Fleas and infectious disease

Definition

Fleas are insects belonging to the order Siphonoptera, which includes 2,380 described species with 15 families and 238 genera. All are obligatory, hematophagous (blood-eating) parasites of warm-blooded mammals and birds. Fleas are wingless and are laterally compressed in shape; they have helmet-shaped heads and simple eyes. They range from light yellow to jet black and from 1 to 5 millimeters. Fleas are remarkably mobile; some can jump as far as 32 centimeters (as much as two hundred times their body length).

Flea species are found on all continents and have adapted to regions as diverse as equatorial deserts and Arctic tundra. The highest diversity of flea species is found in the subtropical to temperate regions of Eurasia. Flea lifestyles are diverse too; fleas can live as nest parasites, waiting to take a meal until the host returns for a rest. Some species, cat fleas (Ctenocephalides felis), for example, live most of their time on the host. In one genus of fleas, the Tungidae, the females become endoparasites by burrowing into human skin of the feet to feed. Although they tend to prefer one or a few host species, most fleas are opportunistic and will feed on what is available.

Fleas are reproductively prolific: C. felis can lay twenty to fifty eggs per day. The wormlike, free-living larvae hatch within two to five days. The larvae feed on adult feces, consisting of partly digested blood. Flea larvae develop into adults by passing through a cocoon-like pupae phase. Development occurs faster in environments with a humidity level of about 75 percent and in places with warm temperatures, optimally between 70 degrees and 90 degrees Fahrenheit. Conversely, eggs and larvae can remain viable at cooler temperatures for as long as twelve months.

Natural History and Risk Factors

As blood feeders, fleas are efficient vectors for disease because they inject and ingest pathogens directly into and from new hosts. Their mobility means they have a large range, which provides for widespread transmission. Also, as opportunistic feeders, they can spread disease among different host species too. Fleas transmit pathogens to their progeny transovarially; that is, female fleas pass pathogens directly into their eggs. Because fleas can persist for long periods in environments that are not ideal, the pathogens they carry are likely to find new, susceptible hosts to infect. The result can be waves of recurring epidemics and epizootics.

As parasites, fleas are resilient and persistent, and they have developed resistance to insecticides. Those species that live in the environment can avoid insecticides on the host; those that live on the host avoid insecticides in the environment. As human parasites, fleas are annoying, at minimum, and can cause skin infection and allergies. Some domestic animals can become overwhelmingly infested and suffer anemia that can be fatal.

Fleas as Vectors

Fleas are vectors for the most devastating human disease in history, plague. Fleas also transmit murine typhus and can infect children with tapeworms of the family Diphylidium; fleas also might be reservoirs for cat scratch fever.

Plague. The oriental rat flea, Xenopsylla cheopis, is the principal vector for Yersinia pestis, the bacterial pathogen that causes plague. The disease takes three forms: bubonic, septicemic, and pneumonic. Bubonic plague is so called because lymph nodes swell and blacken and become buboes. The fatality rate for untreated bubonic plague is as high as 75 percent. Septicemic and pneumonic forms are even more likely to be fatal, especially if antibiotic therapy is not started promptly.

Historically, plague caused three major pandemics: the Justinian plague in the sixth century, which spread around the Mediterranean region; the Black Death that started in Europe in the fourteenth century and continued intermittently for three hundred years; and a more recent pandemic that started in China in the mid-nineteenth century. The most notorious pandemic, the Black Death, is estimated to have killed 40 to 60 percent of Europe’s population in the 1300s. The disease is also blamed for igniting the persecution of whole groups and populations, such as foreigners and Jewish people. The injustice of the disease also led the general population to doubt the authority of the Roman Catholic faith and of established authority, such as the landowning nobility.

The Black Death was spread when rats were transported by ship and over land on trade routes. The rats died of the disease and the fleas left the rats to find new hosts, often humans, and transmitted Y. pestis. Once established in a host, Y. pestis can also be spread between humans either directly through respiratory droplets or through flea bites.

The disease reservoir for plague is in wild rodents, such as ground squirrels and prairie dogs. Rats and domestic animals can carry infected fleas to humans, and humans can be exposed directly from proximity to wild animals.

Murine typhus. Fleaborne, or murine, typhus is caused by rickettsial pathogens. Rickettsia are microorganisms between virus and bacteria in size and lifestyle; they are small and can survive only as intracellular parasites. Murine typhus causes flu-like symptoms, such as headache, muscle aches, and fever, but symptoms can include a rash similar to that of measles. Although it is rarely (less than 2 percent) fatal, murine typhus can cause severe disease in the elderly and in immunocompromised persons.

Internationally, murine typhus is associated with indoor rats and X. cheopis. In the United States, the pathogen is more likely to be found in C. felis. Investigations of recent outbreaks in the Los Angeles region and in southern Texas revealed that the pathogen is more likely to be found associated with opossums and cats than with rodents.

Cat scratch fever. Cat scratch fever is caused by the bacterium Bartonella henselae and is transmitted to people who have been scratched by infected cats. Symptoms include fever and swollen lymph nodes. Although fleas do not appear to transmit the disease to people directly, they do appear to transmit it between cats; fleas also act as a reservoir for the organism.

Diplydium tapeworms. Fleas harbor tapeworm eggs in their gut and transmit the parasite to domestic animals when the domestic animals ingest the fleas as they groom. Children who ingest fleas also can be infected. The parasite is easily treated and does not cause serious disease.

In 2018, the Centers for Disease Control and Prevention (CDC) reported that illnesses passed through insect vectors were on the rise in the United States. Illnesses borne by fleas, ticks, and mosquitoes tripled from 2004 through 2016, although diseases borne by mosquitoes and ticks accounted for most of this increase. Of the sixteen diseases examined in the CDC's study, only one, plague, was carried by fleas. There were eighty-nine cases of plague reported to the CDC between 2004 and 2016. This increase in prevalence of such diseases was attributed to global warming by many scientists in 2021. Warmer waters are more hospitable breeding grounds for fleas, and warmer climates extend the duration of the transmission season of diseases. Existing and newly emerging infectious diseases remained a topic of debate and concern among scientists in 2022.

Prevention and Control

Fighting flea infestations requires using insecticides on hosts and in the environment; intensive, adequate sanitation also is necessary. As common and annoying parasites of domestic animals, fleas have inspired considerable research into new insecticides that are less toxic and more effective. Modern innovations use arthropod hormones that interfere with larval development.

Impact

The World Health Organization estimated in 2021 that, though the number is likely much higher, between one and two thousand individuals are reported to be infected with the plague each year, mostly contracted from exposure to wildlife fleas. Similarly, murine typhus affects thousands of people worldwide.

Bibliography

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