The nuances of hookworm biology in dogs and cats (Proceedings)

Most people consider hookworms the causative agents of hookworm associated anemia in dogs and cats, and that is about where the story ends. Hookworms are fairly easy to treat with modern anthelmintics, and the drugs are well tolerated by the dogs and cats. Without treatment, these worms can live up to 2 years in their canine or feline host depending on the exact species of hookworm involved. So, when an infection is diagnosed treatment leads to resolution of any signs, and onto the next patient. This is the typical presentation most of the time but there are some other things to consider.

In the United States there are four hookworms of dogs and cats. Ancylostoma caninum is the common hookworm of the dog. Ancylostoma tubaeforme is the common hookworm of the cat. Ancylostoma braziliense is a hookworm of the dog and cat shared by cats in the southeastern United States. The worm Uncinaria stenocephala is a hookworm of dogs in the northern United States; this hookworm occurs in dogs and cats in Europe and has been reported from cats in the United States, but it seems to be quite rare in cats in the United States. In Asia, there is a fifth hookworm, Ancylostoma ceylanicum, that is mainly a parasite of cats but which also is capable of growing to the adult stage in dogs and people.

Hookworms cause blood loss by the host. There is one school of workers who believe that this is due to the actual sucking of blood by the worms as they feed. A second school believes that hookworms are plug feeders, and that blood seeps from the lesions the worms create. It could of course be a combination of the two, but in any case the disease caused by hookworms is anemia due to blood los into the bowel. The order of blood loss per worm from most to least is Ancylostoma caninum, Ancylostoma tubaeforme, Ancylostoma braziliense, and Uncinaria stenocephala. A mature female A. caninum will cause about 40 µl of blood to be lost by a dog each day; so, 25 worms means the loss of a ml of blood each day. Thus, 100 worms leads to the loss of 4 ml each day. A. braziliense causes only about 1 µl of blood loss per day or only about one tenth of an ml per 100 worms. U. stenocephala causes only about 0.3 µl of blood lost per worm a day; so just about one, one hundredth the blood lost from A. caninum. In cats, A. tubaeforme is the most dangerous of the worms. There have not been studies measuring the blood lost due to a single A. tubaeforme in cats, but it has been shown that around 200 hookworms will cause acute disease in a cat infected with this species.

Cats and dogs can get infected with hookworms by skin penetration, ingestion of the larvae, or the ingestion of paratenic hosts. Skin penetration may be the most common means of infection for the Ancylostoma species, but dogs and cats do eat grass. It has been shown in the case of U. stenocephala that the infection is much more efficient following oral infection than following skin penetration. These hookworms can also all infect paratenic hosts. A. caninum has been shown to utilize cockroaches and mammals. The other species have all been shown to utilize mammals as paratenic hosts; there have been human cases where larvae have been recovered in muscle biopsies. It seems plausible that the larvae can also persist in birds. It seems highly likely that paratenesis and ingestion are major sources of infection for any dog or cat that hunts.

The same larval stage that persists in the tissues of paratenic hosts will persist in the tissues of dogs, at least in the case of A. caninum. It is unclear or not known to what extent the larvae of the other hookworms persist, because this aspect of their biology has not been examined or not examined in any detail. In the case of the dog form, the worms that are present in the tissues, some say the intestinal wall while some say the somatic muscles, will periodically enter the intestinal lumen and develop to the adult stage. This has been referred to as the "larval leak" phenomenon. The result of this phenomenon is that dogs that are treated have a tendency to be positive again in two weeks or so. There is a good chance that what is occurring is not the suppression of egg laying by affected adults but rather the fact that new worms from inside the dog have repopulated the intestine.

After dogs are infected with larvae, it takes about two weeks before eggs are found in the feces. The appearance of eggs in the feces is about a day quicker for larvae administered via the oral route that larvae that penetrate the skin. Based on oral inoculations: U. stenocephala has the shortest prepatent period, being about 13 days; A. caninum is next at around 14 days; A. braziliense is similar to A. caninum (in the cat it is actually a day shorter following percutaneous infection), and A. tubaeforme is the longest requiring 18 days.

The larvae molt within the dog or the cat at various times after infection until they become adults. This seems highly esoteric; however, worms in different stages have different responses to products. This is well known for heartworms, where the monthlies have no significant effects on the adult worms and where the adulticide has no significant effects on the microfilariae and developing larvae. The same holds for hookworms, where in a similar manner to what happens with small strongyles in the horse, the adult worms tend to be more susceptible to many compounds than are the larvae. Thus, people and the FDA want to know the effects that the products have on the different stages. So, the days are set as to when products are to be tested against the different stages. For the hookworms in the dog, to kill the adults, treatment is not to occur until the worms are at least 21 days of age. For, fourth-stage larvae, the requirement is that the treatment should be administered when the larvae are somewhere around 6 to 8 days old. In the case of Ancylostoma caninum, if a company wants to have a claim against the stages in the female that might be transmitted in the milk, the female is treated (for third-stage larvae) two days before parturition, and then it is determined if the puppies are infected via the transmammary route. Some of the products have larval claims, and some do not. The question is often raised as to the value of a larval claim. It is easy to understand if it stops transmammary transmission, but it is difficult to understand the value of killing developing larvae within the intestinal tract. Drugs, that clear the tract of all stages are going to provide a longer period before eggs can appear again in the feces after treatment. This could be an important point when comparing monthly products.

Australian researchers have raised concern about A. caninum becoming resistant to treatment with pyrantel. In 1987, there was a report of possible hookworm resistance to pyrantel in a dog entering New Zealand from Australia. Larvae from the feces of this dog that were used to infect another dog could not be cleared with a 5X dose of pyrantel. There has also been a controlled trial in Australia that has shown poor efficacy against worms in experimentally infected dogs. This provides another reason why practitioners need to perform post-treatment fecal examinations to verify the efficacy of administered product.

Hookworms seem remarkably simple, but they are quite complicated in their biology. The disease also seems simple, but the long term effects of compensated hookworm disease have never been adequately described in detail. There may be a day when dogs do not have hookworms throughout much of the United States, but this is likely to be a long time away.