Lynn, I believe that Murphy's vet misspoke. Trombiculid infestation such as with the harvest mite/chiggers is independent of my patient's immune status. Debilitated dogs might be more susceptible to being colonized by ectoparasites but Murphy is neither debilitated nor immunocompromised. Most important and contrary to what you may have read, there are no over the counter reliably effective immune system boosters. You'll find wild claims all over the internet to the contrary but there isn't a whit of scholarly evidence that these products do as they're purported to do.
On the other hand, we do know that demodicosis (the Demodex mange mite) can become a problem in immunocompromised dogs - the very young and old, compromised by disease or immunocompromising drugs such as glucocorticosteroids such as prednisone (prednisone should be avoided if immune system compromise is to be avoided in Murphy), chemotherapeutics such as cyclosporin and newer drugs such as oclacitinib. If Murphy's skin has been scraped and Demodex has been identified, an underlying and immunocompromising disorder should be searched for. In general, with adult onset demodicosis I'm looking for hypothyroidism, diabetes mellitus, or Cushing's disease - none of which are likely in an otherwise normal Murphy.
For chiggers and harvest mites, treatment with one or two applications (1-2 weeks apart) of a parasiticidal spray, spot-on, dip, or otic preparation is recommended. Recent studies suggest that a 0.25% fipronil (Frontline or generics, e.g.) pump spray or combination permethrin-pyriproxfen pump spray or spot-on is especially effective when used according to label instructions.
Please respond with further questions or concerns if you wish.