Current Advice on Parasite Control:
Intestinal Parasites - Hookworms
Uncinaria stenocephala (rare in cats in the United States)
Overview of Life Cycle
Ancylostoma caninum wiggling in colon
- Adult hookworms live in the small intestine and shed eggs in feces and into the environment, where they larvate, hatch, and develop into infective third-stage larvae. This development occurs in approximately 2 to 9 days, depending on temperature and humidity.
- Dogs and cats become infected with hookworms via ingestion of the third-stage larvae from a contaminated environment, larval penetration of the skin, and/or ingestion of other vertebrate hosts with infective larvae in their tissues. Dogs and cats may also become infected by eating cockroaches that contain infective larvae.
- Transmammary transmission of larvae from the bitch to pups is an important route of infection for A. caninum. In animals more than 3 months of age, some proportion of A. caninum larvae that enters a dog migrates through the lungs and enters somatic tissue, where the larvae become dormant, or arrested. Arrested development may also occur in the mucosa of the small intestine. These arrested larvae are activated after removal of adult worms from the intestine; activation also occurs during pregnancy, with the larvae accumulating in the mammary glands to be secreted in the milk.
- Unlike A. caninum, A. tubaeforme is not transmitted to nursing kittens but is acquired only after birth through environmental contamination. Migration of hookworms within the host is complex. After penetrating the skin, larvae are carried by the bloodstream to the lungs where they migrate up the respiratory tree to the trachea. Larvae are coughed up and then swallowed, and then make their way to the small intestine, where they mature, mate, and produce eggs. Eggs are first found in the feces 2 to 3 weeks after infection.
- When infective larvae are ingested, some may penetrate the oral cavity and the gastro-intestinal tract mucosae and migrate as described; many of these larvae, however, remain in the alimentary tract and mature to adults in the small intestine.
- Larvae that infect other animals (paratenic/transport hosts) do not develop further in these hosts but become dormant (hypobiosis/arrested development) in various host tissues. When a dog or cat ingests these larvae in animal tissues, the larvae travel to the small intestine, where they are released and mature into adult worms.
- Immature and adult worms attach to the mucosa of the small intestine, digest the tissue, inject anticoagulants, and suck blood. Worms may detach and move to new sites and reattach. Small bleeding ulcers form where the worms once fed.
- Adult worms may live for 4 to 24 months in the small intestine.
Ancylostoma caninum curling in duodenum
- Morulated hookworm eggs are passed in the feces of infected dogs and cats; the most commonly seen species are A. caninum in the dog and A. tubaeforme in the cat.
- Adult hookworms in the small intestine of infected dogs or cats are attached to the intestinal villi by a large mouth cavity (buccal cavity).
- Male hookworms have a posterior copulatory bursa, and it is not uncommon to observe worms in copula in the small intestine.
- The anterior ends of both males and females are bent dorsally, giving them a "fishing hook" appearance. Ancylostoma tubaeforme and A. caninum have three pairs of teeth in the buccal cavity. Ancylostoma braziliense has one pair of teeth (although some references mention a second, inconspicuous pair). Uncinaria stenocephala has cutting plates instead of teeth.
- Dog and cat hookworms range in size from 10 to 20 mm by 0.4 to 0.5 mm.
- Eggs range in size from 55 to 90 μm by 30 to 55 μm, depending on the species. Ancylostoma spp. eggs are 55 to 75 μm by 34 to 47 μm; U. stenocephala eggs are 71 to 93 μm by 35 to 58 µm.
Ancylostoma caninum in patient
- All hookworms suck blood, but A. caninum is a voracious bloodsucker and can actually bleed a puppy to death. Disease in dogs caused by infection with A. caninum is most severe in young pups that are infected through nursing (the transmammary route) shortly after birth.
- Infected pups may present with pale mucus membranes and anemia, ill thrift, failure to gain weight, poor hair coat, dehydration, and dark, tarry diarrhea (melena). Puppies harboring many worms will develop an acute normocytic, normochromic anemia followed by hypochromic, microcytic anemia due to iron deficiency. Without immediate intervention, these animals may die of the infection. Those that survive may continue as "poor doers" with chronic anemia.
- Although some immunity to infection does develop, mature dogs may harbor small numbers of worms and contaminate the environment. If well-nourished and immunocompetent, these dogs will show few, if any, signs of disease. Adult dogs (particularly those that acquire many worms over a short period of time) may develop anemia, anorexia, emaciation, and weakness along with the characteristic dark, tarry diarrhea. These outcomes are more likely to occur in dogs that are stressed and malnourished.
- Ancylostoma tubaeforme, also a voracious bloodsucker, can cause anemia, diarrhea, and weight loss in kittens. Large numbers of these hookworms can be fatal.
- Respiratory disease and pneumonia may occur in puppies and kittens when large numbers of larvae migrate through the lungs. Respiratory signs may also be associated with hookworm-induced anemia.
- Ancylostoma caninum and A. tubaeforme are more pathogenic than U. stenocephala and A. braziliense because they are voracious bloodsuckers and consume much more blood than the latter two species.
