transmission to new hosts tend to cause severe disease and kill their hosts quickly
(Jones 2007). Using the analogy of an uncontrolled wild-fire, large, rapid outbreaks
of these diseases consume all the available fuel and then die out. A disease is thus
more likely to become endemic if it can employ alternative transmission or pathophysiological strategies. For instance, a chronic disease that can be transmitted by its
host for a long period before causing the host’s death may be able to infect the same
number of hosts as a highly virulent disease, by doing so over a longer time and
maintaining host population levels by causing fewer mortalities. This is not to say
that a highly virulent disease cannot become endemic, as this is possible if there are
barriers to rapid transmission of the disease. For instance, in arid habitats where there
is a lower density of susceptible hosts, rabies transmission is stalled and the infection
circulates at a very low level until a threshold is reached, either by an increase in
population size, or by an individual moving out of the area and taking the pathogen
to a new adjacent habitat with a large enough susceptible population (Swanepoel
1995). Other diseases may increase their likelihood of becoming endemic by
infecting an asymptomatic, reservoir species or by utilising an arthropod vector,
such as ticks or mosquitoes, for transmission.
While humans are most often directly or indirectly responsible for introducing
invasive diseases, they also have the unique power to prevent or limit invasion by
instituting control measures that could stop an outbreak from happening, stop the
spread of an outbreak, or stop a new disease from becoming endemic. The diseases
discussed in this chapter illustrate various combinations of the above concepts. Our
discussion is limited to pathogenic bacteria and viruses, as it is not possible to cover
the full range of potential pathogens. However, the reader should be aware that the
other microbes, such as protozoa, fungi and metazoa, are also extremely important.
In this chapter, we do not consider factors such as virulence and the interplay
between invasive and dangerous or pathogenic parasites compared to dangerous
but non-invasive agents, or invasive but not dangerous agents, since that would
require lengthy discourse on its own.
10.2 Animal Diseases
10.2.1 Canine Rabies
Rabies is a viral disease of mammals that is almost invariably fatal once clinical
signs become apparent (Franka and Rupprecht 2011). Transmission is through
infected body fluids introduced through a bite or contact with mucous membranes,
after which the virus spreads along the nervous system to the brain. As the disease
develops it causes brain inflammation, abnormal behaviour and ultimately death
through generalised muscle paralysis or seizures (Murphy 1977; Koyuncu et al.
2013).
Sporadic, unconfirmed cases of rabies in dogs were reported from South Africa
between 1772 and 1861, though several travellers during that time remarked that the
252
L. van Helden et al.
(Jones 2007). Using the analogy of an uncontrolled wild-fire, large, rapid outbreaks
of these diseases consume all the available fuel and then die out. A disease is thus
more likely to become endemic if it can employ alternative transmission or pathophysiological strategies. For instance, a chronic disease that can be transmitted by its
host for a long period before causing the host’s death may be able to infect the same
number of hosts as a highly virulent disease, by doing so over a longer time and
maintaining host population levels by causing fewer mortalities. This is not to say
that a highly virulent disease cannot become endemic, as this is possible if there are
barriers to rapid transmission of the disease. For instance, in arid habitats where there
is a lower density of susceptible hosts, rabies transmission is stalled and the infection
circulates at a very low level until a threshold is reached, either by an increase in
population size, or by an individual moving out of the area and taking the pathogen
to a new adjacent habitat with a large enough susceptible population (Swanepoel
1995). Other diseases may increase their likelihood of becoming endemic by
infecting an asymptomatic, reservoir species or by utilising an arthropod vector,
such as ticks or mosquitoes, for transmission.
While humans are most often directly or indirectly responsible for introducing
invasive diseases, they also have the unique power to prevent or limit invasion by
instituting control measures that could stop an outbreak from happening, stop the
spread of an outbreak, or stop a new disease from becoming endemic. The diseases
discussed in this chapter illustrate various combinations of the above concepts. Our
discussion is limited to pathogenic bacteria and viruses, as it is not possible to cover
the full range of potential pathogens. However, the reader should be aware that the
other microbes, such as protozoa, fungi and metazoa, are also extremely important.
In this chapter, we do not consider factors such as virulence and the interplay
between invasive and dangerous or pathogenic parasites compared to dangerous
but non-invasive agents, or invasive but not dangerous agents, since that would
require lengthy discourse on its own.
10.2 Animal Diseases
10.2.1 Canine Rabies
Rabies is a viral disease of mammals that is almost invariably fatal once clinical
signs become apparent (Franka and Rupprecht 2011). Transmission is through
infected body fluids introduced through a bite or contact with mucous membranes,
after which the virus spreads along the nervous system to the brain. As the disease
develops it causes brain inflammation, abnormal behaviour and ultimately death
through generalised muscle paralysis or seizures (Murphy 1977; Koyuncu et al.
2013).
Sporadic, unconfirmed cases of rabies in dogs were reported from South Africa
between 1772 and 1861, though several travellers during that time remarked that the
252
L. van Helden et al.