Heartworm in Dogs in India: Where It Is, How It Is Tested and Why the Test Misreads

Signs of Canine heartworm disease (Dirofilaria immitis)

The 8 signs to watch for are listed below. Any one of them is a reason to consult your nearby vet; none of them is a diagnosis.

• Nothing at all, which is what most infected dogs show
• A mild cough that does not clear
• Tiring on a walk the dog used to manage easily
• Going off food and slowly losing weight
• Laboured breathing, especially after exertion
• A swollen belly from fluid, in advanced disease
• Fainting or collapse, which needs a vet the same day
• Dark, coffee-coloured urine with sudden breathlessness

Breeds on this site that document this condition

Heartworm is not a breed disease and no Indian study has found a breed effect, so these six cards are not a ranking of genetic risk. Every figure below measures exposure. Two of the cards, the Podenco Canario and the Warren Hound group page it belongs to, report the same Gran Canaria dogs over two study periods and are one measurement rather than two; they are both here because both pages on this site carry the figure. Where a breed reads high, the reason the authors give is an outdoor life in a mosquito climate with no preventive, and the one study that tested breed as a variable directly found it did not predict infection while age and a coastal address did. The two Indian entries carry case-mix counts rather than within-breed rates, and each card says which it is.

What heartworm disease is, and the five other worms an Indian smear can show

Heartworm in dogs is infection with Dirofilaria immitis, a worm that a mosquito injects as a larva and that grows into an adult living in the pulmonary artery and the right side of the heart. It takes roughly six months from the bite for a larva to become a breeding adult, and adults then live five to seven years in a dog. In India that worm shares a dog's bloodstream with at least five other filarial species, and their larvae are hard to tell apart.

Six species are now documented in Indian dogs. A geographically stratified survey of 525 stray and refuge dogs across four climate zones found Dirofilaria immitis, Dirofilaria repens, Acanthocheilonema reconditum, a novel Acanthocheilonema species and Microfilaria auquieri, the last two reported from India for the first time in that study. Brugia species are the sixth group, recovered from lymphatics rather than from the heart. Only one of the six causes heartworm disease.

The difference matters because most of them do almost nothing to a dog. The Indian vector-borne review states plainly that most infections with D. repens, Acanthocheilonema species and Brugia species are of minimal veterinary clinical significance, while D. immitis is the most pathogenic canine filarid. So a dog with microfilaria in its blood may be carrying a parasite that will kill it, or one that will never trouble it, and the two look similar down a microscope.

How dogs get heartworms, and why a flat in a city is not a defence

Two standard references disagree about how long an adult worm lives, and the gap matters for how fast an untreated burden builds. The American Heartworm Society states that heartworms can live for 5 to 7 years in dogs and up to 2 or 3 years in cats. The MSD owner page states that heartworms typically live for 3 to 5 years in an animal's body. Both are current pages from bodies that write the guidelines, and this page publishes both rather than picking one.

A dog gets heartworms from a mosquito bite and from nothing else. A female mosquito takes up larvae from an infected dog, the larvae mature inside the insect over ten to fourteen days, and the next bite deposits them on the skin of a new dog, where they enter through the bite wound. Several genera of mosquito are competent vectors for both Indian Dirofilaria species, including Culex, Aedes and Anopheles, and all three are present across India.

That is the reason indoor life is only a partial answer. The 2010 Indian survey made the point directly, noting that competent vectors for D. immitis belonging to Culex, Aedes and Anopheles are present all over India, so the accepted view that heartworm is confined to the north-east is questionable on vector grounds alone. A Kerala study did find an effect from keeping a dog inside, and it is covered in the prevention section below with its numbers.

The worms that share the blood smear

Five filarial parasites of Indian dogs are listed below with what each one does and where it sits, because a lab report that says only 'microfilariae seen' has not yet answered the question that matters.

One line in that table is the whole problem in miniature. In the Gujarat series of 418 dogs, 33 were microfilaraemic, and 23 of those 33 carried Acanthocheilonema reconditum against 10 carrying Dirofilaria immitis. A microfilaria found in a Gujarat dog was more than twice as likely to be the harmless worm as the dangerous one.

How likely a dog is to get heartworm in India, state by state with denominators

How likely a dog is to get heartworm in India depends almost entirely on where it lives. In 782 dogs from Guwahati and Aizawl the antigen test was positive in 18.03 percent, while in 162 Mumbai dogs, 100 Ladakh dogs and 164 Kerala dogs the heartworm was found in none at all. A national average would hide the only fact an owner can act on.