- Penetration by larval hookworms occasionally causes a dermatitis with erythema, pruritus, and papules. These lesions are most commonly seen on the animal's feet, particularly in the interdigital spaces. Uncinaria stenocephala and A. braziliense and Uncinaria stenocephala are most often the culprits.
- Ancylostoma caninum and A. tubaeforme are common parasites of dogs and cats throughout the United States and are found in most tropical and subtropical environments.
- Both young and adult canine and feline hosts can harbor hookworms and pass eggs in their feces.
- In a survey conducted in 1996 using fecal samples collected from dogs across the United States, 19% of the fecal samples contained Ancylostoma spp. eggs and 1.02% contained Uncinaria spp. Ancylostoma prevalence ranged from less than 5% in the west to 36% in the southeastern United States.
- In ongoing U.S. studies, prevalence of Ancylostoma spp. in ownerless cats ranged from 1.1% in the northeast (New York) to 20% in the southeast (Tennessee).
- The geographic distributions of A. braziliense and U. stenocephala are more limited. In the United States, A. braziliense naturally occurs in warm coastal areas, but it is most common in tropical and subtropical regions of Central and South America and the Caribbean.
- Uncinaria stenocephala, whose common name is the northern carnivore hookworm, prefers colder climates and is most common in the northern United States, Canada, and Europe.
- The transfer of A. caninum larvae from mother to nursing offspring contributes to the high prevalence of infection in puppies, even in pets that are well cared for and routinely treated for intestinal parasites. Because somatic infection is common in adult dogs (even those on routine monthly intestinal parasite control), veterinarians should assume that essentially every nursing pup is at risk for hookworm anemia. Puppies and kittens should be routinely dewormed for hookworms. Deworming does not kill arrested larvae in the tissues (see Treatment ).
- Infection with hookworms occurs in all dogs and cats, but infection rates are likely to be higher in pets kept outdoors or those allowed to cons
Host Associations and Transmission Between Hosts
- Dogs and cats become infected with hookworms through ingestion of infective larvae from a contaminated environment, larval skin penetration, and/or ingestion of larvae in the tissues of vertebrate hosts (usually rodents).
- Puppies are infected with A. caninum when they ingest larvae while nursing. In adult dogs, Ancylostoma spp. larvae become arrested in somatic tissues rather than completing the migration to the small intestine. During pregnancy, these arrested larvae are activated and accumulate in the mammary glands.
- In dogs that are cleared of their adult worm infections, the intestinal tract can be repopulated by preexisting, dormant larvae from the somatic tissues that periodically become activated and resume development.
Prepatent Period and Environmental Factors
- Ancylostoma caninum usually becomes patent in 2 to 3 weeks, but puppies infected while nursing may shed eggs as early as 10 to 12 days after birth. Similar variations are seen in other hookworm species, but in general, the prepatent periods are 18 to 28 days for A. tubaeforme and 13 to 27 days for A. braziliense and U. stenocephala.
- Most hookworm eggs larvate, hatch, and develop into infective third-stage larvae in the environment in approximately 2 to 9 days, depending on temperature and humidity. In general, the environmental stages thrive best away from direct sunlight in warm, moderately moist, aerated soils. Freezing kills Ancylostoma spp. eggs, but Uncinaria eggs are hardier.
- Unlike ascarid eggs, hookworm larvae do not persist in the environment for years. Under optimal conditions, infective hookworm larvae can survive in the soil for a few months until their metabolic reserves are depleted. In addition, larvae usually are killed by freezing temperatures.
Site of Infection and Pathogenesis
- Most hookworm species can penetrate the skin. As noted previously, occasionally the penetration of larval hookworms causes a dermatitis with erythema, pruritus, and papules. These lesions are most commonly seen on the pet's feet, particularly in the interdigital spaces.
- After skin penetration, hookworm larvae move via the lymphatics to the veins and lungs where they penetrate alveoli and migrate up the respiratory tree to the trachea. They are swallowed and return to the small intestine where they attach to the mucosa and mature into adults. Respiratory disease and pneumonia may occur in puppies and kittens when large numbers of larvae migrate through the lungs. Respiratory signs also may be associated with hookworm-induced anemia.
- Hookworms in the small intestine attach and secrete enzymes and anticoagulants to digest intestinal mucosa and facilitate bloodsucking. Hookworm platelet inhibitor inhibits platelet aggregation and adhesion. Intestinal villi are damaged and villus blunting occurs, resulting in malabsorption and diarrhea. Adult hookworms move to new feeding sites, leaving small bleeding ulcerations behind. Enteritis and diarrhea may develop during this intestinal phase.
- The major disease syndrome produced by hookworms is acute or chronic anemia from blood loss.
- Hookworms produce many thin-shelled, morulated eggs that are passed in feces (link to image). The eggs float readily in most flotation solutions. Ancylostoma caninum and A. tubaeforme begin sucking blood before eggs are produced. In puppies infected by nursing their mother's milk, acute anemia and death may result before any eggs have passed in the feces.
