A third hookworm of dogs, A. caninum, is just becoming well recognized as a parasite of humans [1-4]. It is a cosmopolitan species, exceedingly common in both tropical and temperate areas, including North America. It was always considered a host-specific canine parasite that very rarely invaded humans and, when it did, caused essentially asymptomatic infections. The interesting information reported by Croese and colleagues [5] in this issue of Annals indicates that this parasite can develop to adulthood in the human intestine. It causes a new kind of hookworm disease, even in an infection apparently constituted of a single worm. The Australian investigations suggest that the recognition of human infections will become more prevalent [4].

Unlike other hookworms that develop to adulthood in humans, A. caninum is not associated with chronic blood loss and anemia but rather with eosinophilic enteritis [1-4], the pathogenesis of which will be a fertile area for future investigation. Because the worms do not lay eggs, the infections do not become readily apparent, making diagnosis difficult.

Croese and colleagues [5] report nine cases of human infection with Ancylostoma caninum in Australia and describe associated enteric disease, which is frequently, but not invariably, eosinophilic enteritis. Presumably this report will elicit considerable skepticism, given the ubiquity of the parasite and the often dramatic nature of the symptoms of infection. The Australian workers themselves have anticipated a skeptical reaction because parasitism by a common, macroscopic helminth presumably should not have escaped detection in symptomatic infections in human populations with access to good medical care [2]. In fact, a thorough literature review, such as that in the Croese report, indicates that stunted, sexually immature A. caninum had been reported from the human intestine previously, albeit very rarely. Thus, it is the association between this parasite and enteric disease in humans that has escaped detection rather than the mere occurrence of the parasite in the human intestine. This oversight is not surprising, given the diagnostic difficulties: the absence of ova in stools and the inherent improbability of finding very few small hookworms, perhaps just one, in a human intestine.

With the increased awareness that will follow the spate of recent Australian publications [1-4], one anticipates that cases of A. caninum-caused enteritis will be discovered elsewhere and the predicted skepticism will wane. In fact, the first presumptive case in the United States was reported at the 1993 joint meeting of the American Society of Tropical Medicine and Hygiene and the American Society of Parasitologists. The patient, a 7-year-old child, presenting with rectal bleeding, was seen at the Children's Hospital, New Orleans, Louisiana. From clinical, histologic, and endoscopic studies, positive enzyme-linked immunosorbent and Western blot assays for antibodies to A. caninum antigens, household exposure to an infected dog, and the success of specific anthelmintic treatment, the authors [6] concluded that this case of eosinophilic enteritis was caused by A. caninum, even though no hookworm was found by colonoscopy.

The previous cases of human canine-hookworm infection have occurred in tropical and subtropical areas of Australia. Because the free-living life stages of this hookworm need warmth and abundant moisture for development and survival, both human and canine infections will occur most commonly in warm, humid climates. In the early decades of this century, true human hookworm infection (Necator americanus infection) was highly prevalent in the southeastern United States, from Texas and Arkansas in the west to Florida and Virginia in the east. Human A. caninum infection, although possible throughout the broad range of A. caninum in the United States, is most likely to occur in this same southern area. Persons with prolonged, intimate contact with the soil, namely gardeners, workers (plumbers or electricians) who frequent the crawlspaces under buildings, infantry personnel, and sunbathers will be at risk for infection. Additionally, young children who play in sandboxes, yards, or public parks that are open to canine fecal pollution will also be at risk.

Ancylostoma caninum is the most common nematode parasite of mature dogs. Prevalences of infection as high as 60% to 70% have been reported in stray dogs in the Eastern and Midwestern United States, and in the Southeast a remarkable 43 (86%) of 50 dogs examined at a veterinary hospital were found infected [7]. There are now more than 52 million dogs in 38% of American households [8], making the potential for human exposure to this hookworm very high. Fortunately, in dogs receiving good veterinary care, hookworm infection has begun to decrease [9, 10]. This change is attributable to the high efficacy of the benzimidazole anthelmintics that are available [10] and to monthly heartworm chemoprophylaxis, now widely used in endemic areas of North America. Agents of choice for heartworm prevention, namely milbemycin oxime and the combination prophylactic, ivermectin with pyrantel pamoate, expel any adult A. caninum that may have invaded the intestine in the preceding month. The risk for exposure to all canine nematodes, at least in the more affluent communities of the United States, will decrease with time. To hasten this decrease in risk, conscientious physicians should urge their clients, especially those with young children, to initiate nematode control programs for their household pets.