Zhan infection stops a lifeform's ability to breed. Zhan infection can only be transfered through a syrum transfter (bite saliva, injected toxins, etc). So while "an area" can be infected, it eventually dies out and is reclaimed by native flora/ fauna.
Remember zhan themselves are gloppy things that basically breakdown cell walls, and become part of the existing cell.. entering into a lifeform. No true syrum transfer here, it is more like a viral infection- a dna exchange. So a Zhan that can not find an animal to infect, eventually bleeds mass into the plants.. who become grey plants. Greyzones are dangerous because the plants become predatory taking down animals with poisons, spikes, and the occasional trapping mechanism.
When plants are infected, they become more dangerous, but seldom predatory to other plants... therefore no syrum transfer. They do take out the occasional animal, most of which die, but not all (those that survive often become zhani). Animals infected become carnivores; original carnivores become really deadly. They infect other creatures that survive their attacks (where spit or toxin), which is not likely as they are much more efficient as predators.
Note: urine is not normally considered syrum, in terms of transfer... so urinating zhan do not project the infection that way.
For zhan to take over the biosphere, you would need an herbavore to be infected and not become a carnavore. As it eats leaves and grasses, it would infect those things. If the herbavore ate regular plants and zhan infected ones (without succumbing to poisons and such), this herbavore zhani could spread the infection indefinately. (Could happen in your saga)
I think the biosystem's defense against the Zhan is to shut down anything's ability to reproduce. Zhani are efficient predators, so if they could breed they would take over the food chain.
Also the Dragon branch includes Hoppers (our veloceraptors), Gliders, SeaDragons, Mini-dragons, and probably one more. All of them have increased para-sensory abilities because of the amount of nerves in their wing /glider /webbing. These creatures are sensitive to warp (and electromagnetic fields) and things that came through warp (which allows them to know when Zhan are in the area). (That extra neural web in their wings allow them to warp/ phase/ etc.). So the dragons burn these things, but the other ones are known to attack them (and since they are able to sense their presence, more able to avoid zhani suprise attacks.
Everything on Kerren is more sensative to electromagnetic fields than Terran creatures. Perhaps that is another part of the biosystem's defense.