Medical Entomology Intro Chapter

Medical Pests

(adapted with permission from Higley, L. G., L. L. Karr, and L. P. Pedigo. 1989. Manual of Entomology and Pest Management. Macmillan, New York)

Although agronomic and horticultural insects routinely disturb food production, their effects are relatively minor compared to the economic disruption, suffering, and death caused by medical pests. Thus, the medical pests are an extraordinarily important group of insect pests. Insect pests of man had had a tremendous impact on human history and this influence continues. Disease carried by insects dominate many parts of the world, particularly central Africa. In fact, some authorities have suggested that although humans evolved in Africa, civilization instead developed in the Middle East, Europe, and Asia because humans were able to escape disease. Nevertheless, even where continual disease could be avoided, periodic episodes could not. Epidemics of arthropod-born disease undoubtedly contributed to the fall of the Roman Empire. And transmission of plague by fleas led to the pandemic of the Middle Ages which killed a fourth to a third of the population of Europe. But probably the most significant insect transmitted disease is malaria, which continues to be one of, if not the most, important threat to human health worldwide.

Just as human welfare is at risk from insects, so is the health of wild and domesticated animals. Although some insect species attacking humans also attack other animals, many veterinary and medical pests are host specific. However, injuries produced by these pests have many common features regardless of whether a human or other animal host is being injured. In the following sections we will consider these injuries. How medical and veterinary pests affect their hosts is tremendously important in managing those insects. In addition to focusing on injury, we need to recognize how medical and veterinary pests differ. Consequently, we also will examine how pest management is different for these pests, as well as how it differs from the management of plant pests.

Direct Effects

Insects may directly injure an animal host in many ways. Some types of injury may be caused by insect feeding, however, other insect activities may also be damaging. These effects frequently have recognizable economic consequences. However, direct effects, whether on humans, livestock, or other animals, also have less quantifiable results, including pain and suffering. There are six major categories of direct effects from insects:

annoyance (and blood loss) - annoyance comes from disruptive activities of insects, such as flying around or landing on the head, and from feeding, possibly causing a blood loss (called exsanguination). Insects usually do not remove sufficient blood to cause a medical problem, although anemia and significant blood loss caused by insects have been documented with livestock. Nevertheless, annoyance is not a trivial effect of insects. Human activities frequently are disrupted by insects, and in some instances, such as when recreational facilities cannot be used because of insects, annoyance can cause substantial economic losses. With livestock, annoyance is of even greater importance. Continuous irritation from insects may reduce weight gain in cattle, may disrupt milk production, and may contribute to increased susceptibility to other stresses.

dermatosis (and dermatitis) - dermatosis is a disease of the skin, dermatitis an inflammation of the skin. Both dermatosis and dermatitis can be caused by arthropod activities. Many mite species, such as scabies mites and chiggers, produce acute skin irritations. Human scabies, a skin disease caused by infestations of the itch mite (Sarcoptes scabiei) is an important public health problem and periodic outbreaks are common. In livestock, mange, any persistent skin inflammation (often with accompanying hair loss) caused by mites can seriously weaken animals. Serious, debilitating mange conditions in livestock are called scabies.

Figure 1. Itch or scabies mite. (US Public Health Service)

myiasis - is the invasion and feeding on living tissues of humans or animals by dipterous larvae. Fortunately, myiasis is a rare condition in humans, but it commonly occurs in livestock. Besides the detrimental effects of myiasis itself, many additional complications can arise from myiasis, such as secondary microbial infections, secondary infestations by other insects, and debilitation. Myiasis can be fatal.

