**3.1 Biology of the parasite**

Like mentioned before, loiasis is caused by the tissue-dwelling nematode *L. loa*. This worm gets transmitted through a bite of deer fly species *Chrysops silacea*, *C. dimidiate* and *C. langi*, which are restricted to Africa [60, 61]. Similar to other vector species, deer flies transmit infective L3 during a blood meal into their host. Humans are the only known host for *L. loa* although *in vivo* experiments are possible with Drills (*Mandrillus leucophaeus)* and immunocompromised mice [62, 63]. Upon

entering the host, the L3 migrate through the subcutaneous tissues and molt to adult worms. Adult female of *L. loa* can reach lengths of 40–70 mm and are 0.5 mm wide while males are smaller with 30–34 mm lengths and 0.35–0.43 widths. Fecund females release sheathed MF (230–300 μm) that are found in peripheral blood but also in spinal fluid, urine, lung and sputum (**Table 2**). Lifespan estimations of adult worms are rare, but range from at least 9 years to as long as 15–21 years [64]. Adapted to their insect vectors, *L. loa* MF are diurnal, so appear in the peripheral blood during the day. They reside overnight in the lung tissues. During another blood meal, the deer flies ingest the MF. The MF lose their sheaths and migrate from the midgut into the thoracic muscles of the arthropods. Upon developing into the L3, the larvae migrates to the proboscis of the fly to get transmitted during the next blood meal [60, 61].

#### **3.2 Epidemiology**

Loiasis is restricted to the rain forest areas of 12 countries of Western and Central Africa. These are namely Angola, Cameroon, the Central African Republic, Chad, the Republic of Congo and the Democratic Republic of Congo, Equatorial Guinea, Ethiopia, Gabon, Nigeria, Sudan and South Sudan [65]. Although large sections of these countries have low or no prevalence of loiasis, an estimated 14 million people reside in high-risk areas, where the prevalence of *L. loa* is greater than 40%. An estimated 10 million people are currently infected with *Loa loa* [64, 65]. The vector species are more common during the rainy season and usually bite during the day [66]. But these insects can also be found in rubber and palm oil plantations as well as mangrove swamps [67]. Travelers are more likely to become infected if they are exposed to bites for many months.

#### **3.3 Pathology**

The majority of infected individuals remain asymptomatic. However, clinical symptoms of loiasis may take years to develop and due to the lack of severe pathology, this disease is even more neglected [68–71]. One common clinical symptom is the Calabar swelling, a localized angioedema caused by transient subcutaneous swellings which mark the migratory course of the nematode [70]. Interestingly, only around 16% of endemic patients develop this symptom, which are usually located on the face, limbs or joints [27]. It is hypothesized that these swellings are a result of an allergic reaction to the migrating adult filariae or MF [71]. Associated symptoms also include local or disseminated pruritus, urticaria and restricted movement patterns. Symptoms usually resolve after 2–4 days, but they can persist or even reoccur [71]. *L. loa* is known as the "African eye-worm" because of its migration across the eye. This eye migration is found in 10–20% of infected individuals and can result in inflammation, itching, light sensitivity, congestion and severe pain [69]. Similar to the aforementioned symptoms, these signs of infection usually last for several days and the ensuing damage is not permanent. Due to the removal of high MF loads, patients may also present proteinuria and hematuria. Other described, but rare pathologies include inflammation of the lymph glands [72], arthritis [73], scrotal swellings [74], eosinophilic lung infiltrates [75] and endomyocardial fibrosis [71]. Attention to loiasis was raised due to reports about severe adverse effects including fatal cases of encephalopathy after ivermection or DEC treatment during MDA for LF or onchocerciasis [76–79]. These serious events were connected with patients with high peripheral blood MF loads of *L. loa* (>30,000 MF/ml) and the associated inflammatory responses to dying MF [80–82].

*Biology of the Human Filariases DOI: http://dx.doi.org/10.5772/intechopen.102926*

Therefore, in regions where onchocerciasis and LF elimination programmes are ongoing, the co-endemicity with *L. loa* represents a major obstacle [20, 83], leading to a test-and-not-treat scenario. Currently, the focus in these areas is on alternative strategies including a better understanding of the micro-epidemiology, integrated vector management and new *L. loa* tests [84].
