Just Ask the Expert: The patient is receiving preventives, but still picked up worms?

We have a patient that is receiving a monthly ivermectin/pyrantel preventive for heartworm, ascarids and hookworms. The owner says that for the four years she has owned him, she's never missed giving him a dose. He's been experiencing periodic diarrhea, and his fecal test shows he's picked up hookworms somewhere. Is the preventive not effective and is the dog now at risk for heartworm infection? If the owner is unsure of the source of the infection, how do we ensure that he won't pick them up again?

Given that hookworms have about a two- to three-week prepatent time, the most likely explanation for this case is that the dog is ingesting the larvae from the environment and then eggs appear in feces after two to three weeks. Since the preventive is given every month, there is enough time for a dog to be infected and shed eggs in between preventive doses. Also, some hookworm larvae can arrest in muscle. They will remain in the muscle until dog is treated (which only kills the adult hookworms in the intestinal lumen). Only after the adult hookworms are killed will the arrested larvae develop into adults, leading to egg shedding.

The preventive effectiveness and the continued contamination of the premises of hookworm larvae are two separate issues. The monthly preventive drugs hit a high level of effectiveness soon after they are administered, and then a few days later the drug levels drop below a therapeutic level. During the period the drugs are at or above the therapeutic level, the compounds will kill the adult worms and any recently ingested larvae. However, the arrested larvae will not be killed. Once the drug levels fall below the therapeutic level, the arrested larvae can enter the gastrointestinal (GI) lumen, develop to adults and shed eggs. In the case of GI parasites, there needs to be both environmental control of the parasites in addition to the monthly preventive or treatment. The only way to determine if there is drug failure is to do a fecal exam seven to 10 days after administration of monthly preventive. If eggs are seen during this period-and they are indeed hookworm eggs-it would suggest there is incomplete protection of the compound.

Care should be given to try and break the life cycle by changing the area where the dog is housed if it is in an outside kennel, since the kennel environment may be contaminated. Dogs should also be stopped from eating the fecal material of other dogs, which could also lead to infection. Although resistance to pyrantel may occur, this is less likely. It would be worth performing a fecal float exam approximately 10 days after treatment to determine if there is shedding. If shedding of eggs is occurring at this point, it may suggest that the hookworms may be resistant to pyrantel.

One final consideration should include determining if dog has been ingesting wild or domestic ungulate feces. The eggs of the GI nematodes of ungulates may be confused with hookworm eggs. If these are GI nematode eggs, they are just passing through the dog and are not infecting the dog.

There is no evidence to suggest that the dog may be at a higher risk for heartworm infection. However, all dogs, regardless of which preventive is administered, should be tested yearly for heartworms using an antigen test in conjunction with a concentrating microfilariae test (Knott's or filter test).

Richard Gerhold, DVM, MS, PhD

Department of Biomedical and Diagnostic Sciences

College of Veterinary Medicine

University of Tennessee