Parasite and host dynamics are determined by the parasite’s ability to infect the host

Parasites and their hosts have populations that fluctuate over time, much like the dynamics of predator and prey populations that we discussed in Chapter 14. Unlike predators, however, parasites generally have a higher reproductive rate than their hosts and often do not kill them. If we know what causes fluctuations in parasite and host populations, we can predict when parasitism will be prevalent in a host population. This allows us to anticipate population changes in hosts and, in some cases, to intervene and reduce the harmful effects of a parasite on a species of concern. The probability that a host will become infected by a parasite depends on numerous factors that include the parasite’s mechanism of transmission, its mode of entry into a host’s body, its ability to jump between species, the existence of reservoir species, and the response of the host’s immune system.

Mechanisms of Parasite Transmission

The first factor that determines the risk of infection by a parasite is the mechanism of transmission. As illustrated in Figure 15.12, there are many different mechanisms. Parasites can move between hosts through horizontal transmission or vertical transmission. Horizontal transmission occurs when a parasite moves between individuals other than parents and their offspring. For example, horizontal transmission occurs when helminths are transmitted from snails to frogs or from frogs to birds. Horizontal transmission can also occur between conspecifics, such as the transmission of bird flu from one bird to another. Some parasites, such as the prions that cause mad cow disease, cannot be naturally transmitted from one individual to another. The risk of transmission is essentially zero in these cases. As we have seen with mad cow disease, dead infected cows were fed to other cows, which significantly raised the risk of infection. At the other extreme of transmission are influenza viruses, which easily pass between individuals. You may have experienced the rapid transmission of the flu virus if you or someone in your family became ill with flu; most likely, other members of your family quickly became infected. For most hosts, the risk of becoming infected by a parasite generally increases with host population density because higher densities mean that individuals are likely to come into contact with the parasite more often.

Figure 15.12 Mechanisms of parasite transmission. Parasites can be transmitted vertically or horizontally. When transmission is horizontal, the parasite can be transmitted through a vector such as a mosquito, transmitted directly between two conspecifics, or transmitted to other species. When transmission is vertical, a parent host transmits the parasite to its offspring, such as when a mother bird transmits lice to its hatchlings at the nest.

Some parasites require another organism, known as a vector, to disperse from one host to another. For example, the West Nile virus is transmitted from one host to another by a mosquito that picks up the virus from one infected bird and transmits it to an uninfected bird or other animal that it bites. In this case, the mosquito serves as the vector.

Still other parasites require transmission to multiple host species to complete their life cycle, as we saw in the story of the amber snail. Some species of helminths, for example, spend the first stage of their life in a snail, the second stage of their life in an amphibian, and the final stage of their life in a bird. This requirement for multiple hosts poses a substantial challenge for the parasite, which must find all of its required hosts during its lifetime.

Vertical transmission occurs when a parasite is transmitted from a parent to its offspring. In this case, the parasite must evolve in such a way that it does not cause the death of its host until after the host has reproduced and passed the parasite to its offspring. Many sexually transmitted diseases can be passed by vertical transmission. For example, chlamydia is a disease caused by several different species of bacteria within two genera, Chlamydia and Chlamydophila, that infect mammals, birds, and reptiles. In humans, chlamydia causes inflammation of the urethra and cervix, but is typically not lethal. While the bacterium is often horizontally transmitted between individuals, it can also be transmitted vertically from an infected mother to her fetus. When this happens in humans, the newborn baby can suffer eye infections and pneumonia.

Researchers continue to learn about the wide range of pathways that parasites can take to infect a host. The existence of vertical transmission and various methods of horizontal transmission can make it very challenging to predict and control the spread of infectious diseases in humans, crops, domesticated animals, and wild organisms.

Modes of Entering the Host

The mode of entry into the host’s body also affects the ability of the parasite to infect the host. As we have discussed, some species of parasites, such as leeches, are able to pierce the tissues of the host. Other parasites, such as some viruses, bacteria, and protists, rely on another organism to penetrate the host’s tissues and use the damaged tissue as an entry point. The protist that causes avian malaria, for example, depends on mosquitoes to inject the protist into a bird’s body after the protist completes a critical life stage inside the mosquito’s body. Without the mosquito, the malaria life cycle could not be completed.

Jumping Between Species

If a parasite specializes on only one host species and is only able to cause a lethal disease in that host species, then it might eventually run out of hosts and face extinction. One solution sometimes favored by natural selection is for the parasite to be nonlethal to the host; this allows a host population to persist. We will talk more about this strategy later in the chapter.

Alternatively, the parasite might evolve the ability to infect other species. We saw an example of this with bird flu, which infected several species of birds before a mutation occurred that allowed the virus to infect humans. A similar scenario occurred with HIV. For many years it was hypothesized that human HIV originated in chimpanzees, and in 2006 researchers identified a population of chimps in the West African nation of Cameroon that carried a genetically similar strain of the virus. The researchers suspect that the virus jumped from chimpanzees to humans when local hunters consumed the chimpanzees as food. Other examples of parasites jumping between species include the chytrid fungus, which can jump between amphibian species, and the canine parvovirus, which can jump from cats to dogs.

Reservoir Species

One way that parasite populations persist in nature is through the use of reservoir species. Reservoir species carry a parasite but do not succumb to the disease that the parasite causes in other species. Because reservoir species do not die from the infection, they serve as a continuous source of parasites as other susceptible host species become rare. For example, some species of birds can be infected with the protist that causes avian malaria, but they are resistant to developing the disease. However, mosquitoes that feed on these resistant species of birds can pick up the protist and transfer it to other species of susceptible birds that become infected and die. In this way, the reservoir species ensure the persistence of the parasite population over time.

The Host’s Immune System

A host’s immune system can play a role in combatting an infection from endoparasites. As a result, some parasites have evolved the ability to escape the immune system by making themselves undetectable. For example, when HIV enters a human cell, it can hide from the body’s immune system by living in the cytoplasm or by incorporating itself into the chromosomes of the cell. Because the body’s immune system searches for infections on the outside of cells, it cannot detect the virus. Other parasites have evolved additional strategies to escape the immune system. For instance, parasitic worms known as schistosomes produce a protective outer layer around their bodies that prevents them from being detected by the host’s immune system. Still others, such as the protists that cause African sleeping sickness, are able to continually change the compounds present on their outer surface so they become a moving target that stays one step ahead of the immune system as it tries to respond to the infection.