Brugia malayi microfilariae transport alphaviruses across the mosquito midgut

Abstract

Concurrent ingestion of microfilariae (MF) and arboviruses by mosquitoes can enhance mosquito transmission of virus compared to when virus is ingested alone. Within hours of being ingested, MF penetrate the mosquito midgut and introduce virus into mosquito hemocoel, creating a disseminated viral infection much sooner than normal. How virus is actually introduced is not known. In this report, we present experimental evidence that suggests that certain alphaviruses may adhere or otherwise associate with sheathed Brugia malayi MF in the blood of a dually-infected host and that the virus is carried into the mosquito hemocoel by the MF during their penetration of the mosquito midgut. The mechanism of MF enhancement may be more complex than simple leakage of viremic blood into the hemocoel during MF penetration. The affinity of arboviruses to adhere to or otherwise associate with MF may depend on the specific combination of the virus and MF involved in a dual host infection. This in turn may determine the relative importance that MF enhancement has within an arbovirus transmission system.

Reproducibility and relevance in insect-arbovirus infection studies

“…Evidence to date indicates that the presence of parasites such as microfilarial nematodes is likely to be of greater importance than the presence of other arboviruses in a bloodmeal, with studies of simultaneous co-ingestion of multiple arboviruses resulting in similar levels of infection, dissemination and transmission to those seen during single-virus studies [ 70 ], suggesting that considering each virus in isolation for epidemiological purposes may be entirely valid. However, several studies have shown that simultaneous ingestion of an arbovirus and a nematode may result in elevated susceptibility to the arbovirus [ 71 , 72 , 73 ], initially assumed to be a result of the mechanical damage to the gut caused by the microfilariae allowing the virus to bypass the midgut infection and escape barriers in a similar way to intrathoracic inoculation, although more recent evidence has suggested that certain arboviruses may specifically adhere to the microfilariae and be actively transported within the insect [ 74 ]. Simulation studies suggest that this interaction could result in macro-scale epidemiological differences in outbreak frequency [ 75 ].…”

Nodding syndrome research revisited

“…Laboratory research has shown that arboviral transmission is enhanced in haematophagous insects, such as mosquitoes, that concurrently ingest microfilariae (Vaughan and Turell, 1996;Vaughan et al, 1999). It has been shown that microfilariae penetrating the mosquito midgut may introduce viruses to the hemocoel if coinfected, resulting in dissemination and enhanced transmission (Vaughan et al, 2007;Vaughan and Turell, 2017). This phenomenon, known as 'microfilarial enhancement of arboviral transmission' (Vaughan and Turell, 2017), has not been investigated in blackflies.…”

Mosquitoes as Suitable Vectors for Alphaviruses

“…Mosquitoes are frequently infected with parasites that may play a role in alphavirus-mosquito interactions [ 39 ]. Microfilariae are an early developmental stage of certain parasites which are known to live in vertebrates.…”

“…This facilitation of viruses across the midgut barrier is known as microfilarial enhancement of arboviral transmission. It is hypothesised that this results in greater vector competence, as less susceptible mosquitoes are infected by viruses, increasing the number of infected mosquito vectors during a transmission cycle [ 39 ]. Additionally, the shorter virus incubation period allows the virus to be transmitted by the mosquitoes quickly, leading to enhanced arboviral transmission [ 39 ].…”

“…In a recent study done by the same group, it was found that EEEV and VEEV binds to Brugia malayi microfilaria, allowing it to be transported across the midgut epithelial barrier and suggesting this as the main mechanism for microfilarial enhancement [ 39 ]. Comparing co-infection of virus and microfilaria with direct virus inoculation into the haemocoel, the authors found a delay of 24 h in the growth kinetics of co-infected virus in the haemocoel, and postulates that this delay is due to the time required for microfilaria penetration, as well as the amount of virus delivered [ 39 ]. Another set of experiments were performed using multiple vigorous washings of microfilaria after virus incubation and infectious virus was still detected, suggesting viral attachment to the microfilaria [ 39 ].…”