10.4 Infectious Agents That Have Moved Out of Africa
Although this chapter discusses species introduced into southern Africa, pathogens
are also introduced from southern Africa to other regions (see also Pyšek et al. 2020,
Chap. 26; Measey et al. 2020, Chap. 27, for a discussion of South African species
that have become invasive elsewhere). Two examples are discussed below.
10.4.1 West Nile Virus
West Nile Virus infection is caused by a mosquito-borne Flavivirus, which originated in Africa. A mosquito-bird cycle is the maintenance mechanism, and birds are
considered to be amplifying hosts for the virus. This disease subsequently spread to
the Middle East and then into Europe where it continues to cause sporadic outbreaks.
However, the most dramatic course of events occurred when this virus was introduced into the United States of America in 1999. It is thought to have arrived with an
infected mosquito by aircraft or ship, and was first seen in New York, when many
birds began dying quite dramatically, some dropping out of the sky. This was
followed within a few years by an unprecedented and well-documented spread
right across continental North America, killing millions of birds and also affecting
thousands of horses and many humans. Although 80% of infections in humans are
sub-clinical, symptomatic infections range from a self-limiting fever to severe
neurological disease with long-term sequelae and death (Suthar et al. 2013). The
2002 and 2003, West Nile Virus epidemics were the largest recognised arbovirus
meningo-encephalitis epidemics in the western hemisphere, with more than
500 human deaths (Sejvar 2003). During these 2 years, a total of 13,278 human
cases were reported in the USA, with a mortality rate of between 3 and 7% (Bengis
et al. 2004). Many infected horses also died of neurological disease. Clinical disease
and deaths were also recorded in 155 resident avian species.
This disease has now become endemic in North America, with focal outbreaks in
birds, humans and horses occurring annually. West Nile Virus infection in the USA
is a classic example of an alien vector-borne infection being introduced into a naïve
ecosystem.
10.4.2 African Swine Fever
In the natural African environment, the African Swine Fever (ASF) virus circulates
between soft ticks (tampans) and wild African suids such as warthogs and bush pigs,
which become sub-clinically infected. However, in domestic swine, ASF infection
becomes directly contagious and causes a severe, usually fatal, haemorrhagic disease. In the African context, ASF presents a severe limitation to commercial pig
farming in areas where tampans and native wild porcines co-occur. African Swine
10 Pathogens of Vertebrate Animals as Invasive Species: Insights from South Africa
265
Although this chapter discusses species introduced into southern Africa, pathogens
are also introduced from southern Africa to other regions (see also Pyšek et al. 2020,
Chap. 26; Measey et al. 2020, Chap. 27, for a discussion of South African species
that have become invasive elsewhere). Two examples are discussed below.
10.4.1 West Nile Virus
West Nile Virus infection is caused by a mosquito-borne Flavivirus, which originated in Africa. A mosquito-bird cycle is the maintenance mechanism, and birds are
considered to be amplifying hosts for the virus. This disease subsequently spread to
the Middle East and then into Europe where it continues to cause sporadic outbreaks.
However, the most dramatic course of events occurred when this virus was introduced into the United States of America in 1999. It is thought to have arrived with an
infected mosquito by aircraft or ship, and was first seen in New York, when many
birds began dying quite dramatically, some dropping out of the sky. This was
followed within a few years by an unprecedented and well-documented spread
right across continental North America, killing millions of birds and also affecting
thousands of horses and many humans. Although 80% of infections in humans are
sub-clinical, symptomatic infections range from a self-limiting fever to severe
neurological disease with long-term sequelae and death (Suthar et al. 2013). The
2002 and 2003, West Nile Virus epidemics were the largest recognised arbovirus
meningo-encephalitis epidemics in the western hemisphere, with more than
500 human deaths (Sejvar 2003). During these 2 years, a total of 13,278 human
cases were reported in the USA, with a mortality rate of between 3 and 7% (Bengis
et al. 2004). Many infected horses also died of neurological disease. Clinical disease
and deaths were also recorded in 155 resident avian species.
This disease has now become endemic in North America, with focal outbreaks in
birds, humans and horses occurring annually. West Nile Virus infection in the USA
is a classic example of an alien vector-borne infection being introduced into a naïve
ecosystem.
10.4.2 African Swine Fever
In the natural African environment, the African Swine Fever (ASF) virus circulates
between soft ticks (tampans) and wild African suids such as warthogs and bush pigs,
which become sub-clinically infected. However, in domestic swine, ASF infection
becomes directly contagious and causes a severe, usually fatal, haemorrhagic disease. In the African context, ASF presents a severe limitation to commercial pig
farming in areas where tampans and native wild porcines co-occur. African Swine
10 Pathogens of Vertebrate Animals as Invasive Species: Insights from South Africa
265