Veterinary Services

West Nile virus (WNV), a flavivirus spread by mosquitoes, hit the news headlines when it unexpectedly appeared in the United States in 1999, causing the death of thousands

of wild birds and disease in humans and horses. Since then over 29,000 people have been affected in the US and over 1100 deaths have occurred, with human cases reported from almost every US state. Canada has seen over 4500 human cases since it appeared there in 2002. Most infected humans show no clinical signs, but a small proportion develops mild fever, headaches and sometimes a skin rash. Less than one percent of infected humans show neurological signs associated with meningitis or encephalitis. The virus is spread between wild birds by biting mosquitoes, which can also bite and infect other hosts such as geese, horses and humans. Some species of mosquito found in Great Britain are capable of transmitting WNV between birds and other hosts, raising concerns that disease caused by WNV could arise in this country.

West Nile virus in wild birds

When WNV reached New York State in 1999, thousands of wild birds died, including at least 3000 American crows. Some birds exhibited weakness and recumbency, and some had neurological signs such as ataxia. In the following years many other species of bird were affected, including raptors and owls. Mortality has also been described in wild birds in Italy and Austria. Disease in wild birds may occur before cases are seen in humans and horses, and so monitoring wild bird deaths may provide early warning of the appearance of this disease in a new area.

Monitoring wild birds in Great Britain

Between 2001 and 2006, as part of wild bird disease surveillance in Great Britain, tissue samples were collected from over 1800 wild birds by the laboratories of SAC in Scotland and the Veterinary Laboratories Agency (VLA) in England and Wales. The samples were collected between April and October, when mosquito activity would be greatest, and tested by VLA for the presence of WNV by virus isolation and PCR. No evidence of WNV infection was found. This surveillance continues and to date the results remain negative. In contrast, research workers at the Centre for Ecology and Hydrology in Oxfordshire and Cambridgeshire have reported the detection of seroconversion of sentinel chickens in Great Britain to three mosquito-borne flaviviruses – WNV, Usutu virus and Sindbis virus. The threat from WNV or related viruses cannot therefore be discounted.

What should you do if you find dead wild birds?

Members of the public in Scotland are asked to remain vigilant and report to the GB Helpline (08459 335577) if they find five or more dead birds in one location at the same time (a “mass mortality incident”). Depending on the location, species of bird involved and time of year, further tests may be carried out on the carcases including screening for avian influenza and WNV. Even smaller numbers of carcases can be reported to the local Disease Surveillance Centre of SAC Consulting Veterinary Services if the mortality appears to be unusual, for example if the birds show nervous signs before they die.

WNV in horses

Horses are considered to be “dead-end hosts” if bitten by infected mosquitoes, because any viraemia that develops is not high enough to infect other mosquitoes that feed on the horse. Clinical disease may however be seen, sometimes resulting in death. Most equine cases occur in late summer or autumn. For example, in southern France in 2000, 76 clinical cases were seen in horses from early September to early November. Similarly, neurological disease caused by WNV occurred in 76 horses in Israel between August and October 2000. Italy reported 266 outbreaks in horses between September 2008 and January 2009. An epidemic of 1698 cases was reported in Texas between June and December 2002, peaking in October. Horses may act as sentinels, giving early warning of the arrival of the virus in the area.

Clinical signs in horses are unfortunately not pathognomonic for WNV but can be seen in a number of neurological conditions. Signs may include:

• Ataxia or abnormal gait
• Muscle twitching
• Depression
• Hind limb paresis or recumbency
• Death or euthanasia
• Fever may be seen in less than 25% of cases

What should you do if you suspect the presence of WNV in horses?

If WNV infection is suspected in horses, the local Animal Health Office must be immediately informed because this is a notifiable disease in horses. In such circumstances a full investigation will be carried out. What if the condition is not suspected but the attending veterinary surgeon wishes to rule out WNV infection because of the nature of the clinical signs exhibited? In those circumstances it has been agreed that, subject to a discussion with the local Animal Health Office and their agreement, a blood sample may be submitted to VLA Weybridge for WNV serology at the owner’s expense (see Drummond 2008. Surveillance for West Nile virus in horses. Veterinary Record 162, 763).

Funded by the Scottish Government as part of its Public Good Veterinary and Advisory Services Programme.