Botflies inhabit every continent except Antarctica, ranging from the Amazon rainforest to the Norwegian Arctic. The family Oestridae includes roughly 150 species, and their habitats track closely with the mammals they parasitize. The human botfly lives in the warm lowlands of Central and South America, livestock botflies infest sheep and cattle across Europe, Africa, and Asia, and reindeer warble flies thrive above the Arctic Circle. Understanding where each type lives matters whether you are planning a trip to Bolivia, raising livestock in North Africa, or wondering what that lump on your backyard mouse is.
The Human Botfly in Central and South America
The species most people picture when they hear “botfly” is Dermatobia hominis, the human botfly. It is native to tropical and subtropical Central and South America, from southern Mexico through most of Brazil.1PubMed. The Botfly, A Tropical Menace: A Distinctive Myiasis Caused by Dermatobia hominis Dense, humid forests at low to mid elevations are its stronghold. The adult fly does not land on people directly. Instead, it captures a blood-feeding insect like a mosquito, glues its eggs onto that insect’s body, and lets the mosquito do the delivery work. When the mosquito lands on a warm-blooded host, body heat triggers the eggs to hatch, and the tiny larvae burrow into the skin.
This indirect egg-delivery system is unusual even among botflies. Most botfly species either deposit eggs or larvae directly onto their host or onto surfaces the host will contact. Ancestral-state analyses suggest that laying eggs directly on the host was the original strategy for the family, and that depositing on non-host surfaces evolved independently at least three times across the four main botfly subfamilies.2PubMed Central. Reproductive behavior and early immature morphology of Portschinskia magnifica: implications for evolutionary biology in bot flies (Diptera: Oestridae) The human botfly’s trick of hijacking a vector insect is an especially creative twist on the non-host strategy, and it keeps the adult fly safe from being swatted while ensuring reliable skin contact with a mammalian host.
Because the human botfly depends on warm, humid conditions for its pupal stage in soil and for the availability of mosquito vectors, its range does not extend far into temperate zones. You will not find established populations in the continental United States or Europe. Cases that show up in those regions are imported, almost always in travelers returning from endemic areas.
Livestock Botflies Across Europe, Africa, and Asia
Sheep and goat farmers across much of the world contend with Oestrus ovis, the sheep nasal botfly. The adult fly deposits live larvae around a sheep’s nostrils, and the larvae migrate up into the nasal cavities and sinuses, causing a condition called oestrosis. A large meta-analysis of published studies estimated that roughly half of sheep in Africa and Asia, and over half in Europe, carry these parasites at any given time. Pooled prevalence ran about 48% in Africa, 44% in Asia, 57% in Europe, and 34% in the Americas.3PubMed Central. The global and regional prevalence of oestrosis in sheep and goats: a systematic review of articles and meta-analysis Those numbers are striking: in some European flocks, more sheep are infected than not.
Seasonal patterns vary by region but follow a general warm-weather peak. In Egypt’s Nile Delta, for example, the greatest infestation rates occur from August to October, declining through winter and then gradually climbing again through spring and early summer.4Veterinary Parasitology. The distribution and seasonal dynamics of Oestrus ovis linne infesting the nasal cavities and sinuses of sheep in egypt Within the same country, prevalence differed by geography: the eastern Nile Delta had lower rates than the central delta and its western governorates, likely because of differences in flock density and local climate.
Cattle have their own botfly problem in the form of warble flies, mainly Hypoderma bovis and Hypoderma lineatum. These species cause bovine hypodermosis, a condition where larvae migrate through the animal’s body and eventually form visible lumps under the skin of the back. Cattle warble flies have been documented throughout the Northern Hemisphere, including across Europe, Central Asia, and parts of North Africa and North America.5SpringerLink / Trop Anim Health Prod. Bovine hypodermosis–a global aspect Extensive eradication campaigns in western Europe have greatly reduced their numbers in some countries, but they persist in areas where control programs are less intensive.
