Warble Fly

Warble fly is an umbrella term that is used to define insects and flies that belong to the genus called Hypoderma. The term warble fly is used to define all the members of the family Oestridae, under which the genus is classified. Various species of insects are classified as warble fly, the only common link between all these species is the family under which they are categorized. The two most common and pathologically important species that are considered as warble flies include, Hypoderma lineatum (the cattle warble fly) and  Hypoderma bovis (the ox warble fly). Apart from these two species of warble flies, the insect species called Hypoderma tarandi  (the reindeer warble fly) is also very important from the host-pathogen perspective.

The term warble flies is also commonly known as cattle grub, bot fly, or heel fly. All these terms are used interchangeably. It is important to note that the term warble fly is not a scientific term nor is it a term that is used in the taxonomical classification. Warble fly is a general term that is used in day to day conversation to describe any insect that belongs to the family of Oestridae. Another important note is that two insects might be categorized as warble fly but they are not necessarily related species or have the same life cycle or host interaction pattern. This will be evident further in the article. 

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The term warble has an Anglo-Saxon origin, which essentially refers to “boil”. The infection of cattle by warble fly renders the skin damaged giving it a vaguely boiled appearance. Warble fly holds importance in research in areas such as parasitology and veterinary medicine because warble larvae are the most abundant insect that harms the livestock throughout the world, it infests cattle like cows and buffalos. The most common disease caused by warble larvae is hypodermosis. Some of the species of warble fly like reindeer warble fly, rabbit bot fly larvae infect animals like deers, reindeers, ox. The disease caused by these warble flies causes heavy economic losses to animal products like the milk and leather industry. It also affects the general health status as well as the immune system of the body of diseased animals. 

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The article focuses on the general description of warble fly, the taxonomic classification, host-pathogen interaction that includes the life cycle of the fly. The article also briefly covers the management techniques of warble fly infestation. The two main species that are described in the article is warbles in cattle and ox warble fly.

General Description of Warble Fly

As discussed earlier warble fly is an umbrella term, used to define various species of insects that are classified under the family called Oestridae. The warble fly undergo metamorphosis, the most important part of their life cycle is the warble larvae stage, during this stage, they reside inside the body of the host animal like cow, deer e.t.c.

Warble flies during adult stages can grow up to the size of a bumblebee. The adult warble fly similar to other insects has compound eyes, completely developed hind leg and foreleg. They also have wings which are common to all insects. The colour of an adult warble fly ranges from brown, black yellow and orange colour. Similar to most of the members of the Insecta family the warble fly adults have a hairy appearance. The hair-like extensions are majorly present on the legs of the insect. An important feature of these warble flies in the presence of the mouthpart, the mouth part in these flies are vestigial. It refers to that the organ is not functional during the adult stage of life. 

The female warble fly lays eggs on the body of the host animal, the specific site of infection is most commonly seen in the hind limb of the animal. The eggs hatch and produce larvae. The warble larvae travel inside the host body,  a unique feature of the larvae is that they have a hole in their body during the early developmental stage. The essential role of this hole is to allow the gaseous exchange in the larvae. The residing time of the larvae can range up to several months. Upon the development of warble eggs, the skin develops protrusion and lumps, these are generally seen on the back of the animal. It is the development of these protrusions that attributes to the generic name of the fly.

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Taxonomical Classification

Taxonomical classification is defined as a method used to categorize different animals based on their similarities and dissimilarities. The functional unit of the classification system is the taxa. Taxa can be defined as the different hierarchical levels that are used to classify an organism. An important question that arises here is the need for taxonomical classification? Taxonomical classification provides the evolutionary history, the phylogenetic relationship of the organism with other species. Apart from these insights, the classification system is also very useful to understand the typical characteristics and the features of the animals. As in the case of warble fly, the taxonomy plays an important role in understanding the host-pathogen interaction based on the general features and pathogenicity of the family. 

It is again very important to understand that warble fly is an umbrella term that is used to define many pathogenic insects, the classification mentioned here encompasses the general taxonomy of the warble fly, which can further be classified based on different genera and species. The taxonomical classification is as follows.

Warble fly belongs to the kingdom Animalia. This kingdom consists of every eukaryotic multicellular organism. The characteristic feature required to be classified under the kingdom Animalia require the following, the organism must uptake organic nutrients to survive, the organism must produce carbon dioxide as respiratory waste. The kingdom is further classified into phylum. Warble fly belongs to the phylum called Arthropoda. 

Arthropoda is among the largest phyla that are categorized under the kingdom Animalia. The characteristic feature of this phylum is the presence of a strong exoskeleton. The main constituents of the exoskeleton are chitin and calcium carbonate. Another characteristic feature of this phyla is the presence of a segmented body and paired jointed appendages. Although most of the warble fly lacks a strong exoskeleton but the segmented body and paired jointed appendages are prominent in all warble fly. The body segmentation generally results in 3 parts namely the head, thorax and abdomen region. The joined appendages are prominent in the adult warbles but are not present in warble larvae.

Warble flies are further categorized into an order called Diptera. The word Diptera has a Greek origin which refers to “wings”. The winged structure is prominent in every adult warble fly. A unique adaption seen in the hind wings of the members is that the hindwings are particularly acting as mechanosensor wings. This also true in the case of warble fly. Another very important feature of this family is the presence of a metamorphic developmental stage. Dipterians undergo true metamorphosis. Almost all the warble flies undergo metamorphosis, that they have a warble larvae stage. It is seen that the warble fly larvae have more life span compared to the adults. It is essentially the warble fly larvae that cause diseases in the animal. Most of the Diptera members are oviparous that is they lay eggs. All of which holds true for the warble fly species (ox warble fly, bot fly warble).

