The Danger
Here’s what happened: One in three dogs entering shelters in Northern Italy carried filarial worms, which are parasites transmitted by mosquitoes. Of these, 15.7% were actively infected with heartworm (Dirofilaria immitis), and 6.9% had the skin worm D. repens. Dogs acquired these infections before shelter intake and can transmit the parasites quickly if not screened and treated. Mosquitoes spread heartworm from untreated dogs to others in the same area within a single season.
Who’s at risk
Specific numbers show: In a survey of 510 stray or surrendered dogs at shelter intake, about one in three harbored filarial worms. This means thirty-three out of every one hundred shelter dogs in Northern Italy act as a parasite reservoir. Dogs living near high stray populations—especially in regions like the Po Valley—face increased exposure even when kept indoors. If you adopt or foster a dog from a shelter, you should consider these risks before introducing the animal to your home. Untreated strays can maintain ongoing transmission in the local dog community.
What the study found
Shelter intake screening revealed that 15.7% of dogs tested positive for heartworm (D. immitis). This means nearly one in six dogs had active, transmissible infection. In plain terms, your dog could contract heartworm from a single bite if exposed to this population. Skin worm D. repens infected 6.9% of dogs on arrival. This means almost seven in one hundred dogs carried another mosquito-borne parasite. Co-infection with both parasites occurred in 11.4% of cases. Practically, this suggests that many dogs in the same shelter environment may carry multiple parasites simultaneously, increasing transmission risk. Unchecked stray populations serve as a reservoir for these parasites. This means local mosquito populations can pick up and spread infection even if most owned dogs receive preventive care. Owners who skip monthly heartworm prevention or delay shelter intake screening risk exposing their dog to these threats. In two mini-scenarios, a newly adopted shelter dog tested positive for heartworm at its first vet visit, requiring expensive treatment; another owner delayed starting prevention for their indoor-only dog, which later developed heartworm after a single mosquito bite from a stray-infected area.
Signs to watch for
Watch for:
- Cough lasting longer than five days, especially when exercise increases it
- Sudden reluctance to run or walk distances your dog normally tolerates
- Abdominal swelling measurable as a visible roundness or distension
- Labored breathing observed at rest or with minimal activity
- Weight loss greater than one pound in two weeks without dietary change
- Pale gums when checked in daylight, indicating possible anemia
- Unexplained skin lumps or nodules persisting more than seven days
- Visible fatigue after short walks, not previously seen
Preventing exposure
Prevent exposure by keeping your dog on year-round heartworm prevention, regardless of local incidence reports. Mosquitoes can transmit filarial worms from untreated shelter or stray dogs to yours within weeks. Administer preventive medication on the same date each month, without skipping doses. If you adopt a shelter dog or foster a stray, request a heartworm and skin worm screening before bringing the animal home. This step reduces risk to other pets in your household.
Indoor dogs are not exempt from risk. Mosquitoes enter homes through open doors, torn screens, or during summer months when windows stay open. Even one bite from an infected mosquito can transmit heartworm. Check for standing water sources around your home—birdbaths, buckets, and clogged gutters attract mosquitoes and should be emptied weekly. If you board your dog or take them to group play, confirm the facility requires heartworm prevention for all dogs.
Discuss preventive options with your veterinarian, especially if you live in or travel to regions with high stray populations. Ask about annual screening tests, as some infections remain undetectable for months. If you delay starting prevention after adoption, you risk exposing both your new and existing pets to filarial worms. Owners who maintain prevention and screen new arrivals reduce parasite transmission in their home and community.
When to call your vet
- Persistent cough lasting longer than seven days—call within 24 hours and mention possible heartworm or D. repens exposure
- Sudden abdominal swelling noticed overnight—schedule an urgent appointment and request heartworm testing
- Visible skin nodules present for more than a week—ask for D. repens screening at your visit
- Refusal to exercise or walk distances previously tolerated—call within 48 hours and describe recent shelter or stray contact
- Weight loss exceeding one pound in two weeks—request immediate parasite screening
- Pale gums detected during a daylight check—call within 12 hours and mention possible anemia from filarial infection
- Difficulty breathing at rest—seek emergency care and ask for heartworm evaluation
Study limitations
- Survey limited to shelter intakes in Northern Italy’s Po Valley region
- Only stray or surrendered dogs screened; owned dogs not included
- No data reported for seasonal mosquito activity or incidence outside shelters
- Asymptomatic carriers complicate detection without laboratory screening
- What we don’t know yet: prevalence rates in other regions or among owned dogs
Bottom line
Start heartworm prevention and screen new shelter intakes before introducing them to your home. Dogs kept on monthly prevention and tested before arrival avoided infection and secondary transmission.
Quick steps
Check your dog’s monthly prevention schedule and set reminders for every dose. Ask your veterinarian to test for heartworm and skin worm when adopting or fostering a shelter dog. Inspect screens and doors for gaps, repairing tears before mosquito season starts. Remove standing water from your yard twice a week and use pet-safe mosquito repellents when outdoors. Confirm group dog facilities require prevention for all attendees before booking. Review your pet’s weight and exercise tolerance every week, noting sudden changes for your vet. Keep a written record of all parasite screenings and medication dates for reference during emergencies.
Disclaimer
This brief is for educational use and does not replace direct veterinary care. Consult your veterinarian for diagnosis, prevention, and treatment tailored to your dog’s health and exposure risks.
