How to define an eradication program?

Nathues, Heiko (12 May 2022). How to define an eradication program? In: 13th European Symposium of Porcine Health Management. Budapest. 11.05.2022 - 13.05.2022.

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Eradication of infectious diseases in pigs
Lessons learned & challenges in the future
In recent years, the eradication of infectious production diseases in pig farms has become more and more popular all
over the world. This development is partially based on good experiences that were made elsewhere and have been
published in journals or presented in conferences numerous times. Other reasons to eradicate infectious production
diseases and to obtain a ‘specific pathogen free’ (SPF) status include higher revenue for animals that are sold to
other farms or even access to new markets. In any case, eradications remain costly and challenging, whereby the
confirmation of the SPF status on herd level is still lacking an international and sometimes national standard that
allows reciprocity of recognition. This paper highlights the lessons we have learned in previous attempts to eradicate
infectious diseases in pig herds. Moreover, a critical review of recent methods to confirm successful eradication and/
or SPF status will be presented.
Understanding the epidemiology of an infection
Thorough understanding of the epidemiology of the infection in question is of utmost importance and is not the same
in every individual case. An attempt to eradicate a disease from one herd might be successful, whereby the same
protocol does not lead to success in another herd, region, or country (Neirynck et al., 2020). Ideally the transmission
of the pathogen into the herd, the spread of the infection within the herd, the persistence of infection in individual
animals or groups of animals and, finally, the time and way of elimination of the pathogen from animals and/or their
environment is known. However, this complete understanding of the epidemiology is not valid for many infectious
SESSION: ERADICATION OF INFECTIOUS DISEASES:
THE PRRS EXAMPLE
Biosketch
Prof. Dr. med. vet. Heiko Nathues graduated from the University of Veterinary Medicine in Hannover, Germany
in 2004. He spent some month in a specialised pig & poultry practice in Northern Germany.
From late 2004 until 2011 he was employed at the Field Station for Epidemiology of the University of Veterinary
Medicine Hannover in Bakum, Germany, where he also obtained his Dr. med. vet. in 2007 and his PhD in 2011.
Awarded with a Marie-Curie Intra-European-Fellowship he joined the Veterinary Epidemiology, Economics
and Public Health Group of the Royal Veterinary College in London, from 2011 to 2013. He also obtained a
Postgraduate Certificate in Veterinary Education.
In 2013 he finalized his habilitation and received the venia legendi for ‘Pig diseases and population medicine’
from the University of Veterinary Medicine Hannover. Since the same year, he is a professor at the Vetsuisse
Faculty of the University of Bern, Switzerland, where he is heading the clinic for swine. His primary areas of
interest are diagnosis, epidemiology and control of porcine virus- and mycoplasmal-diseases.
Prof. Nathues is a Diplomate of the European College of Porcine Health Management (ECPHM), certified as
EBVS™ European Veterinary Specialist of Porcine Health Management and currently the President of the
ECPHM. Since April 2017 he is representing the PHM college in the EBVS, and since April 2021 he is the Vice-
President of the EBVS and a member of the Executive Committee.
Abstract
How to define an eradication program?
Heiko Nathues, Prof. Dr. med. vet.
Clinic for Swine, Vetsuisse Faculty, University of Bern, Switzerland
12
production diseases in pigs or at least is not present in a given farm setting where the herd is infected, e.g. with Porcine
Reproductive and Respiratory Syndrome Virus (PRRSV) (Arruda et al., 2019).
The lack of information about the susceptible subpopulation, the uncertainty regarding the time that an infected
individual remains infectious and thereby exposes others, and the level of protection of acquired immunity in recovered
individuals lead to failure, not only in combating a pandemic such as SARS-CoV 2 (Muñoz-Fontela et al., 2022), but also
in managing infectious diseases in pigs (Dorjee et al., 2013). In order to correctly assess the epidemiological situation of
an infection in a given herd, veterinarians should be well aware of the susceptible group of pigs, the incubation period,
and the persistence of the infection in individual pigs. For PRRSV, numerous studies describe such parameters (Corzo
et al., 2010; Pileri and Mateu, 2016), although important facts such as the persistence of PRRSV for more than 250 days in
single animals (Wills et al., 2003) is often neglected in the field.
Evaluating the socio-economic value of an eradication
Before starting any eradication programme, a spatial and socio-economic analysis (Figure 1) should highlight the
feasibility and the monetary benefit. A partial budget analysis supports an informed decision regarding a particular
eradication concept that is most applicable to the individual herd.
FIGURE 1: OUTLINE OF A SOCIO-ECONOMIC ANALYSIS IN LIVESTOCK FARMS
Such spatial analyses have been conducted for PRRSV in areas of different pig density (Fahrion et al., 2014), where the
significant increase in the risk for re-infection was linked not only to the distance of neighbouring pig herds, but also
their production type (i.e. sow herd vs. fattening herd vs. 1-site production system).
Economic analyses have been published for Brachyspira hyodysenteriae (Cadetg et al., 2019), Porcine Circovirus Type
2 (Alarcon et al., 2013) and PRRSV (Nathues et al., 2017). These studies have in common that in addition to health and
performance data also livestock and market data were considered in the partial budget analysis. This process warrants
a better estimation of the expected value, when eradication is selected as the combat tool for the particular infection.
If spatial (i.e. geographic) or economic specifications put the ‘long term success’ of an eradication programme in
