The nerve ring 21·4 to 21·8 per cent. Excretory pore 30·4 to 31·8 per cent. Excretory cell 34·8 to 37·3 per cent. G1 cell 68·2 to 68·5 per cent. Anal pore 81·6 to 82·4 per cent. of total length. For other details cf. Filaria bancrofti.
Fig. 297.—Mf. loa: in thick film, dried and stained with hæmatoxylin. × 1,000. (After Fülleborn.)
Larvæ in Blood.—These from their diurnal periodicity are known as Mikrofilaria diurna. The evidence that these larvæ are the young of the adult worm Loa loa is: (1) They are identical in structure with larvæ taken from the uterus of L. loa; (2) their geographical distribution is the same as that of L. loa; (3) they eventually occur in the blood of patients suffering from Calabar swellings, a condition due to L. loa. Their occurrence in the blood in this latter condition and in L. loa infections we shall consider later.
Periodicity.—Here, as in the case of the larvæ of Filaria bancrofti, the larvæ that appear in the blood are probably the overflow simply of the larvæ which we assume, on analogy, to have their principal site in the lungs. They appear in the blood about the time of getting up, 6 to 8 a.m. (10 in 20 mm.3), at 12 noon there are twenty-four, at 8 p.m. the number has fallen to eighteen, and at midnight to one, while from 2 a.m. to 6 a.m. none, or one only, may be found. This periodicity is, as a rule, a very constant one, but there are exceptions, and in certain cases more have been found at midnight than at 9 a.m. The periodicity is also lost in pathological conditions, e.g., sleeping sickness (vide also under Filaria bancrofti). The possibility of non-periodic Loa loa larvæ should also be considered.
Pathology.—The parasite wanders about the body, and may be seen under the skin in thin parts. Their advance is in some cases at the rate of an inch in two minutes. During their progress they give rise to creeping sensations and to a condition of transient œdematous areas known as Calabar swellings on various parts of the body, e.g., arm. These vary in diameter from 1 to 10 cm., and often shift their position an inch or so a day. They give rise to a certain amount of redness, tension and heat, and their development is promoted by muscular action of the part. They disappear to reappear elsewhere. The condition is associated with a high eosinophilia, 50 per cent. being not uncommon. Patients known to harbour L. loa, e.g., native children, frequently show no larvæ in their blood, but they may do so after years of infection. Again, in patients having an infection of Mikrofilaria diurna, there is frequently at the time no evidence of the presence of Loa loa adults. Here again they may appear later, but the conditions which determine whether persons infected with L. loa show larvæ in the blood, or persons infected with Mikrofilaria diurna also show L. loa, are unknown, though explanations unsupported by facts abound. Likewise also the mode of production of the swellings is unknown.
Not uncommonly Mikrofilaria perstans occurs in the blood together with M. diurna.
Duration of Life.—This is long, as some cases have been observed five to six years after leaving Africa. The incubation period is about a year.
Life-history.—Development of the larvæ takes place in the salivary glands of Chrysops sp. as shown by Leiper.