‘Transmission Tracker – Dirofilaria’– a public dashboard to assess in real‐time the temperature‐bounded transmissibility of canine heartworm across Australia

Abstract

The causative agent of canine heartworm disease, Dirofilaria immitis, requires specific temperature conditions to mature within its mosquito vector, and therefore (re‐)infect a canid host. Suitable temperature conditions are not continuously met for locations where most (>97%) Australians and their pet dogs live. The length of the disruption in the transmissibility of D. immitis varies greatly across Australia, and to some degree, between years. We developed an online dashboard ‘Transmission Tracker – Dirofilaria’ that processes near real‐time temperature records across Australia and allows users to enquire about historical and current weather suitability for canine heartworm transmission at any Australian postcode of their interest. This information allows veterinarians to access when, and for how long, heartworm may be transmitted at a specific location, assess the associated risk of infection and advise on a patient‐dependent dirofilariosis prevention plan for their canine patients and guardians. Our dashboard is publicly accessible at: https://heartworm-mapping.adelaide.edu.au/shiny/.

Canine heartworm disease, or dirofilariosis, is caused by infection with Dirofilaria immitis and ranges from mild exercise intolerance to right‐sided congestive heart failure which can be fatal, although subclinical infections are most common. ¹ The severity of dirofilariosis is related to adult worm burden, ² but also host‐specific factors such as exercise requirements and size, as well as chronicity of infection and the associated structural changes to host tissues where adult worms live. ³

Macrocyclic lactones (MLs) are the only licensed veterinary drug class used to prevent canine heartworm disease. ⁴ MLs are primary administered to kill acquired D. immitis L3 and L4 larvae before they develop into pathogenic adult worms, thereby preventing potential clinical disease. ⁵ It takes approximately 6 months from the time of inoculation of the dog for the larvae to develop into adults, although the larvae only remain susceptible to MLs for approximately 2 months (depending on the ML formulation and dose used). ⁶ MLs can be administered either as a periodic preparation (orally or topically) labelled for monthly administration, or an injectable formulation that progressively releases ML for up to 12 months. ⁴ Whilst MLs have been described to have some long‐term adulticidal properties, they are not currently labelled to treat dirofilariosis. ⁷ Current dirofilariosis prevention guidelines in Australia, released in 2014, recommend year‐round prevention across Australia, regardless of location. ⁸

Transmission of D. immitis between dog hosts requires passage through a mosquito vector, ³ and the two most common mosquito species involved in Australia are the common backyard or urban mosquito, Ochlerotatus notoscriptus, and the more rural banded mosquito, Culex annulirostris. ⁹ Intrinsic replication of adult worms in the dog is not possible, so any (re‐)infections require the bite of an infective mosquito. ³ Although the presence of mosquitoes is often thought as the primary necessary factor for D. immitis infection, the proximity to a reservoir (infected canids) and sustained warm weather for the parasite to develop within the mosquito vector are equally important to enable the parasite to circulate. ¹⁰ The rate of development from the first larval stage (L1) to the third (infective) stage (L3) within the mosquito vector is bounded by the ambient temperature, with a minimum development threshold of 14°C. ¹¹ For development to complete (and transmission to be possible), the larvae must accumulate at least 130 degree‐days above 14°C. ¹¹ For instance, on a day with an average temperature of 27°C (over a 24‐h period), D. immitis larvae would experience 13 degree‐days, and would therefore require nine additional days with the same ambient temperature for the L1s to mature into L2s and then L3s. The time available for degree‐days to accumulate is restricted by the mosquito's lifespan which unlikely exceeds 30 days for the longest‐lived mosquito species of concern. ¹² Just as infections cannot occur in the absence of the mosquito vector, they also cannot occur if these temperature conditions are not met, and therefore the administration of MLs is not necessary to prevent dirofilariosis.

A retrospective analysis of the last decade of temperature records reported that the weather could enable D. immitis larval development (and therefore transmission) all year round in the northern most part of Australia, covering 17.0% of the territory and 2.7% of the population. ¹³ Indeed, dirofilariosis is more commonly reported in northern Australia. ¹ For the rest of the country, the parasite's lifecycle and transmissibility are disrupted for part of the year, and the length of this disruption is highly variable both according to the location and year. According to the duration of the disruption at a given location, practitioners may want to adjust their infection risk assessment and customise their advice to dog guardians on a dirofilariosis prevention plan, after taking into consideration their patient's clinical circumstances and the guardian's values and preferences. The previously reported mapping did not allow veterinarians to observe location‐specific transmissibility data, indicating only whether transmission of D. immitis was disrupted, or not, at a location throughout the year. ¹³ We have therefore designed an online dashboard called ‘Transmission Tracker – Dirofilaria’ to help veterinarians assess the seasonality of D. immitis transmissibility at their practice location, or patients’ place of residence or travel (within Australia). The dashboard is publicly accessible at: https://heartworm-mapping.adelaide.edu.au/shiny/.

