Our environment is changing and the average temperature has increased by 1⁰C on a global level. That one seemingly insignificant degree had profound effects on seasons and weather. Namely, the 1⁰C rise resulted in shorter and warmer winters, fewer cold days per year, decreased snow covers, earlier spring arrivals, more prolonged and hotter summers, accelerated sea level rises, and increased frequency of extreme weather events like floods, droughts, thunderstorms, tornadoes, and temperature extremes.
When talking about seasonal changes, the aforementioned consequences first come to mind. However, when reviewing the seasonal change consequences, there is more than what meets the eye – there are significant threats we commonly overlook. One of those threats is the increased presence and distribution of external parasites, which act as vectors for infectious pathogens.
How seasonal change affects vector parasites?
The seasonal change affects the vectors’ activity, the geographic spread, the transmission pathways, and the emergence and re-emergence of vector-borne diseases.
Seasonal change exerts its impact on multiple levels – it affects the infectious disease-causing pathogen, it affects the vector necessary for the pathogen’s transmission, and ultimately, it affects the definite host.
In general, seasonal change impacts vectors through two pattern alterations:
- Longer seasons – because of the more favourable weather conditions, vectors can become active earlier and remain active for longer than normally. For example, becoming active 15 days earlier than normally and remaining active for 15 days longer results in a 1-month longer season.
- Range extensions – once again, because of the more favourable conditions, vectors expand their geographical range. For example, a vector that was once considered exotic, such as the Tiger mosquito can now be found in the northern parts of Europe.
All in all, longer seasons plus range extensions equal increased density of vector populations.
Other factors affecting vector parasites
Human and animal travel
Up until the Covid-19 crisis, people traveled worldwide, and often in the company of their beloved pets. Travelling with a pet can contribute to the spread of vectors and pathogens.
What is more, pets can also travel alone – international purchases, adoptions, and animal welfare organizations contribute to the problem. Perhaps, the most crucial factor in pet travel is the increasing trend of adopting dogs and cats from abroad. From a disease spreading aspect, adult pets are potentially more dangerous than youngsters. Namely, adults are more likely to carry an infectious pathogen simply because they have been exposed for longer and therefore had more time to come into contact with a pathogen-carrying vector.
This trend, combined with the lack of regulatory oversight of pet travel, increases the risk of uncontrolled vectors and pathogens spread.
The “urban heat island effect”
The “urban heat island effect” manifests with slightly higher temperatures in urban areas than in rural areas. Therefore, specific vectors, such as for example the sandflies, become active earlier and remain active later into the year in urban areas.
The fact that in urban areas, the host populations are denser only adds to the problem – more hosts means more vectors.
The “naturban” environment
While well-developed, ultra-urban areas deal with the so-called “urban heat island effect”, in less developed regions, the distinction between urban and rural is blurring which creates an intermediate environment, popularly known as “naturban”.
These “naturban” environments are of paramount importance to vector and pathogen transmission because, in them, people and pets overlap with wild animals that act as reservoirs.
Are all factors seasonal change-related?
Factors like human and pet travel are not directly related to seasonal change. However, they enable vector distribution. Once the otherwise absent vector is introduced in the new area, its ability to establish itself, thrive, and eventually cause harm is definitely determined by seasonal change and its tendency to create vector-favorable weather conditions.
For example, let us assume that an infected vector is introduced to a new environment. If the weather conditions are not favorable, the infected vector will die before becoming capable of transmitting the infection.
On the other hand, if the conditions are favorable, the vector will thrive long enough to spread the disease, even if the pathogen is slowly developing and needs time to reach its infectious potential or even if it has to wait until finding a suitable host. If the conditions are favorable, the vector might even establish itself and become permanent in that area.
Basically, it all comes down to the weather conditions – if they are right, the vector will overcome all other obstacles. Seasonal change alters and molds entire ecosystems, eventually creating a habitat that supports the vectors’ establishment.
Frequent and emerging vectors
The milder autumns and winters paired with early spring arrival shifted the nature of tick seasonality. It is safe to say that there is no such thing as tick season, in many regions – they are present all year round.
The seasonal change impact on tick activity is more pronounced in colder regions. Usually, the frost arrival would coincide with the end of tick season. Now, frost arrives later and lasts shorter, which favors tick activity and promotes longer tick seasons.
There is blunting of seasonality in warmer regions, which promotes all year round tick activity. For example, in 2017, an experiment was conducted in Germany. Namely, citizens were encouraged to report tick findings, and unexpectedly, such reports arrived at all times, regardless of the season.
Ticks also mark an accented geographical range expansion. There are three critical aspects to tick migration:
Exotic tick species can now thrive further north than ever because now even the north offers moderate weather conditions suitable for tick activity.
Ticks spread vertically as well as horizontally. Today, you can find ticks in the Austrian Alps at altitudes over 2000 meters above sea level. It is believed that the vertical spread results from the decreased snow coverage and presence of heavier vegetation that ensures wider distribution of wild hosts and reservoirs like, for example, wild deer.
Migratory birds arrive earlier – following the early spring arrival. When birds migrate, they can introduce new vectors.
Sandflies are biting phlebotomines widely distributed throughout the Mediterranean region, where their activity is all year long. Recently, sandflies mark a northward spread. However, in the northern areas, their movement is seasonal.
In addition to spreading horizontally, sandflies are starting to extend vertically. Namely, now it is not rare for sandflies to be found on attitudes higher than 1000 meters above sea level. Plus, if the snow cover is not particularly deep, sandfly larvae can survive the winter in the soil.
