Mansonellosis refers to diseases caused by the filarial nematodes Mansonella perstans, M. ozzardi, and M. streptocerca. Transmission is by biting midges or blackflies. Infections are often asymptomatic, but symptoms can occur. The diagnosis of M. perstans and M. ozzardi is confirmed by identifying microfilariae in blood smears; neither species displays significant periodicity so blood can be drawn any time. The diagnosis of M. streptocerca is based on identifying microfilariae in skin snips or biopsies. Treatment depends on the infecting Mansonella species.
(See also Approach to Parasitic Infections and Overview of Filarial Nematode Infections.)
Three species of Mansonella cause most human disease. As with other filarial diseases, their life cycle is complex (see DPDx - Laboratory Identification of Parasites of Public Health Concern: Mansonellosis).
Mansonella perstans
Mansonella perstans is transmitted by the bites of midges (Culicoides) in sub-Saharan Africa and in the Americas from Panama to Argentina. It is estimated that more than 100 million people are infected worldwide. Adult worms reside in pleural, pericardial, and peritoneal cavities and in the retroperitoneum and mesentery. Unsheathed microfilariae are found in the bloodstream. Midges acquire them when they take a blood meal from an infected human.
Most infections are asymptomatic or mild. Migrating adult worms can cause transient subcutaneous swellings similar to those of Loa loa (Calabar swellings), inflammation of the pericardium or pleura, conjunctival nodules, retinal damage, and periocular inflammation. Nonspecific symptoms including pruritus, urticaria, arthralgias, and malaise. Neuropsychiatric manifestations, meningoencephalitis, and hepatitis have been attributed to M. perstans on rare occasions.
The diagnosis of M. perstans is made by identifying non-sheathed microfilariae in a blood smear. Microfilariae can be identified in blood both day and night. Eosinophilia is often present.
M. perstans is relatively resistant to medications, including diethylcarbamazine, used to treat other filarial diseases. Targeting the endosymbiotic Wolbachia bacteria in adult M. perstanswith doxycycline (200 mg daily for 4 to 6 weeks ) which is presumed to interrupt the reproduction of adult worms can be curative. Wolbachia has been demonstrated in M. perstans from Mali, Cameroon, Ghana, and Gabon. There are divergent reports of the presence of Wolbachia in Uganda, possibly due to methodological constraints or the presence of multiple strains of M. perstans. Doxycycline has been shown to be effective in Mali and Ghana, but the utility of doxycycline for treatment of infections in regions where Wolbachia are absent has not been systematically studied. In cases where doxycycline is ineffective, combination therapy with diethylcarbamazine and mebendazole for 3 weeks has improved activity relative to monotherapy.
Mansonella ozzardi
Mansonella ozzardi is endemic in Central and South America and in a number of Caribbean islands. Humans are the major reservoir of infection. M. ozzardi is transmitted by biting midges (Culicoides) and blackflies (Simulium amazonicum). The life cycle is similar to that of M. perstans except adult worms reside in lymphatics as well as in the thoracic and peritoneal cavities. Microfilariae are present in both the blood and skin.
Most individuals with M. ozzardi infection are asymptomatic, but some have skin rash, lymphadenopathy, arthralgias, fever, headache, or pulmonary symptoms. M. ozzardi microfilaremia induced-keratitis has been reported from the Amazon region of Brazil.
The diagnosis of M. ozzardi is confirmed by identifying microfilariae in blood or skin specimen. Blood can be drawn at any time. Eosinophilia is common. Serologic tests may also be useful in identifying infection, but they are not specific.
Treatment is with ivermectin (200 mcg/kg orally, as a single dose). Before treatment with ivermectin, patients should be assessed for coinfection with Loa loa, another filarial parasite, if they have been in areas of central Africa where both parasites are transmitted because ivermectin can cause severe reactions in patients coinfected with heavy Loa loa infections. Endosymbiotic Wolbachia are present in M. ozzardi, but the efficacy of doxycycline for therapy has not been assessed.
Mansonella streptocerca
M. streptocerca is transmitted in tropical rainforests in west and central Africa and Uganda. The prevalence of M. streptocerca is not known. Nonhuman primates are occasionally infected, but are not major reservoirs of infection. The life cycle of M. streptocerca is similar to that of M. perstans except that adult worms reside in the dermis of the upper trunk and shoulder areas. Microfilariae are found in the skin. Most people with M. streptocerca are asymptomatic. Thickening of the dermis, hypopigmented macules, and bilateral axillary or inguinal lymphadenopathy may be observed. Unlike O. volvulus, adult M. streptocerca do not form subcutaneous nodules.
The diagnosis of M. streptocercais made by identifying microfilariae in skin snips or biopsies. Eosinophilia is common. Serology may also be useful in identifying filarial infection, but it is not specific.
Diethylcarbamazine (DEC) 2 mg/kg orally 3 times a day for 12 days is used to treat M. streptocerca. DEC kills both microfilariae and adult worms. However, it can exacerbate skin and systemic symptoms due to the release of antigens from dying microfilariae. DEC should not be administered to patients with concurrent O. volvulus or heavy Loa loa infections because of the potential for severe adverse effects.
Ivermectin (150 mcg/kg as a single dose) can reduce the microfilarial loads of M. streptocerca, but its impact on the course of infection is uncertain. Before treatment with ivermectin, patients should be assessed for coinfection with Loa loa, another filarial parasite, if they have been in areas of central Africa where both parasites are transmitted because ivermectin can cause severe reactions in patients with heavy Loa loa coinfections. It is not known whether M. streptocerca harbors Wolbachia or doxycycline has a role in treatment.
