Different canine gastrointestinal parasites peak at different times of the year, but not all dogs have year-round protection. A recent study, which shows seasonality of canine whipworms for the first time ever, helps drive home the importance of year-round parasite control strategies (1).

Seasonality of Worms in Dogs

In the U.S., hookworm, whipworm, and roundworm are the three most common gastrointestinal nematodes found in dogs. After analyzing changes in prevalence and seasonal fluctuations for these particular GI nematodes from 2012 to 2018, researchers determined that the prevalence of both whipworm and roundworm peaked in the winter, while prevalence of hookworm peaked in the late summer and early autumn (1).

“While seasonality of whipworm and roundworm was surprising, the higher prevalence in the winter provides support for the need of year-round protective measures against a wide variety of parasites,” says one of the study’s authors, Dr. Jason Drake, DVM, DACVM-Parasitology, Elanco Animal Health.

Though the study was specific to the U.S., these species of worms are seen all around the world in dogs. “It is likely that seasonality is seen in these worms, but the actual timing of peak prevalence may vary by location,” Drake says. “Since dogs are threatened by a wide variety of parasites, year-round use of broad-spectrum parasite protection protocols are likely warranted in many parts of the world.”

Diagnosis, treatment and control of intestinal parasites have been a hallmark of veterinary preventive care for a long time. By evaluating the seasonal prevalence of worms, we can enhance our ability as veterinarians to control internal parasites.

Whipworm Prevalence in Dogs

According to the study, whipworm infections in dogs peak in the winter and are lowest in May and June. A number of factors are likely responsible for the seasonality of whipworms, Drake says.

First, we should consider the whipworm life cycle in dogs. When a dog ingests embryonated whipworm eggs and becomes infected, larvae hatch from eggs in the gastrointestinal tract, penetrate the mucosa, and develop for 2 to 10 days. They then move to the cecum and mature into adult worms. Adult whipworms release eggs inside the dog’s large intestine. Once the prepatent period has elapsed (74 to 90 days), the infected dog will pass unembryonated eggs in his feces and contaminate the soil. The eggs will embryonate in 9 to 21 days, putting other dogs at risk of infection. (2).

“Dogs may be exposed to eggs in the soil while spending a higher amount of time outdoors during summer, then not begin shedding eggs for three months after infection,” Drake says.

The discontinuation of broad-spectrum protection during the winter may also play a role in the seasonality of whipworms. “The seasonality seen in dogs may be amplified by drops in parasite [protection] during colder months of the year, if people stop giving heartworm preventives that also provide whipworm protection during the winter,” Drake says.

Whipworms can be difficult to eradicate because of their life cycle and their stability in the environment. Infective whipworm eggs are hardy and can survive for years. Once an environment is contaminated, it will continue to be a source of infection and re-infection. Whipworm infections are also challenging for us to diagnose in dogs.

“Whipworm eggs are quite dense compared to roundworm and hookworm eggs, so they are more difficult to float using passive floatation in sodium nitrate solution,” Drake says. “Centrifugation with sucrose solution with a specific gravity of 1.25 is more sensitive, but whipworms produce a relatively small number of eggs compared to roundworms and hookworms, and may not produce eggs every day.”

In fact, one study reported that one-third of whipworm infections were missed using traditional, passive floatation fecal examination (3). The recent availability of fecal antigen testing for whipworms (as well as hookworms and roundworms) should help improve our ability to diagnose infections. Reference laboratories also provide floatation with centrifugation, which is also better than passive float.  

“Fecal antigen testing is able to detect protein produced by whipworms, even if eggs are not being produced at the time,” Drake explains. “A combination of fecal antigen testing and centrifugal floatation provides better sensitivity for detecting whipworms than fecal floatation alone.”

Roundworm Prevalence in Dogs

Roundworms follow a similar seasonal pattern to whipworms, with peak prevalence in winter and lowest prevalence during the summer. “This provides additional support for the need to continue parasite control through the winter and not stop parasite protection as the temperatures drop,” Drake says.

Because of their ability to transmit in utero or via nursing, roundworms are commonly diagnosed in puppies. The seasonality of roundworms could partially be associated with changes in the population signalment (like age or breed) each month, if certain times of year have higher populations of puppies than others. However, the intestinal parasite data analyzed for the study did not include signalment information, Drake points out.

“While roundworms are common in puppies as they are passed to puppies by their mothers, whipworms are not passed directly to offspring and must be acquired from the environment, yet both roundworm and whipworm peak in prevalence in the winter,” Drake adds.

Hookworm Prevalence in Dogs

Unlike whipworms and roundworms, hookworms are diagnosed most often in the late summer and early autumn. Factors like climate and weather changes may have an impact on hookworm prevalence. Their seasonality is mostly based on the nematode’s preference for moisture and warm weather.

“Hookworm larvae hatch from eggs in the environment, and the larvae are then infective to dogs. Warm, humid weather helps these larvae survive, while cold, dry environments are detrimental to larval survival,” Drake explains.

Hookworms are transmitted through the fecal-oral route or direct penetration through skin or mucous membranes. Approximately 10 to 21 days after infection, dogs start to release eggs in feces, which mature to infectious filariform larvae in about a week. Once ingested, the larvae mature into adult worms or can enter into tissues where they can lay dormant in a larval stage for a period of time as well.

“It can take less than three weeks from the time a dog becomes infected until they are passing eggs in their feces, so seeing highest prevalence in the fall, at the end of the warm season, makes sense based upon the biology and life cycle of hookworms,” Drake says.

The Need for Broad-Spectrum Parasite Protection

While several factors exist as to why we may be diagnosing more roundworms and whipworms in the winter and more hookworms in the summer and fall, the seasonal prevalence is most important in its impact on your parasite control protocols. 

Year-round protection with a broad-spectrum deworming product is imperative for controlling intestinal parasite infections in dogs. The recent proof of seasonality of whipworms should add more weight to the conversation when we discuss year-round broad-spectrum protection protocols with our patients.

By recommending a suitable broad-spectrum dewormer that covers multiple types of worms (including roundworm, hookworm, and whipworm, as well as heartworm and tapeworm), we can help pet parents protect their dogs (and themselves) all year long.