Top 3 myths veterinarians believe about parasites

Some species of ticks are active and questing for hosts in the cooler weather of fall and winter, contrary to popular belief. | Shutterstock.com

You talk with your clients all day long about the risks of vector-borne disease and why parasite preventives should be used year-round, so you're pretty well-versed when it comes to parasitology-right? Maybe it'd surprise you to know there are some facts that trip up even the most experienced veterinarians. Check your memory as we look at the most common parasite myths I've heard from veterinarians' own lips.

Myth No. 1: Heartworm preventives are able to kill adult heartworms in infected animals.

This is false. Macrocyclic lactones kill only third- and fourth-stage larvae of Dirofilaria immitis. This means a heartworm preventive eliminates larval forms that have been acquired by the pet since the last dose was administered a month earlier. Once the D. immitis parasite reaches the adult stage in dogs, the only approved and effective drug that kills it is melarsomine dihydrochloride.

This underscores three important points: (1) monthly administration of heartworm preventive is imperative for preventing heartworm disease, (2) the preventive should be given year-round, regardless of geographical location or perceived risk, and (3) “slow-kill” adult treatment regimens (giving a preventive that controls larvae but is ineffective against adult worms) should be avoided for many reasons, including increased risk of selecting for D. immitis strains that are resistant to preventives.

Myth No. 2: Microfilaria identification testing should be performed only in dogs with a positive heartworm antigen test results.

This is also false. Recent research from the American Heartworm Association has demonstrated that veterinarians should test for microfilariae using a concentrating method such as a Knott's test or filter test in routine exams as well as in dogs with suspected heartworm disease. The antigen test detects the adult female uterine antigen and, as with all serology testing, is subject to false-positive and false-negative results. This is due to a number of reasons, including positive and negative predictive value, laboratory error and available antigen within the blood of infected dogs. Given this information, experts are now recommending antigen and microfilariae testing on a routine basis and in suspected cases.

If microfilariae are present, it's imperative to distinguish D. immitis microfilariae from those of the nonpathogenic nematode Acanthocheilonema reconditum. This can be performed by measuring the length and width of the parasite, as well as observing its anterior and posterior regions. This can be done in-clinic as long as a micrometer from a microscope eyepiece is available. Reference laboratories can do it as well. Typically D. immitis has a tapered anterior region, whereas A. reconditum has a blunted anterior end. In addition, A. reconditum has a curved to spiral tail, often forming what is referred to as the head of a shepherd's staff. This information, along with clinical signs and imaging techniques, can greatly assist veterinarians in making the proper diagnosis.

Myth No. 3: Ticks are active and questing for hosts only during spring and summer.

Once again: false. Although ticks may appear to be most abundant in specific geographical areas during the warmer parts of the year, certain species are active during fall and winter. One example is the black-legged tick (Ixodes scapularis), which is a known vector for pathogens including Borrelia burgdorferi (the causative agent of Lyme borreliosis), Anaplasma phagocytophilum and Babesia microti.

In most temperate climates where I. scapularis exists, the adult stage emerges in autumn and can be found questing or attached to hosts throughout winter. Adult I. scapularis may parasitize domestic and wild animals as well as people. It's important for veterinarians to educate their clients about the need for year-round tick prevention for their pets as well as routine examination of pets for ticks to minimize pathogen transmission.

Now that these myths are busted ...

You're even more prepared to discuss the importance of year-round prevention with veterinary clients. And if these myths didn't surprise you, you can feel confident in your smartest-veterinarian-in-the-room status.

Richard Gerhold, DVM, MS, PhD, works in the Department of Biomedical and Diagnostic Sciences in the College of Veterinary Medicine at the University of Tennessee.