Physiological basis of host susceptibility of Florida mosquitoes to Dirofilaria immitis.

Young females of seven species of Florida mosquitoes were fed a meal of dog blood infected with Dirofilaria immitis to repletion to study the physiological mechanisms which control susceptibility and resistance in these mosquitoes. Various species of mosquitoes showed different grades of susceptibility. In all mosquitoes, microfilariae reached the midgut immediately after ingestion. Their movement from midgut to the specific host tissue—the Malpighian tubules—was either facilitated or inhibited depending on the presence or absence of anticoagulins in the salivary glands of these mosquitoes. In Anopheles quadrimaculatus, Aedes taeniorhynchus, and Aedes sollicitans, microfilariae move freely from the midgut to the Malpighian tubules, because of the presence of substantial amounts of anticoagulins in their salivary glands, and 30 to 60 mf/female developed normally to an infective stage. Very few microfilariae reached the tubules of Mansonia titillans as most of them were defaecated within a very short time after ingestion. In Aedes aegypti, Culex nigripalpus, and Culex pipiens quinquefasciatus movement of microfilariae from the midgut to the Malpighian tubules was obstructed by the coagulation of blood soon after ingestion. Coagulation of blood was followed by formation of oxyhaemoglobin crystals in C. nigripalpus and C. p. quinquefasciatus. It is suggested that secretions from symbiotic bacteria in the midgut of these mosquitoes lyse ingested red blood cells, and the released haemoglobin is oxidized to oxyhaemoglobin crystals which hinder the further movement of microfilariae and kill them.Microfilariae developed normally in A. quadrimaculatus, thus making them potentially the most susceptible mosquitoes, even though these mosquitoes did not survive to be effective potential vectors. A few microfilariae or their later developmental stages were melanized in the tubules of most A. sollicitans and A. taeniorhynchus, but the numbers of melanized stages were too few to affect the vectoring potentials of these species. In 20 per cent of A. sollicitans, 60 per cent of M. titillans, and ca. 80 per cent A. aegypti substantial numbers of the microfilariae after reaching the Malpighian tubules did not advance beyond the prelarval stage, and very few microfilariae developed successfully in the remaining mosquitoes. Very few microfilariae reached the Malpighian tubules of a small percentage of C. nigripalpus and C. p. quinquefasciatus and developed normally. The vectoring potentials of A. sollicitans, M. titillans, A. aegypti, and both Culex species were greatly hampered. These studies suggested that host-specificity of mosquitoes to D. immitis infection is controlled by the presence or absence of secondary physiological factors in their digestive tracts or in the Malpighian tubules.