The dissection of a mosquito for malaria parasite (1953)

mosquitoes are the genus enophiles are the transmitters of malaria to man this is a female aniles atropus feeding on blood drawn from the capillary bed in the wrist of its victim the blood meal is necessary to the female anopheline before the eggs can mature but it is also the means whereby malaria is transferred from man to insect and then on to man again the female May feed about every 48 hours or so ingesting each time slightly more than her own weight of blood so that the stomach becomes greatly distended at the same time clear serum which is separated from the blood in the stomach is passed along the gut to the exterior at the start of any antimalarial work one of the First Essential steps is the investigation of the local anene mosquitoes to find which species are the important vectors of malaria to do this the mosquitoes must be dissected and malaria parasites looked for and either in the stomach or midgut of the mosquito or in the salivary glands the gyes or sexual forms of the malaria parasite enter the stomach of the mosquito with the blood from an infected human fertilization occurs there and oines are form formed which penetrate the stomach wall the blood meal is digested in about 48 Hours by which time the oines have developed into oos cysts on the outer surface of the stomach the oos cysts grow and by the 10th or 12th day after infection they are mature they then rupture and liberate thousands of sporozoites into the body of the mosquito many of these enter the salivary glands and when the mosquito next feeds they are carried with the saliva into the body of the victim thus sporozoites in the salivary glands and O cysts on the stomach wall are the evidence of infection this stomach is exceptionally heavily infected seldom would such a large number of O cysts be found during field surveys these are 7 days old and a fairly large about 40 microns in diam Dieter sporozoites are long thin organisms which are seen moving here near to an unruptured oosy they are about 14 microns long and to show their structure they should be stained this is not however done during field work since they are readily recognizable unstained if the stomach of the captured mosquito is filled with blood the insect should be kept for a day or two until the blood has been digested this is because the distended stomach ruptures easily and the earthists may be obscured by Blood special dissecting needles with very sharp cutting edges are used and they are made of stainless steel because the dissection is carried out in Saline the mosquito is killed before Dissection the live insect is shaken to the bottom of a narrow test tube and is stunned by by giving the tube a few sharp wraps then a light pressure on the thorax kills it without crushing it it is then held by a wing while the legs are pulled off one at a time followed by one of the Wings the other Wing is cut off and so is the head and the mosquito is ready for Dissection the salivary glands are removed first a dissecting microscope magnifying 10 times is used and the insect is placed on the stage with its thorx on the operator's right a drop of saline is put on the slide close to the thorax but not touching it then the thorax is held with a needle at a point just behind where the glands lie and with another needle the glands are gently pressed out and into the saline this is as the operator would see the process each gland has three loes the length of these varies but it averages about a third of a millimeter the sporozoites are present in the cells and ducts of the glands a microscope cover slip is now dropped Corner wise onto the glands just heavily enough to rupture them without dispersing them which makes for easy examination the sporozoites are observed with a 4mm objective they are relatively inactive organisms drifting slowly in the saline and body fluids to remove the stomach the body of the mosquito is placed on a slide in a drop of saline with its thorax to the operator's left the thorax is held with a needle while the kitus abdominal covering is nicked above and below at the level of the sixth or seventh segment the detached segments are then gently pulled and the gut comes away with them at higher magnification it is seen that first the malpan tubes and the hind gut emerge then the stomach or mid gut and finally part of the esophagus for convenience the slide is turned round and the migan tubes are cut off then a cover slip is lowered very gently onto the stomach and if the correct amount of saline has been used the stomach is just flattened without being ruptured the ear tsts can be seen with a six 15 mm objective these specimens are fairly heavily infected the oos cysts are well developed and easily identified the percentage of mosquitoes found to have sporozoites is termed the sporozoite rate for the species the oosy rate may be similarly defined however oos assists on the stomach wall only show the susceptibility of the mosquito to infection they do not confirm it as a vector the sporozoite rate on the other hand is the most important measurement of field malariology its average value in highly endemic areas is about 4% but for one species the rate may vary from 30% during an epidemic to 6% where the malaria transmission is occurring in the presence of a high mosquito population it is therefore the combination of the sporozoite rate with the density of the species that determines the importance of that species as a vector in a particular locality