Do mosquitos vary in toxicity?
This is more a biological (immunology, serology) question, than one about gardening. It is possible that mosquitoes vary in 'toxicity', however I think the greatest part of the reaction is caused by yourself (no offense, I am prone to allergic reactions to mosquito bites too). So the chemicals that enter your body can vary per mosquito species (or even individual), which can trigger a milder or worst reaction. Just like different pathogens (bacteria, viruses) can also trigger different reactions (influenza can be innocent or deadly). But it is more likely that your own immunological 'state' is causing a worst reaction. For instance if you also have hay fever, levels of immunological molecules (etc. cytokines, chemokines) are already high in your blood and cause a larger or more intense reaction than in periods when less of these molecules are present in your blood.