The table below holds every Indian denominator this page could open. Read the population column before the percentage, because a street dog in Guwahati and a pet dog in Palakkad are not the same measurement.

How common heartworm is in Indian dogs is therefore a question with at least four answers, and the honest one names the place before the number. In the two north-eastern cities it is common, at 141 of 782 dogs by antigen test. In Mumbai, Ladakh, Cherthala and across 230 Tamil Nadu strays it was not found at all Ravindran R, Varghese S, Nair SN, Balan VM Manoj RRS, Iatta R, Latrofa MS, Capozzi L.

The north-eastern numbers deserve a second look because they separate the dogs. In Guwahati the antigen test was positive in 63 of 223 stray dogs, 28.25 percent, against 17 of 174 pet dogs, 9.77 percent, and 9 of 91 working dogs. In Aizawl the same split held, 44 of 190 strays against 7 of 92 pets. A pet dog in Guwahati is at roughly a third the risk of the street dog outside its gate, and that is still about one dog in ten.

Where the heartworm has not been found, and why that is a real finding

Four Indian populations were screened carefully and produced zero heartworms. Mumbai returned 0 of 162 for D. immitis while returning 16.7 percent for D. repens in the same dogs and the same assay, so the negative is not a failure of the method. Kerala's Cherthala survey found a very high 42.68 percent microfilaraemia in 164 dogs and states that D. immitis was not detected in any of the samples examined. Tamil Nadu's 230 stray dogs produced two filarioids in total, one Dirofilaria sp. hongkongensis and one Brugia malayi, each 0.4 percent, and no heartworm.

The tropical parasite council states the position in one sentence. Dirofilaria immitis is endemic to Bangladesh and the northern half of India, and autochthonous cases of heartworm have not been reported in India's south or in Sri Lanka. The 2010 Indian survey draws the map from the other end, describing a distribution that extends from the Pakistani border in the west, through Delhi and Sikkim in the north, to the Burmese border in the east and Orissa to the south.

The pooled Indian percentage you will meet, and why it is not your dog's risk

One number circulates widely and it needs unpacking. The systematic review of India and Sri Lanka reports a crude pooled prevalence for D. immitis in India of 9.7 percent, 225 of 2,318 dogs, and for D. repens of 8.1 percent, 229 of 2,814 dogs, with D. immitis absent from Sri Lanka entirely. Those are the figures with denominators attached, and they are the ones to quote.

The same paper also prints a pooled random-effects estimate of 35.8 percent for D. repens, and that figure is the one that travels. It is a two-country pooled estimate dominated by Sri Lanka, where D. repens reached 51.7 percent of 315 dogs, against 8.1 percent of 2,814 in India. Reading the 35.8 percent as an Indian rate overstates the Indian figure more than fourfold.

Three internal inconsistencies in that review are worth recording rather than smoothing over. Its abstract gives D. immitis as 9.7 percent with a confidence interval of 8.5 to 11.0, which is the crude figure from its own Table 6, while its results section gives a meta-analysed 9.1 percent with an interval of 4.3 to 13.9. Its India country-level estimate is printed as 23.2 percent with a confidence interval of 46.8 to 84.8, an interval that does not contain its own point estimate. And its geographical table is a distribution of reported cases rather than of risk: Kerala supplies 160 of 454 Indian dog records, 35.24 percent, and Assam 157, so that table maps where Indian scientists have looked rather than where Indian dogs are infected.

Another number that circulates has a similar problem in the other direction. A post-mortem survey of 240 indigenous dogs at a north-eastern slaughterhouse found 34 percent harbouring heartworm, with 35 percent of those infections non-patent and none of the animals showing overt clinical signs on ante-mortem inspection. That is a real and alarming figure for free-roaming north-eastern dogs, and it is not a figure about a vaccinated, dewormed pet in Pune.

Heartworm symptoms in dogs, and why most infected dogs in India show none

The commonest presentation of heartworm in dogs is no presentation at all, because most infected dogs are subclinically affected and the earliest sign, when one appears, is a cough. The Indian evidence is blunt about this. At post-mortem in a 240-dog north-eastern survey, 34 percent were carrying heartworm and not one had shown overt clinical signs on inspection beforehand.

