Research reinforces possible participation of domestic cats in the epidemiology of VL

Publicação: 10 de September de 2022

This was the first study to report experimental transmission of L. infantum from cats to another host, with the sandfly as vector

Leishmaniasis can be contracted by both dogs and cats, although the incidence in cats is lower

Caused by the protozoan Leishmania spp, visceral leishmaniasis (kala-azar, in humans) is a parasitic disease of protracted characteristic, which, when left untreated, leads to death. Contrary to what many imagine, domestic cats (Felis catus) can also be infected with Leishmania spp, although domestic dogs (Canis familiaris) are considered the main reservoir of the parasite. The first report of feline visceral leishmaniasis (FVL) was, as early as 1912, in Algeria, in a four-month-old animal, which lived with a dog and a child, with visceral leishmaniasis. The diagnosis was based on the finding of amastigotes of Leishmania spp. in the bone marrow of this cat.

Leishmaniasis in cats, zoonosis caused by protozoan L. infantum and transmitted by the sandflies (Lutzomyia longipalpis), has been reported in several countries. In Brazil, the first case of domestic cat infected by L. infantum was registered by the Federal University of Minas Gerais (UFMG). The animal, an eight-year-old male, castrated, presented as clinical signs weakness, lymphadenopathy, onychogryphosis, cachexia, fatigue, anorexia and a wound near the left pinna. Recently, the Zoonosis Control Center of Ribeirão Pires, metropolitan region of São Paulo, carried out a mapping (epidemiological survey) to diagnose cases of the disease. Since the first report of FLV, more than 40 clinical cases have been reported in Europe, including Portugal, Italy, France, Spain, Greece, as well as Israel and Palestine, Egypt, Switzerland, Iran, French Guiana, Venezuela and Brazil, the latter with areas endemic to Leishmania spp. Studies in Italy and Syria have shown, through DNA typing, identical parasites in the blood of cats and in the intestines of sandflies, which were naturally infected when carrying out blood meal in cats, proving the transmissibility of the parasite. This evidence of proven transmissibility of feline parasites to a vector suggests that cats may play the role of domestic reservoirs of the disease rather than simply an accidental host.

However, transmission of FVL to another host had not yet been demonstrated. To try to clarify the subject, a study entitled Transmission of Leishmania infantum from cats to dogs, verified the possibility of transmission of L. infantum from cats to dogs by xenodiagnosis in a cat with the disease, using 55 females of Lu. longipalpis. DNA sequencing of the parasite isolated from the cat confirmed infection by L. infantum and showed 99% similarity to DNA sequences of L. infantum from dogs. From this study, it was possible to confirm the experimental transmission of L. infantum from a domestic cat to a domestic dog by its biological vector, Lu. longipalpis. This was the first study to report experimental transmission of L. infantum of cats to another host having the sandfly as a vector and with it was possible to demonstrate that Lu. longipalpis feeds on cats, which is infected with L. infantum while feeding on infected, domestic cats and transmits L. infantum to the dog.

Asked why cats seem more attractive to Lu. longipalpis, Dr. Ivete Lopes de Mendonça, from the Department of Clinic and Veterinary Surgery of the Federal University of Piauí (UFPI), one of the authors of the study, explains that in the xenodiagnosis performed on cats, sandflies had a very fast attraction and became engorged with blood in approximately 25 minutes. “To get an idea, a serological survey carried out in Teresina (PI) in 2018 showed that 5% of cats would be infected, a rate close to canine infection. This means that both cats and dogs can act as reservoirs of the disease,” she says. The researcher believes that cats are very attractive in nature, perhaps even more so than dogs, due to the observation that in xenodiagnosis they quickly attracted insects. “Cats really are candidates to be urban reservoirs of VL, just like dogs, and this has to be studied more broadly under penalty of the control program, focused only on dogs, is doomed not to work,” she points out.

The parasites were observed in the skin and lymph nodes of the dog of the experiment, but not in the bone marrow, spleen and liver, perhaps it can be answered due to the pattern of the efficient immune response developed by the animal, which may have controlled the infection in these organs. It is possible that in the cat, the L. infantum has more predilection for the skin. “Each animal has its immune response. I don’t think it’s possible for cats to have the same response as dogs. In these, the response of T lymphocytes is the one that exerts the greatest influence on the infection. As Leishmania is an obligate intracellular parasite, host defenses depend on the activity of these cells, which are reduced during infection. However, there is intense proliferation of B lymphocytes and the production of antibodies is immense, but not protective,” explains Dr. Mendonça. According to her, it is also necessary to consider the possibility that only a small number of parasites has been transmitted to the dog, delaying the dissemination in sufficient quantities to be detected in the methods used. “Further studies are needed to better understand behavior of L. infantum in dogs, which have different immune response profiles when infected with parasite originated from cat,” she adds.

Debatable form of vector control x human cruelty against  animals

No risk to humans from living with an animal infected with L.  infantum have been demonstrated in the absence of transmitting insects, although direct transmission has been demonstrated in people who use intravenous drugs. This is because visceral leishmaniasis, as well as dengue, requires the presence of the vector, thus being a vector-borne disease. Therefore, the fact that the animal is in the same environment as humans does not indicate risk of the disease in the absence of the disease transmitter. The main focus of disease prevention and control should be vector control. However, the debatable Brazilian norms for the elimination of dogs with the disease cause strangeness when compared to those of other countries. Although the care of infected dogs is common in Europe, where there are efficient treatments, in Brazil, the understanding is that the use of drugs for humans with VL is considered a risk. Of the 88 nations in the world where the disease is endemic, Brazil is the only one that uses the sacrifice of dogs as a public health instrument.

The question now is whether cats play any important role in the transmission of VL? What if the measures that apply to dogs should be extended to cats as well? Dr. Carlos Henrique Nery Costa, coordinator of the Leishmaniasis Laboratory of the Federal University of Piauí (UFPI), a reference for the world on the subject, recalls that the fight against the vector is used only as a complement to the strategy of eliminating dogs. “Several studies show that sacrificing animals is not efficient. Sacrificing dogs or cats does not and will not reduce transmission to humans, as the degree of competence to infect the vector is a less important parameter. The most important is always the vectorial capacity, which involves more complex parameters, and the fight against vectors must be part of the main strategies for controlling the disease,” he justifies. In the Indian Subcontinent, for example, which does not have VL in animals, it adopts as one of its main strategies the use of dichlorodiphenyltrichloroethane (DDT) and other insecticides for control. “There the disease is ending, there was a reduction of more than 90% of cases with the use of various public health measures and, perhaps the disease can be eradicated in a few years. This demonstrates that, in fact, the fight against the vector is more efficient than the sacrifice of animals. Unfortunately, the culture of sacrificing animals still persists in Brazil,” concludes Dr. Costa.

Dr. Mendonça agrees. “Animal and environmental management are fundamental to interrupt the chain of transmission. It is essential to educate the population about the responsible possession of the animal, proper destination of the garbage, use of leashes impregnated with insecticides, use of vaccine,” she signs. Regarding the development of a specific vaccine for cats, the researcher does not see this possibility in the short term, since studies need to be carried out to clarify the immunological aspects of feline leishmaniasis. However, according to her, vaccines that exist for dogs may be tested on cats to assess the impact on transmission to humans.

In Brazil, there is little government mobilization around kala-azar, which despite killing more people than dengue, remains with little political vision and interest on the part of the administrators of power, making this tropical disease remain with little visibility, which translates into lack of investments for control, knowledge and research, as well as lack of investments for drugs capable of treating it. Perhaps this is because kala-azar affects the poorest and most neglected portion of the population.