- There are no available clinical assays capable of detecting prepatent infections. The high prevalence of infection in young pups and kittens and the inability to detect infections in animals before eggs are passed in the feces necessitate presumptive, routine deworming of all animals (see Treatment).
- In severely affected animals, anthelmintic treatment must be combined with supportive therapy to keep the animal alive until the drugs can kill the worms. In addition to keeping the patient warm, supportive treatment can include electrolyte and fluid therapy, iron supplements, a high-protein diet, and perhaps blood transfusions.
- Routine anthelmintic therapies do not kill arrested third-stage larvae in tissues.
Dogs (click here to view treatment chart for dogs)
- Adult A. caninum: fenbendazole, milbemycin oxime, moxidectin, and pyrantel pamoate are approved.
- Adult U. stenocephala: pyrantel pamoate, fenbendazole, and moxidectin are approved.
- Adult A. braziliense: pyrantel pamoate is approved.
- Fourth-stage and young adult A. caninum and U. stenocephala in the intestine: moxidectin is approved.
Cats (click here to view treatment chart for cats)
- Ancylostoma tubaeforme: emodepside, ivermectin, milbemycin oxime, moxidectin, pyrantel, and selamectin are approved.
- Fourth-stage and immature adult A. tubaeforme in the intestine: emodepside and moxidectin are approved.
Control and Prevention
- Puppies and kittens require more frequent anthelmintic administration than adult dogs and cats because (1) they often are serially reinfected via nursing and from the environment and (2) they often harbor migrating parasite larvae that later mature and commence laying eggs.
- Hookworm infections in puppies and kittens may cause serious illness or even death before a diagnosis is possible by fecal examination.
- In light of the hookworm prepatent periods, puppies, kittens, and their mothers should be treated with appropriate anthelmintics when the young are 2, 4, 6, and 8 weeks of age. Puppies and kittens should be put on monthly preventives as soon as label recommendations allow. If puppies and kittens are not treated until 6 to 8 weeks of age or later, they should be put on a monthly preventive according to label recommendations, dewormed again in 2 weeks, and then maintained on monthly preventives thereafter.
- Beginning treatment when puppies and kittens are 2 weeks of age will help minimize environmental contamination.
- Pregnant bitches may be treated during pregnancy with daily fenbendazole (from the 40th day of gestation through the 14th day of lactation) or 2 to 4 times with a high dose of ivermectin to prevent transmammary transmission of hookworm larvae to the pups. Both protocols involve off-label use of anthelmintics.
- Efficacy of the initial dewormings, effectiveness of the monthly control product, and client compliance should be monitored by performing a fecal examination 2 to 4 times in the first year and 1 to 2 times annually thereafter, depending on the age of the animal and its prior history of infection.
- Preventing predation and scavenging activity by keeping cats indoors and dogs confined to a leash or a fenced yard will limit the opportunity for cats and dogs to acquire infection with hookworms through ingestion of vertebrate hosts (usually rodents) or from an environment contaminated with feces from untreated animals.
- Prompt removal of feces from the yard or litter box will also help prevent hookworm eggs from hatching and larvae developing and dispersing into the environment.
- Enforcing leash laws and requiring owners to remove feces deposited by their pets can protect public areas from contamination with hookworm larvae.
Public Health Considerations
- Animal hookworms are well-documented zoonotic disease agents and are the most common cause of cutaneous larva migrans (CLM) in people. The larvae migrate in the skin, producing intensely pruritic, serpentine lesions. Usually these infections are self-limiting. Ancylostoma braziliense usually migrates more extensively in the skin than do the other hookworms.
- Humans are most likely to be infected with the hookworms of dogs and cats via direct skin penetration of infective larvae. Ancylostoma caninum can penetrate into deeper tissues and may arrest. On occasion, A. caninum can cause visceral larva migrans symptoms or may migrate to the intestine where it produces an eosinophilic enteritis. The worms do not mature to adults and no eggs are produced, but infected people may experience severe abdominal pain and eosinophilia.
- Most CLM and other hookworm-associated syndromes are diagnosed in the southeastern and Gulf Coast states. The most likely victims are individuals in contact with larvae-contaminated soils such as electricians, plumbers, exterminators, and other workers who crawl beneath raised buildings; farmers and gardeners with close soil contact; sunbathers who recline on larvae-contaminated sand; and children who play in contaminated areas.
- CLM is the most common travel-related skin infection in tourists to tropical areas.
- Early and regular deworming is essential in preventing contamination of the environment with hookworm eggs and larvae.
- Enforcement of strict adherence to leash laws and prompt removal of feces from environments where pets defecate are key to protecting public areas from contamination; receptacles for feces disposal should be provided.
- Children's sandboxes should be covered when not in use; outbreaks have occurred where larvae have developed to the infective stage in sandboxes and caused CLM in children.
- To avoid larval penetration, individuals should wear shoes and gloves when gardening.
- People whose occupations occasionally require contact with moist soil for extended periods of time should cover the contact area with a waterproof liner.