicks prior to exposure to any possible environmental source. Cultural isolation of Campylobacter from commercial hatching cabinets (hatching debris and fluff), commercial tray liners and from interior egg contents of commercial breeder eggs is very strong evidence that egg transmission of Campylobacter does indeed occur.
As laboratory-based cultural methods for detecting Campylobacter continue to improve, our understanding of the ecology of this bacterium will also improve including developing a better understanding of the role of transmission of Campylobacter to chickens through fertile eggs. This fundamental information will provide a basis for developing and implementing the most effective intervention strategies for mitigating Campylobacter carriage and dissemination by poultry. The best evidence to support the concept of vertical transmission of Campylobacter will be reduction in the transmission of Campylobacter to eggs or progeny birds by an intervention applied at the breeder level.
Cox, N. A., M. E. Berrang, N. J. Stern, and M. T. Musgrove. 2001. Difficulty in recovering inoculated Campylobacter jejuni from dry poultry-associated samples. J. Food Prot. 64: 252-254.
Cox, N. A., L. J. Richardson, J. J. Maurer, M. E. Berrang, P. J. Fedorka-Cray, R. J. Buhr, J. A. Byrd, M. D. Lee, C. L. Hofacre, P. M. O'Kane, A. M. Lammerding, A. G. Clark, S. G. Thayer, and M. P. Doyle. 2012. Evidence for horizontal and vertical transmission in Campylobacter passage from hen to her progeny. J. Food Prot. 75: 1896-1902.
Rettger, L. F. 1914. Ovarian infection in the domestic fowl and direct transmission of disease to the offspring. J. Exp. Med.19:552-561.
Poppe, C., R. P. Johnson, C. M. Forsberg, and R. J. Irwin. 1992. Salmonella enteritidis and other Salmonella in laying hens and eggs from flocks with Salmonella in their environment. Can. J. Vet. Res. 56:226-232.