West Nile Virus Infection: Cohort Study in Nursing Home

This report presents a comprehensive analysis of a West Nile Virus (WNV) outbreak in a Scottish nursing home, focusing on a cohort study to investigate the association between mosquito bites and the resultant development of the infection. The study included elderly patients with neurological illnesses and recent exposure to mosquitoes. Methodology involved a cohort study design, PCR analysis of patient samples, and environmental surveillance. The results revealed 17 WNV cases, with positive RT-PCR results in the initial patients. Histological analysis showed characteristic features of WNV meningoencephalitis, and environmental surveillance identified infected mosquitoes near the nursing home. The discussion establishes a positive association between WNV infection and mosquito bites. The conclusion suggests a WNV outbreak in Lochwinnoch County, Scotland, considering the virus's transmission features and environmental data.

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Background: West Nile virus (WNS) is an enveloped RNA virus and a neurotrophic pathogen
belonging to the Flaviviridae family. Despite majorly maintaining a enzootic cycle between
mosquitoes and birds, WNV is often also responsible for causing diseases among horses and
humans. It has been estimated that approximately 80% of individuals inflicted with this virus
will be asymptomatic whereas, an estimate 1% of the victims acquiring pathogenic infection are
likely to suffer from a disease which is potentially fatal or of the non-invasive, severe type. It is
often difficult to obtain a specific diagnosis in case of an infection caused due to WNV, since
this pathogenic viral strain is often vector-borne (mosquito), avian, equine and human borne.
Additional reasons underlying the complications associated with WNV fever, is due to the
possibility of this viral strain to cross react with other arthropod borne flavivirus. Taking insights
from existing clinical trials, transmission of WNV infection can be prevented via a sustainable
and organized control of the vector agents such as the mosquito.
Aims and Objectives: To explore, identify and evaluate an association between exposure to
infected mosquito bites in a nursing home and the resultant development of the infection caused
due to the West Nile virus, using a cohort study and the descriptive findings generated.
Methodology: Elderly patients residing in a Scottish care home, who had presented with an acute
or febrile neurological illness, two weeks prior to the manifestation of the index case and had
encountered a recent exposure to blood transfusion, mosquitoes or organ transplantation
especially during the months of summer were included. Cases of the primary type were reported
via notifications from Porto Down, whereas secondary cases were reported with the aid of
telephonic notifications from acute emergency departments in hospitals and the staff residing in
the nursing home. A cohort study design was utilized for the purpose of establishing exposure

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groups which were responsible for the outbreak. Patient groups with illness suspected to be due
to WNV infection, that is, fatigue, fever, headache, night chills, nausea, and additional specific
symptoms such as disorientation, skin rash, muscle weakness, confusion and seizures were
compared to other nursing home residents. Samples of cerebral spinal fluid and blood obtained
from the patients were analyzed for the presence of pathogens coupled with conductance of PCR
due to increased sensitivity compared to serological analysis alone. Geographical information
mapping and field based research were performed for the purpose of quantitatively assessing the
risk of primary vector borne illnesses – which involved assessment of the number of occupying
mosquitoes in surrounding areas of the care home.
Results: A total of 17 cases of WNV infected were identified among the residents of the
Lochwinnoch nursing home. However, total number of WNV infection cases across the county
of Lochwinnoch were unidentified. The samples of the first eight patients which were tested for
West Nile virus using RT-PCR were found to be positive. There was however, no data
concerning the results of the samples acquired from rest of the 9 patients of the nursing home.
However, serological as well as RT-PCR analysis proved to be efficient for the purpose of
establishing a diagnosis. Indeed, the efficiency of PCR has been attributed to be more than that
of ELISA in terms of specificity and sensitivity due to the latter being associated with increased
incidence of cross reactivity with other flaviviruses. Histological analysis of deceased patient
samples reported presence of lymphocytic cuffing microglial nodules and neural cell necrosis –
which are underlying characteristic features of WNV meningioencephalitis. Results from the
environmental surveillance reported the presence of ponds infected with aedes punctor
mosquitoes near a bird sanctuary and the Lochwinnoch nursing home. WNC RT PCR testing

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from samples obtained from deceased Sedge Warblers and migratory grasshoppers were revealed
to be WNV positive.
Discussion: Taking insights from the above data, a positive association between development of
WNV infection and exposure to bites from mosquitoes infected with WNV virus can be
established, for the disease cases occurring at Lochwinnoch nursing home during the period from
21st August, 2018 till present date. However, the strength of this study is difficult to validate due
to the lack of sufficient data obtained from the analytical studies.
Conclusion: Considering the characteristic transmission features of West Nile virus coupled
with the acquired environmental data and the recent negative travel history of the index case, the
findings from this study are strongly suggestive of an outbreak of West Nile virus infection in
Lochwinnoch County, Scotland.