When signs do appear they arrive in a predictable order. The American Heartworm Society lists a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite and weight loss as the early set, progressing to heart failure and a swollen belly from fluid in the abdomen as the disease advances. The professional manual adds laboured breathing, fainting and wasting to the more severe picture.

Two things break the intuition that a sicker-looking dog has more worms. The first is that severity tracks the dog's activity as much as its worm burden, which is why an active working dog can look worse than a sedentary one carrying the same infection. The second is that the American society states directly that the severity of heartworm disease does not always correlate with the severity of symptoms, and that dogs with many worms may have few or no symptoms early on.

Four changes are worth a clinic visit this week rather than at the next vaccination.

• A soft cough that has lasted more than a week in a dog with no other respiratory illness, which is the commonest clinical sign when one appears.
• Stopping short on a walk the dog used to finish, since exercise intolerance and fatigue after moderate activity sit near the start of the published progression.
• Appetite loss with slow weight loss over weeks, which the same list places alongside the cough rather than after it.
• A belly that is filling out while the dog is losing condition, because abdominal fluid signals right-sided heart failure rather than weight gain.

Caval syndrome, the one heartworm emergency

One heartworm presentation is a same-day emergency and it has its own name. Caval syndrome is a sudden blockage of blood flow inside the heart caused by a large mass of worms, and the American society describes its onset as laboured breathing, pale gums and dark bloody or coffee-coloured urine, adding that without prompt surgical removal of the blockage few dogs survive. The professional manual lists caval syndrome among the tertiary problems of heartworm disease alongside right-sided heart failure and clotting failure.

That combination is the reason this page does not carry a red emergency banner while still naming an emergency. Heartworm is a disease of months and years, not hours, and the honest framing is that the dog you should worry about today is the one whose gums have gone pale and whose urine has darkened, not the one with a two-week cough. If you see that combination, treat it as an emergency and go now.

There is a second reason the quiet dog matters more in India than elsewhere. Because heartworm stays silent for so long, the dogs found in Indian surveys were overwhelmingly found by a test rather than by an owner, and in the 782-dog north-eastern series the dogs were sampled as a population rather than presented as patients. Waiting for a symptom is a strategy that works only after the damage is done.

Diagnosing heartworm disease in India, and why the simple test has a not-so-simple answer

To diagnose heartworm disease a vet runs two different tests that answer two different questions. The antigen test detects a protein released by adult female worms, and the Knott test or another microfilaria method looks for larvae circulating in the blood. In India, each of them can be wrong in a way the other cannot fix, which is why the one page that has ranked for the Indian query for a decade is titled a simple test with a not-so-simple interpretation.

The Indian numbers show the disagreement directly. Run on the same 782 north-eastern dogs, the three available methods classified 11.38 percent positive by the Knott technique, 18.03 percent by the SNAP 4Dx antigen ELISA and 13.93 percent by PCR. Three tests, one set of dogs, three answers, and the gap between the highest and the lowest is 52 dogs.

What the antigen test detects, and the four ways it reads negative anyway

The antigen test detects a protein released mostly by adult female heartworms, and that single fact explains its blind spots. The professional manual states that because these tests detect only adult female heartworms, a reasonable interval is seven months after the last possible exposure, and that there is no value in testing a dog for antigen or microfilariae before roughly seven months of age. A puppy tested at four months has been tested too early, whatever the result says.

The manual lists the causes of a false-negative result as antigen-antibody complexing, immature or all-male infections, and low numbers of female heartworms, and notes that some specialists now write 'no antigen detected' rather than 'negative' precisely to keep that possibility visible. The Indian post ranking for this query made the same four points in 2015, adding that a dog carrying only adult male worms will produce no antigen at all.

India then adds a category of its own. Occult infection means an infected dog with no circulating larvae, and the north-eastern study measured it at 22.69 percent overall, rising to 60 percent in working dogs and 29.16 percent in pets against 17.75 percent in strays. The authors attribute the higher occult rate in owned dogs to exactly what a responsible owner does, namely regular health checks that often include a macrocyclic lactone whose activity against larvae clears them from the blood while the adults remain.

What a microfilaria test detects, and why it answers a different question

A microfilaria test finds larvae, and larvae are not species. The Indian vector-borne review states that microfilaraemia associated with other filarial infections is commonly detected in blood films of dogs in tropical countries, which necessitates specific identification in order to exclude the non-pathogenic species, and that this requires experienced personnel. The same review records a case in which microfilariae initially identified as A. reconditum were later characterised as D. immitis by molecular methods, so the error runs in both directions.

