Karen McGlothlin, DVM, PhD
Hookworms (Ancylostoma caninum, Ancylostoma braziliense, Ancylostoma tubaeforme, and Uncinaria stenocephala) are some of the most common intestinal parasites in dogs and cats. They are so common that a large percentage of new puppy owners and some kitten owners get the news that their new companions are positive for hookworm on their very first visit to the veterinarian.
Photo: Lifelearn
HOOKWORM LIFE CYCLE
Adult hookworms reside in the small intestine of an infected individual, where they attach themselves to the wall of the intestine using an array of 6 razor-sharp teeth. Once attached, adult hookworms nourish themselves by sucking blood from the well-vascularized wall of the small intestine. The dog hookworm (Ancylostoma caninum) is such a voracious blood sucker that a sizeable infestation in a puppy can lead to death.
Adult hookworms reproduce in the dog or cat host, shedding eggs in the host’s feces and into the environment. Over the course of the next 2-9 days, those eggs hatch, go through a process of development called larvation, and become third-stage larvae that are ready to infect their hosts.
There are multiple ways in which larval hookworms may infect a new host:
- Ingestion of the infective third stage larva. This can be done by sniffing or licking the ground where contaminated feces are or have been; licking the feet or coat that may have come in contact with third stage larvae in contaminated areas; ingesting infected tissue of another vertebrate host (including rodents and birds); or ingesting an organism, such as a cockroach, that may contain infective larvae.
- Direct penetration of the skin. This may occur through the skin of the feet, belly, or any other area of the body that may touch contaminated ground.
- Larvae may cross the placenta of a pregnant female and infect the puppies or kittens (prenatal transmission).
- Larvae may be transmitted to puppies and kittens through an infected mother’s milk (transmammary transmission).
Once in the host’s body, most larvae migrate to the small intestine, where they attach to the wall, start consuming blood, and mature into reproductive adults. The life cycle is completed when those adults start shedding eggs into the environment to undergo larvation, as mentioned above. Some larvae migrate down the trachea, where most are coughed up and swallowed, ending up in the intestinal tract, where they attach and start feeding and reproducing.
LARVAL LEAK
There is yet another option that some hookworm larvae may take – enter into dormancy or arrested development! Some of the larvae that migrate down the trachea actually make it to the lungs and may tunnel out of lung tissue and enter other types of tissue, encyst, and enter a state of dormancy. A few larvae in the small intestine actually tunnel through the intestinal wall to enter other types of tissue, encyst, and become dormant, as well. These inactivated, encysted larvae then serve as reserves and become reactivated in waves when adult worms are cleared from the small intestine (by deworming, for example) and during pregnancy, when reactivated larvae can either migrate across the placenta or accumulate in the mammary glands where they are secreted in the milk and infect puppies and kittens.
This phenomenon is known as “larval leak” and it allows hookworms to repopulate the small intestine, typically about 2-3 weeks after deworming. This is why you are typically asked to administer additional doses of dewormer for your pet about 3 weeks after an initial deworming, unless your pet receives a dose of heartworm preventative on a regular basis.
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GREYHOUNDS AND HOOKWORM RESISTANCE
This is where the story gets very complicated for greyhounds – and for greyhound owners! For several years, it has been noted that greyhounds adopted from racing facilities in Florida very often came with large hookworm infestations, of the species Ancylostoma caninum. These dogs would have undergone regular deworming during their racing careers; however, they would arrive at adoption facilities and be found to be shedding very large numbers of hookworm eggs in their feces. Even after adoption and multiple rounds of deworming, the numbers of hookworm eggs remained high.
While it is known that these greyhounds continue to shed due to larval leak syndrome, it is not definitively known why such great numbers of hookworms are able to persist and recur in this particular breed. Is it a result of their history of being in highly contaminated environments, such as greyhound breeding facilities and racing kennels? Is it a result of abnormally elevated dormant larvae encysted in their bodies? Do greyhounds have decreased immune function, leading to compromised ability to fight parasite infestations? Has the Ancylostoma caninum hookworm developed a resistance to the deworming products that are routinely used in racing and breeding facilities?
The honest answer is: researchers are not entirely sure. There is some recent evidence indicating that the persistent hookworms in greyhounds are multidrug resistant (Jimenez Castro, et al., 2019). It may be a combination of any or all of the above, but parasitologists agree that more research is needed to find the answers.
At Lawndale Veterinary Hospital, we see a substantial number of greyhounds that are hookworm positive on adoption and continue to test positive for hookworm eggs, even after multiple doses of a variety of dewormers. There have been a number of different protocols developed using different deworming products on an alternating schedule. Although there is no single, reliable protocol for treating these patients, there are combinations that work and result in cleared infestations. The frustrating part of this issue is that not every individual responds to the same protocol and it takes time to determine the right combination of medications to solve the problem for a particular individual.
Photo: Pixabay.com
IMPORTANCE OF MONITORING AND TREATMENT
If you have a greyhound with persistent hookworms, it is important to find the combination of medications that work for your pet – we can help. However, no matter what kind of dog or cat you own, it is important that you deworm your pets regularly. The cost of hookworm infestation can be high, with consequences such as profound, sometimes even fatal, anemia; chronic diarrhea; weight loss; and more.
There are public health consequences to consider, as well. Hookworm larvae can burrow through the skin of humans (usually across the skin of bare feet) and attempt to migrate to the intestine, resulting in a condition called cutaneous larval migrans. Some larvae may even migrate across the cornea, resulting in ocular larval migrans.
It is of utmost importance that pet owners clean up after their companions to decrease environmental contamination and decrease the chances that pets and their humans become infected while out enjoying dog parks, trails, and even their own back yards. Annual fecal evaluations, with recheck fecals as needed, are a necessary part of your pet’s yearly exam; don’t skip these important tests. And, finally, keep your pet on a heartworm preventative every 30 days, for life. These monthly medications not only help prevent heartworm infestation, they also provide protection against the most common intestinal parasites in dogs and cats (and some of them are even flea preventatives, as well!).
REFERENCES:
General Biology and Life Cycle:
https://capcvet.org/guidelines/hookworms/
Resistance:
Pablo D. Jimenez Castro, Sue Howell, John. J. Schaefer, Russell. W. Avramenko, John. S. Gilleard, Ray M. Kaplan. 2019. Multiple Drug Resistance in the canine hookworm Ancylostoma caninum: an Emerging Threat. https://www.biorxiv.org/content/biorxiv/early/2019/06/21/676007.full.pdf
Greyhound Hookworm Information:
http://greyhoundcrossroads.com/index.php?page=resistant-hookworms