# A cross-sectional investigation of the ophthalmological impact of loiasis in Cameroon, Central Africa

## TL;DR

This study shows that loiasis, a parasitic infection, is linked to severe eye problems like chorioretinitis and vision impairment, suggesting it should be classified as a neglected tropical disease.

## Contribution

The study provides the first evidence linking high Loa loa microfilarial loads to severe ocular lesions and unilateral ametropia.

## Key findings

- High Loa loa microfilarial loads are independently associated with chorioretinitis.
- People with recent eye worm passage are three times more likely to have unilateral ametropia.
- Chorioretinitis was most prevalent in individuals with high microfilarial loads.

## Abstract

Current knowledge of ocular manifestations of loiasis is limited to the transient subconjunctival passage of the adult filaria and anecdotal reports of posterior segment lesions. Therefore, the ocular burden of loiasis is likely underestimated since it has never been systematically assessed at the population level. We aimed to evaluate the relationship of Loa loa microfilaremia and recent eye worm passage with chronic ocular lesions identified through comprehensive ophthalmological assessment in an endemic area.

Subjects aged ≥ 15 years, residing in Akonolinga for ≥ 5 years, without filaricidal treatment for ≥3 years, were screened for filariases. After excluding participants with onchocerciasis lesions, a subset of randomly selected participants was assessed by ophthalmologists blinded to blood test results then allocated to four groups defined by microfilarial load (MFL) on calibrated thick blood film: G1 (Loa MFL = 0), G2 (MFL < 8000/mL), G3 (MFL ≥ 8000/mL), G4 (co-infestation with Mansonella MFL > 100/mL). The ophthalmological assessment comprised distance visual acuity, examination of the anterior segment with a slit lamp, and fundoscopy. The primary analysis consisted of univariable comparisons of the frequency of abnormal findings across four groups (G1 – 4) or two groups defined by history of eye worm passage. The secondary analysis consisted of a multivariable logistic regression analysis of the relationship of high Loa MFL (≥8000/mL) with chorioretinitis and eye worm passage with unilateral ametropia, adjusting for confounders. Of 1511 subjects screened, 200 underwent ophthalmological assessment, including 65, 69, 35, and 16 in G1 to 4. History of eye worm passage in the previous year was reported by 121 participants (65.4%). Unilateral ametropia was more prevalent in people with history of eye worm passage in the previous year (26.5% versus 10.9%, p = 0.014). Chorioretinitis was the most frequent posterior segment lesion (n = 11, 6.1%) and was most prevalent in G3 (14.3%). The frequency of chorioretinitis was higher in participants with moderate-to-severe visual impairment (27.3% versus 4.4%, p = 0.002). High Loa MFL was an independent predictor of chorioretinitis (adjusted OR=5.28; p = 0.01). History of eye worm passage in the previous year was independently associated with unilateral ametropia (adjusted OR=3.27, p = 0.0088).

This study has, for the first time, provided evidence of independent association between history of eye worm passage and unilateral ametropia, and between high Loa MFL and severe chorioretinal lesions. This suggests that loiasis should be classified as a neglected tropical disease.

Loiasis is endemic in Western and Central Africa and affects more than 10 million individuals. Currents knowledge of its ocular manifestations is limited to the transient subconjunctival passage of the adult worm and anecdotal case reports of severe lesions such as optic neuritis and retinal hemorrhage. Consequently, loiasis has repeatedly failed to meet WHO’s morbidity and mortality criteria for inclusion in the strategic plan for neglected tropical diseases. We aimed to provide additional evidence to support this inclusion by studying the relationship of Loa loa microfilaremia and recent eye worm passage with chronic ocular lesions by performing a systematic ophthalmological assessment of community-dwelling adults in an endemic area. For the first time, we provide evidence that, contrary to prior knowledge, severe ocular lesions such as chorioretinitis are highly prevalent (14.3%) in people with high Loa loa microfilarial loads (MFL) and significantly associated with moderate-to-severe visual impairment. Specifically, people with a high MFL (>8000 mf/mL) are 5 times more likely to have chorioretinitis than those with low MFL or without infestation, independent of age, sex, co-infestation with other filaria, and cardiovascular risk factors. We also report that people with recent eye worm migration were 3 times more likely to have unilateral ametropia. Our findings contradict perceptions that loiasis is a benign disease and suggest that it should be classified as a neglected tropical disease. Such reclassification will boost the design and funding of cost-effective public health strategies to eliminate loiasis.