Myiasis in dogs: a parasitic disease
Myiasis in dogs: what is it?
Myiasis is a disease that occurs when larvae of fly species from the Diptera genus infect a wound. Fly larvae are laid in the wound and feed on the animal’s dead tissue, producing an enzyme that severely damages the animal’s skin.
Depending on the affected area of the body, myiasis may be classified as cutaneous, ocular, auricular or urogenital.It can affect any vertebrate animal, including humans, and is found worldwide, predominantly in the wettest months.
Myiasis only occurs when the dog has a wound, that is, when the tissues of the animal’s skin have been breached. The flies lay eggs in the wound and these develop into larvae. The larvae are very small but begin to grow and feed on the tissue. As they penetrate the animal’s skin, the wound increases in size, so more flies may be attracted and lay more eggs in the wound. It is commonly known as fly strike.
Aetiology
Myiasis is a fairly common disease in stray dogs or those with wounds that are not managed correctly. It can also develop in animals with a constantly wet body part, caused by lacrimation, incontinence, salivation, and so on.
Clinical signs of myiasis in dogs
The disease is easily diagnosed because the larvae are visible in open wounds. Eggs hatch within 1–3 days and the white maggots can rapidly increase the size of the wound. Over the next 2 weeks or so, the larvae grow and transform into maggots that produce a salivary enzyme that digests the host’s skin, causing deeper wounds and allowing them to penetrate.
The larvae grow for 2 weeks and invade the wound, which may develop into a very large and severe lesion. The dog’s skin may then suffer a bacterial infection. If the infection is severe the dog may go into shock as a result of the enzymes and toxins produced by the larvae.
The larvae may initially affect the skin but can subsequently migrate to different gastrointestinal, urogenital, auditory and ocular tissues and organs. They temporarily feed on the host’s living or necrotic tissues, as well as on its body fluids.
Clinical forms
Myiasis can be divided into cutaneous, cavitary, wound and systemic forms.
Clinically, there are three types of cutaneous myiasis: furuncular, rampant or creeping and traumatic. In systemic forms, although the initial infestation is in the skin, the larvae may migrate and ultimately attack gastric, intestinal, rectal, urinary, auricular and ocular tissues.
The lesion looks like an abscess or insect bite; there is usually only one in the exposed area and it causes little discomfort at first. It is an erythematous papule that grows slowly, becoming pustular and/or discharging a bloody, serous fluid. If the larva penetrates deeper, subcutaneous nodules of 1–2 cm may form, which sometimes cause painful abscesses. In addition to pruritus, there may be pain, at times stabbing, and perception of the larvae’s movement. Little or no infection or necrotic tissue can be observed when the larvae are present. However, secondary infections often occur after the larvae leave the wound. Regional lymphadenopathy and lymphangitis may occur. Another form of presentation of obligatory myiasis is the creeping, rampant or serpiginous eruption consisting of a threadlike red line, terminating in a vesicle, which marks the larva’s path through the skin. The larva is located near the vesicle, which is surrounded by apparently healthy tissue.
- Diagnosis: as mentioned previously, myiasis in dogs is easily diagnosed with a simple physical examination. When an animal is developing myiasis, the wound progressively increases in size, creating lesions with extensive holes due to tissue loss. There is also a foul odour and dark secretion that usually attracts more flies.
- Differential diagnosis:pyogenic staphylococcal forunculosis, abscesses, epidermal cysts, cellulitis, granuloma due to retention of the arthropod’s mouth parts, tungiasis, foreign-body reaction, insect bites, allergic reactions, lesions caused by Sarcoptes scabiei, infected sebaceous cysts.
TREATMENT
Treatment of myiasis in dogs is based on shaving the area and cleaning the wound (the skin and hair trap bacteria which may reinfect the wound) and applying various products to kill the larvae. Povidone-iodine should be applied and the larvae removed manually with tweezers taking care not to break them in the process.
Once the wound is clean, a local antibiotic should be applied and the rest of the coat sprayed with an insecticide.
Antibiotics and support measures are indicated in the event of systemic involvement with bacterial superinfection.
A number of points need to be considered:
- The areas of skin infested by maggots may be very painful, so patients should be prevented from biting in those areas.
- It can be treated with nonalcoholic products containing pyrethrins which eliminate infestation and control the remaining larvae until they are completely eliminated.
- Make sure that topical antibiotics are nontoxic, as the animal might consume some of the medicine.
How to prevent the onset of myiasis in dogs?
Prevention is the best strategy because, although easily treated, maggot-infested wounds can be life-threatening if not addressed promptly. It is suffice to inspect wounds or areas which are constantly wet given their predisposition to this disease.
Besides basic hygiene care for dogs, certain climatic conditions that can favour the onset of myiasis should be avoided, since heat and environmental humidity attract flies. Check out the features and composition of Advance Atopic Care Shampoo here.