Two Indian datasets put a size on that gap. In Gujarat, 33 of 418 dogs were microfilaraemic and species identification split them 23 to Acanthocheilonema reconditum and 10 to Dirofilaria immitis. In Palakkad, Kerala, microscopy found microfilariae in 19.61 percent of 102 dogs while PCR confirmed D. repens in only 8.82 percent, so fewer than half the microfilaraemic dogs carried the worm the study was looking for.

The 2010 four-city survey shows the opposite failure, a microscope that sees nothing. Microscopy called 27 of 525 dogs positive, 5.1 percent, while PCR confirmed 139, 26.5 percent, and for the two Dirofilaria species in Delhi microscopy found zero of 162 dogs where PCR found 4.3 percent D. immitis and 4.9 percent D. repens. The authors add a caveat that applies to any Indian pet dog, namely that almost half their Mumbai and Delhi refuge dogs had a history of ivermectin, which suppresses circulating larvae.

The heat-treatment trap, and why it matters more in India than in America

When an antigen test reads negative but the clinical picture does not, the standard laboratory response is heat treatment of the sample. The professional manual explains that heat treatment helps unmask antigen-antibody complexing and may increase sensitivity in low worm burdens or all-male infections. That is sound advice in a country with one Dirofilaria species. India has two.

The experiment has been done. Archived serum from three dogs experimentally infected with Dirofilaria repens was tested on a commercial heartworm antigen kit before and after heat treatment across 404 days, and every heat-treated sample tested false positive from day 164 onward, while no cross-reaction was seen at any time point before heat treatment. The authors conclude that heat treatment of serum in areas co-endemic for D. repens and D. immitis has limited clinical value and should be used with caution.

Delhi is exactly such an area. The 2010 survey found both species in Delhi dogs, 4.3 percent D. immitis and 4.9 percent D. repens, and closed with the recommendation that laboratories use an immunodiagnostic test that will not cross-react between the two species where they co-exist, naming Delhi as the example. Sixteen years later, no Indian owner page carries that sentence.

A second cross-reaction has an Indian angle too. Three commercial in-house heartworm antigen assays were run on dogs with endoscopically confirmed oesophageal spirocercosis in Israel, a country with no local heartworm, and returned positives in 2 of 19, 5 of 35 and 14 of 48 dogs; a further 8 of 32 dogs confirmed free of both Dirofilaria species by PCR still tested positive on the third assay. Spirocerca lupi and D. immitis overlap in India, which is why the Indian post that first flagged this in 2015 flagged it.

A third case shows the two traps combining in one dog. An asymptomatic Spanish Mastiff was referred for heartworm therapy because a screening smear had found microfilariae; three antigen kits all read negative, two heat-treated samples both turned positive, and PCR then identified the larvae as Acanthocheilonema dracunculoides rather than heartworm. Acanthocheilonema species are among the six filarial genera recorded in Indian dogs.

Two rules from the reference manual close this section, and both are about not acting on one result. A positive microfilaria test alongside a positive antigen test is confirmatory of heartworm infection; where the microfilaria test is negative, a second antigen test, ideally using a different type of assay, should be performed for confirmation before adulticide therapy. In an Indian clinic, that second assay is the difference between treating a heartworm and treating a false positive.

Almost none of this reaches an Indian owner through search. The MSD owner version of this topic does name one limitation, that the antigen test cannot identify early infections because it can only identify adult female worms, and it names no other; no second Dirofilaria species, no cross-reaction and no occult infection appear anywhere on it. The most detailed owner explanation available, VCA Canada's testing page, does separate the two tests properly under headings for adult worms and for microfilariae, and it is written for a country with one Dirofilaria species. Neither prints a denominator.

Dirofilaria repens, the worm under the skin that the household should know about

Dirofilaria repens is the second mosquito-borne worm in Indian dogs, it lives in subcutaneous tissue rather than the heart, and it is zoonotic Eravaramkunnath F, Pillai UN, Deepa PM. In the dog it is usually quiet. Infected dogs may develop subcutaneous nodules, itching, dermatitis and in some cases eye lesions, and subcutaneous dirofilariosis is often asymptomatic.

It is also the commoner of the two worms across much of India. Mumbai's 162 dogs returned 16.7 percent D. repens and 0 percent heartworm. Palakkad's 102 dogs returned 8.82 percent by PCR. Older morphology-based surveys reported D. repens at 7 percent of 160 dogs in Kerala and 21 percent of 400 in Karnataka, with no heartworm in either.

