Dogs as a reservoir
Canine leishmaniosis (CanL) is caused by the protozoan Leishmania infantum. Dogs are the most important reservoir of the pathogen and are mainly responsible for the persistence of CanL and also human visceral leishmaniosis in the Paleartic and Neotropical regions. Other hosts have been found naturally infected (foxes, cats, rodents, horses), or serologically positive, but it is unlikely that they can act as effective reservoirs. The dog is such an excellent reservoir for L. infantum for three reasons:
- a very long pre-patent period of infection
- a high concentration of protozoan amastigotes in the skin
- a high percentage of relapses together with uncertain parasitological sterilization after treatment
More than 50% of dogs with proven established infections are apparently healthy on clinical diagnosis (asymptomatic). They are either progressing towards overt disease (pre-patent cases), remaining symptomless for prolonged periods (even for life), or exhibit spontaneous healing; the latter groups are regarded as resistant.
The long pre-patent period of infection produces a high percentage of asymptomatic dogs; however, they are able to infect the sand fly vector. Furthermore, depending on the type of cellular immune response, dogs may not show any clinical signs or develop only cutaneous nodules at the site of inoculation. Nevertheless, the ability to infect sand flies has been observed to be similar in both asymptomatic carriers and animals with different degrees of disease, although it was shown that the rate of infected sand flies increased with the appearance and severity of the clinical signs. About half of the seropositive dogs identified in field studies are asymptomatic. In a conclusion, apart from untreated diseased dogs and free-ranging ones, asymptomatic dogs are one of the most important reservoirs of CanL: they are able to infect sand flies and they might remain asymptomatic for years and therefore may not be recognized by pet owners and veterinarians (if diagnostic tests fail).
Generally it can be distinguished between stable and unstable endemic conditions of CanL. Stable CanL is viewed as an endemic situation in which dog-to-dog transmission of Leishmania and so the appearance of new cases invariably occurs every season.
In Europe stable transmission is usually from May-June to September-October and associated with optimal ecological conditions:
- rural and peri-urban environment;
- hilly territory, commonly with southerly exposure;
- altitude from 0 to 6-900 m a.s.l.;
- lack of extreme conditions (strong winds, absence of vegetation etc.).
Unstable is defined as an endemic condition in which autochthonous cases, usually in low numbers, occur sporadically or periodically, but not every year. It is generally found at the border of a stable endemic distribution. For prevalence studies serology is supposed to remain the method of choice for large-scale screening of dogs. Using this method not all infections are detected so that seroprevalence studies only represent a fraction of all infected animals. Prevalence rates are not uniform in any of the territories investigated. Large variations have been observed in dog populations a few kilometres apart, a situation that is presumed to be caused by strict ecological requirements for Leishmania transmission.
Infections in non-endemic countries
Numerous case reports and personal communications about Leishmania infected dogs exist in North America and Northern Europe. In Germany, a total of 20,000 infected dogs is calculated by third party organizations. These are mainly imported animals, but also a lot of dogs travelling with their owners to endemic areas.
Autochthonous cases of CanL in non-endemic countries have been described in Switzerland, Germany, England, Belgium, Netherlands and in the USA. Cases with no travel history or any source of infection and vector identification in the USA have been reported from the following states: Texas, Ohio, Oklahoma and Maryland.
In the USA the emergence of visceral leishmaniosis in a relatively large and widespread population of foxhounds was reported during the last 28 years. Even though sand fly species are present in most of the respective areas, vector transmission has not been demonstrated and is furthermore not supported by the fact that solely foxhounds in distinct kennels are infected. Starting with single foxhounds in 1980, nowadays several thousand dogs in foxhound hunt clubs in some 21 US states and two Canadian provinces have been detected to be infected. In some of the respective areas transmitting vectors are even lacking. Non-vectorial transmission (see below) and a possible unique susceptibility of foxhounds and/or subtle behavior that maximizes the possibility of exchange of blood and secretions with other foxhounds are suggested. Other potential sources of transmission as congenital transmission and direct transmission by contact or reuse of needles have also been considered. Nevertheless the epidemiology of canine visceral leishmaniosis in the USA remains unclear. Besides the foxhounds, sporadic cases in dogs other than foxhounds have been reported in the USA.
Slappendel and Teske (1999) judge the uptake of amastigotes by an intact gastrointestinal tract as questionable, but presume that it seems quite possible that macrophages may pick up ingested amastigotes via a mucosal lesion or a bite wound. They propose that transmission from dog to dog in the absence of sand flies is not merely a theoretical possibility and may occur more often than recognized in endemic regions, but the possibility of humans being directly infected by dogs is according to them virtually negligible, provided immunological resistance is normal. They advise contact of infected dogs with babies, toddlers, and HIV or otherwise immunodeficient people to be avoided.
From mother to offspring
Transmission of infection from a bitch to a puppy has been described or presumed. In man, diaplacental transmission in a child from Sudan, respectively intrauterine transmission has been reported as well.
Via infected blood
Transmission via blood transfusion has been reported. Furthermore transmission via contaminated needles between drug addicts has been observed.
Via other body secretions
In canine semen and urine as well as in human urine samples leishmanian parasites have been isolated. In human saliva and urine of patients with the Indian type of visceral leishmaniosis (kala azar), amastigotes of Leishmania have been detected.
Via other insects
Mechanical contamination by biting flies has been documented, but there is no evidence for or against a spreading of the disease through this type of transmission. Ticks have been suspected of transmitting leishmaniosis but, though their gastrointestinal tract may harbor infectious promastigotes, their role in transmitting the disease is believed doubtful due to their life cycle.
- Slappendel, R.J., and E. Teske in: Killick-Kendrick, R. (ed.): Canine leishmaniasis: an update. Proc. Int. Can. Leishm. Forum, Barcelona, Spain, 1999, Intervet Int., Boxmeer, The Netherlands, 1999, 54-59