Figure 2. Example of myiasis, a cattle grub larva living and feeding under the skin of a cow. (Courtesy USDA).

envenomization - is the introduction of a poison into the body of humans and animals. Few arthropods have sufficiently toxic poisons to kill humans outright. However, humans and other animals do die from arthropod venoms, and envenomization: biting, as occurs with spiders; stinging, as occurs with scorpions and some Hymenoptera (such as ants, bees, and wasps); contact - passively or inadvertently touching a poisonous feature, such as urticating hairs (hairs that produce wheals and itching) which are found on many Lepidoptera larvae and some spiders (such as tarantulas); active projection - contacting poisons that are secreted or expelled such as vesicating fluids (acid or alkaline liquids causing skin irritation or blistering), that occur in blister beetles; and ingestion - accidentally eating poisonous insects (e.g., horses can be killed by ingesting hay containing dead blister beetles).

Allergic reactions (anaphylaxis) - a hypersensitive response to insect proteins. All of the mechanisms associated with envenomization can also cause exposure to allergens. In fact, human deaths from bee and wasp stings usually are associated with hypersensitive reaction rather than direct effect of a toxin. Additionally, allergies to insect proteins may be expressed in other ways. For example, on study of individuals allergic to chocolate discovered that 37% of people tested actually were not allergic to pure chocolate but were allergic to cockroaches (cockroach parts are a common contaminant of cocoa - something to think about the next time you eat a candy bar).

entomophobia - an irrational fear of insects. This may range from unwarranted fears of innocuous insects to sensory hallucinations. One extreme form of entomophobia is delusory parasitosis, in which individuals become convinced they are infested with insects when no infestation exists. Delusory parasitosis may even be manifested by physical symptoms such as skin irritations and welts. Entomophobia may cause undue alarm and anxiety, leading to unwarranted use of insecticides, and, in severe cases, requiring professional treatment. To a certain extent, the common dislike and repulsion most people have towards insects also is an unwarranted fear and has the unfortunate consequence of increasing intolerance to insects and insect injury which leads to increased, and even unnecessary, use of insecticides and other management tactics.

Indirect Effects

The primary indirect effect of medical and veterinary insects is disease transmission. Indeed, disease transmission is more important than any other effect produced by medical and veterinary pests. Underlying the relationship of arthropods to disease requires consideration of many concepts and much terminology.

Organisms that produce disease are called pathogens, and disease itself is a stress condition produced by the effects of a pathogen on a susceptible host. Arthropods capable of transmitting pathogens are called vectors. Some diseases may depend on only a single host and a vector; however, other diseases may include multiple host species, and even multiple vectors. In any of these instances, an organism that maintains the infective agent (the pathogen source) when active transmission does not occur is termed a reservoir. For example, the reservoir for malaria is human populations, with transmission occurring when a mosquito feeds on an infected individual and later feeds on an uninfected individual. With plague, the most common reservoirs are rats and other rodents, with transmission occurring when fleas feed on rats or rodents and then feed on humans. Often the infection in the reservoir species is less severe than in the primary host, however, this is not always the case (e.g., plague is as deadly to rats as it is to humans).

The study of the nature of disease, especially how a pathogen produces disease by altering host physiology, is the province of pathology. Another fundamental consideration in characterizing any disease is epidemiology, the study of the incidence, distribution, and determinants of disease in a population. In considering epidemiology we can recognize different levels and distribution of disease: endemic refers to disease being native to a region or population, epidemic refers to disease outbreaks affecting a high proportion of a population, and pandemic refers to disease outbreaks affecting a wide geographical area and a high proportion of a population or populations.

Epidemiology is particularly important in describing the involvement of arthropods in disease transmission. In particular, understanding host/pathogen, vector/host, and vector/pathogen relationships is central to most epidemiological questions.

Host/Pathogen Relationships
Fundamentally, disease is a manifestation of interactions between host and pathogen. An array of environmental and physiological factors may influence these interactions. Additionally, qualities of the host and pathogen influence disease development. Resistance refers to a host’s ability to prevent infection and disease; virulence refers to a pathogen’s ability to produce disease. These terms apply equally to plant pathogens as to animal pathogens; however, practical implications of resistance are different for plants and animals. Whereas genetic resistance to disease is an important component for managing plant disease, selecting resistant genotypes has more limited applicability with livestock and is impossible for humans. However, conferring resistance through the use of vaccines is possible for humans and other animals and is a primary mechanism of disease management.