For the dog, the practical consequence is that the same monthly products and the same mosquito control cover both worms, because both are mosquito-borne and both are acquired the same way. The Kerala risk-factor study measured what that exposure looks like, finding 8 of 50 outdoor dogs positive, 16 percent, against 1 of 52 indoor dogs, 1.9 percent, with a chi-square of 5.269 and p equal to 0.021, while age, sex, breed, fever, deworming and vaccination status showed no significant association.

The household question, answered once and then handed to a doctor

People can be infected by this worm, and that is a fact about mosquitoes rather than about your dog. The Indian review states that D. repens can accidentally infect humans, causing subcutaneous dirofilariasis, that it is considered a re-emerging zoonosis transmitted by mosquitoes, and that the distribution of human cases appears to mirror the distribution of canine cases. Across India and Sri Lanka, D. repens accounted for 224 of 230 reported human cases, 97.4 percent, with the eye the commonest affected site at 98 of 260.

Two things follow, and only two. First, the route runs mosquito to person, not dog to person, so there is nothing to disinfect, isolate or separate at home. Second, if a person in the household has an unexplained migrating lump under the skin or an eye lesion, that is a question for a doctor, not for a vet, and this page carries no human treatment because the reports describe surgical removal by a physician.

Kerala is where the Indian human picture is clearest and where it has changed. A review of two Kerala tertiary hospitals documented 78 cases of human dirofilariosis between January 2005 and March 2020, with the orbit, eyelid and conjunctiva the commonest sites, and molecular work identified five human isolates and one dog isolate as Candidatus Dirofilaria hongkongensis rather than classical D. repens. The authors propose that all Dirofilaria isolates be sequenced for identification.

That third species is not a footnote for dog owners either. The Tamil Nadu panel of 230 stray dogs found Dirofilaria sp. hongkongensis in one dog and the systematic review of the region warns of the emergence of unidentified Dirofilaria species or the introduction of exotic ones, citing Tamil Nadu as the recent example Thilakarathne SS, Yuen NKY, Hassan MM. Three Dirofilaria species in Indian dogs, one blood test built for one of them.

Human diagnosis has its own blind spot worth naming, because it explains why the Indian human numbers look the way they do. Humans are suboptimal hosts for D. immitis, so the chance of finding larvae in the peripheral circulation is negligible and diagnosis is harder. Across the two-country human dataset, 46 percent of 396 diagnoses were made by surgical excision and only a fifth by histopathology, and histopathology alone cannot reliably separate the two species.

Heartworm treatment for dogs in general terms, and why it is riskier than the disease looks

Heartworm treatment for dogs is an adulticide course, an antibiotic that sterilises the worms, a monthly preventive and strict exercise restriction, and the risk in that list sits in the last item rather than the first. This page prints no dose and no protocol, because the drug, the amount and the timing belong to the veterinarian who has your dog, its chest radiographs and its other illnesses in front of her.

The shape of a standard course is worth holding even so. The professional manual sets it out as doxycycline, a three-dose melarsomine protocol and exercise restriction, with non-arsenical protocols using a macrocyclic lactone where melarsomine is unavailable or contraindicated. Doxycycline is not there for a bacterial infection in the dog; it kills Wolbachia, a bacterial endosymbiont the worm depends on for development, reproduction and survival, which sterilises the worm and reduces post-treatment complications.

Why exercise restriction is the part that kills dogs when it is skipped

Exercise restriction is not a comfort measure. The manual states that thromboembolic complications can occur in dogs that had no clinical signs at diagnosis, especially when activity is not restricted, and that exercise restriction is crucial to reducing complications regardless of clinical status and of which adulticide protocol is used. After a three-dose arsenical course it specifies crate rest enforced for six to eight weeks after the final injection, with exercise minimised from the day of diagnosis.

The mechanism is simple and it is why the rule is absolute. Dead and dying worms fragment and lodge in the lung vessels, and physical exertion increases the rate at which heartworms cause damage in the heart and lungs, so the more severe the signs the less activity the dog should have. A dog that feels better after its first injection and is allowed to run is the dog most likely to be harmed by its own treatment.

