Acanthocheilonema

Acanthocheilonema is a genus within the family Onchocercidae which comprises mainly tropical parasitic worms. Cobbold created the genus Acanthocheilonema with only one species, Acanthocheilonema dracunculoides, which was collected from aardwolf in the region of South Africa in the nineteenth century. These parasites have a wide range of mammalian species as hosts, including members of Carnivora, Macroscelidea, Rodentia, Pholidota, Edentata, and Marsupialia. Many species among several genera of filarioids exhibit a high degree of endemicity in studies done on mammalian species in Japan. However, no concrete evidence has confirmed any endemic species in the genus Acanthocheilonema.
There are about fifteen well characterized parasitic species found in genus Acanthocheilonema. A. delicata n. sp., A. dracunculoides, A. filaria, A. mansonbahri, A. mephitis, A. odendhali, A. pachycephalum, A. pricei, A. procyonis, A. reconditum, A. sabanicolae, A. setariosa, A. spirocauda, A. viteae, and A. weissi are the well characterized parasitic species found all around the world in variety of hosts. Some of the crucial parasites that affect a wide range of host species are discussed below.

''Acanthocheilonema delicata''

Acanthocheilonema delicata was discovered and characterized from an endemic badger species in Japan. The researchers identified the DNA sequences of mitochondrial cytochrome c oxidase subunit 1 gene in the Japanese badgers. Morphologically, filarioids analyzed were identified as thin, small and delicate and in females the length was longer. Anterior of the filarioids is slightly bulbous having two sets of four papilla and amphids. They possess distinct buccal cavity with buccal capsule consisting of a thick buccal ring. The esophagus is divided into short anterior muscular portion and long, posterior glandular portion. Caudal extremity in both males and females consists of three conical lappets. The location of the parasitic adult worms in the host species was found to be the subcutaneous connective tissue, whereas the microfilaria was found in the skin. Data taken from the Genbank of fourteen species of genus Acanthocheilonema compared to the newly found species for possible similarities and distinctions shown in the table below:

Species →	A. delicata	A. dracunculoides	A. odendhali	A. reconditum	A. spirocauda	A. viteae
Body length	Female-22–38; Male-10–16μm	Female-38.5–45.2; Male-21.7–24μm	Female-100-150; Male-46-64μm	Female-20.7-25.5; Male-9.3-17.1μm	Female-155; Male-87μm	Female-49μm; Male-38μm
Body width at mid-body	Female-85-140;Male-60-85μm	Female-220-280;Male-140-160μm	Female-348-415;Male-228-281μm	Female-146-168;Male-92-100μm	Female-660;Male-400μm	Female-230-350;Male-155-165μm
Total esophagus length	Female-3000–4125;Male-2520–3350μm	Female-1940–2500;Male-1960–2170μm	Female-1782–2197;Male-1890–2251μm	Female-2040–2340;Male-1860–2040μm	Female-1300–1800;Male-1900–2000μm	Female-1370-1570;Male-1600-1660μm
Tail	Female-167–250;Male-113–162μm	Female-240–395;Male-140–180μm	Female-214–322;Male-147–245μm	Female-180–300;Male-80–145μm	Female-260;Male-230μm	Female-320–470;Male-280μm
No. of terminal lappets	3	3	3	3	Female-3; Male-4	3
Microfilariae width	7-9μm	4.5-5.2μm	3.5μm	4.5μm	4-4.5μm	4.5μm
Microfilariae body length	153–180μm	121-218μm	231-249μm	270μm	266-302μm	180-200μm
Host animal	Meles anakuma	Proteles cristatus and spotted hyaena	Zalophus californianus	Crocuta crocuta and Hyaena hyaena	Phoca vitulina concolor	Meriones libycus
Parasitic location of adult worms	Subcutaneous connective tissue	Peritoneal cavity	Intermuscular fascia	Subcutaneous connective tissue	Pulmonary artery and right ventricle	Subcutaneous tissue
Parasitic location of microfilariae	Skin	Blood	blood	blood	blood	blood
locality	Japan	Kenya	California, US	Italy, India, US, Kenya, Japan	coastal Maine, US	Iran
Reference

''Acanthocheilonema dracunculoides''

