Loa loa

Loa loa
Loa loa microfilaria in thin blood smear (Giemsa stain)
Scientific classification
Kingdom:	Animalia
Phylum:	Nematoda
Class:	Chromadorea
Order:	Spirurida
Superfamily:	Filarioidea
Family:	Onchocercidae
Genus:	Loa
Species:	L. loa
Binomial name
Loa loa (Cobbold, 1864)
Synonyms
Filaria loa Cobbold, 1864

Loa loa is the filarial nematode (roundworm) species that causes Loa loa filariasis. Loa loa actually means "worm worm" but is commonly known as the "eye worm" as it localizes to the conjunctiva of the eye. Loa Loa is commonly found in Africa and India.^[1] It mainly inhabits rain forests in West Africa and has native origins in Ethiopia.^[2]

L. loa is one of three parasitic filarial nematodes that cause subcutaneous filariasis in humans. The other two are Mansonella streptocerca and Onchocerca volvulus (causes river blindness).

Maturing larvae and adults of the "eye worm" occupy the subcutaneous layer of the skin – the fat layer – of humans, causing disease. The Loa loa adult worm which travels under the skin can survive up to 10–15 years causing inflammations known as Calabar swellings. The adult worm travels under the skin where the female deposits the microfilariae which can develop in the host’s blood within 5 to 6 months and can survive up to 17 years. The young larvae, or microfilariae develop in horseflies of the genus Chrysops (deer flies, yellow flies), including the species C. dimidiata and C. silacea, which infect humans by biting them. After bites from these infected flies, the microfilariae are unique in that they travel in the peripheral blood during the day and migrate into the lungs at night. ^[3]

Epidemiology

Reports of Loa loa microfilaremia have been found in Angola, Benin, Cameroon, Central African Republic, Congo, the Democratic Republic of Congo, Equatorial Guinea, Gabon, Nigeria and Sudan, and possibly rare cases in Chad, Ghana, Guinea, Liberia, Uganda and Zambia.^[4] Of the ten countries that have high rates of infection, about 40% of the people that live in the area have reported being infected with the worm in the past. The population in high risk areas is about 14.4 million, in addition, 15.2 million people live in areas were around 20–40% of people admitted to having the worm in the past.^[5]

Risk Factors

People at the highest risk for acquiring loiasis are those that live in the rainforests of West/Central Africa. Furthermore, the probability of getting bitten by a deer or mango fly increases during the day and during rainy seasons. The flies are also attracted to smoke from wood fires. It is not common for these flies to enter houses, but they are attracted to the houses that are well lit and will congregate outside.^[5]

Regarding travelers, they can be infected in less than 30 days after arriving in an affected area, although they are more likely to be infected whilst being bitten by multiple deerflies over the course of many months. Men are more susceptible than women due to their increased exposure to the vectors during activities such as farming, hunting and fishing.^[6]

Biology

Morphology

Loa loa worms have a simple body consisting of a head, body, and tail. Males range from 20mm to 34mm long and 350μm to 430μm wide. Females range from 20mm to 70mm long and can be about 425μm wide. They vary in color.^[1]

Life cycle ^[7]

The human is the definitive host, in which the parasitic worms attain sexual maturity, mate, and produce microfilariae. The flies serve as intermediate hosts in which the microfilariae undergo part of their morphological development, and then are borne to the next definitive host.

Two species of Chrysops deerflies, Chrysops silacea and Chrysops dimidiata are the main vectors for Loa loa filariasis disease.

1. A fly bearing third-stage filarial larvae in its proboscis infects the human host through the bite wound.
2. After entering the human host the larvae mature into adults, commonly in subcutaneous tissue. Adult females measure about 40 to 70 millimeters in length and 0.5 millimeters in diameter. Males measure some 30 to 34 millimeters by 0.35 to 0.43 millimeters.
3. The adult female produces large numbers of microfilariae about 250 to 300 micrometers in length and six to eight micrometers in width. She continues to do so continuously for her lifetime, which typically spans several years.
4. Microfilariae tend to reside within spinal fluids, urine, and sputum; by day they also circulate in bloodstream. Apart from their presence in bodily fluids however, microfilariae in the non-circulation phase also occur in the lungs.
5. The vector fly ingests microfilariae while feeding on the host's blood.
6. Once inside the vector, the microfilaria sheds its sheaths and escapes through the walls of the midgut into the fly’s haemocoel.
7. It then migrates through the haemolymph into the wing muscles in the fly's thorax.
8. In the thoracic muscles the microfilaria develops successively into a first-stage larva, second-stage larva, and finally into the infectious third-stage larva.
9. The third-stage larva migrates to the fly’s proboscis.
10. Once the larva is established in the proboscis and the fly takes its next human blood meal, the cycle of infection continues.