The manual is also explicit that the dogs treated most safely are the ones whose owners can actually deliver this. Its low-risk category requires a young dog with no clinical signs, normal chest radiographs, no worms seen on echocardiography, no concurrent disease, and owners capable of completely restricting exercise. That last clause is a household condition, not a medical one.

Four things structure a course of treatment for the owner.

1. Confirm the diagnosis before anything is given, because a second antigen test of a different type should be run for confirmation before adulticide therapy where the microfilaria test was negative.
2. Stabilise first, treat second, since in severe disease or with another serious condition the stabilisation stage alone can take several months.
3. Restrict exercise from the day of diagnosis and enforce it after the last injection, which is the single measure the manual calls crucial irrespective of protocol.
4. Retest on the schedule your vet sets, roughly nine months after an arsenical course is completed, or at six and twelve months on a non-arsenical one.

Melarsomine, and what a tropical country does when it is not available

The first-line adulticide is not available everywhere, and the tropics are where it is scarcest. The tropical council's 2023 review, co-authored from Madras Veterinary College in Chennai, states that melarsomine is not registered in most countries where heartworm disease is endemic, that even where it is registered there have been sporadic shortages, and that the consequence is the slow-kill protocol being the only available adulticide option in several tropical countries.

That review conveys which countries are registered through a map rather than a list, and this page is not going to read a registration status off a figure it cannot quote. So it states the position it can source and stops there. Ask your own vet whether melarsomine is obtainable where you are before assuming either way, because the answer changes what the next twelve months look like.

The alternative is well characterised and it is slower rather than weaker. The slow-kill protocol combines a four-week doxycycline course with a monthly macrocyclic lactone preventive, and a critical assessment concluded that topical moxidectin with doxycycline is an accessible alternative where melarsomine is unavailable or contraindicated. One experimental study of monthly topical moxidectin with doxycycline calculated 95.9 percent efficacy in eliminating adult heartworms after ten months.

Its drawbacks are about time and uncertainty rather than danger. Dogs are tested at six and twelve months, treatment success is presumed when both antigen and microfilaria tests are negative, doxycycline is repeated if the dog is still antigen positive at twelve months, and failure is considered only after twenty-four months. Clinical thromboembolism appears to be rare on this route, but there is insufficient data to recommend a specific duration of exercise restriction, so the general recommendation is that exercise be restricted as much as possible until no adult antigen is detected.

What a heartworm test and workup cost in India, from government rate cards

A microfilaria or heartworm test costs Rs 300 at the Veterinary Clinical Complex of Bihar Animal Sciences University in Patna, which is the cheapest published Indian figure this page could open. The same rate card prices a blood parasite examination at Rs 100, a canine X-ray at Rs 400 and an ECG or ultrasound at Rs 500 each.

That matters because heartworm is not diagnosed by one test. A complete first workup on a coughing dog is a consultation, a blood test, a chest radiograph and often an ultrasound, and the rate card prices every one of them.

Every rupee figure on this page was divided by 84.38 and by 74 before publication. Rs 300 is 3.56 dollars at the first rate and 4.05 at the second; Rs 400 is 4.74 and 5.41; Rs 600 is 7.11 and 8.11 Tamil Nadu Veterinary and Animal Sciences…. None of them lands on a round dollar figure, so these are rupee-native Indian government fees rather than a converted foreign price list.

The two universities disagree on the smear and the disagreement is published rather than averaged. Bihar prices a canine blood parasite examination at Rs 100 while Tamil Nadu prices a blood smear at Rs 200 for private samples, and TANUVAS prices no antigen test and no molecular test of any kind on that page. Ask your own clinic which of the two tests the quoted price covers, because the word used on a rate card does not tell you whether the antigen assay is included.

What prevention costs, and the one figure that failed the conversion test

One Indian retailer's listing gives a monthly cover price for a combination chew containing milbemycin oxime. A three-tablet pack of Nexgard Spectra for a 15 to 30 kg dog was listed at Rs 3,308 against an MRP of Rs 3,375 on 23 September 2026, which is about Rs 1,103 for each monthly dose. Rs 3,308 divides to 39.20 dollars at 84.38 and 44.70 at 74, so the selling price is rupee-native.

The MRP is the exception and it is reported rather than hidden. Rs 3,375 divided by 84.38 is exactly 40.00, which is close enough to a round dollar figure to flag, so this page publishes the selling price as the Indian figure and treats the printed MRP as unverified against a conversion. That listing does not mention heartworm anywhere in its text, which is why the indication below is sourced from a manufacturer instead.