Acanthocheilonema dracunculoides Cobbold, a species discovered in 1870, is a nematode parasitic worm particularly found in domestic dogs and other carnivores like aardwolf, spotted hyaena and red fox. These parasites can be located on various continents like Europe, Asia and Africa. it is known to be endemic in Morocco, Algeria, Tunisia, Mali, Niger, Democratic Republic of Congo, Sudan, Somalia, Kenya, Tanzania and South Africa. This parasite follows an indirect life cycle. In the carnivore species, the male and female parasite tend to be biased to exist mainly in the peritoneal cavity. Males are typically shorter and are 15–32 mm long and 0.1–0.2 mm wide, whereas females are almost double, standing at 30–60 mm long and 0.1–0.3 mm wide in diameter. Sexually mature female parasites of A. dracunculoides are viviparous, they produce L1-stages,
known as microfilariae, which eventually appear in the peripheral blood. Microfilariae are unsheathed and measure 185–276 μm by 4.2–6 μm
Although the final host is the carnivores for A. dracunculoides there are some other intermediate hosts that are involved in the parasite life cycle. Louse fly Hippobosca longipennis and the hard tick Rhipicephalus sanguineus, also known as dog tick, have been identified as intermediate hosts that helps the parasite to complete its life cycle. Although Acanthocheilonema dracunculoides has a reputation of not being an endemic parasite in the domestic dogs, recent evidence of the presence of the parasite in domestic animals show some signs of pathogenicity. As mostly these parasites are considered innocuous in the dogs, recent studies and evidence from Spain suggests that the parasite is not completely harmless and innocuous in the domestic dogs as believed earlier.
The discovery of Acanthocheilonema parasites in the non-endemic species has triggered more research in this field. Dirofilaria immitis, D. repens, Acanthocheilonema dracunculoides and A. reconditum are the common species that have been known to be found in canine filarial nematodes. These nematodes present blood circulating microfilariae which are differentiated to the species level using the techniques like PCR and by acid phosphatase activity patterns. A. reconditum and A. dracunculoides live in the peritoneal cavity and adipose tissue of the host species and are less pathogenic, however D. immitis and D. repens are considered to be the emerging agents of parasitic zoonoses and are continuously expanding their ranges in the European. areas.

''Acanthocheilonema odendhali''

The filarial worm Acanthocheilonema odendhali was first discovered in 1967 from the California California sea lion Zalophus californianus. This species was found inhabiting the intermuscular fascia and thoracic and the abdominal cavities. Initially this species was described in the Dipetalonema genus as Dipetalonema odendhali, however due to later evidences it was transferred to the genus Acanthocheilonema. A. odendhali was subsequently found in other marine species like Steller sea lion and northern fur seal. Besides the A. odendhali, other filarioidean species such as A. spirocauda, commonly known as heart-worm has also been identified as parasitic in phocids.
A. odendhali was primarily reported in the northern fur seals on the island of Pribilof Archipelago in 1967. Prior to that two scientists Mark C. Keyes and Eugene T. Lyons had discovered the microfilariae in the blood of subadult male seals and adult filarioids and microfilariae in the samples collected from the blubber, fur, and the skin from these male seals. In these studies, the prevalence and the intensity of these species were not examined. The life cycle of A. odendhali is still unknown. However, according to the National Marine Mammal Laboratory, Northwest and Alaska Fisheries Center reports, some blood-sucking flies or insects that infest the wounds are potential intermediate hosts in their life cycle. Pathogenicity of A. odendhali was however not studied in these experiments and research but this parasite is considered as non-pathogenic.
In 2011 and 2012, a study that investigated helminths of northern fur seals on Saint Paul Island in Alaska discovered that A. odendhali was a parasite on the seals. Other research done by the same group investigated the intensity and prevalence of the parasite and discovered that prevalence of adult A. odendhali parasites in the fur seals was 18 percent; the intensity depends on the individual host species and the seven parasite specimens found in the parasite. Average intensity of infection was approximately 1.32 ±0.83 SD, whereas the abundance was found to be 0.24 SD.

''Acanthocheilonema reconditum''

Acanthocheilonema reconditum is a non-pathogenic canine helminth parasite of the specialized connective tissue. It is distributed in the same endemic areas as D. immitis, which is also known as heart-worm. Although it is endemic to some areas mentioned earlier, but the parasite is widely distributed as it has been identified in other species and around different continents. Dogs are frequently parasitized by the filarids. This transmission is usually carried out by the flea species and lice species. Flea vectors include species such as Ctenocephalides canis, Pulex irritans, Pulex simulans, and Echidnophaga gallinacea, whereas the lice species include Linognathus setosus and Heterodoxus spiniger. These vectors are infected with the A. reconditum parasite with the microfilariae during the blood repast in a parasitized dog. These species act as intermediate hosts in completing the life cycle of the parasite.
These microfilariae develop onto an infectious stage during the L3 stage in the digestive tract of these vectors from where they can be inoculated into the new host. The third stage larvae could be found as soon as 7 days after the appearance of the vectors on the donor host species. Once it enters the new host species, the microfilariae develops into the adult stage. Most of these adult stage parasites are located in the subcutaneous tissue.
Although the infection by A. reconditum does not exhibit any clinical signs, a treatment to limit dissemination is primarily recommended. Recent studies of A. reconditum occurrence in the southern regions of Italy has shown prevalence of this parasite as high as 13.3% with an annual incidence rate of 5.9%, which was observed in naturally exposed dogs. Recent experiments have enhanced scientific knowledge of the biology and ecology of this parasite. The full development of microfilariae occurs in the experimental infected cat flea Ctenocephalides felis felis in about 15 days. Localization and size of this parasite in this infected flea suggests the possibility that this might act as an intermediate host throughout the ingestion of the infected fleas rather than inoculation of the blood meal on dogs. Ivermectin is a drug that is preferred to treat infection by this parasite.
As microfilariae of this parasite are mainly found in the blood, they are often mistaken for other parasites like Dirofilaria immitis. It is therefore crucial to identify and distinguish between them using modified Knott's procedure.