Host/pathogen relationships also are disrupted with various therapeutic agents. For example, plague infections can be treated with tetracycline and related antibiotics. Unfortunately, just as we may observe ecological backlash by insect populations to insecticides, so do many pathogen populations develop resistance to various drugs. Strains of the plasmodium causing malaria, for example, are resistant to antimalarial drugs such as chloroquine.

Vector/Host Relationships
Many aspects of insect behavior and life history are important in disease transmission, especially those relating to relationships between vectors and hosts. Generally, the closer the association between vector and host, the greater the suitability of the vector to transmit disease. Different degrees of association are possible. Species that live on or in a different species are called parasites; external parasites are called ectoparasites, and internal parasites are called endoparasites. If a parasite can only live on a given host species the relationship is called obligate, e.g., head lice are obligate ectoparasites of man. Alternatively, if a parasite does not live exclusively on a given host species, then the relationship is said to fluctuate, e.g., cat fleas are facultative parasites of humans. Additionally, some parasites may be continuous on a host (like lice) but others may be temporary (like fleas).\

The association of a vector species to humans is crucial to the importance of medical pests. Animals living in close association with people are said to be synanthropic. Species that "like" (usually feed on) humans are called anthropophilic. Behavioral relationships to man can greatly influence the medical importance of a vector species. Both ticks and mosquitoes are facultative, blood-sucking parasites that vector a tremendous array of human pathogens. However, ticks are only incidentally associated with humans, whereas many mosquito species are anthropophilic and routinely feed on humans. These differences help explain why the incidence of tick-borne disease is relatively trivial compared to that of mosquito-borne disease.

Vector/Pathogen Relationships
The ability of a pathogen to survive and remain infective in or on a vector species is a critical factor in disease transmission. Two mechanisms of transmission are possible. Mechanical transmission is the transfer of a pathogen from an infectious source to a susceptible host by a vector, without any reproduction or developmental changes in the pathogen. Generally, mechanical transmission is an inefficient mechanism for disease transmission. Many insects carry disease producing pathogens on their body parts, but relatively few are known to be associated with disease outbreaks. Table 1 summarizes important mechanically-transmitted diseases with arthropod vectors.

The other transmission mechanism is biological transmission, in which the pathogen either reproduces, undergoes developmental changes, or both in the vector. Biological transmission is the most effective and significant mechanism for disease transmission by arthropods. Table 2 presents important biologically-transmitted diseases arranged by arthropod vectors.

Frequently, the relationships between vectors, pathogens, and hosts are complex, and the challenge in epidemiology is to resolve these complexities. For example, we mentioned the relationship between rats, fleas, and humans in plague transmission, but the plague pandemic of the 1300’s resulted from more than transmission of pathogen from rat by flea to humans. In the Middle Ages rats were the reservoir for plague but also were susceptible to the pathogen. As rats were killed by plague, rat fleas left their hosts and looked for alternative hosts, usually humans. The plague pathogen (a bacterium, Yersinia pestis) was highly virulent and the European populace highly susceptible. Although these points account for how plague was introduced to human populations, they probably cannot account for the extremely rapid spread of plague through Europe. Probably many of the human plague victims developed a form of the disease called pneumonic plague, in which the infection is centered in the lungs and is easily transmitted by coughing. Thus it is likely that once the disease was established, humans were themselves the most important vectors of plague.

