Expert answers the main questions about fowl typhoid

Fowl typhoid is a disease of extreme importance for the laying poultry industry: due to the severity of the disease, it leads to lower productivity and huge economic losses. The infection caused by the bacterium Salmonella enterica subspecies enterica sorovar Gallinarum biovar Gallinarum (Salmonella Gallinarum) can lead to a mortality rate of 80% of the flock – or more— in more susceptible chicken (broiler breeders, medium and heavy female breeders), while it can be negligible, but does occur, in white layers^{5 e 3}.

Dr. Angelo Berchieri Junior

For this reason, fowl typhoid is a constant cause for concern in farms and still generates many doubts in the industry. Among them: what are the most effective measures, whether the surviving chickens remain carriers of the disease and other issues such as the reversal of vaccine virulence.

To answer these and other extremely pertinent questions about fowl typhoid, we invited Dr. Angelo Berchieri Junior, full professor in Ornitopathology at the Department of Pathology, Reproduction and Single Health of the Faculty of Agricultural and Veterinary Sciences (FCAV) of the Universidade Estadual Paulista (Unesp), Jaboticabal campus.

Expert in avian salmonellosis, Dr. Berchieri Junior is the author of several scientific articles and has helped us detail the main differences between diseases caused by these bacteria: fowl typhoid, Pullorum disease, and avian paratyphoid. Now, he comes to contribute with one more important content for the Brazilian poultry industry. Check out the interview!

Interview Dr. Angelo Berchieri Junior

Biocamp – Could the most recent cases of fowl typhoid have been caused by reversal of vaccine virulence?

Dr. Angelo Berchieri Junior – Fowl typhoid is not a recent disease. So much so that the PNSA (National Poultry Health Program) in Brazil, since its inception in 1994, has already established the obligation to eliminate breeding flocks affected by the disease. Around the year 2000, cases of fowl typhoid began to appear more frequently.

The attenuated strain SG 9R was developed by Dr. W. Smith in the 1950’s and since then I am unaware of any scientific information that actually proves this comment.

Salmonella Gallinarum

Biocamp – When there are no clinical cases, what would be the most appropriate vaccine program when we are facing a “status” control challenge against Salmonella Gallinarum?

Dr. Angelo Berchieri Junior – For long-lived chickens, the combination of live bacterial vaccine plus inactivated bacterial vaccine in oil emulsion would be the most appropriate⁶. Given the intensive, high-density, long-term rearing system and the fact that we do not know when the chickens will become infected, protection by mixing live and inactivated vaccines can be considered a good strategy.

Biocamp – Can vaccination with the SG 9R strain lead to a momentary drop in the chicken´s immunity?

Dr. Angelo Berchieri Junior – No. Like any vaccine, it causes an immune response, that is, the organism develops a reaction in order to defeat the invading agent. In healthy animals, the reaction to the vaccine would not cause complications.

Biocamp – The vaccine has the role of producing an immune response to the disease agent. However, are there more consistent ways to achieve this response depending on the route of application?

Dr. Angelo Berchieri Junior – The vaccine strain is considered as such because it can multiply in the chicken’s organism and cause the immune response to develop – without, however, having the aggressiveness of a wild-type strain. This means that it is more susceptible to reaction in the chicken’s organism.

So, when offered orally, the bacteria have to overcome the digestive barriers in order to proceed. When it is injected, it doesn’t need to cross these barriers and falls straight into the bloodstream. Both produce immunity, but it is clear that the injectable has a chance to produce more consistent immunity, considering the amount of viable SG9R cells that will stimulate the immune response.

It is also important to remember that, because these are nucleus with a huge number of chickens, often in places where there are several farms with different sanitary management, movement of vehicles and personnel, the efficiency of the vaccine program will depend on the robustness of the sanitary management in the region.

Biocamp – Does the strategy of vaccinating flocks at the beginning of a fowl typhoid outbreak work?

Dr. Angelo Berchieri Junior – This is a step that should be taken with caution, as it may favor misinterpretations. It is necessary to remember that the vaccine strain is alive and is an attenuated SG strain. Therefore, like any vaccine strain, it needs to reach the bloodstream and spread in the animal, reaching the spleen, liver, heart, among others.

It is considered attenuated because it is less resistant to fighting by the chicken’s immune system. That is why it is used as a vaccine strain. If the animal has the disease in progress, receiving another “load” of bacteria can potentiate the condition. It is worth noting that the changes of fowl typhoid are due to the immune response of the chicken’s body to the invading microorganism.

Having said that, considering the epidemiology of fowl typhoid, in which the disease spreads gradually through the flock, with pecking of carcasses of chickens killed by the disease being preponderant⁵, the immune response to vaccination can contribute to protect other chickens that have not yet been infected.

It must be emphasized that vaccination is one more tool and should always be recommended to add, within a biosecurity program.

Biocamp – Why does fowl typhoid appear even in vaccinated flocks?

Dr. Angelo Berchieri Junior – The control of fowl typhoid should not be based only on the vaccination process. The set of attitudes that make up the biosecurity program must be respected and vaccination must be one more ally. Besides reviewing and strengthening the biosecurity program (which should include microbiological monitoring), it is necessary to evaluate the vaccination program and its execution.

Biocamp – Do sick chickens that survived fowl typhoid remain carriers of the disease agent?

Dr. Angelo Berchieri Junior – Based on our experience, I do not believe so. Attempts to reproduce the carrier condition had no effect. The Salmonella Gallinarum (SG) bacteria either killed the chicken or disappeared within a few days after infection. Even trying to keep the chicken alive, either with non-lethal infective doses (the development of the disease is dose dependent) or using drugs, it was not possible to observe prolonged persistence of the bacteria. Furthermore, all eggs produced in these experiments were examined and found not to carry SG^{1, 2 e 3}. 

It is worth mentioning that white layer chickens are very resistant to clinical disease. However, as for the host-parasite relationship, the persistence of SG in these animals is longer than in semi-heavy and heavy chickens³. But they also do not persist as Salmonella Pullorum (SP) ^{1,2 e 8}.

Biocamp – How does antibiotic therapy work for the control of fowl typhoid?

Dr. Angelo Berchieri Junior – Antibiotic therapy is not a measure to fight fowl typhoid. Anyone who has faced the problem in the field knows this. Besides not solving the problem, it may intoxicate the chickens and hinder microbiological tests, making it difficult to isolate and identify the etiologic agent and, thus, masking the disease.