question, then farmers shall be thoroughly informed and also advised to carefully consider the risk of failure or reinfection.
In certain scenarios, tailor-made vaccination programmes in combination with other prevention measures
might be more beneficial compared to eradication (Nathues et al., 2018).
Applying sustainable eradication concepts
Next to the concept of total depopulation and repopulation, which nearly always results in an elimination of the pathogen
from the herd, the ‘Swiss method of eradication’ has been proven very successful in the field. In Norway and Switzerland,
the Swiss method has been used to eradicate Enzootic Pneumonia on the national level (Stark et al., 2007; Gulliksen et al.,
2021); both countries are considered free of the disease today. Remarkably, these efforts on national level achieved SPF
status for both pig populations, where today even no more infection with Mycoplasma hyopneumoniae can be found,
although the eradication programmes aimed at combating the (clinical) disease rather than the pathogen itself.
Apparently, the Swiss method, which is based on completely eliminating the susceptible subpopulation from a herd
(i.e. the number of animals neither ‘infectious’ nor ‘recovered’ equals zero) can be used in case of other infections
as well. The ‘Close & Roll-over’ concept that is applied to eradicate from PRRSV can be seen as a modification of the
Swiss method, where the subpopulation of susceptible pigs is reduced to zero by not introducing new PRRSV uninfected
pigs into the herd during six months, while waiting for all other pigs to become ‘infectious’ and thereafter becoming
‘recovered’.
13
Other eradication concepts that are based on mass treatment with antimicrobials instead of partial depopulation
and elimination of the susceptible subpopulation have been used for Brachyspira hyodysenteriae and Mycoplasma
hyopneumoniae, but they were either unsuccessful on the spot or characterized by another breakdown in a very short
period (Yeske et al., 2020). Taking this evidenced low rate of success and the concept of prudent use of antimicrobials
into account, such concepts do not reflect state of the art and cannot be recommended in future.
In any case, the eradication concept must be well selected based on the individual situation in the pig herd, the farmer’s
social and economic capabilities, the epidemiological situation, and other aspects (Table 1).
TABLE 1: PROS AND CONS OF THE DIFFERENT ERADICATION CONCEPTS
Defining appropriate sampling strategies to control the success
Noteworthy, appropriate sampling strategies being necessary to control the success at the end of the eradication and
to continuously monitor the new status of the herd should be designed in advance. Otherwise, farm owners might be
upset about the cost of controlling and maintaining the new SPF status of their herd. Once the eradication has been
achieved, only monitoring programmes with highest sensitivity will truly evidence the accomplishment and build up
trust among business partners. Inadequate sampling schemes, convenience-driven sampling sites, and ‘low-budgettesting’
increase the risk of false negative results and extended periods to detect breakdowns, i.e. re-infection of the
herd, which might poses highest risk along the whole production chain.
The World Organization for Animal Health (OIE) still recommends whole blood samples or serum for the direct and
indirect detection of most viral pathogens. This is also valid for PRRSV, where other sample sites such as oral fluids or
processing fluids are not mentioned at all. The problem of ‘alternative’ sampling sites is not only the fact that others
than veterinarians, who are able to examine herd health in parallel, collect those samples, but also the significant
reduction of sensitivity and specificity of such samples when correctly compared to serum samples (Prickett et al.,
2008; Kittawornrat et al., 2010; Steinrigl et al., 2014; Pepin et al., 2015; De Regge and Cay, 2016). Nowadays, some studies try
to compensate the lower quality of such samples (oral fluid or processing fluid from docked tails or tongues of dead
animals) by increasing the number of sampled animals (De Regge and Cay, 2016; Baliellas et al., 2021). However, this
approach only works in very large pig herds and does not account for inevitable future developments such as ban of
tail docking, etc., where the ‘alternative material’ will no longer be accessible.
The sample size and frequency of testing needs to be adopted to the individual herd, taking the risk of infection /
re-infection into account. For most infectious production diseases, a maximum possible prevalence of 10% and
a confidence of 95% is accepted, when calculating the exact sample size. However, in case of PRRSV the maximum
possible prevalence might be reduced to 5% in case of significant impact of an infection, e.g. in a boar stud, in a PRRSV
free country, etc. As soon as the population is dynamic (i.e. regular introduction of new animals such as gilts, etc.), the
testing should be repeated at least every three months (Holtkamp et al., 2011).
Designing tailor-made programmes
Summarizing all mentioned necessities for a successful eradication of an infectious production disease in a given pig
herd, it becomes obvious that tailor-made programmes are required on herd level and, in case this is favoured, on
regional and national level. This not only applies to the verification of the initial success of an eradication programme,
but also to the period thereafter, when SPF status is maintained.
It is strongly recommended to decide for the best options, which often does not correspond to the cheapest. This
regards to the selection of the concept of eradication and also to the sampling site, sample size and testing interval
once a SPF status has been achieved. Nevertheless, open and transparent communication of these efforts will underline
credibility and thereby facilitate trust among farmers, veterinarians and other stakeholders involved in the production
of healthy pigs.

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