WEST NILE VIRUS AND OTHER MOSQUITO BORNE VIRUSES ASSOCIATED WITH NEUROLOGICAL DISEASE IN HUMANS AND HORSES IN SOUTH AFRICA.

Prof Marietjie Venter, Head of the Respiratory and Zoonotic disease programme, Department of Medical Virology, University of Pretoria and Respiratory virus unit, National Institute of Communicable Diseases, Sandringham, South Africa.

Prof Marietjie Venter (PhD, Medical Virology) Cell: 0829020412, E-mail: marietjiev@nicd.ac.za; www.up.ac.za/medicalvirology

Summary: West Nile virus (WNV) may cause severe neurological disease in horses with a high death rate. Cases are frequently reported in the USA where horses are now vaccinated annually. We investigated the situation in South Africa to determine if cases are being missed in horses. We found that WNV may be an important under detected cause of neurological disease in South Africa and local strains can cause fatal disease in horses. We also tested horses for other viruses that are carried by mosquitoes that could co-circulate with WNV and identified Wesselsbron virus, Middelburg virus and Shuni virus as causes of neurological disease in South Africa and Sindbis and Equine encephalosis virus as a co-circulating causes of fevers.

West Nile virus (WNV) is a mosquito-transmitted virus in the flavivirus genus that is widely distributed throughout Africa, the Middle East, Asia, parts of Europe, Australia, North and South America and the Caribbean. It was first isolated in the West Nile province of Uganda from a woman with fever and was named accordingly. WNV circulates between birds as replication hosts and mosquitoes usually without causing disease. Humans and horses are incidental hosts and do not have a high enough level of virus in the blood to infect mosquitoes. The mosquito that transmits WNV in Africa belongs to the Culex genus (Figure 1) and is found in the vicinity of dams or standing water across the continent. Outbreaks of WNV occur in late summer and autumn during the rainy season. Certain other viruses that are also carried by mosquitoes may co-circulate with WNV and also cause neurological disease in horses and humans and will also be discussed here.

Humans and horses may experience severed disease following WNV infection. In humans 20% of infections develop West Nile fever with symptoms including a rash, joint and muscle pain, fever, nausea and headache while less than 1% of cases will develop WNV neurological disease which includes meningitis, encephalitis, polio-like paralysis in rare cases and death (Figure 2). Symptoms include a stiff neck, sore back, severe headache, eye sensitivity to light, weakness and changes in metal status in certain cases. In horses 20-30% of infections develop symptoms with as many as 90% of these developing severe neurological diseases as described below and 30-40% being fatal or euthanized for humane reasons.

WNV emerged as a new pathogen in the USA in 1999 and may have been imported from birds or mosquitoes from the Middle East where identical virus strains was identified the previous year. Since then it spread across the continent causing more than 30 000 human cases of which 40% were neurological and more than 1000 fatal. In horses, the largest outbreak occurred in 2002 when 15 000 cases were reported in horses over 41 states. Cases decreased in horses since the virus became endemic and a vaccine was introduced but severe disease is still reported with ~200 cases reported every year in horses in Texas alone.

WNV is endemic in Southern Africa. The largest outbreak occurred in humans in the Karoo in 1974 affecting ~10 000 people with thousands visiting their local doctor with mainly WNV fever. An outbreak in humans and animals also occurred in the 1980’s in the Witwatersrand affecting hundreds of people. Over the past decade 5-15 cases are reported every year although only a proportion of cases are thought to be submitted for laboratory investigation and may be largely missed as a cause of neurological disease. In the past year, following the Rift Valley fever (RVF) outbreak, more than 100 cases of fever and a number of neurological cases were identified in humans in South Africa, only because cases were submitted for RVF investigations. Cases of severe WNV disease have been identified including fatal hepatitis and several non-fatal encephalitis cases in humans as well as deaths in ostrich chicks, horses and a dog. Screening of blood from horses at the annual yearling show in 2001 indicated that 11 % had been exposed to WNV over a period of a year and up to 75% of their mothers had antibodies suggesting that WNV is still widely distributed across South Africa. Experimental infection of 2 horses with a local WNV strain did not cause disease leading to the misperception that South African WNV strains are not pathogenic in horses. Today we know that in only 20% of WNV infections develop symptoms and many horses in the USA get asymptomatic infections.

Research on WNV in South Africa: Genetic sequencing has shown that local strains all belong to genetic lineage 2 rather than lineage 1 found in the USA. However all cases of severe disease were caused by lineage 2 strains and certain lineage 2 strains were just as capable of causing severe disease in laboratory animals as the lineage 1 strain from the USA.

