Emerging Exotic Viral Diseases of Global Importance

Emerging Exotic Viral Diseases of Global Importance present a growing threat to human and animal health and jeopardise food security.

Increases in human and animal populations, with accompanying environmental degradation and globalized trade and travel, enhance opportunities for transfer of pathogens within and between species. The resulting diseases pose enormous challenges now and in the future.

Emerging and re-emerging animal diseases have in recent years been associated with outbreaks that have serious consequences for animal and human health. The World Organisation for Animal Health (OIE) defines an emerging disease as “a new infection or infestation resulting from the evolution or change of an existing pathogenic agent, a known infection or infestation spreading to a new geographic area or population, or a previously unrecognised pathogenic agent or disease diagnosed for the first time and which has a significant impact on animal or public health.” A known or endemic disease is considered to be re-emerging if it shifts its geographical setting, expands its host range, or significantly increases its prevalence.

Factors responsible for emerging and re-emerging diseases includes:

• Increasing human population
• Increasing numbers of food producing animals
• Human and domestic animal encroachment into wildlife habitat and resulting exposure to wild animals
• Environmental degradation
• Climate change
• Interspecies transfer of pathogens and
• Globalization of travel and trade

Emerging disease of global importance:

1. Rinderpest
2. Foot and Mouth Disease
3. Blue Tongue
4. West Nile Virus
5. Rift Valley Fever
6. Influenza (H5 & H1)
7. Nipah Virus in Malaysia
8. Ebola Virus in Africa

(1) Rinderpest

Rinderpest is an acute, highly contagious, viral disease of cattle, domesticated buffalo, and some species of wildlife. It is not a zoonotic disease, yet its introduction into Africa in the 19^th century caused starvation and resulted in massive human fatalities. In 1889, cattle shipped from India carried the rinderpest virus to Africa, causing an epidemic that established the virus on the continent. Initially, approximately 90 percent of the cattle in sub-Saharan Africa died as well as many sheep and goats. Wild buffalo, giraffe, and wildebeest populations were decimated. This rinderpest epidemic is considered by some to have been the most catastrophic natural disaster ever to affect Africa. The Global Rinderpest Eradication Program is a large-scale international collaboration involving vaccination, local and international trade restrictions, and surveillance. This effort may be one of veterinary medicine’s greatest achievements and rinderpest, the second disease (after smallpox) have been eradicated globally.

(2) Foot and Mouth Disease

Foot and mouth disease (FMD) is perhaps the most important disease of livestock worldwide. It is a highly contagious viral disease that primarily affects cloven-hooved livestock and wildlife. FMD was once found worldwide, but has been eradicated from 70 countries. However, more than 100 countries are still considered either endemically or sporadically infected with the disease. Where the disease is endemic, it affects animal production and performance, including the capacity of draft animals to help farmers produce their crops. Countries which are infected are unable to access international markets. Outbreaks of FMD in countries that were previously free of the disease can be devastating to the economy due to loss of trade and the cost of eradication. An outbreak of FMD in the United Kingdom in 2001 is estimated to have cost $10 billion. More than four million cattle and sheep were destroyed in order to eliminate the virus and re-establish trade. The World Organization for Animal Health (OIE) and the Food and Agriculture Organization of the United Nations (FAO) have recently launched a global campaign to eradicate FMD.

(3) Blue tongue

Bluetongue is a non-zoonotic, non-contagious vector-borne disease, caused by a virus of the Reoviridae family with 25 known serotypes worldwide. It affects all domestic and wild ruminants, with sheep and cattle experiencing the highest rates of infection. The severity of the disease depends on the strain and morbidity can be very high in susceptible animals. Bluetongue outbreaks also cause direct economic losses through disease and mortality, loss of production, loss of milk yield, and declines in fertility. As bluetongue is not zoonotic, it poses no risks to human health and cannot be contracted or spread through food. Bluetongue is transmitted by biting midges that are infected with the virus after ingesting blood from infected animals. The spread of the disease depends mainly on those ecological and climatic factors that favour biting midge populations; outbreaks are often seasonal, occurring at or shortly after the season of peak midge activity. Although bluetongue emerged in Africa and is endemic in many tropical areas, it has experienced dramatic geographical expansion in recent years, due to environmental factors and the growth of international trade links. A series of bluetongue outbreaks in Europe starting in 1998 illustrate the role played by a warming climate and the increased risks of spreading disease vectors (insects or other living carriers that transmit an infectious agent) through trading routes. As climate change becomes more of a concern in the future, the risks of bluetongue causing an epidemic will continue to increase. According to a leading expert on vector-borne diseases, “through changes to climate, one of the most competent vector species of midge has spread around southern Europe and further north than it had before” while at the same time midge vectors have become “better at being able to transmit the virus.”  The bluetongue control measures include the surveillance of susceptible animals, quarantine, zoning, insect control and vaccination. Since controlling midge populations is not possible — they are too numerous — vaccination is the most effective practical measure to minimise losses related to the disease in endemic regions. A voluntary vaccination programme, combined with movement restrictions, successfully led to the control of the bluetongue virus-8 outbreak in the United Kingdom in 2008.