‘Transmission Tracker – Dirofilaria’ dashboard functionality.

The primary function of the dashboard is for the user to enter an Australian postcode of their choice and access a calendar plot classifying the D. immitis transmissibility status of each day since 1 January 2013 (Figure 1). The dashboard is updated daily to display records up to 6 days prior to the dashboard access date.

Figure 1.

‘My Location’ functionality of the ‘Transmission Tracker – Dirofilaria’ dashboard. Calendar plots accessed on 12 August 2024, for the user‐selected postcode 5371 (The University of Adelaide School of Animal and Veterinary Sciences, South Australia) (A), 7000 (Hobart, Tasmania) (B) and 4670 (Bundaberg, Queensland) (C). The years from 2013 are shown on the vertical axis, and calendar days shown on the horizontal axis. Days when transmission was possible are shown in red, and days when transmission was unlikely are shown in blue.

To illustrate the functionality of our dashboard, we use by default the postcode corresponding to The University of Adelaide's School of Animal and Veterinary Sciences (5371 – Roseworthy, South Australia) (Figure 1A). In 2023, the calendar plot indicated D. immitis transmissibility was disrupted for almost 7 months (from 17 April to 11 November). We can also appreciate how variable the length of this period is by comparing with previous years’ disruption (the length of the blue band). Although, the cut‐off dates may move between years, the duration of the disruption was stable over the last decade at Roseworthy. This may help a user to anticipate what may happen in a current year. For instance, we cannot yet know when transmission may become possible in this postcode in 2024, however, in the previous 11 years, transmissibility resumed between 15 October (in 2014) and 4 December (in 2022). This would prompt users to monitor transmissibility from October to identify the date when the transmissibility status changes. This seasonality contrasts with other postcodes in Australia, for instance with Hobart (postcode 7000, Tasmania; Figure 1B) or Bundaberg (postcode 4670, Queensland; Figure 1C).

In Hobart, there was a substantially longer disruption of transmissibility, even extending to the full calendar year in 2015. For the other years, transmission became possible only during summer and early autumn, ranging between 5 days (in 2021) to 10 weeks (in 2013). Both, Roseworthy and Hobart, were previously reported as belonging to the same area of Australia where D. immitis transmissibility is supported for less than half of the year. ¹³ However, a veterinarian may assess the risk of D. immitis infection and plan its prevention very differently at each of those locations.

On the other end of the spectrum, Bundaberg was previously reported as belonging to the region of Australia where D. immitis transmissibility is supported for more than half of the year. ¹³ At this location, the disruption was short, and in both 2017 and 2023, the transmissibility was not disrupted at all. Otherwise, some disruption did occur during July and August, ranging from 5 days (in 2021) to 10 weeks (in 2022). The risk assessment of a Bundaberg veterinarian should be less likely dictated by the weather conditions compared to the other necessary factors for D. immitis infection (i.e. mosquito activity and prevalence of an infective reservoir).

Our dashboard provides an indication of when the temperature conditions are most likely disabling the transmissibility of D. immitis for the 83% of the Australian territory where transmission is not possible year‐round. Considering this new information, veterinarians may decide to divert from the 2014 national dirofilariosis prevention guidelines that recommended year‐round ML preventative everywhere in Australia and, instead, consider alternative strategies adapted to the temperature‐Abounded risk of infection at their patient's place of residence or travel. For instance, they may recommend an interrupted ML plan, in which MLs are used only during times when transmission is possible. In this instance, a veterinarian need not predict exactly the date when transmission becomes possible after a period of disruption. Utilising the monthly labelling of MLs, they simply need to recommend guardians to commence ML usage within 1 month when transmission becomes possible. ¹⁴ Ultimately, however, the decision to administer any medication, or not, lies with a pet guardian (primarily relying on their veterinarian's recommendations to make an informed decision). All health recommendations require nuanced conversations with stakeholders, considering factors beyond the perceived and real risk of infection and/or disease, such as the effectiveness, cost, safety and convenience of an intervention, guardian/user compliance and chemical stewardship.

Despite interruptions to transmissibility across Australia, it is still unclear how long this disruption needs to be before endemicity of D. immitis is impeded. One recent report found no evidence of endemicity of D. immitis in South Australia, ¹⁵ and further investigation into this is necessary. Therefore, this dashboard provides evidence to improve the assessment of a patient's risk of D. immitis infection when making an evidence‐based, patient‐specific dirofilariosis preventative plan. A review of the current dirofilariosis prevention guidelines to reflect on the temperature‐bounded transmissibility of the parasite in Australia is warranted.

Conflicts of interest and sources of funding

The authors declare no conflicts of interest for the work presented here.

Atkinson, PJ. , Stevenson, M. , O'Handley, R. , Nielsen, T. and Caraguel, C. , ‘Transmission Tracker – Dirofilaria’– a public dashboard to assess in real‐time the temperature‐bounded transmissibility of canine heartworm across Australia. Aust Vet J. 2024;102:626–629. 10.1111/avj.13379

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.