Warbles are further classified in a family, they belong to the superfamily called Oestridae. The common term for this family includes bot fly warble, heel flies, and gadflies. The very important distinguishing feature of this family is the parasitic nature of the members. The larvae of this family are essentially parasitic in nature. This is very true for warble fly larvae as they are responsible for causing diseases like hypodermosis in cattle. Warbles in deer and other animals also cause disease. The larvae are internal parasites of mammals, some species larvae grow on the host's flesh while others in the alimentary tract. The warble fly larvae grow on the host flesh, but they travel inside the host body. The resident time of the larvae varies from species to species. An important point to note here is that although many members of the family Oestridae lay eggs on the host directly there are some exceptions. Some of the species of this family use an intermediate vector to lay eggs on the host.

The family is further classified into genus and genus into species. There are various different genus that is classified under this family but most of the warble flies belong to the genus called Hypoderma. There are several species of warble flies classified under this genus, some of the most common examples of species with their common name are mentioned below.

1.  Hypoderma lineatum (the cattle warble fly)

2.  Hypoderma bovis (the ox warble fly)

3. Hypoderma tarandi  (the reindeer warble fly)

4. Cuterebra buccata (the rabbit bot fly larvae)

Among these, the Hypoderma lineatum (the cattle warble fly) Hypoderma bovis (the ox warble fly) are primarily responsible for causing hypodermosis in cattle livestock.

Global Distribution

Warble flies are a cosmopolitan group of insects, they are found in almost all parts of the world. The geographic distribution range of these insects is calculated based on the occurrence of hypodermosis in animals. Although they are found globally, their occurrence is highly recorded in tropical and subtropical countries. The countries that have recorded the maximum population of these flies include the following, India, Pakistan. Turkey, Kazakhstan, Albania, United Kingdom (England, Scotland, Wales and Northern Ireland.) and North America and North Africa.

Reproduction

Warble flies are sexually reproducing animals,n the process of mating occurs during the adult stage of life. The process of fertilization is internal, that is the male gamete fuses with the female gamete inside the female body. The female is considered oviparous that refers to the ability to lay an egg. It is interesting to note that the eggs are laid on the body host. Because of this, they have also termed ectoparasites. It is important to note that some species of warble flies lay eggs on the vector instead of laying eggs directly on the host. This is because the actual host has evolved a defence mechanism against the adult warble fly.

The egg is laid by a special appendage called an ovipositor. The ovipositor is an anatomical modification of stingers, that are commonly found in many insect species.

The Life Cycle of the Warble Fly

The life cycle of any animal is defined as the complete set of events that is typical to the species. The unique feature of the life cycle of warble flies is their larval stage. The warble larvae are the most important stage of the life cycle. It is very important to note that the larval stage is also the most prominent and the longest stage of the life span. The adult stage lasts up to 7 to 8 days leading to the death of the animal. The complete life cycle of the warble fly species like  Hypoderma lineatum (the cattle warble fly), Hypoderma bovis (the ox warble fly), Hypoderma tarandi  (the reindeer warble fly) can be divided into 3 stages, they are as follows

• Warble Egg

• Warble larvae

• Adult warble fly

The complete life cycle events common to all the warble fly species are mentioned below.

The adult female warble fly upon sexual fertilization develops eggs. The female warble flies lay about 100 to 150 eggs over the host body. They specifically lay eggs on the hair extensions of the cattle. The specialized appendage that is used to lay an egg is called an ovipositor, these appendages are present on a warble fly. The eggs get attached to the hair.

The first stage is the hatching of the larvae from the eggs, the newly hatched larvae moves crawls to the base of the hair and penetrates into the skin. It takes about 4 days to hatch from the egg. The penetration into the skin is mediated by several proteolytic enzymes and the mouthpart of the warble larvae.

The exact mechanism of reaching to the final site is still unknown but it takes several months to reach the final site. The final site of infection is the epidural fat of the spinal canal. It may vary from species to species Hypoderma lineatum larvae travel through the submucosal

connective tissues of the oesophageal wall of the body and then the back of the animal. While the Hypoderma bovis (the ox warble fly) travels along the nerve paths and reaches into the spinal canal, then finally at the back of the body. At this age, the body size of the warble larvae is 15mm.

After this, the warble larvae move to the mid-dorsal area of the host, at this site they moult and increase their body size.  The moulting results in the second larval stage. At the second larval stage, the moult reaches the epidermal layer of the skin. At this stage, characteristic raised skin nodules are formed, which are called warbles. Here the warble cuts the hole in the skin. This hole is used for breathing. After residing for several days the second larval stage proceeds to the third larval stage of  30-mm-long prepupa.

After several weeks when the prepupa is completely developed it force breaks the skin and drops on the floor. The morphological appearance can be described as a yellowish-brown coloured pupa. The pupal stage needs 3 to 10 weeks for development. Upon development, they break from the cocoon and reproduce. During this time it burrows under the soil. The overall life of an adult warble fly is 3 to 7 days depending upon the species.

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Summary at a Glance

• Warble fly belongs to the family called Oestridae.

• They are large bugs with hairy legs, compound eyes and heavy heads.

• They specialized organ called an ovipositor to lay an egg.

• They undergo metamorphosis

• The warbles in cattle are responsible for causing diseases like hypodermosis.

• The life span of warble fly larvae is several weeks while the life span of warble adults is 3 to 7 days.

• The economic impact on cattle includes damage to hides, gadding, decrease in milk production, carcass depreciation resulting in the butcher’s jelly or licked beef.

• A drug named ivermectin is widely used to control the population of warble flies on cattle.