Some vaccines drive the evolution of more virulent pathogens, according to Dr Andrew Read of Pennsylvania State University.

Invited to give the Milton Y. Dendy keynote address as part of the International Poultry Scientific Forum (IPSF) in Atlanta yesterday, 27 January, Dr Read said that conventional wisdom is that natural selection will remove highly lethal pathogens if death of the host greatly reduces transmission.

However, he explained, vaccines that keep hosts alive but still allow transmission of the virus – in other words, they are “leaky” - allow very virulent strains to circulate in a population.

Dr Read said: “Vaccines protect virulent pathogens from themselves.”

He went on to describe experiments which show that immunisation of chickens against Marek’s disease virus enhances the fitness of more virulent strains, making it possible for hyperpathogenic strains to transmit.

History supports this view.

Dr Read said that Marek’s disease was originally a mild disease, causing transient paralysis in chickens. A vaccine was developed to combat it but this started to fail after about 10 years as the virus appeared to become more virulent.

This led to a new vaccine, which was effective for another decade or so but then emerged what is known as the vv+MDV (very virulent-plus Marek’s disease virus), which invades multiple organs.

The current vaccine is of the Rispens type. It has been working satisfactorily for around 20 years but Dr Read said that it may only be a matter of time before the virus evolves into an even more virulent form. It is unlikely to have reached the limit of its virulence.

He and his colleagues have investigated where this viral evolution occurs in nature and how vaccination affects viral fitness using Marek’s disease in poultry as a model for human diseases such as malaria, diphtheria and pertussis (whooping cough).
Immunity elicited by direct vaccination or by maternal vaccination prolongs host survival but does not prevent infection, viral replication or transmission, thus extending the infectious periods of strains otherwise too lethal to persist.

Dr Read said his data show that anti-disease vaccines that do not prevent transmission can create conditions that promote the emergence of pathogen strains that cause more severe disease in unvaccinated hosts.

These characteristics of the Marek’s disease virus are also seen in other poultry viruses, such as avian influenza, according to the Dr Read. He expressed concern that frequent vaccination of poultry against avian influenza in some countries is allowing the silent circulation of “hot” strains which will be even harder to control in future. In his view, it would be better to cull flocks that become infected rather than risk the virus becoming ever more virulent.

In conclusion, Dr Read stressed that it is critical for successful vaccination to stop transmission of the virus and this bring to a halt viral evolution.