Recently, a new and hazardous mosquito species migrated to Europe – it is the Asian Tiger Mosquito. This formerly exotic mosquito is now well-distributed not only in the southern parts of Europe but in certain northern parts like the Netherlands.
Many mosquito species can be found in northern Europe, but the warm months are not long enough, and the cold months are not warm enough to support pathogens to reach the infectious stage inside the mosquito. The pathogens they carry cannot reach their infectious stages in such conditions.
Another seasonal change supporting mosquitoes’ wider geographical distribution is the weather extreme – heavy rains. Heavy rains cause floods, and flooded areas are the perfect breeding environment for mosquitoes.
Fleas are less prominently affected by the seasonal change because they prefer and are well-adapted to living indoors where the conditions are always controlled and moderate regardless of the environmental conditions.
When a flea finds its host, it does not really have a reason to leave. It feeds whenever it wants and droops poop wherever the host spends time. The blood-containing fleas feaces is a great food source for the flea larvae.
In such perfect conditions, female fleas lay around 40 eggs per day. Over time, the eggs hatch and feed on the widely distributed poop. It is an enchanted and never-ending cycle.
Frequent and emerging vector-borne diseases
Leishmaniosis is an insect-borne disease caused by the protozoa Leishmania infantum and transmitted by phlebotomes or sandflies. Phlebotomes are nocturnal insects widely distributed in the Mediterranean region. However, some sandfly species can be found in northern France, southern Switzerland, and Germany.
Infected sandflies transmit the pathogen while taking blood meals in hosts – dogs, cats, other mammals and humans. Leishmaniasis is a potentially life-threatening disease that manifests with skin lesions, weight loss, nail hyperplasia, conjunctivitis, muscular atrophy and kidney problems.
Dirofilariosis is transmitted by mosquitoes and caused by the pathogenic worm Dirofilaria immitis and D. repens. The vector is distributed in southern parts of central Europe. It affects dogs, cats, and humans.
When infected mosquitoes feed, the worm’s larval stages enter the wound and invade the host’s organism. Inside the host, they develop into incredibly long worms that live in the right heart and pulmonary arteries, thus causing heart failure. In the case of D. repens, the worms live under the skin and form painful lumps.
Bartonellosis, is a zoonosis popularly known as “cat scratch disease,” is caused by the bacterium Bartonella henselae. Cats are the main reservoir, and cat fleas are the primary vector. Both the bacterium and the vector are distributed worldwide. Bartonellosis manifests mostly in immunosuppressed humans with fever, enlarged lymph nodes, and inflammations of the eyes, gingivae, and heart.
Babesiosis or piroplasmosis is caused by the protozoa Babesia canis and other Babesia species transmitted by hard ticks. The protozoa must spend some time inside the tick to achieve infectious potential, which means not all protozoa-containing ticks are infectious. Babesia carrying hard ticks such as Dermacentor reticulatus are distributed in western, southern, and central Europe up to the Baltic.
The protozoa invades the host’s red blood cells in which it multiplies. As a result of the protozoa’s lifestyle, the red blood cells rupture eventually causing anemia. Unless quickly and adequately treated, the disease is lethal for dogs.
Borreliosis or popularly known as Lyme disease is caused by the bacteria belonging to the Borrelia burgdorferi group. The bacteria from this group are transmitted by ticks. The vector ticks are distributed all over Europe except in extremely cold and extremely hot areas. Lyme disease seldomly affects dogs and cats, but for humans it can be a serious disease and it manifests with fever, rashes and muscle and joint issues.
Ehrlichiosis is caused by the bacteria Ehrlichia canis and transmitted by the tick Rhipicephalus sanguineus. The vector is present in southern and some parts of central Europe and it affects dogs and, sometimes, cats. Depending on the conditions, the bacteria can overwinter in infected ticks.
The bacteria lives in blood cells and causes significantly low blood cell numbers – low white, low red and low platelets.
Learning how to cohabitate
We must be proactive and take the necessary steps to deal with changing parasite risks. The process of learning how to coexist with these vectors is long and complex. It requires specifically crafted strategies and targeted approaches.
First, we need to raise awareness and enable free and easy access to accurate information. Basically we need adequate education. We must know everything about the enemy.
Secondly, pet owners must provide their pets with adequate protection. In the past, vets recommended using anti-parasite products starting from March up until October. Now, the vet needs to assess the pet’s parasitic risk and provide anti-parasite protection based on lifestyle risk.
When making sure our pets are adequately protected, we must debunk a popular myth – indoor pets do not require protection. This popular belief cannot be further from the truth. The strict indoors is not a parasite-free environment – mosquitoes and sandflies can reach the highest floors of skyscrapers. Ticks can hide in our clothes and emerge when a more appealing host comes into insight. Fleas can live in our beds, sofas and furniture without our knowledge.
The next step involves close collaboration with veterinarians, especially when it comes to accepting advice and choosing suitable protection.
The ultimate step would be to ensure governments apply strict regulations and oversights to pet travels.
Because of the many uncertainties and affecting factors, predicting how future seasonal changes will affect vectors and vector-borne disease is quite challenging. However, it is safe to say that future events are mainly determined by how aggressively we react.
Our strong reaction should basically consist of several vital aspects. First of all, we must slow down the progression of seasonal changes by protecting the environment. Secondly, we must control the vectors’ activity by being vigilant and protecting our pets all year round. Ultimately, we must raise awareness of the changes, what they entail, and prevent their impact. This last aspect will protect everything and everybody – most importantly our pets and us.
The mean global temperature is expected to increase by 4 to 5⁰C by the end of the century. The balance between the environment, vectors and hosts has never been so fragile. Therefore, in the face of ongoing changes, this is the time to act.