This paper describes clearly and thoroughly all the epidemiological control steps carried out in relation to the avian influenza outbreak in a hen farm in Castilla-La Mancha, caused by the highly virulent strain of the H5N1 subtype.  

This virus has caused unprecedented multiple outbreaks of infection in Europe and America during the period 2021-2022. As discussed in the article, in Spain there have been 193 outbreaks of infection with this H5N1 strain in poultry and wild birds up to January this year. Exceptionally, one of these outbreaks has occurred in a mink farm in Galicia, causing particular alarm due to the singularity of the transmission of this virus from birds to mammals.  

This "species jump" constitutes a qualitative change that could imply a certain adaptation of the virus to proliferate not only in birds, but also in mammalian species, with all that this implies for humans.  

Therefore, in this case, as in all outbreaks of highly virulent avian influenza in which people - such as farmers or personnel involved in the slaughter of birds and disinfection of affected farms - are directly involved, official regulations require them to be closely monitored for infection. This surveillance includes monitoring the health of these persons and, in the event of any suspicious symptoms, virological (PCR) and/or serological (presence of H5N1-specific antibodies) testing to confirm or rule out possible infection.  

In this case, the PCR of one of the samples (nasopharyngeal swab) taken from one of the farmers two days after the outbreak was positive. Fortunately, a repeat test 11 days later was negative, as was another antibody test. In addition, the worker was symptom-free throughout this time, so H5N1 infection was ruled out. Another worker involved in the dismantling of the farm was also PCR positive with a sample taken 25 days after the outbreak, but the second sample, taken 6 days after the first one, was already negative. It also had no suspicious clinical signs. The positive PCR values of both persons indicated a low viral load.  

In summary, it is concluded that positive PCRs are the product of environmental contamination and not of a complete or "productive" infection. In other words, there has apparently been no significant proliferation in human cells and the positivity is a consequence of the very high sensitivity of the test. It should be noted that the concentration of virus particles in the air or on the premises of the affected farm can be immense given the large number of affected birds, tens of thousands, which release virus to the outside. It should be noted that the operators were equipped with the required uniforms and biosecurity equipment and that they complied with the established protocols. 

The outcome of this viral outbreak shows that, for the time being, this H5N1 virus has little capacity to transmit efficiently to humans and that continuous and systematic surveillance in wild and domestic birds is necessary to detect in time the possible mutation of the virus into forms capable of effectively infecting humans or other animal species. 

These results raise the question of the appropriateness of PCR testing of people associated with the outbreak when they do not present specific clinical symptoms. This would avoid misleading results and difficulties in interpretation and implementation of appropriate measures. In fact, this is not advised by European regulations and has been applied by the Spanish health authorities since March 2022. 

This episode illustrates the good functioning of public and veterinary epidemiological operations in Spain, which is able to detect, sanitise and control outbreaks of highly virulent zoonotic pathogens with potential risk for humans and other animal species.

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