Biological agents (viruses, insects, bacteria, etc) can become resistant to countermeasures (engineered host resistance, drugs, pesticides, etc), but the resistance often comes with a cost. In the absence of the countermeasure, resistant mutants are often less fit and less competitive than wild-type counterparts.
Consequently, I'm wondering if there are any real-world examples of a previously resistant organism becoming susceptible over time in the absence of the countermeasure.
For instance, an insect pest that develops resistance to a pesticide but then loses resistance when that pesticide isn't used for several years.
Or, a chronic infection that is initially treated with a drug, develops resistance, but then becomes susceptible to that drug after a prolonged period of no treatment.
The best example is the acquired resistance of insects to insecticides. Insecticides place a selection pressure on insect pests of agricultural crops to evolve resistance, and resistance has developed in many pest species against the major insecticidal modes of action currently on the market. If there is selection of pressure on a population of a pest insect through the continued application of the same insecticidal molecule with the same mode of action, more than 80% of the population acquires resistance in a few generations, but if the insecticide is no longer applied, the populations will become susceptible again in a few generations. You can find several examples with insects in the literature:
https://www.ucl.ac.uk/taxome/jim/pap/mallet89tree.pdf
This paper discusses how resistance can change in insect populations. Check the references and I'm sure they will have a real example.
Article Identifying circumstances under which high insecticide dose ...
In the case of a plasmid-borne antibiotic resistance gene, it is quite possible to loose the plasmid (and the resistance), particularly in the absence of selection
Resistance is a hereditary property. Thus initial occurrence of resistance (in an extremely non apparent percentage of the population) is mainly due to either an alteration of the site of action of the agent or to an enhanced metabolic rate of the agent in the resistant part of the population. Both processes are genetically (e.g. mutagenic) mediated. From this point on the determining factor for either the reduction or the growth of the percentage of the resistant organisms in the population is the selection pressure of the chemical agent. Consequently if one stops using the chemical agent susceptible individuals will be selected against the resistant ones. Thus the resistant part of the population tends to gradually reduce in favor of the susceptible one. Yet this does by no way mean that resistant individuals have been modified to susceptible ones. To my knowledge this could only happen either through a spontaneous mutation to the opposite direction (zero practical probability) or through mechanical (molecular) engineering.
The best example is the acquired resistance of insects to insecticides. Insecticides place a selection pressure on insect pests of agricultural crops to evolve resistance, and resistance has developed in many pest species against the major insecticidal modes of action currently on the market. If there is selection of pressure on a population of a pest insect through the continued application of the same insecticidal molecule with the same mode of action, more than 80% of the population acquires resistance in a few generations, but if the insecticide is no longer applied, the populations will become susceptible again in a few generations. You can find several examples with insects in the literature:
https://www.ucl.ac.uk/taxome/jim/pap/mallet89tree.pdf
The reversion of drug or chemical resistance seems to be possible. Have a look at:Regional reversion of insecticide resistance on Helioverpa armigera is associated with the use of Bt cotton in northern China . Pest Management Science 61(5) 491-8, 2005. and
Reversion in insecticide resistance to fenobucarb in the Philippines (in line)
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