Resistance of Moniliformis dubius to the defense reactions of the american cockroach, Periplaneta americana

Localization of phenoloxidase in the midgut of Periplaneta americana parasitized by larvae of Moniliformis moniliformis (Acanthocephala)

In cockroaches infected with Moniliformis moniliformis, the melanogenic enzyme phenoloxidase (PO) was histochemically localized in the posterior midgut and in haemocytes. Midguts were incubated with either 3-hydroxytyramine-HCl (dopamine) or 3-(3,4-dihydroxyphenyl)-L-alanine (dopa), and the resulting electron-dense reaction products of PO activity were found to be homogeneously distributed in the cytoplasm of both midgut cells and haemocytes. Following experimental infection, the first acanthors that reached the outer surface of the gut elicited a haemocyte response similar to that observed during wound healing. Larvae that remained attached to the gut became melanized or developed successfully. PO activity gradually decreased as the course of infection proceeded (10-50 days post-infection) but was apparently not inhibited in either midgut cells or haemocytes that were closely associated with the parasites. PO was lacking in the midgut cells of uninfected cockroaches. The results of the present study are discussed with respect to the defence reactions of the host and the survival mechanisms of the parasite.

Evasion of the insect immune response by Moniliformis dubius (Acanthocephala): further observations on the origin of the envelope

The envelope around larvae of Moniliformis dubius appears to protect the parasite against immune recognition and encapsulation by the insect host's haemocytes. The origin of this envelope has been the subject of controversy although most evidence suggests it is parasite-derived. If host-derived, the envelope would be expected to share surface properties with host tissue. Thus, experiments were undertaken, transplanting parasites and host tissue to other insects and using haemocytic encapsulation as an assay for immune recognition, in order to compare the response to host tissue and to the parasite's envelope. Parasites without their envelopes, and pieces of tissue (ventral nerve cord) from the experimental host (the locust Schistocerca gregaria) were recognized as foreign and encapsulated in the cockroach, Periplaneta americana. The majority of parasites with their envelopes were unencapsulated or only partially encapsulated on transfer to their normal host, P. americana, indicating that the envelope does not have surface similarity to locust tissue. Cockroach-derived parasites with or without envelopes were not encapsulated in S. gregaria, suggesting that the larva itself can evade or inhibit the locust's recognition mechanism. However, since larvae which develop in S. gregaria are enclosed in an envelope, the formation of the envelope would seem to be an inherent feature of the parasite's development.

The cystacanth of Moniliformis moniliformis (Bremser, 1811) and its relationship with the haemocytes of the intermediate host (Periplaneta americana)

The nature of the capsule of the cystacanth of Moniliformis moniliformis was studied. Fat body, tracheal system, Malpighian tubules and haemocytes of the host are intimately associated with the cystacanth. The capsule of the cystacanth is cellular. The cells encapsulating the middle and late acanthellae are granular haemocytes of the host, whereas those encapsulating the cystacanth are agranular. Degranulation of haemocytes after encapsulation of the cystacanth recalls the common method for encapsulation of parasites of insect haemocytes, but a difference between the two kinds of encapsulation is that the capsule around the cystacanth does not bring about melanization nor destroys the parasite. It is suggested that the surface of the parasite may have inhibitors against enzyme systems causing melanization. Host haemocytes are also found in the capsular fluid. The infiltration of haemocytes into the capsule may be in response to the release of metabolites by the parasites. It is also suggested that the haemocytes may detoxify the parasitic metabolities. Rarely, capsules containing two parasites were observed. In one of the cases, one of the two cystacanths already possessed its own capsule. This finding, together with the cellular nature of the capsule, suggests that the capsule is of host origin.