I'm so sorry about your loss! The geographic distribution is thought to be world wide. Advances in diagnostic testing and surveillance is improving and we will know more in the future about all vector-borne diseases! The vector is not known but Rhipicephalus sanguineus is suspected, and this tick is expanding in its geographic range possibly due to climate change (it likes hot weather). It also may be spread by biting and other vectors and possibly vertically. Hemotropic mycoplasmas in cats have a fair bit of evidence that a pathogenic species called Mycoplasma hemofelis causes IMHA. In dogs the hemotropic mycoplasmas seem to be less pathogenic and the evidence was not strong that it can cause IMHA in the evidence review for the consensus statement, but hemolytic anemias (not necessarily immune mediated) and more severe disease has been reported in dogs that have had their spleens taken out or have other diseases or are immune compromised / immunosuppressed. Coinfections and the individual immune milieu at the time of infection may theoretically contribute to disease manifestations. 

Please see the answer to the question about genetics.  Its likely that Cocker spaniels have an overall “genetic burden” that predisposes them to being very cute and also having a propensity to developing immune mediated disease.  There have been a few studies that have looked at Cocker spaniels with and without a history of IMHA but those studies are difficult because we don’t know if the ones without IMHA at the time of the study will develop it in the future and they just haven’t had the “trigger” exposure to instigate the disease. It appears that the diversity of genes that code for a family of molecules that present foreign antigen to the immune system and another that  down regulates the immune response is limited in cocker spaniels. (Trelfall AJ et al. Analysis of DLA-DQB1 and polymorphisms in CTLA4 in Cocker spaniels affected with immune-mediated haemolytic anaemia.)  More studies are needed to determine if genetics will determine which ones get it and how many genes are involved and if epigenetics and environment play a role. Note that studies of twins show some develop immune mediated disease and some don’t, so its not all genes.

You won't see evidence of IMHA but you will see evidence that may suggest the presence of some diseases that could be associated with its development. 

Interesting question. Do you see a lot of atopy in dogs with IMHA? More than in dogs without? It’s a different kind of immune “hypersensitivity” but immune dysregulation or treating it with drugs that modify the immune response could theoretically contribute to the development of immune mediated disease. 

Regarding: If my dog goes into remission, what can serve as a trigger for it to return?   There unfortunately is no way to know on a case by case basis at this time, except to say that if a trigger was identified in association with its development it should be avoided. Avoiding triggers such as Babesiosis that have a high level of evidence for causing IMHA in many patients would also be important. As mentioned in responses to the other questions, the evidence for trigger factors is summarized in a systematic way in the upcoming ACVIM Consensus statement on diagnosing and reviewing the evidence for trigger factors for IMHA. As mentioned above this will be available open access.  Regarding: Is pregnancy a trigger?  Pregnancy has been associated with IMHA in people. The mechanism is unknown.  Regarding: What % of patients develop IMHA secondary to a trigger like a tick bite?   There is evidence that the infections the ticks carry, particularly Babesia, can trigger IMHA as reviewed in the lecture and in the upcoming consensus statement mentioned above. To find out the prevalence of tick-borne disease in patients with IMHA compared to those with IMHA and no tick borne disease, we need to do controlled studies where all the patients are tested for infections in the same and comprehensive manner. This will likely vary with exposure risk and geographic locale. For example see Kidd L, Qurollo B, Lappin M. et al. Prevalence of vector-borne pathogens in southern California dogs with clinical and laboratory abnormalities consistent with immune-mediated disease. J Vet Intern Med 2017 29(3):908-16.