And no wonder; they’ve been desperately wanting a dog for a while now. So why not take this one home with them from their vacation in Spain?

There are number of good reasons. Professor Georg von Samson-Himmelstjerna, a veterinarian and the director of the Institute of Parasitology and Tropical Veterinary Medicine at Freie Universität Berlin, strongly urges vacationers to leave strays where they find them. In many southern European countries that attract vacationers from more northerly climes – including Italy, Spain, and Greece – stray animals are infected with parasites that are not yet “at home” in Germany: worms that infect the skin, heartworm, or Leishmania

The latter group is made up of single-celled organisms that can live in various parts of the body, including the blood. “One of the most important ways of preventing the spread of these pathogens – some of which can also spread to humans – is by making sure we don’t import infected animals,” von Samson-Himmelstjerna warns.

Unlike worms of the intestinal tract, infections with the skin worm Dirofilaria repens are not an issue of lack of hygiene, for example. Instead, they are the result of tourism and climate change. These parasites, which have been widespread in Southern Europe for some time now, are not transmitted directly, through contact with animal feces, for example, but rather by an intermediate host, or “vector”: mosquitoes. When a mosquito bites an infected animal, it ingests the tiny worm larvae with the animal’s blood. These larvae continue their development inside the mosquito, reaching the third larval stage. The next time the mosquito bites, they are transmitted to another animal.

“For a long time, it was thought that these parasites didn’t stand a chance of survival north of the Alps, since they need a certain temperature to be able to continue their development in the intermediate host,” von Samson-Himmelstjerna explains. But studies performed in recent years, including some in which veterinary researchers from Freie Universität participated, have shown that there are now indeed enough hot summer days in Germany for one, two, or even three generations of larvae to mature in their mosquito hosts.

There is also a risk of further spread of Leishmania infections. These single-celled life forms, which are only about ten micrometers across, are known as protozoa. They are transmitted by tiny sand flies or drain flies. The flies stow away, probably in trucks from southern regions, and now that summers are heating up, they can increasingly survive in Germany. Leishmaniasis is a serious chronic infection in dogs and humans alike. Symptoms include inflammations of the skin and joints, and later also changes in internal organs.

The fact that the mosquito bite was not a “normal” one, but might have carried parasites, isn’t immediately obvious. “The site of the bite gets infected, and this infection comes with discoloration and becomes chronic,” von Samson-Himmelstjerna says. A test for antibodies in the blood shows whether someone is infected. Leishmaniasis can be cured in humans. Infected dogs can be treated as well, but only for the clinical symptoms – the pathogens remain inside the animal’s body. As a result, they need lifelong treatment, which is very costly, and there is always a high risk of relapse.

Von Samson-Himmelstjerna and his colleagues aim to raise awareness of these new parasitic diseases through education. They expect the diseases to continue to spread. Prevention in the form of mosquito control helps reduce the risk of infection. Dogs can also be immunized against Leishmania in areas where the pathogens are found, providing them with some protection against the disease.

Reliable protection from transmission is not available for either humans or animals, however. Since the intermediate hosts are increasingly able to survive in Germany, the number of primary hosts must be kept down. As von Samson-Himmelstjerna says, “It’s crucial to make sure there isn’t a large enough pool of infected dogs in this country. Otherwise, we run the risk that skin worms and heartworm, and possibly also Leishmania, will take up residence here in the long term.”

The United States offers a cautionary example of what happens when these efforts fail. The heartworm, Dirofilaria immitis, was initially found in only a few isolated cases in Florida. It has now spread to all of North America in just 50 years. Dirofilaria worms have traced a similar path in Ukraine and other countries of the former USSR in recent decades. More than 4,000 human infections with these worms have been documented there by now.

In light of the situation, it was no wonder that in late March, when about 500 experts came together in Berlin for the 28^th annual meeting of the German Society for Parasitology, the areas of focus included spreading parasitic diseases in animals that are transmitted by vectors.

Dirofilaria repens, which infects the skin, grows to a length of ten to 15 centimeters. It forms nodules under the skin and can move as much as 20 centimeters in the subcutaneous tissue on a single day, causing severe itching. Hardly a pleasant notion, but at least this worm is relatively benign and can be removed surgically. In rare cases, the worm can also infect the chambers of the eye, internal organs such as the lungs, or, in women, the breast tissue. There have already been several documented infections in dogs in Germany. The first human infection occurred in 2013, when a worm was removed from a nodule in the skin of the patient’s cheek.

Infection with the heartworm, Dirofilaria immitis, is much more serious; it affects the internal organs of dogs and cats. Full-grown specimens are 20 to 30 centimeters long. They form a mass like a ball of spaghetti, blocking the artery that runs from the heart to the lungs, which causes serious cardiac problems. There are over 400 known cases in which humans have been infected with heartworm, most of them in the United States.

To prevent the larvae of these worms from developing into fully grown parasites inside the animals’ bodies, treatment with a low dose of anthelmintic, or anti-worm, drugs during the first four weeks following infection is highly effective, since the parasite develops very slowly. These medications are not effective when the worm reaches later stages, however, which is why prophylactic monthly treatment for worms is advised for pets worldwide in areas where Dirofilaria immitis is endemic.

“It’s best not to take animals along to areas where Leishmania or heartworm is found, for example. Pet owners who take their animals with them on vacation anyway should definitely consult a veterinarian and get as much protection for their animals as possible,” von Samson-Himmelstjerna says.

He advises travelers from Germany who are planning to visit southern Europe with their pets to visit the website of an association of European veterinary parasitologists (www.esccap.de) beforehand and take a “travel test” using the interactive map of Europe. It shows what health risks pets face, and what kinds of protective measures are needed.

Those who, in spite of all the warnings, simply cannot resist bringing a dog home with them from another country or adopting one from a local animal welfare organization there can find a checklist for dogs from other countries on the website. In Berlin alone, incidentally, there are hundreds of dogs and cats waiting for a new home – and they are definitely free of skin worms, heartworm, and Leishmania.