Worm Prevalence in Stray Dogs and Cats Dropped in Drought-Affected Areas
Worm risk in dogs and cats is not always predictable from season to season — and a new observational study suggests that large climate events may be part of the reason why. Researchers in southern Thailand observed stray dogs and cats through the 2014–2016 El Niño event, which brought a 14-month drought, and found that soil-transmitted helminth prevalence decreased after the drought in affected areas among animals that were not receiving deworming treatment. Geohelminths are soil-transmitted parasitic worms — including hookworms, Strongyloides (strongyloidiasis), Toxocara (toxocariasis), and Spirometra species — that can infect animals through contact with contaminated environments.
The finding reinforces something vets already emphasize: locally informed, regular fecal testing and prevention matters more than assuming a fixed parasite season.
Which Parasites Were Measured in This Study?
“Geohelminth” is a scientific term for parasitic worms that spend part of their life cycle in soil. They are among the most common internal parasites in dogs and cats worldwide.
Here is a simple breakdown of the organisms tracked in this research:
- Hookworms (Ancylostoma species) — small worms that attach to the intestinal lining and feed on blood. Can cause anemia in puppies.
- Strongyloides (strongyloidiasis) — tiny intestinal worms that can be hard to detect and may persist in contaminated environments.
- Toxocara (toxocariasis) — roundworm-type parasites that can spread via eggs shed in stool.
- Spirometra species — tapeworm-related parasites tracked as part of the surveillance findings.
These worms pass eggs in an infected animal’s stool. The eggs land in soil, develop to an infectious stage, and can then be picked up by other animals — or, in some cases, people — through contact with that soil. Think of them as a chain: infected animal → contaminated ground → new host.
What the El Niño Study Found
El Niño is a large-scale weather pattern that periodically warms parts of the Pacific Ocean. That warming shifts rainfall and temperatures across wide regions — changes that ripple all the way down to the soil conditions where parasite eggs and larvae survive.
This study observed stray dogs and cats during and after the 2014–2016 El Niño in southern Thailand and tracked geohelminth prevalence in animals that were not receiving anthelmintic treatment (the class of medications used to kill internal worms). Because these animals had no deworming treatment in place, researchers could observe how parasite prevalence shifted around the climate event itself, without treatment masking the signal.
The key findings were directional and area-specific. In stray cats from the unaffected wet sandy-soil area, prevalence was unchanged before and after the drought. In the drought-affected dry sandy-soil area, hookworm prevalence in stray cats decreased after the drought; strongyloidiasis was unchanged in 2016 but decreased by 2020 compared with 2014; and toxocariasis did not change. In stray dogs from study area 2, hookworm prevalence decreased after the drought, while strongyloidiasis and toxocariasis were eliminated. Spirometra spp. prevalence stayed unchanged in stray dogs and in cats from the unaffected area.
This was an observational study, meaning researchers watched and recorded what was happening rather than running a controlled experiment. That design is useful for identifying patterns and associations, but it cannot prove that El Niño directly caused the worm changes — other environmental factors could also have played a role.
Why This Matters for Pet Owners
Local Conditions Shape Parasite Risk
The main takeaway for everyday pet owners is that parasite risk is not the same everywhere, or every year. Regional climate, weather patterns, local soil conditions, and the density of stray animals all influence how much exposure your dog or cat has to soil-transmitted worms.
A rainy, warm season can create ideal conditions for worm eggs and larvae to survive in soil longer. A dry period might reduce survival. Large climate swings — like those associated with El Niño — can scramble those expectations further.
This is why blanket assumptions like “my area doesn’t have many parasites” or “the season hasn’t started yet” can miss the mark. Conditions change.
Regular Fecal Testing Is the Most Reliable Way to Know
A fecal test (also called a fecal exam or stool test) is a simple test your veterinarian runs on a small stool sample. It checks for worm eggs or larvae that your dog or cat may be shedding — even when they look perfectly healthy.
Many pets with low-level worm infections show no obvious symptoms. They eat normally, play normally, and seem fine. But they may still be passing worm eggs into your yard or home environment.
Your vet can recommend how often to test based on your pet’s lifestyle, your region, and local parasite trends.
Deworming Plans Should Reflect Where You Live
There is no single universal deworming schedule that fits every pet in every location. What makes sense in a dry, arid climate may not be right for a humid, tropical region. And as this study suggests, what worked during a typical year may not apply during an unusually warm or wet one.
Discuss your pet’s deworming plan with your veterinarian — especially if you live in an area prone to climate swings, if your pet spends time outdoors in soil or grass, or if your pet has contact with stray or free-roaming animals.
The Human Health Connection
Several geohelminths are zoonotic — meaning they can spread from animals to people. Toxocara larvae can cause a condition called visceral larva migrans (larvae migrating through body tissues) in children who accidentally ingest contaminated soil. Hookworm larvae can burrow through human skin — a condition called cutaneous larva migrans, or skin-wandering larvae — in people who walk barefoot in contaminated areas.
This makes regular parasite prevention in pets a household health issue, not just a pet health issue. Simple habits help:
- Pick up pet waste promptly
- Wash hands after handling soil or pets
- Keep sandboxes covered when not in use
- Keep children away from areas where stray animals may have defecated
Asking Your Vet About Parasite Prevention
This study is a reminder that parasite control works best when it is tailored to local conditions — and those conditions can shift with the climate. Good questions to bring up at your next vet visit:
- How often should my dog or cat have a fecal test?
- Is there a deworming plan that makes sense for where I live?
- Are there any local parasite trends I should know about this year?
- Could recent weather shifts affect my pet’s parasite exposure?
Your vet has access to local and regional parasite surveillance data that can help personalize your pet’s prevention plan.
Study Limitations
This was an observational study of stray dogs and cats during one climate event. The stray population studied may differ from household pets that receive regular veterinary care. The findings reflect the population and region studied; parasite patterns in other areas may vary. Because this is observational research, it shows associations — not direct cause-and-effect relationships.
The Bottom Line
An observational study found that, in southern Thailand, a 14-month El Niño drought was associated with lower soil-transmitted helminth prevalence in drought-affected stray dogs and cats, while prevalence in cats from an unaffected wet-soil area remained unchanged. Soil-transmitted gut worms appear sensitive to the kind of environmental shifts that large climate events bring.
For pet owners, the practical message is clear: don’t assume parasite risk is constant. Climate variability may shift it. Regular fecal testing and a locally informed prevention plan — guided by your veterinarian — is the most reliable way to protect your dog or cat throughout the year.
This article summarizes peer-reviewed research for educational purposes. Always consult with your veterinarian for personalized advice about your pet’s health and parasite prevention.