A high seroprevalence of West Nile Virus among equids in some regions of Africa shows that a workable regional or sub-regional surveillance protocol is required, according to researchers.
Olaolu Olufemi, Marta Barba and Janet Daly, in a review published in the journal Pathogens, said their work highlighted the scarcity of seroprevalence studies among African equids for the virus.
West Nile Virus was first identified in Africa and is endemic. It is transmitted between birds by biting mosquitoes, with equids and humans being incidental hosts.
Most infected incidental hosts show no or only mild clinical signs, but a fraction develop life-threatening brain inflammation.
The review team set out to identify and evaluate primary research on the presence of antibodies to the virus among African equids.
Of 283 articles identified in three databases, only 16 satisfied all the inclusion criteria.
The researchers found that the overall seroprevalence reported ranged from 17.4% to 90.3%, with 1998 of the 5746 horses, donkeys and mules across the 16 studies having screened positive for antibodies to the virus, representing 35% of the animals.
Several articles determined that seroprevalence increased significantly with age.
The authors found that variations in study design posed challenges when trying to determine risk factors for, and trends in, West Nile seroprevalence.
Overall, they found an uneven spread of research on West Nile seroprevalence across the five regions of Africa. More studies were reported in North and West Africa, with only one in Southern and Eastern Africa, while another study had equid representation from central and eastern Africa.
East Africa, where the virus originated, was under-represented, despite the burden of the disease in the region.
Despite speculation that climate change is leading to increased circulation of vector-borne viruses such as West Nile, there was no discernible trend in seroprevalence across the years covered by the studies reviewed. “Levels of seroprevalence were likely to be strongly influenced by location, timing of sampling, and other factors,” they said.
Some studies, they noted, were evidently performed in the rainy (wet) season when mosquito vectors are more abundant, while some were conducted over a longer period. The abundance of mosquito vectors suggests increased transmission and more infection.
“This,” they said, “might be the reason for the very high seroprevalence observed in equids sampled in the wet season in Senegal (78.3% and 85%) and Nigeria (90.3%). A high seroprevalence, 74.2%, was also observed among equids sampled at the end of the dry season.
“It is challenging to describe the climate of the sampling period for studies that reported a sampling period based on a calendar month. With climate change, it is often difficult to describe the climate based on the calendar month, especially for Africa.”
One reason for the few studies across some regions might be the cost of the commercially available ELISA test kits, coupled with the difficulty of confirmatory laboratory tests. Development of cheaper but reliable and specific antibody tests are required, they said.
The authors said that, as equids are often used as draft animals and also in sport or leisure, the seroprevalence rate may indicate human exposure to West Nile.
“The high seroprevalence in some regions is a call to institute a workable regional or sub-regional surveillance protocol for Africa.”
Governmental and non-governmental agencies need to be involved in developing strategic plans for routine surveillance and reporting of West Nile among equids and other animals as a public health strategy to curb this zoonotic disease.
“These are very pressing issues that require a collaborative regional One Health approach,” they said.
Olufemi, O.T.; Barba, M.; Daly, J.M. A Scoping Review of West Nile Virus Seroprevalence Studies among African Equids. Pathogens 2021, 10, 899. https://doi.org/10.3390/pathogens10070899