. Animal parasites and human disease. Parasites; Medical parasitology; Insects as carriers of disease. 300 FILARIjE AND THEIR ALLIES worms reach the blood by way of the lymph stream and these grow to about 300 fi (a little over T^ of an inch) in length. They are delicate colorless worms (Fig. 124A), blunt at the anterior end and tapering to a slender point at the tail end, and are entirely enclosed in a remarkably delicate transparent sheath, which, although it fits as tightly as a glove over a finger, is too long for the animal and can be seen projecting at either end. The sheath may be looke
. Animal parasites and human disease. Parasites; Medical parasitology; Insects as carriers of disease. 300 FILARIjE AND THEIR ALLIES worms reach the blood by way of the lymph stream and these grow to about 300 fi (a little over T^ of an inch) in length. They are delicate colorless worms (Fig. 124A), blunt at the anterior end and tapering to a slender point at the tail end, and are entirely enclosed in a remarkably delicate transparent sheath, which, although it fits as tightly as a glove over a finger, is too long for the animal and can be seen projecting at either end. The sheath may be looked upon as a wonderful adaptation to prevent the worms from being able to bore through the bloodvessels and escape from the blood, in which case they would miss their chance for " ; The internal organs are in a very rudimentary condition. The most remarkable cir- cumstance connected with the life of .these microfilariae is the periodical appearance and dis- appearance of them in the blood of the peripheral vessels. If the blood of an infected person is examined during the day few if any worms can be found, but as evening ap- proaches they begin to appear and continue to increase until about midnight, after which they decrease again until morning. During the night when they are most abun- dant there may be as many as 500 worms in a single drop of blood. If the parasites are assumed to be evenly distributed throughout the peripheral circulation, this would imply the presence of several million worms in the body. The periodic appearance and disappearance of microfilariae in the blood is not invariable. When an infected person is made to sleep in the daytime instead of at night, the appearance and disappearance of the parasites in the peripheral bloodvessels can be reversed, implying that the distribution of the parasites may be dependent. Fig. 124. Comparison of microfilariae; A, mf. bancrofti (large with sheath); B, mf. perstans (small, blunt tail, no sheath);
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Keywords: ., bookcentury1900, bookdecade1920, bookpublishernewyo, bookyear1922