Horse Stomach Botflies in China and Beyond
Equids carry their own group of botflies in the genus Gasterophilus, whose larvae develop inside the horse’s stomach and intestines. Six species have been documented in northwest China’s Kalamaili Nature Reserve alone, with Gasterophilus pecorum overwhelmingly dominant, accounting for 89 to 98% of all botfly infections in the area. In other parts of China, G. intestinalis and G. nasalis are the more common species.6PubMed Central. The impact of temperature on the life cycle of Gasterophilus pecorum in northwest China
The dominance of different Gasterophilus species in different regions reflects both climate and the identity of the local equid hosts. Wild Przewalski’s horses and Asiatic wild asses in arid northwest China face extremely high infection pressure, while domestic horses in moister, more temperate parts of the country deal with a different species mix. Horse stomach botflies are found wherever horses, donkeys, and zebras live, spanning Africa, Europe, Asia, and the Americas, but species composition shifts depending on the continent and the local environment.
Rodent Botflies in North America
People in the eastern and central United States and Canada sometimes notice a large swelling on a wild mouse or chipmunk. The culprit is usually a Cuterebra species, a genus of botflies that parasitizes rodents and rabbits. Unlike the human botfly, Cuterebra adults deposit eggs near rodent burrows or along runways the animals use. A mouse brushing past the eggs picks them up, and the larvae enter through a natural body opening or thin skin.
In Kansas, researchers tracked botfly infections in white-footed mice over multiple years and found that Cuterebra fontinella was the primary species. The larvae typically develop in the groin area, though neck infestations occur too. Other small mammals on the same study grids picked up Cuterebra as well, including woodrats, prairie voles, deer mice, harvest mice, and meadow jumping mice.7Journal of Mammalogy. Demography of botfly (Cuterebra fontinella) parasitism in white-footed mice (Peromyscus leucopus) in Kansas Woodrats carried a different species, C. americana, which was common in the region. The non-primary hosts are generally considered incidental, meaning the botfly can survive in them but did not specifically evolve to target them.
Cuterebra species span much of North America, from southern Canada through the United States and into Mexico. Pet cats and dogs occasionally turn up with a Cuterebra larva if they explore rodent habitat. These cases are incidental too, and they are far more common in late summer and early fall when rodent botflies are in their active larval phase.
Arctic and Subarctic Botflies
It surprises many people to learn that some botflies thrive in some of the coldest regions on Earth. Hypoderma tarandi, the reindeer warble fly, parasitizes reindeer and caribou across most of the northern hemisphere where those animals range, including Alaska, Canada, the Nordic countries, Greenland, and Russia.8PubMed Central. Human myiasis caused by the reindeer warble fly, Hypoderma tarandi, case series from Norway, 2011 to 2016 The larvae burrow under the skin of the reindeer’s back and develop over winter, emerging in spring as the weather warms. Heavy infestations can seriously damage hides and weaken the animals.
The same paper that described reindeer warble fly distribution also documented human cases in Norway, where people who handle reindeer or spend time in caribou calving areas have occasionally been infested. Human infection is rare but not unheard of, and it highlights that even subarctic environments are not free from botfly risk if the right host species is nearby.
Wild ungulates beyond reindeer also carry their own botfly species in northern latitudes. Moose across Scandinavia are parasitized by Cephenemyia ulrichii, a nasal botfly whose larvae develop in the animal’s nasal passages and throat. In Norway, researchers found that the prevalence of moose nose botfly infection tracked with moose density: areas with more moose per square kilometer had higher infection rates.9PubMed Central. Distribution, prevalence and intensity of moose nose bot fly (Cephenemyia ulrichii) larvae in moose (Alces alces) from Norway That finding makes biological sense: the adult fly needs to find a host to complete its life cycle, and denser host populations mean shorter search distances and better odds.
How Climate and Host Density Shape Where Botflies Appear
A botfly’s range is not just a function of geography. It is shaped by temperature, rainfall, and how many hosts are packed into a given area. This is because every botfly species spends part of its life outside its host, usually as a pupa developing in soil or leaf litter. That free-living stage is vulnerable to weather.
A study of botfly parasitism in howler monkeys in the Neotropics found that the likelihood of new skin nodules increased when temperatures dropped in the three weeks before nodules appeared, and also when rainfall decreased in the days just before. The researchers concluded that climate affects pupal development and the ability of larvae to reach hosts.10PubMed Central / Wiley Online Library. Bot fly parasitism in mantled howler monkeys (Alouatta palliata): General patterns and climate influences That might seem counterintuitive, since you would expect warmer weather to speed up insect development. But the timing lag matters: what happened weeks ago during the pupal stage determines how many adult flies are buzzing around now. A period of cooler temperatures might concentrate pupal development into a narrower emergence window, leading to a burst of parasitism.