(4) West Nile Virus

In 1999, the New York City area of the United States experienced an outbreak of encephalitis in humans of unknown etiology. At the same time, increased deaths were observed in a variety of bird species. Dr. Tracy McNamara, a veterinarian at the Bronx Zoo, hypothesized a link between the disease in birds and humans, and submitted samples from dead zoo birds to the USDA National Veterinary Services Laboratories (NVSL). NVSL, the U.S. Centers for Disease Control and Prevention (CDC), and the University of California, Irvine identified the virus as West Nile, which had never previously occurred in the Western Hemisphere. This virus progressed across North America in the next few years. It is now endemic in North, Central and South America and there is no feasible way to eradicate it. West Nile (WNV) virus infects wild birds, which are the primary reservoir hosts. It is mainly transmitted by mosquitoes, although it can spread between some birds and reptiles by direct contact. Although most humans are infected asymptomatically or develop flu-like symptoms, WNV can cause encephalitis, meningitis, or a paralytic syndrome in less than one percent of infected people. This virus can cause fatal encephalitis in horses and occasionally in other mammals. It also affects alligators, causing severe outbreaks among farmed animals, and it can kill some species of birds. The veterinary vaccine industry, working in cooperation with the USDA Center for Veterinary Biologics, quickly developed an effective vaccine to prevent the disease in horses. WNV vaccine is now considered one of the core equine vaccines in the U.S. and in WNV endemic areas of Canada. The vaccine has also been used off label to protect some endangered birds, such as California condors.

(5) Rift valley fever

Rift Valley fever (RVF) is a zoonotic, mosquito–borne viral disease which affects ruminants. Sheep, cattle, and goats may be severely affected and serve as the primary host and amplifiers of the virus. Dogs are highly susceptible to infection and cats can be infected as well. In all affected species, a very high rate of abortion and death in neonates is observed. RVF is endemic in sub-Saharan Africa. Epidemics occur after heavy rainfalls when infected mosquito eggs hatch near large numbers of susceptible animals. RVF first appeared outside Africa in 2000, when outbreaks were reported in Saudi Arabia and Yemen. The virus can also produce mild to severe disease in humans. Between November 2006 and February 2007, RVF caused more than 200 human deaths in Kenya. Control efforts included restricting the movement or slaughter of animals, vaccination of livestock, insect vector control, and public education. RVF could spread to countries outside of Africa via an infected mosquito, animal, or human, and could become established in a new region following a pattern similar to that of the spread of WNV throughout the Americas.

(6) Influenza (H5 and H1)

Influenza viruses are highly prone to mutation and transfer between species. Expanding human and animal populations provide greater opportunities for influenza viruses to mutate and emerge in new species. In 1997, a highly virulent H5N1 influenza virus emerged in domestic waterfowl and poultry in Asia and has been sporadically transmitted to people. Novel influenza viruses also have recently emerged to cause severe disease in dogs (H3N8) and cats (H5N1/H1N1) for apparently the first time. In 2009, a novel H1N1 virus with genetic elements from swine, human, and avian influenza viruses emerged and caused a worldwide pandemic in humans.

(7) Nipah Virus in Malaysia

In 1998, a never-before-observed virus emerged in the Malaysian pig population, causing severe respiratory and neurologic signs. The virus spread to swine caretakers resulting in more than 265 cases and over 105 deaths in Malaysia and Singapore. A novel paramyxovirus (Nipah virus) was isolated from a human patient. Both the U.S. CDC and the Australian Animal Health Laboratory sent teams of veterinarians and other specialists to Malaysia within days of isolation of the virus. Working closely with the Malaysian government and scientists, these teams developed diagnostic tests and control strategies (including the culling of more than one million pigs), that resulted in eradication of the virus from the swine population. Between 2001 and 2009, there were no new cases of Nipah virus infection in Malaysia. Scientists have discovered that the virus is carried by healthy fruit bats. The virus is still present in fruit bats in Southeast Asia. Human cases of Nipah virus encephalitis have been reported repeatedly since 2000 in Bangladesh, in some cases due to infection from fruit or fruit juices contaminated by fruit bat saliva or urine. Subsequent human-to-human transmission has also occurred.

(8) Ebola Virus in Africa

Ebola virus is a non-segmented RNA virus, which, together with Marburg virus, makes up the filovirus family. This now notorious group of viruses was discovered in 1967 when Marburg virus was identified as the etiologic agent of a hemorrhagic fever outbreak in research facilities in Europe, which handled tissues from African green monkeys imported from Uganda. Subsequently, Ebola viruses were shown to be the cause of simultaneously occurring hemorrhagic fever outbreaks in 1976 in the Democratic Republic of Congo (DRC, formerly Zaire) and Sudan. These outbreaks were shown to be caused by two different subtypes of Ebola virus, which became known as the Zaire and Sudan subtypes. Mortality rates of up to 80% were recorded in these and more recent outbreaks in DRC and Gabon in 1995–1996. Epidemiologic data from recent outbreaks indicate that close contact is necessary for efficient transmission of Ebola virus from one individual to another, and little evidence can be found for aerosol transmission of the virus (5). Despite considerable efforts to identify the natural reservoir for Ebola and Marburg viruses, the host species remains an enigma. Although non human primates have been implicated as the source of introduction of the virus into humans during several of the identified outbreaks, they are not considered likely to represent reservoir species because of their susceptibility to high mortality hemorrhagic disease similar to that seen in humans. Little genetic difference has been detected between Ebola-Zaire viruses isolated 20 years apart and from locations over 1,000 km from one another, suggesting that ecological rather than genetic factors may play the dominant role in initiation of Ebola hemorrhagic fever outbreaks.

Control

Development of international cooperation; effective local, regional, and global networks for strong infectious disease surveillance and research collaboration to enable sharing of biological and study materials to enhance antimicrobial product development and vaccine trials; collaboration between animal and human health sciences to strengthen capacity for identification of microbial agents with epidemic potential so as to prevent their emergence.