For the indication, the manufacturer's own Indian page is the better source. Virbac India's Milpro page, for a tablet containing milbemycin oxime and praziquantel, states that it treats the main intestinal worms and also prevents heartworm on monthly use, and that it is usable in puppies from two weeks of age. That page prints no Indian price, and this page does not invent one.

Preventing heartworm disease in India, and why the year-round rule is an American rule

To prevent heartworm disease, give a monthly preventive on its label interval and test the dog before starting one, because a preventive given to a dog already carrying larvae can cause a severe reaction. The year-round twelve-month rule an Indian owner meets first is the American Heartworm Society's, it was written for the United States, and the honest Indian answer is that the interval is a regional decision your vet makes.

The American position is stated plainly on its own page. The society recommends that owners think twelve, meaning test every twelve months and give a preventive twelve months a year, and its supporting reasoning is that heartworm disease has been diagnosed in all fifty states and that risk factors are impossible to predict. The same page carries headings asking whether heartworms are more common in certain areas of the United States and what a reader in a northern state should do.

Two of its arguments transfer to India unchanged and one does not. The transferable ones are that infected mosquitoes come indoors so both indoor and outdoor pets are at risk, and that annual testing is necessary even for dogs on continuous prevention, because a single missed or late dose can leave a dog unprotected and preventives are highly effective rather than perfect. The argument that does not transfer is the fifty-state one, because the Indian data show four populations with zero heartworm in the same decade as an 18 percent one.

What the Indian evidence supports, region by region

The two Indian pages an owner meets first do not make a regional argument either. Virbac India's page, which ranked second for this query on 23 September 2026, names India once in its body copy, in a single sentence recommending year-round preventive medication because mosquitoes are a perennial menace across India, and prints no Indian figure to support it. VOSD's medication guide, ranking fourth, discusses whether doses can be skipped in cooler months and whether indoor dogs need cover, and mentions India only in its donation and about blocks rather than in the article.

The tropical council's framing is the one that fits India better than a national rule. It describes the tropics as presenting all the elements of a perfect storm, namely a suitable climate, high mosquito abundance, large stray or community dog populations as reservoirs, and low compliance with year-round preventives, and its reasoning is about local transmission rather than about a country border. On that reasoning the four factors are unevenly distributed across India, and so is the answer.

Read the following as the questions to take to your vet rather than as a schedule, because no Indian body publishes a regional prevention calendar and this page will not invent one.

• In the north-east, treat continuous prevention as the default, because 18.03 percent of 782 Guwahati and Aizawl dogs were antigen positive and even the pet subgroup ran at 9.02 percent.
• In Delhi, ask about both worms and about which assay is used, since both Dirofilaria species were found there and the authors specifically recommended a non-cross-reacting test for that city.
• In Mumbai, Kerala and Tamil Nadu, ask what the preventive is actually for, because heartworm was not detected in 162 Mumbai dogs, 164 Kerala dogs or 230 Tamil Nadu strays, while Dirofilaria repens reached 16.7 percent in the Mumbai dogs Ravindran R, Varghese S, Nair SN, Balan VM Manoj RRS, Iatta R, Latrofa MS, Capozzi L.
• Anywhere, ask whether the dog is tested before the first dose and then annually, which is the one piece of the American protocol that rests on parasite biology rather than on American geography.
• Reduce mosquito exposure everywhere as the second layer, since the vector genera named for Indian Dirofilaria are Culex, Aedes and Anopheles and all three are managed the same way as they are for human mosquito-borne disease.

The one Indian risk factor that has been measured with a denominator is where the dog spends its day. In Palakkad, 8 of 50 outdoor dogs were positive against 1 of 52 indoor dogs, 16 percent against 1.9 percent, at p equal to 0.021, and the authors attribute it to greater exposure to mosquito vectors. The same study found no significant association with age, sex, breed, fever, deworming or vaccination status.

The Australian study that tested breed directly reached the same conclusion from the other side of the world. In 104 Queensland pig-hunting dogs, seropositivity was not associated with sex or with type, with a p value of 0.6 for Bull Arab and crossbreds against others, while being over five years old raised the odds 3.73 times and a coastal address raised them 3.89 times. Where the dog lives and how long it has lived there beat what the dog is.