This example illustrates many aspects of epidemiology. Although rates were a preferred host of the rat flea, as the plague bacillus killed the rats the fleas were forced to seek alternative hosts. Because rats are synanthropic, the most available alternate hosts were humans. Additionally, the relationship of the pathogen to the vector contributed to the effectiveness of rat fleas in transmitting plague. Once fleas ingested the pathogen, the bacillus multiplied in the gut. Eventually, bacillus would almost block the gut, and the infected fleas began to starve. In feeding, starving fleas would suck so forcefully that when the sucking muscles relaxed, recoil in the esophagous shot bacillus-laden blood back through the feeding tube into the host’s blood stream. Thus, the combined vector, pathogen, and host relationships associated with plague were all conducive to plague epidemics and pandemics. Indeed, these conditions remained sufficiently favorable for plague epidemics to occur periodically through to the 1900’s. Plague still occurs, but now the disease is easily treated with antibiotics if caught early on.

Pest Management for Medical and Veterinary Pests

Although pest management for medical and veterinary pests shares common features with pest management of plant pests, there are more differences than similarities. The basic distinction between the two systems is that in most instances fewer medical or veterinary pests can be tolerated than plant pests. In other words, the economic injury levels (EIL's) for medical and veterinary pests are lower than for plant pests. Additionally, more management techniques are available for plant pests than for medical and veterinary pests. Moreover, the use of a given technique may be more limited with medical and veterinary pests than with plant pests. For example, although host plants can be treated with insecticides to control pests, usually humans cannot be so treated (although insecticides are used for louse control on humans). And because EIL's are so low for medical and veterinary pests, the value of biological control is diminished, because biological control agents may not suppress pest populations sufficiently.

Management may differ for pests producing direct effects versus those with indirect effects (disease transmission). For pests with direct effects some level of pests may be tolerable, even if only a small number. Two approaches available for managing these pests are to avoid pests, through use of physical barriers or chemical repellents, and to reduce pest numbers. Although barriers are useful for medical insects, they are less useful for veterinary pests. Reducing pest populations also presents problems. Attacking active, flying insects may require treatment of large areas or developing methods to expose parasites to insecticide while on the host. Often immature pest stages may provide better opportunities for pest population reduction, because immatures are less mobile and may be less widely dispersed. Additionally, sanitation may be a useful method for some species, depending on manure or decaying material as larval habitats. Environmental manipulations, such as draining larval mosquito habitats, have been valuable in controlling some medical pests.

For indirect injury the situation is analogous to that of insects vectoring plant diseases few or no pests can be tolerated. The objective in managing arthropod vectors is to prevent disease transmission and development. One approach to interrupting disease transmission is to change the vector/pathogen relationship. For example, releasing mosquito strains incapable of transmitting malaria has been suggested as one method to reduce malaria incidence. More commonly, disease transmission can be disturbed in one of two ways: by disrupting the activities of the vector or by disrupting the activities of the pathogen. Approaches for reducing vector activities are the same as discussed for reducing direct effects, i.e., barriers or reducing vector populations. These approaches pose some particular problems in that barrier techniques must completely exclude vectors and pest populations must be reduced to extremely low levels. If possible, disrupting the activities of the pathogen is a more favorable approach. Vaccines and therapeutic agents are the best techniques, but these are not available for many arthropod-borne diseases. Even when vaccines are available epidemics are still possible, because money, facilities, and staff to provide vaccine and vaccinations are not available in many parts of the world.

Medical Pest Management
Because the fundamental threat posed by medical pests is disease transmission, management of many of these arthropods presents virtually intractable problems. In temperate regions, some diseases have been eliminated or greatly diminished through reducing vector populations. For example, malaria no longer occurs in most of Europe and North America. But malaria and many other arthropod-borne diseases persist in the tropics. Although attempts have been made to eradicate insect vectors, these efforts have not been very successful. In fact, by subjecting pest populations to heavy, continual insecticide pressure, insecticide resistance has developed in a number of medically-important species. More balanced, rational management tactics may circumvent resistance problems, but the dilemma we face is that unacceptably high levels of disease may persist even after our best efforts at vector management. Potentially, focusing efforts on the pathogen through vaccine development or similar directions may offer more promise in controlling infectious disease. However, we are far from having a vaccine for many important infectious diseases.

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