Susceptibility of a Male Mosquito to Malaria

Abstract

Only female mosquitoes are malaria vectors because males do not feed on blood. It has been unknown if the male shares the specific susceptibility to Plasmodia of the female. Weathersby (1954, Expl Parasit. 3: 528-543) showed that blood infected with Plasmodium gallinaceum injected directly into the hemocoel of female Aedes aegypti could develop oocysts and sporozoites ectopically. This has also been found to be true when zygotes of P. gallinaceum fertilized in vitro and suspended in Medium 199 were injected into females (Rosenberg et al., 1984, Expl Parasit. 57: 158-164). Using this same procedure, I injected each of 30 male and 30 female sibling Ae. aegypti with 1.4 x 104 zygote-macrogametes of P. gallinaceum. A second group of females was fed on 1.4 x 107 zygotes/ml whole blood. At 5 and 14 days after injection 10 of each sex were fixed in formalin, dehydrated in graded ethanol, cleared in chloroform, embedded in Paraplast, sectioned at 5 4, and stained with Giemsa. Oocysts developed in the males at the same pace and in the same approximate density as in females. Females ingesting about 2.8 x 104 zygotes in whole chicken blood developed 154 oocysts per gut, which appeared to be substantially larger than either injection group. Thirteen days after injection sporozoites were seen in wet preparations of the salivary glands of 2 of 4 females and 2 of 7 males. Numbers of sporozoites were light to moderate in both sexes. Sporozoites were not seen, however, within the glandular acini or ducts in the wet or stained preparations of either sex. There is no doubt that sporozoites from ectopic oocysts can invade the glands of females (Rosenberg et al., loc. cit.) and the failure to find them there in this experiment may owe to the light infection and imperfect, excessively dark staining. There is, however no proof yet that sporozoites developed in males actually enter male salivary glands. It is physically impossible for a male mosquito to become naturally infected with malaria: its mouthparts are unable to pierce skin. It is not known if infected blood artificially introduced into the mid-gut lumen will produce oocysts. Malaria exflaggellation factor is present in male Ae. aegypti gut epithelium, as well as other organs (Nijhout, 1980, Expl Parasit. 48: 75-80), but may not be secreted. It has been shown that blood digestion by trypsin-like enzymes is necessary for P. gallinaceum to infect Ae. aegypti females (Rosenberg et al., loc. cit.) but in one of the few studies done of digestion in male mosquitoes (Wagner et al., 1961, Navy Med. Res. Rpt. No. 6) it was found that male Ae. aegypti lacked at least 1 protease present in females. Circumventing these obstacles by injecting zygotes suspended in Ml 99 directly into the hemocoel, oocysts develop normally. This appears to confirm that once beyond the gut barrier the processes of blood digestion and ova production are not essential for survival of the parasite and that the male hemolymph contains nutrients sufficient for parasite growth and not elements harmful to it. The methods used here could be used to determine whether any genes controlling susceptibility are sex-linked. I thank L. C. Koontz for preparing the zygotes and D. Seeley and J. Edelin for maintaining the mosquitoes and parasites.