Host density is the other major lever. The moose nose botfly data from Norway showed this clearly. And the sheep oestrosis data demonstrated geographic variation within a single country that likely reflects flock density and management practices. In general, botflies are more common wherever their preferred host species congregates in large numbers, because the adult flies are short-lived and need to locate a suitable host quickly. Sparse host populations make that harder, which is one reason some botfly species are now at risk of disappearing entirely.
Where Travelers Pick Up Botflies
If you are a traveler worried about botflies, the answer to “where should I be careful?” is overwhelmingly Latin America, and within Latin America, forested lowland areas. A study of 90 travelers diagnosed with myiasis (fly-larva infection) found that 80% had acquired it in Latin America, with over half of those cases traced to Bolivia’s Madidi National Park in the Amazon basin. The remaining 20% came from Africa.11Journal of Travel Medicine. Myiasis in Travelers
The concentration around Madidi is probably a combination of high human botfly density in that ecosystem and the park’s popularity with eco-tourists who spend extended time in the forest. Africa’s cases involve different species and different fly behaviors, but the principle is the same: prolonged exposure in rural or wild areas where host animals and botflies already coexist raises the risk.
Travelers to northern temperate zones, including most of Europe and the northern United States, face essentially zero risk from the human botfly. Cuterebra in North America only very rarely infests humans, and even then the infection is usually a single larva that does not develop fully. The reindeer warble fly in Scandinavia has caused a handful of documented human cases but only among people with close contact with reindeer herds. In practical terms, the human botfly risk map is a bullseye centered on tropical and subtropical Latin America, fading as you move into drier, cooler, or more urban areas.
How Botflies Spread Across Continents
Botflies did not pop up independently on every continent. Their current global distribution reflects millions of years of co-evolution and movement alongside their mammalian hosts. Phylogenetic analysis of stomach botflies in the genus Gasterophilus and their relatives suggests that the family originated in Africa, parasitizing elephants. From elephants, the lineage shifted to rhinoceroses, and from rhinoceroses to equids. As equids dispersed from Africa into the Palearctic and Oriental regions, their botflies came along for the ride.12Systematic Entomology. Evolutionary history of stomach bot flies in the light of mitogenomics
Related species in other botfly subfamilies show parallel patterns of host tracking. Hypoderma warble flies spread with ungulates across the northern hemisphere, and Cuterebra diversified alongside New World rodents. The tight relationship between botfly and host means that as host populations expand or contract, the botfly’s range shifts with them. This has obvious modern consequences: wherever livestock are introduced to new regions, their parasites tend to follow, sometimes within a few generations.
Wild ungulate botflies tell the opposite story. Hypoderma species that parasitize roe deer, red deer, and reindeer each track their host’s range fairly precisely.5SpringerLink / Trop Anim Health Prod. Bovine hypodermosis–a global aspect Hypoderma diana follows roe deer across Europe, H. actaeon follows red deer, and H. tarandi circles the Arctic with reindeer and caribou. Each of these host-parasite pairs is geographically bounded by the host’s range, not by any intrinsic climatic limit of the fly.
Botflies Facing Extinction
Most conversations about botflies focus on how to avoid or eliminate them. But some parasitologists have raised a counterpoint: certain botfly species may be headed toward extinction, and losing them would erase evolutionary lineages that have no close relatives.
Two forces are pushing botflies toward the edge. First, the widespread and highly effective use of anti-parasitic drugs in domestic livestock has virtually eliminated some botfly species from regions where they were once common. Cattle warble fly eradication campaigns in western Europe have been remarkably successful, which is great for the cattle industry but has brought some Hypoderma populations close to local extinction. Second, botfly species that depend on endangered wild hosts face co-extinction. If a wild ungulate species declines to the point where botfly adults cannot reliably find a host, the botfly population collapses alongside it.13PubMed. Oestrid flies: eradication and extinction versus biodiversity
This puts the botfly habitat map in an unusual light. The places where botflies live are not just a function of climate and host presence. They are increasingly shaped by human decisions about pest control and wildlife conservation. A botfly species that evolved over millions of years in tandem with a particular host can disappear in a few decades if that host is treated with ivermectin or driven to the brink by habitat loss. Whether that loss matters depends on how you weigh biodiversity against the real harm parasites cause to animals and, occasionally, to people. It is a genuine tension with no clean answer, and it is one that researchers who study these flies think about more than the public might expect.