Role of the surface coat of Romanomermis culicivorax in immune evasion

Interactions of the mermithid nematode Romanomermis culicivorax with the immune system of mosquito larvae were examined by scanning electron microscopy. The host immune system rapidly recognised invading parasites, as granulocytes and discharged granules were observed attached to parasitic nematodes within 5 min. Melanin deposition was infrequently observed. As a counter measure, the parasites secreted and shed an extracellular surface coat which aided immune evasion. During the first 4 days of infection, when parasite growth was limited, the coat served as a disposable, renewable barrier between parasite and host that was intermittently shed to cleanse the nematode of adhering host immune products. In the later infection phase the parasite grew rapidly and was beyond the effect of the depleted host immune response. The broad host range of R. culcivorax within culicines may be partly a function of the nonspecific defence it mounts against the host immune system. In summary, shedding of the surface coat is an adaptive counter response by R. culicivorax to the mosquito immune reaction to infection and provides a classic example of host-parasite coevolution.