Chicken spotty liver disease – a potential vaccine on the horizon

BY Dr. Deni S e He A r D

TUCKER, Ga. — Spotty Liver Disease (SLD) manifests as acute infectious hepatitis in layer chickens. The disease occurs mostly in barn housed or free-range layer flocks and is characterized by an acute drop in egg production and high mortality.

With the production practice changes in the egg industry from conventional cages to cage-free housing systems, the incidence and economic impact of SLD is expected to become more prominent. Due to the great economic losses incurred by SLD, it has become an important concern for the egg industry.

Recently, Campylobacter hepaticus was identified as the causative agent for chicken SLD and was confirmed to infect the liver via the transmission of fecal-oral route and subsequent systemic spread. The discovery of C. hepaticus as the causative agent has led to a better understanding of the etiology of SLD. However, the virulence mechanisms of C. hepaticus remains unknown. Moreover, U.S. poultry producers have recently ranked SLD as a top disease problem that needs to be addressed. There are no vaccines available to protect layer chickens against C. hepaticus infections, and essentially, there is a void in knowledge of immune responses to C. hepaticus in chickens.

In SLD outbreaks, individual birds and flocks have been reported to be able to recover from SLD without treatment, suggesting that the infected birds eventually developed protective immunity. Enzyme-linked immunosorbent assays (ELISA) have also shown that C. hepaticus can induce robust specific antibodies in sera samples from both naturally and experimentally infected birds.

However, it remains unknown whether the antibodies protect against SLD or whether recovered birds remain susceptible to recurring infection. Studies have shown that Campylobacter bacterins are efficient to protect against Campylobacter-induced systemic infections and the associated clinical diseases in animals. Thus, it is necessary to examine whether C. hepaticus bacterins can induce protective immunity against SLD in layers and consequently reduce the loss of egg production.

In response to the identified need from the poultry industry, USPOULTRY and its Foundation funded a research project during their 2022 Fall research competition to develop bacterins for the control of poultry SLD. The project will be conducted by scientists at Iowa State University where preliminary studies determined the complete genome of a representative C. hepaticus strain (USA52) from the U.S. outbreaks of SLD and confirmed its virulence in chicken SLD models. The previous work found that the C. hepaticus strains from the U.S. outbreaks are genetically diverse, represented by multiple sequence types and genetic lineages.

The research will consist of two very specific objectives. First, the study will evaluate homologous protection of experimental bacterins against SLD. An experimental bacterin will be prepared using a clinical C. hepaticus strain in an oil adjuvant. The bacterin will be assessed for the efficacy of protection against SLD by homologous challenge (with bacterin strain) in layers. In addition, one-dose and two-dose immunization schemes will be compared by homologous challenge.

The second objective will evaluate heterologous protection of experimental bacterins against SLD. The efficacy of the bacterin (the same one used in Objective 1) will be assessed by heterologous challenges (with three phylogenetically different representative isolates from the bacterin strain) in layer chickens.

In both objectives, development of immune response (specific antibodies in serum), induction of disease (via culture and histopathology), and degree of protection against SLD will be assessed.

The vaccination and challenge trials will evaluate the efficacy of C. hepaticus bacterins. Researchers expect to determine if one dose can produce similar protection to two doses of vaccination and if C. hepaticus bacterins can produce broad protection.

These results will provide a foundation to guide the improvement of vaccines for practical use on poultry farms in future studies.