Disease

Signs and Symptoms

It usually takes about 5 months for larvae (transferred from a fly) to mature into adult worms—in which they can only do so inside the human body. The most common display of infection is the localized allergic inflammations called Calabar or Cameroon swellings that signify the migration of the adult worm in the tissues away from the injection site by the vector. The migration does not cause significant damage to the host and is referred to as benign. However, these swellings can be painful as they are most found near the joints. ^[5]

Although most infections with Loa loa are asymptomatic, symptoms generally do not appear until years, or even more than a decade, after the bite from an infected fly. However, it has been shown that symptoms can appear as early as 4 months after a bite.^[8] This parasite has a diurnal periodicity in which they circulate in the peripheral blood during the daytime, but migrate to vascular parts of the lungs during the night, where they are considered non-circulatory. Therefore, the appearing and disappearing characteristics of this parasite can cause recurrent swelling that can cause painful enlargements of cysts in the connective tissue surrounding tendons. Additionally, chronic abscesses can be caused by the dying worms.^[3]

The most visual sign of an adult worm infections with Loa loa is when the worm crosses the sclera of the eye. The migration to the eye causes significant pain to the host and is usually associated with inflammation and less likely, blindness. Eye worms typically cause little eye damage and last a few hours to a week.^[5] Other tissues in which this worm can be found includes: the penis, testes, nipples, bridge of the nose, kidneys, and heart. The worms in these locations are not always possible to visualize.^[4]

Diagnosis

The main methods of diagnosis include the presence of microfilariae in the blood, the presence of a worm in the eye, and the presence of skin swellings. However, in cases where that is not the case, a blood count can be done. Patients with infections experience a higher number of blood cells, namely eosinophils as well as high IgE levels that indicate an active infection with helminth parasites. Due to the migration of microfilariae during the day, the accuracy of a blood test can be increased when samples are taken between 10 am and 2 pm.^[4] A Giemsa stain is most commonly used diagnostic test that uses a thick blood smear to count the microfilariae. Other than blood, microfilariae can also be observed in urine and saliva samples.^[6]

Treatment

Adult worms found in the eye can be surgically removed with forceps after being paralyzed with a topical anesthesia. Studies have shown that the worm is not paralyzed completely, therefore if it is not extracted quickly, the worm can vanish upon extraction.^[6]

Ivermectin has become the most common antiparasitic agent used worldwide but can lead to residual microfilarial load when given in the management of loiasis. Treat with ivermectin has shown to produce severe adverse neurological consequences in some cases. These treatment complications can be increased in co-infected individuals with onchocerciasis.^[4] Some of these patients experienced cases of coma and resultant encephalopathy, parkinsonism, and death. After about 12 hours the first signs start to appear and include fatigue, pain in joints, mutism, and incontinence. Severe disorders of the consciousness start to develop after about a day.^[9]

High microfilarial loads should be decreased by a course of Ivermectin, a prolonged administration of albendazole, or cytapheresis sessions to prevent occurrence of serious adverse events, including fatal encephalopathy induced by dying microfilariae. Cytapheresis is helpful in decreasing very high microfilarial loads up to 75%. Diethylcarbamazine kills both microfilariae and adult worms but has more severe side effects and can be fatal.

Prevention

Currently, there are no control programs or vaccines for loiasis. However, if you plan to be in a high-risk area where this parasite inhabits, it is suggested that you take diethylcarbamazine (DEC)—about 300 mg each week—to reduce your risk. Another way is to avoid areas where the vectors, deer and mango flies, are found. This includes swamps, bogs, and shaded areas near rivers or near wood fires. Ways in which you can minimize fly bites is by using insect repellents such as DEET (N, N-Diethyl-metatoluamide) and to wear clothing that has long sleeves and pants during the daytime. Permethrin treatment on your clothes is an additional repellent that could be used. Also, using malaria nets can also reduce the number of fly bites acquired. ^[5]

Clinical Case

In a village in Katsina State, Nigeria, a 25-year old woman came to the eye clinic complaining of three-month-old symptoms that included redness, pain, photophobia, tearing and itching. She also mentioned experiencing two-month-old symptoms of gradual vision loss accompanied by a moving sensation in her left eye. She had no history of fever and no itching or swelling in parts of her body. Although she had visited a primary healthcare facility multiple times over the last few months, she was only prescribed topical eye treatments that resulted in no change in her symptoms. When in the eye clinical and upon examination, swelling was absent and her chest and heart were normal. During an examination on her left eye, her cornea was clear but conjunctival hyperemia was present, as well as the presence of a live, thread-like worm in the anterior chamber of her eye. Multiple blood samples were taken during the day and revealed 52% eosinophilia that were negative for microfilaria and a stool sample was also negative for ova and parasites. The patient then was admitted for surgical removal of the worm in which her symptoms eventually subsided.^[10]

References

• Taxonomy Browser: Loa Loa. National Center for Biotechnology Information (NCBI).