To determine if cases of severe disease are being missed in horses in South Africa the Zoonosis research group, department of Medical Virology, University of Pretoria have been working with the veterinary community to test horses with unexplained neurological disease for WNV over the past 3 years. Veterinarians were invited to submit blood or post mortem specimens from horses that suffered of acute neurological signs and /or fevers from 2008-2010. We received a total of 194 cases from horses over the 3 years. We tested these specimens for WNV, as well as for mosquito-borne viruses that may co-circulate with WNV including Wesselsbronvirus; Middelburg virus, Sindbis virus and Shunivirus. To rule out common horse pathogens specimens were also tested for equine encephalosis virus (EEV); African horse sickness virus and equine herpes virus and rabies.

WNV was detected in 47 cases of which 42 had neurological symptoms. Of these 23 were fatal or euthanized (48%). Wesselsbron virus was detected in 2 horses in 2008 with neurological disease, one that was fatal and both with neurological symptoms similar to WNV. We also detected 8 Sindbis cases and 12 Middelburg virus infections over the 3 years. Sindbis cases were only associated with fevers while many of the Middelburg case had neurological symptoms of which 3 were fatal. Finally, Shunivirus was identified for the first time in 2009 and 2010 in 6 horses with neurological signs 4 of which were fatal. Coinfection with African horsesickness virus (AHSV) was detected in 2 WNV cases both with neurological disease, which is atypical for AHSV and that may enhance disease while WNV and Sindbis co-infections were also detected in 2 fatal cases. A number of EEV cases were detected, but all of these only had fevers. So far in 2011, most of the neurological cases identified in horses were Middelburgvirus many that were fatal. We did confirm one fatal case of WNV in the Karoo in March.

Affected horses were 0 to 19 years of age and included thoroughbreds, Arabians, Lipizzaner, Welsh ponies, warmbloods, and mixed breeds. Cases were identified in Gauteng, Northern Cape, North-West, Natal and the Western Cape and occurred from March to early July.

Typical symptoms in horses included stumbling in all cases, weak hind and/or forelimbs; partial loss or impaired movement, complete paralysis, partial blindness and jaundice in certain cases. Severe cases were unable to get up, had quadriplegia, limb paddling, teeth grinding, muscle twitching; chewing fits, seizures and coma before death. Fever was not always noted. Two cases that survived were sick for 21 days and had to be rested for several months, but recovered fully. The WNV season co-insides with the AHSV season which may be the reason that cases were missed in the past.

Gene sequencing identified lineage 2 as the major strain associated with neurological and fatal cases in the country.

Conclusion: This study indicated that WNV, Wesselsbron, Middelburg and Shunivirus may be important under reported cause of neurological disease in horses in South Africa and should be considered in animals with any of the described symptoms in late summer and autumn. Sindbisvirus and EEV may be responsible for fever cases seen during the rainy season. We are in the process of conducting a vaccine trail to determine if the WNV vaccine used in the USA will protect against South African strains and hope to have the Pfizer vaccine licensed for use in South Africa soon. We are also developing a local WNV vaccine that may be used in the future and form the basis for vaccines against the other viruses.

We thank horse owners and veterinarians who let us know about cases and sent us specimens from horses with neurological disease to test. This helped us determine the true disease burden of WNV in horses in South Africa and help us make recommendations for vaccination in the future. We next plan to investigate the mosquito vectors for the different viruses and to look at antibody levels to WNV in horse owners in the country since we may be exposed to more mosquitoes due to our outdoor life style. We hope you will participate in this study and donate a small amount of blood for us when we start to test for antibodies against the virus. This will help to determine the importance of WNV in humans in South Africa.

Points to note:

• You can protect your animals with insect repellents containing pyrethroids or DEET.
• Although the virus will not be transmitted to humans from infected horses, care should be taken when handling brain tissue or blood of infected animals.
• No specific treatment exists and symptoms are mainly treated with anti-inflammatory drugs and preventing self injury.
• A horse vaccine is not currently available in South Africa and a human vaccine does not yet exist. The Pfizer vaccine maybe licensed soon at which time it may be imported for use in horses.
• In the first few days of symptoms we can test for the virus by genetic tests in the blood or in fatal cases in the brain and spinal cord
• Later in disease antibodies can be detected in the blood, but since the virus is endemic is have to be confirmed with follow up blood.
• Cases occur in late summer and autumn
• Neurological signs include stumbling; weak hind and/or forelimbs; partial loss or impaired movement, complete paralysis, partial blindness and jaundice in certain cases, severe cases were unable to get up, had quadriplegia, limb paddling, teeth grinding, muscle twitching; chewing fits, seizures and coma before death

Further reading:

• Venter, M., S. Human, D. Zaayman, G. H. Gerdes, J. Williams, J. Steyl, P. A. Leman, J. T. Paweska, H. Setzkorn, G. Rous, S. Murray, R. Parker, C. Donnellan, and R. Swanepoel. 2009. Lineage 2 west nile virus as cause of fatal neurologic disease in horses, South Africa. Emerging Infectious Diseases 15:877-84.
• Venter, M., and R. Swanepoel. 2010. West Nile virus lineage 2 as a cause of zoonotic neurological disease in humans and horses in southern Africa. Vector Borne Zoonotic Dis 10:659-64.

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