West Nile virus (WNV) is a member of the Flaviviridae family, transmitted through an enzootic bird–mosquito cycle (Figure 1). It can accidentally infect horses and humans, causing neurological disease (Long, 2018).
There are two strains of WNV:
- Lineage 1 has been predominantly detected in outbreaks in southwest Europe following its initial detection in France in the 1960s. It was introduced to North America in 1999, where it spread rapidly. Now it is considered endemic in the USA
- Lineage 2 is currently the most dominant strain in the old continent, causing outbreaks throughout east and southeast Europe (Bessell et al., 2016)
The primary reservoir hosts of WNV are birds, in particular passerines, raptors and corvids (Browne and Medlock, 2019). In some cases, the virus is transmitted to “dead-end” hosts, such as humans, dogs and horses. Additional modes of transmission include contaminated blood transfusion or organ transplant if the donor is viraemic (Long, 2018). In these “dead-end” hosts, most infections are asymptomatic, with 10 percent of infected horses showing neurological signs while only 1 percent of infected humans develop severe neurological symptoms (Browne and Medlock, 2019).
The WNV season in Europe is from June until November, with peak infection coinciding with the peak in mosquito population (Dauphin et al., 2004). In 2018, there was a significant increase in the number of WNV cases in Europe, associated with the higher-than-usual summer temperatures. Indeed, higher temperatures can result in increased viral replication within the insect vector and an increased rate of mosquito breeding (Paz, 2015).
Current risk of West Nile virus in the UK
The emergence of new diseases or outbreaks of previously described diseases is the product of globalisation and climate change (Long, 2018). The likelihood of WNV entering and establishing in the UK is deemed low but given recent cases and the geographical distribution in Europe, there are now cross-government recommendations to monitor its vector and host populations (Browne and Medlock, 2019).
The likelihood of West Nile virus entering and establishing in the UK is deemed low but given recent cases and the geographical distribution in Europe, there are now cross-government recommendations to monitor its vector and host populations
The migratory route known as the East Atlantic Flyway, which starts from northwest Africa and leads to the Iberian Peninsula, France and the UK, is the route for summer avian visitors, including passerines. To date, no WVN-positive birds have been detected in the UK because mosquito activity and climatic temperatures are too low for extrinsic incubation when birds arrive (Bessell et al., 2016). However, due to increased temperatures during summer months, environmental changes and strategies to expand wetlands to mitigate coastal flooding and enhance habitats for biodiversity, it is important to regularly consider the risk (Medlock et al., 2005).
The main putative vector for WNV in the UK is Culex modestus. This species was considered absent from the UK before 2010, but since then, it has been detected in various wetlands in southern England (Vaux et al., 2015).
Clinical signs in horses
When clinically apparent disease occurs, both systemic and neurological signs are observed in horses (Long, 2018). A mild increase in rectal temperature, anorexia and depression are the most common initial systemic signs. Abdominal pain and gait abnormalities are not uncommon before the development of an obvious neurological syndrome (Porter et al., 2003). However, the onset of neurological signs is frequently sudden and progressive.
The major hallmarks of equine WNV encephalomyelitis are muscle fasciculations and personality changes. Some horses appear to have abnormalities of sudden sleep-like activities resembling narcolepsy (Gonzalez-Medina et al., 2017; Long, 2018). The fasciculations are most notable around the muzzle and eyelids. Eyelid activity is enhanced with light, and affected horses appear quite photophobic. Spinal abnormalities are characterised by ataxia and paresis, which can be symmetrical and asymmetrical and involve both the forelimbs and the hindlimbs. Cranial nerves are also frequently affected, with weakness of the tongue, muzzle deviations, head tilts and an inability to swallow most commonly reported (Goehring, 2008; Browne and Medlock, 2019).
Overall, the clinical picture of WNV can be highly variable, but most affected horses will improve within three to seven days after showing clinical signs. A small percentage of affected horses progress to complete paralysis, recumbency and coma. These horses usually die spontaneously or are euthanised for humane reasons. If the horse demonstrates significant improvement, then full recovery can be expected in 90 percent of patients (Long, 2018).
What to do with suspected cases
WNV is listed as a “Category E” disease and is thus notifiable and subject to surveillance across Europe (Figure 2). If you suspect WNV, you must contact the Animal and Plant Health Agency (APHA) via the Defra Rural Services Helplines to discuss the case before submitting any sample. If the APHA suspects that the case is WNV then a serum sample should be submitted. However, you can still send a sample to exclude the disease if the APHA does not suspect WNV (Browne and Medlock, 2019).
Serological testing, which is based on the detection of the IgM antibody response, is the method of choice for WNV diagnosis (Long, 2018). Horses develop a very intense IgM response on exposure to WNV that lasts approximately six weeks (increases in IgM rarely occur after vaccination). The APHA offers two accredited diagnostic tests (cELISA and mELISA) that detect antibodies against WNV (Browne and Medlock, 2019).
Serological testing, which is based on the detection of the IgM antibody response, is the method of choice for West Nile virus diagnosis
Other ancillary diagnostic tests for horses with suspected WNV should include complete blood count (CBC), serum biochemistry and cerebrospinal fluid (CSF) analysis. However, CBC and serum biochemistry profiles are usually normal. Mild lymphopenia and elevated muscle enzymes secondary to trauma or prolonged recumbency may be present. CSF cell counts and protein concentration may be elevated (Long, 2018).
Brain material collected during a post-mortem examination can also be used for viral culture, PCR and immunohistochemistry (Goehring, 2008).
Treatment for West Nile virus
There is no effective antiviral medication currently available, so supportive care is the only treatment option for WNV. Therapy should aim at decreasing inflammation by administering non-steroidal anti-inflammatory drugs, such as flunixin meglumine (Goehring, 2008).
Therapy of recumbent horses tends to be more aggressive and may include corticosteroids such as dexamethasone and intravenous osmotic agents such as mannitol to decrease cerebral oedema and spinal cord swelling. Controversy remains as to whether corticosteroids enhance viral replication and propagation in the tissues (Goehring, 2008; Long, 2018). Low doses of acepromazine provide excellent anxiety relief in both recumbent and standing horses.
Other supportive measures may include oral and intravenous fluid therapy and antibiotics for infections that can occur in recumbent horses (Long, 2018).
Epidemiological and experimental evidence exists regarding the effectiveness of vaccinations against WNV (Seino et al., 2007). There is one licensed vaccine commercially available in the UK (Equip WNV; Zoetis), which provides 12 months’ duration of immunity. Vaccination before mosquito season is critical in endemic countries, and any owner intending to take their horse abroad from the UK should consult their vet about a vaccination schedule (Browne and Medlock, 2019). Horses that have recovered from clinical disease have long-term immunity and should not require annual boosters (Long, 2018).
Horses that have recovered from clinical disease have long-term immunity and should not require annual boosters
Current surveillance measures for WNV in the UK by animal health and public health agencies include:
- Reporting of human cases of autochthonous WNV infection to national surveillance centres
- Measures for the safety of blood products taken in line with the EU Directive for blood safety
- Passive surveillance for Equidae relying on testing of horses with neurological signs
- A passive surveillance system for wild birds in the UK operating between April and October
- Active surveillance for mosquitoes carried out by Public Health England in areas with suitable habitat
- Monthly meetings between human and animal health risk assessors to share information (Gossner et al., 2017)