Hookworm Dermatosis
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Description
Hookworm dermatosis is a dermatological disease caused by hookworm larvae, of which Ancylostoma caninum and Uncinaria stenocephala are the most common species in the UK. The L3 larvae can complete their life-cycle by penetrating the dermis of the canine definitive host and entering the blood. The larvae then move into the lungs, are coughed up and swallowed to complete their development in the small intestine. As animals grow older and become exposed to hookworm larvae, a strong Th2 immune response develops so that new L3 larvae are trapped within eosinophilic granulomata and killed in the skin before they can reach the blood. In severe infestations, secondary bacterial infection may occur.
Signalment
U. stenocephala is the most common cause of hookworm dermatosis in the UK and it it usually acquired by dogs kept in confined moist areas, such as grass or wet concrete runs. Greyhounds (in racing yards), foxhounds and farm dogs are therefore most often affected. Signs of severe dermatitis are seen in older animals that have developed some resistance to the parasites. Since A. caninum is usually found in warm or subtropical areas, it should be suspected in animals that may have travelled abroad.
Diagnosis
Clinical Signs
The disease has a sudden onset and pressure points are the most commonly affected sites, including:
- The palmar/plantar aspects of the digital and metacarpal/metatarsal pads.
- The nailbeds.
- The skin over the elbows and hocks.
- The skin of the ventral abdomen.
The pads are most commonly affected and these may be swollen and, due to self-trauma, there may be areas of excoriation where the superficial cornified layer is absent. The lesions are intensely pruritic. In areas of haired skin, red papules may be observed, progressing to areas of alopecia and lichenification. The owner of the dog may be affected by cutaneous larva migrans.
Diagnostic Tests
Skin scrapings should be obtained because, although they are often negative for hookworm larvae, they can be used to rule out infection by Demodex mites, which is a major differential diagnosis for this presentation. Similarly, impression smears or scrapings from nail bed can be used to rule out infections by bacteria (paronychia) or fungi such as Malassezia pachydermatis (onychomycosis). Atopic dermatitis may cause animals to chew and lick their feet but this is unlikely to have a sudden onset and rarely causes lesions on the cornified palmar/plantar surfaces of the pads.
Histological examination of biopsies from affected areas may show:
- Hyperplastic spongiotic perivascular dermatitis.
- Presence of large numbers of eosinophils.
- Migration tracts of inflammatory cells showing the path of larvae through the skin.
Treatment
The disease can be prevented by practicing good sanitation (removing all faeces from areas frequented by other dogs and humans) and housing dogs in dry conditions that prevent the development of hookworm eggs and L1-2 larvae.
Infections with adults hookworms (in the small intestine) can be cleared with several of the anthelmintics licensed for use in dogs, including praziquantel and pyrantel.
Animals affected by hookworm dermatosis can be treated with oral ivermectin (but this should not be used in collies or Shetland sheepdogs) and glucocorticoids may also be necessary to reduce the associated pruritis.
Prognosis
Hookworm dermatosis is a mild disease that will resolve spontaneously and can be treated and prevented easily.
Zoonotic Potential
Hookworm larvae may cause cutaneous larva migrans in humans.