Just a few references regarding Ich with the appropriate links, some of these are well worth reading... The last one is pretty dry but it is interesting to see the tie in with columnare. Read what will into them.
"Ich is so widespread that many experts feel that it is present in the environment of most aquariums, especially in larger holding tanks, rearing ponds of breeders, collectors, and wholesalers. In fact, just about every aquarium fish will come into contact with this protozoan at several times in its life. Because it is so widespread, most fish have developed a good immune response against the disease to allow them to fight off the protozoan infection before it ever causes any symptoms. " Cause, Treatment, and Prevention of Ich in Freshwater Fish
"Endosymbiotic bacteria were identified in the parasitic ciliate Ichthyophthirius multifiliis, a common pathogen of freshwater fish. PCR amplification of DNA prepared from two isolates of I. multifiliis, using primers that bind conserved sequences in bacterial 16S rRNA genes, generated an ∼1,460-bp DNA product, which was cloned and sequenced. Sequence analysis demonstrated that 16S rRNA gene sequences from three classes of bacteria were present in the PCR product. These included Alphaproteobacteria (Rickettsiales), Sphingobacteria, and Flavobacterium columnare. DAPI (4′,6-diamidino-2-phenylindole) staining showed endosymbionts dispersed throughout the cytoplasm of trophonts and, in most, but not all theronts." Endosymbiotic Bacteria in the Parasitic Ciliate Ichthyophthirius multifiliis
"As Ichthyophthirius multifiliis is an obligate parasite, it is thought that its survival between outbreaks is through low-level infection in the population. There may also be some fish that carry encysted parasites despite having developed immunity." Ichthyophthirius multifiliis - WikiVet
"When a fish has had Ich eradicated, it may develop partial resistance to reinfection. Partially treated fish may initially harbour low numbers of unseen trophozoites, often in the gills. This subclinical carrier will cause another outbreak weeks later, most likely when stresses occur or uninfected fish are introduced to the aquarium." Ichthyophthirius multifiliis - Wikipedia, the free encyclopedia
"The scientific name of the parasite is translated as "the fish louse with the many children" reflects its reproductive capacity."
Lost this link reference
"Thus, the association of F. columnare with I. multifiliis appears to be extracellular through interactions with cilia and enmeshment in mucus secreted by theronts and tomonts. Because of the close physical association of Flexibacter columnare with the surface of I. multifiliis, it is possible that the parasite serves as a carrier of bacteria to fish, including individuals that are susceptible to bacterial infection....
...The DNA used to generate these clones was isolated from two different strains of I. multifiliis: G5, passaged in our laboratory since 1995, and G13, recently isolated from a wild fish and minimally passaged in our laboratory. However, DNA isolated from G5 tomonts and G13 theronts contained the same two endosymbionts in the same relative abundance. This suggests that these two endosymbionts are commonly found in I. multifiliis populations and are not lost even after long-term passage in the laboratory.
The physiological relationship between these endosymbionts and I. multifiliis is not understood. As all theronts do not contain detectable endosymbionts, the endosymbionts do not appear to play a critical role in supporting the growth of I. multifiliis, but their presence must also not be particularly detrimental to the growth of I. multifiliis. It is not known whether they play a role in the pathogenesis of I. multifiliis infections or if they affect the immune response of infected fish." Endosymbiotic Bacteria in the Parasitic Ciliate Ichthyophthirius multifiliis