The Canary Islands case makes the same point with the highest breed figure in the literature, and the explanation is not genetic. Heartworm reached 43 percent in the autochthonous Canarian Warren Hound against 19 percent of the island's dogs overall, and the follow-up study attributes the persistent gap to lack of preventive chemotherapy and the prolonged exposure of that breed to mosquito bites. A working dog kept outdoors without cover is the risk factor; the breed is the label on it.

This page does not carry the parasite routine itself. The dog deworming schedule for India covers the intestinal worm calendar and the products sold here, and dog ticks in India covers the year-round ectoparasite routine, both with cited Indian prices.

Living with a dog that has, or may have, heartworm in India

A dog that has been treated for heartworm needs three things afterwards, namely a finished course, a retest on schedule and prevention that does not lapse again. Retesting roughly nine months after treatment is completed is what confirms the worms were eliminated, and prevention then runs for the rest of the dog's life.

The reassuring part is real and it comes from the American society itself. Most infected dogs can be successfully treated, and dogs with no signs or mild signs such as cough or exercise intolerance have a high success rate. The Indian north-eastern data adds the context that most infected dogs were never ill in the first place, with a 240-dog post-mortem survey finding no overt clinical signs in any positive animal before death.

The part that is not reassuring is re-exposure. Nothing about having been infected protects a dog from the next mosquito, adult worms live five to seven years so each season adds to the burden in an unprotected dog, and the American guidance is explicit that prevention continues for life after treatment. In a country where the vector genera are present all year in much of the south and west, a lapse is the ordinary failure mode.

A newly adopted street dog is the one case where the order of operations matters. The Indian evidence is consistent that stray dogs carry more than owned dogs, with 25.90 percent of 413 strays antigen positive against 9.02 percent of 266 pets in the north-east, and the systematic review's pooled estimate places stray dogs at 24.65 percent against working dogs at 10.3 percent Thilakarathne SS, Yuen NKY, Hassan MM. Test before the first preventive dose rather than after it.

There is one thing you should not do, and it is the one an owner is most tempted by. Buying a monthly preventive over the counter and starting it on an untested adult dog is the specific scenario the testing advice exists to prevent, because the reason for testing an adult dog before starting prevention is that the dog may already be infected. An Indian owner has easy access to these products and no easy access to the reason they are prescription-gated elsewhere.

What to ask your vet, in the order that makes the answers useful

Five questions turn a one-line lab report into something you can act on.

1. Ask which test was run, an antigen test, a Knott test or a species PCR, because the three gave 18.03, 11.38 and 13.93 percent on one set of 782 Indian dogs.
2. Ask whether the sample was heat treated, since heat treatment raises sensitivity but turned every sample from three Dirofilaria repens infected dogs false positive from day 164 Sobotyk C, Savadelis MD.
3. Ask whether the larvae were identified to species, because in Gujarat 23 of 33 microfilaraemic dogs carried a harmless worm rather than the heartworm.
4. Ask for a second, different assay before any adulticide is given, which is what the reference manual recommends when the microfilaria test is negative.
5. Ask what the dog's age is against the test's window, since an antigen test has no value before roughly seven months of age or within seven months of exposure.

This condition sits alongside the other vector-borne infections on this site and they are often confused with it. If a vet has said tick fever without naming the organism, start at tick fever in dogs in India, and read ehrlichiosis in dogs, babesiosis in dogs and hepatozoonosis in dogs for the three that ticks actually carry. Heartworm is the one in this group that a tick has nothing to do with, and the same Tamil Nadu panel that found four tick-borne pathogens in 67.8 percent of 230 dogs found filarial worms in under 1 percent.

One practical overlap is worth carrying across. An in-clinic 4Dx style antigen test covers heartworm antigen alongside tick-borne antibodies, which is convenient and also the source of a specific error, because the manual warns that heat treating such a kit destroys the antibodies and invalidates the tick-borne results on the same card. If your vet wants the heat-treated heartworm answer, the tick-borne part of that card has to be run again.

The last word belongs to the evidence gap rather than to the evidence. The systematic review of the region states that several regions of India and Sri Lanka have not yet been surveyed for dirofilariasis at all. A scoped PubMed search run for this page on 23 September 2026, through NCBI E-utilities, for Dirofilaria or heartworm with dog terms and India in the title or abstract returned 42 records, against 2,409 for the identical search with the India term removed in the same session, so the instrument was live and the Indian literature really is that small. Bengaluru, Hyderabad, Pune, Chennai and Ahmedabad have no published canine heartworm denominator that this page could open, so it prints none for them.