West Nile fever: health professionals are not prepared to identify the disease

There is difficulty in diagnosing West Nile virus due to the cross reactivity between it and other flaviviruses such as dengue, yellow fever, Zika, and others

Brazil confirmed this year a second case of West Nile fever. The first case was registered in 2014, in Aroeiras do Itaim, and another in Picos, both in the state of Piauí (PI). Neurologist at the Natan Portella Institute of Tropical Diseases and member of the Epidemiology Management of the Teresina Municipal Health Foundation (FMS / PI), Dr. Marcelo Adriano Vieira, who diagnosed the first Brazilian case together with Dr. Pedro Vasconcelos, a virologist and director of the Evandro Chagas Institute (IEC) in Pará (PA), admits that in recent years there has been a relatively stable number of reports of encephalitis in Piauí, with some seasonality. The fact that the state is the only one to register the disease is explained by the fact that it pioneered the implementation of a viral encephalitis surveillance system, initiated in 2012, under the auspices of the IEC, involving research for herpesviruses, enteroviruses and arboviruses, including West Nile Virus (WNV). In addition, there are migratory migratory bird routes that pass through the state. However, we need more studies to verify what characteristics of the local biome have favored the emergence of the cases, he adds.

But to Dr. Pedro Vasconcelos, more cases should be diagnosed in the coming years. And to make matters worse, he said, health professionals are not prepared to identify the disease and the main reason is the absence of suspicion and lack of information. The virologist argues that in addition to engaging in surveillance and research to better understand the epidemiology of the virus in the country, it is still necessary to disseminate information on arboviruses with details about the clinical conditions that WNV can cause by focusing on Nile fever and encephalitis it causes. In addition, he points out that as the Nile virus is still little known, the suspicion of clinical cases caused by it may not be performed, perhaps by the absence in thinking of it as a disease agent in Brazil. Another point also considered crucial by the specialist is the limitation of laboratories capable of diagnosing WNV in the country. He emphasizes the need to increase the network of laboratories capable of making the specific diagnosis especially of the neurological cases.

The difficulty in diagnosing Nile fever due to the possibility of cross-reactions to similar viruses is another problem. The clinical presentation of encephalitis by WNV may closely resemble those caused by other viruses, and thus may go unnoticed if systematic investigation is not performed. This reactivity occurs because flaviviruses share a very close genome, similar even in many regions; the viral RNA encodes the production of certain proteins that will induce the immune response of the host (for the production of antibodies), which causes the person to respond serologically, in a way that serological crossing is observed. For example, if one is vaccinated against yellow fever and has already been infected by dengue, when infected by another serotype of dengue or other flavivirus circulating in Brazil (13 different flaviviruses are known to circulate here), the serological reactivity will be of the secondary type, which means that by serology, the reaction in laboratory tests will be for multiple flaviviruses and not only for the that responsible for the disease.

This becomes a serious problem in defining the cause of the disease, and is a problem observed in all serological methods including the enzyme-linked immunosorbent assay (ELISA), the main serological test used to diagnose dengue, yellow fever, Zika and also WNV. This cross-reactivity greatly limits the use of serological methods to make the specific diagnosis of flavivirus in countries endemic to the circulation of several flaviviruses, as in Brazil. From 2014 and 2018, 31 suspicious cases of the disease had an indeterminate result. Neutralization tests are recommended, but the serological cross-check in the ELISA-IgM and HI tests has persisted in the neutralization tests because of the antigenic similarity between the flaviviruses and because the majority of the cases correspond to patients with secondary flavivirus infections – which greatly limits the discriminative capacity of the neutralization tests.

There is no difficulty in virological diagnosis, for example, with molecular methods – especially reverse tran ion followed by real-time polymerase chain reaction (RT-qPCR) or cell culture for viral isolation. But these latter laboratory procedures should be tested on blood samples collected up to five days after the onset of symptoms. For serology, the ideal is to obtain blood samples from the sixth day of illness.

But is Brazil at risk of an epidemic of the disease? Following the line of reasoning that previous infections by other flaviviruses (dengue, for example) may exert a relative protective effect on the Brazilian population against WNV infection, the risk would be low, unless extremely favorable environmental conditions in certain ecological niches, at certain moments, surpass this relative protection.

In last June, a horse death caused by West Nile Fever put the state of Espírito Santo on alert. At the time, the Ministry of Agriculture, Livestock and Supply (MAPA) issued a technical note confirming the presence of the disease in Brazil and recommended the intensification of surveillance for the detection of animals with symptoms. Serological investigations carried out in animals in Brazil point to a broad distribution of the virus in Brazil, keeping the limitations inherent to the method (serological), for more concrete inferences. More information can be found in the article First isolation of West Nile virus in Brazil, which presents the application of the molecular clock technique on the virus isolated from the equine brains in the state of Espirito Santo.

Serological evidence of WNV circulation has also been demonstrated in certain areas of the Pantanal and the Amazon. Whether the WNV will become enzootic in these areas is difficult to know. All conditions for it to establish are present. The basic elements of arbovirus cycles, such as WNV exist, are hematophagous arthropods and wild vertebrates that act as vectors and amplified hosts, respectively. Whether it will become endemic or not only time will tell. However, no WNV epidemic will have the same impact of Aedes aegypti-transmitted diseases, even in countries with little or no previous large-scale circulation of other flaviviruses. The best way to raise WNV awareness is to inform the public about the disease it can cause in humans, but also and especially in animals. To Dr. Pedro Vasconcelos, bird mortality is an alert and sentinel of the most important for WNV circulation in the US and Europe and should also be considered in Brazil. Bird death is a powerful sign of the circulation of an infectious agent, which may be the WNV or another arbovirus. This is because birds here in the Americas do not have immunity to WNV and this virus basic maintenance cycle has wild birds as its main vertebrate host.

The neuroinvasive disease by WNV is a diagnostic challenge in Brazil, emphasizes Dr. Marcelo Adriano when detailing that usually, when the patient presents neurological symptoms that suggest the disease, he is already outside the period of viremia. Therefore, viral isolation and genomic detection techniques applied to blood or cerebrospinal fluid at the time suspected encephalitis are poorly sensitized. At this stage, in the thesis, the serological tests would be the solution for the diagnosis. However, a significant proportion of the population has already been exposed to some other flavivirus (wild or vaccinated). This causes the serological tests performed to be influenced by the phenomenon of original antigenic sin, which combines with the antigenic similarity between flaviviruses to result in serological crosses, even in the neutralization tests. Finally, the expert recalls that some studies indicate that genomic detection in urine samples may be a solution since viruria seems to be present for longer and with higher levels in the symptomatic (neurological) phase of the disease.…