phylum Acanthocephala

Introduction

The acanthocephala, or “spiny-headed worms” (acantho: spiny; cephala: head) are a small, monophyletic phylum of unsegmented obliga­tory parasitic worms (Kennedy 2006); all are characterized by bilateral symmetry (Crompton and Nickol 1985).

The adult body is divided into two major regions (Fig. 3.10). The praesoma comprises a retractile armed proboscis (spiny head) and unarmed neck. The metasoma (trunk) includes the body wall around the pseudocoel, which in turn includes sex organs and excretory organs. The outside tegument includes a set of chan­nels (lacunar system); this lacunar system is organized differently among the classes of Acanthocephala (Crompton and Nickol 1985). Acanthocephala have no digestive tract and, like cestodes, obtain their nutrients through their surface integuments (Roberts and Janovy 2000); however, acanthocephala do have a central ligament which often has a superficial similarity to an intestinal tract.

The acanthocephalan proboscis is invagin- able and retractile, and covered with spines (Miller and Dunagan 1985). In the definitive host, it is embedded in the intestinal wall, and can lead to severe granulomatous, hemorrhagic enteritis. A fatal peritonitis can occur when the proboscis penetrates through the intestinal wall. Commonly, small numbers of worms are tolerated well, but heavy infections can be fatal.

All acanthocephalan species for which life cycles have been confirmed require vertebrates as definitive hosts and arthropods as inter­mediate hosts (Nickol 1985). They also share the characteristic with nematodes of having no asexual reproduction among larval forms, in contrast to many Platyhelminthes. Acan- thocephalans parasitic in terrestrial defini­tive hosts usually use insects, often species of Coleoptera or Orthoptera, for intermediate hosts. Acanthocephalans parasitic in aquatic definitive hosts commonly use microcrusta­cean species such as Amphipoda, Copepoda, Isopoda, Decapoda, or Ostracada (Nickol 1985). Some acanthocephalans also use paratenic hosts between the arthropod and the defini­tive host (Nickol 1985); this may occur when an intermediate host is eaten by a vertebrate host which is unsuitable as a definitive host; in that case the cystacanth penetrates the vertebrate’s gut, encysts, and ceases development until eaten by a suitable definitive host (Cole 1999).

Acanthocephala rarely dominate the helminth communities of their definitive hosts (Kennedy 2006). Diet appears to be an important deter­minant by which terrestrial vertebrate hosts are infected. Acanthocephala normally are not found in strict herbivores and are somewhat uncommon in strict carnivores. They are more common in species with an omnivorous diet or in some pred­ators that ingest paratenic hosts (Kennedy 2006).

Adult acanthocephala are parasites of the small intestine of their definitive hosts. Only eggs exist outside of the host, free in the envi­ronment, and transmission from one definitive host to another requires that eggs be ingested by appropriate invertebrate hosts (Fig. 3.11). Eggs pass out with the feces and may overwin­ter in the environment, where they commonly become embryonated. The acanthor is the larval stage that emerges from the egg after ingestion by an arthropod. Often spindle-shaped, with a row of rostellar hooks, it penetrates through

FIGURE 3.10 Generalized anatomy of acanthocephala (McDonald, 1988; Courtesy of Malcolm McDonald and the U.S.

the alimentary canal wall into the hemocoel and develops into an acanthella. The acan- thella often develops the proboscis and other basic organs. The acanthella often is capable of changing the behavior of the intermediate host in such a way as to make it more noticeable and even more attractive to the definitive host for which it serves as food (Nickol 1985; Moore 1995, 2002). Once the acanthella is ingested, it encysts; the cystacanth (juvenile) is the final stage in the invertebrate or paratenic host and is infective for the definitive host.

Acanthocephala infect all classes of verte­brates (Cole 1999). By one assessment, about 2,600 species of acanthocephala were esti­mated to exist (Poulin and Morand 2004). A considerable number of acanthocephala have been reported from wild mammals (DeGiusti 1971) and wild waterfowl (McDonald 1988); most acanthocephala do not cause serious pathology in their definitive hosts, and only a limited number of groups are of concern to wildlife studies (App. 1: Table 2). Most com­monly, the spiny proboscis of the acantho- cephalan may penetrate the intestinal mucosa of the host, causing some inflammation and sometimes hemorrhagic enteritis. Each point of attachment is a potential source of secondary bacterial invaders such as Staphylococcus spp., Streptococcus spp., or Pasteurella multocida. Smaller numbers of the worms can be tolerated relatively well, but with heavier infections some may penetrate the intestinal wall, leading to a peritonitis and likely death.

One set of organisms will be addressed as an example. Several other species will be summarized briefly at the end of this section. Additional information about wildlife parasites is available at other sources (DeGiusti 1971, Crompton and Nickol 1985).