Hookworms in Small Animals
Ancylostoma caninum is the principal cause of canine hookworm disease in most tropical and subtropical areas of the world. A tubaeforme of cats has a similar but more sparse distribution. A braziliense of dogs and cats is sparsely distributed from Florida to North Carolina in the USA. It is also found throughout Central and South America and Africa. A ceylanicum of dogs, cats, and people is widely distributed throughout Asia, the Middle East, and parts of South America. Uncinaria stenocephala is the principal canine hookworm in cooler regions; it appears to be the predominant canine hookworm in Canada and the northern fringe of the USA, where it is primarily a fox parasite. U stenocephala also is seen in cats. A caninum males are ~12 mm long, females, ~15 mm; the other species are somewhat smaller. The infective larvae of canine hookworms, particularly those of A braziliense, may penetrate and wander under the skin of people and cause cutaneous larva migrans.
The elongate (>65 μm), thin-walled, hookworm eggs in the early cleavage stages (2–8 cells) are first passed in the feces 15–20 days after infection; they complete embryonation and hatch in 24–72 hr on warm, moist soil. Transmission may result from ingestion of infective larvae from the environment and additionally, in the case of A caninum, via the colostrum or milk of infected bitches. Infections with A caninum, A braziliense, A tubaeforme, or A ceylanicum can also result from larval invasion through the skin, but this route is of little significance for U stenocephala. Skin penetration in young pups is followed by migration of the larvae through the blood to the lungs, where they are coughed up and swallowed to mature in the small intestine. However, in animals >3 mo old, A caninum larvae, after migration through the lungs, are arrested in the somatic tissues. These arrested larvae are activated during pregnancy, then accumulate in the mammary glands. Arrested development may also occur in the mucosa of the small intestine; activation may occur after removal of adult worms from the intestine.
An acute normocytic, normochromic anemia followed by hypochromic, microcytic anemia in young puppies is the characteristic, and often fatal, clinical manifestation of A caninum infection. Surviving puppies develop some immunity and show less severe clinical signs. Nevertheless, debilitated and malnourished animals may continue to be unthrifty and suffer from chronic anemia. Mature, well-nourished dogs may harbor a few worms without showing signs; they are of primary concern as the direct or indirect source of infection for pups. Diarrhea with dark, tarry feces accompanies severe infections. Anemia, anorexia, emaciation, and weakness develop in chronic disease.
Anemia results directly from the bloodsucking and the bleeding ulcerations that result when A caninum shift feeding sites. The amount of blood loss due to a single worm in 24 hr has been estimated to be up to 0.1 mL. There is no interference with erythropoiesis in uncomplicated hookworm disease. The liver and other organs may appear ischemic, and some fatty infiltration of the liver may occur. Hemorrhagic enteritis with a swollen intestinal mucosa that shows red, small ulcers and attached worms is usually seen in acute, fatal cases. A braziliense, A tubaeforme, A ceylanicum, and U stenocephala are not avid blood feeders, and anemia rarely develops. However, hypoproteinemia is characteristic, and serum seepage around the site of attachment in the intestine may reduce blood protein by >10%.
In dogs, dermatitis due to larval invasion of the skin may be seen with any of the hookworms but has been seen most frequently in the interdigital spaces with U stenocephala; skin infections with U stenocephala rarely mature. Pneumonia and lung consolidation may result from overwhelming infections in pups.
The characteristic thin-shelled, oval eggs are easily seen on flotation of fresh feces from infected dogs and cats (Ancylostoma spp 52–79 × 28–58 μm; Uncinaria sp 71–92 × 35–58 μm). Acute anemia and death from infections acquired via milk may be seen in young pups before eggs are passed in their feces, ie, as early as 1–2 wk of age.
Bitches should be free of hookworms before breeding and kept out of contaminated areas during pregnancy. Sanitary quarters should be provided for whelping and nursing. Concrete runways that can be washed at least twice a week in warm weather are best. Sunlit clay or sandy runways can be decontaminated with sodium borate (1 kg/2 m2).
In dogs, fenbendazole, moxidectin, and pyrantel are approved for treatment of A caninum and U stenocephala infections. Milbemycin is also approved for treatment of A caninum infections (see Table: Drugs for Intestinal Helminths of Dogs Approved in the USA and UK). Nitroscanate is also approved for both hookworms at 50 mg/kg in some countries (eg, Canada). When anemia is severe, chemotherapy may have to be supported by blood transfusion or supplemental iron, followed by a high-protein diet until the Hgb level is normal. Heartworm prevention with products containing milbemycin control A caninum, whereas ivermectin/pyrantel, ivermectin/pyrantel/praziquantel, moxidectin, and moxidectin/imidacloprid control A caninum and U stenocephala. Heartworm preventives containing pyrantel also have activity against A braziliense (see Table: Drugs for Intestinal Helminths of Dogs Approved in the USA and UK) and are approved for this purpose. Finally, the injectable formulation of moxidectin for heartworm prevention in dogs also has significant efficacy against infection with A caninum and U stenocephala for at least 3 mo. For A ceylanicum, the combination product containing pyrantel embonate/febantel/praziquantel is approved for treatment in Australia.
In cats, drugs approved for treatment of A tubaeforme include emodepside, fenbendazole, ivermectin, milbemycin, moxidectin, pyrantel, and selamectin (see Table: Drugs for Intestinal Helminths of Cats Approved in the USA and UK). Heartworm prevention with ivermectin, milbemycin, milbemycin/praziquantel, moxidectin/imidacloprid, or selamectin controls A tubaeforme, whereas ivermectin also controls A braziliense (see Table: Drugs for Intestinal Helminths of Cats Approved in the USA and UK).
When neonatal pups die due to hookworm infection, subsequent litters from the bitch should be treated weekly for A caninum for ~12 wk beginning at 2 wk of age. In addition, fenbendazole (25 mg/kg, PO) given daily to pregnant bitches from day 40 of pregnancy to day 2 after whelping greatly reduces transmammary transmission to the pups (approved in the UK). Likewise, treatment of the bitch with ivermectin (0.5 mg/kg) on two occasions (4–9 days before whelping and 10 days later), or with moxidectin/imidacloprid spot-on on day 56 of pregnancy, has the same effect (extra-label use).
Resistance of A caninum to pyrantel has been reported in parts of Australia.