Epidemiology of zoonotic tick-borne diseases in Latin America: Are we just seeing the tip of the iceberg?

Over the past decades there have been significant achievements in the understanding of tick borne diseases (TBDs), which are mostly zoonoses and classed as neglected diseases^1–5. Their occurrence is significant in tropical and subtropical areas, leading to an important impact on public health as well as the economy, as they affect humans, domestic animals and livestock, among others⁶. Knowledge of the occurrence of these diseases in animal species is of utmost importance for the understanding of the risk for human infection⁷. Ticks, and animals, including human beings, interact with nature, and their environmental and ecological interactions regulate the populations of ticks and vertebrates, determining their contact rates and the circulation of the diseases⁸. Moreover, although most TBDs are tick-borne, other ways of transmission can occur. For instance, bovine anaplasmosis, caused by Anaplasma marginale, can be spread through mechanical means, e.g. biting arthropods or by contaminated fomites, like needles, ear tagging, dehorning and castration equipment⁹. A. marginale can be also transmitted transplacentally, which has contributed to the occurrence of bovine anaplasmosis in some areas¹⁰.

In Latin America, there is a lack of studies about TBDs. However, data from Panama, Brazil, Mexico, Peru, Colombia, and Venezuela clearly show that these pathogens are prevalent when they are assessed. Recently, two lethal cases of rickettsiosis caused by Rickettsia rickettsii were reported in rural and urban of Panama, probably transmitted by the tick Rhipicephalus sanguineus s.l., accounting for the first molecular detection of this bacteria in this tick in Panama and Central America¹¹. TBDs caused by rickettsial species are life-threatening infections that in the tropical Americas have an emerging and reemerging trend. Until some years ago, R. rickettsia was the only tick-borne species of rickettsia present in Latin America. Nowadays, multiple other species, such as R. parkeri and R. massiliae, are causing infections in humans in this region. In Peru, different ectoparasite samples have tested positive for a Rickettsia genus-specific qPCR, with strong evidence that active searching of TBDs etiological agents is required in order to improve reporting and detection¹². Additional, new species are being reported; although their pathogenicity has not been definitely confirmed, they should be considered as potential pathogens¹³.

Ehrlichiosis is another TBD caused by rickettsial organisms of the genus Ehrlichia^14,15. Canine monocytic ehrlichiosis caused by Ehrlichia canis apparently is highly endemic in Brazil and has been also detected in Mexico, where it is closely related to strains from the USA¹⁴. In Brazil, E. canis is the principal Ehrlichia specie found in canines and has been also detected in felines, although the prevalence has not been estimated yet. In addition, in Brazil, E. ewingii has been recently detected, as well as E. chaffeensis in marsh deers, and there is immune-epidemiological evidence that suggests the occurrence of ehrlichiosis in humans^16,17, but its etiologic agent has not yet been established. Improved molecular diagnostic resources for laboratory testing will allow better identification and characterization of ehrlichial organisms associated with human ehrlichiosis in Brazil¹⁶.

In Colombia, three outbreaks of human rickettsiosis have been reported in the Northwestern region during 2006–2008, with a lethality up to 54%¹⁸. And later, in 2010–2011 a cross-sectional study revealed the presence of three different Rickettsia species: R. felis in fleas, and R. bellii and Rickettsia sp. strain Atlantic rainforest, both in Amblyomma ovale ticks¹⁸. Additionally, in Venezuela, detection of Anaplasma platys has been described in humans. Those patients were bitten by Rhipicephalus sanguineus and suffered chronic non-specific clinical signs, including headaches and muscle pains, supporting A. platys as a zoonotic tick-borne pathogen¹⁹. Other studies from Venezuela have found a high proportion of positivity of antibodies against Babesia caballi and Theileria equi in horses and other animals²⁰, suggesting that probably the real frequency and importance of these hemoparasites are overlooked.

Babesiosis is caused by any of a group of vector-borne, protozoal hemoparasites of the phylum Apicomplexa. There are more than 110 described Babesia spp. worldwide, identified from mammalian and also avian hosts^21,22. This group of TBDs is transmitted by ixodid ticks and they infect a wide variety of vertebrates that maintain transmission cycles. Till today, there are multiple species of the genus Babesia that can infect people, and the regional distribution of Babesia microti is the most prevalent. In the USA, babesiosis is caused primarily by B. microti, whereas cases in European countries are commonly caused by B. divergens²². In countries such as the USA its incidence has increased 260% between 2005–2010²³, with a proportion of 40% of cases of Lyme disease reporting co-infections with Babesia. In Latin American countries, particularly Colombia, the seroprevalence of Babesia has been reported has high as 30% from people of urban and rural locations²⁴. Additionally, recently, new Borrelia burgdorferi sensu lato strains or new related species have been described in countries such as Uruguay, Brazil and Chile²⁵.

In the USA, some authors suggest that approximately 95% of ~50 thousand cases of locally acquired vector-borne diseases in humans reported annually to the Centers for Disease Control and Prevention are caused by organisms that were vectorized by ticks (Table 1)^6,26. Beyond the Americas, in other regions of the world, like in Europe, ticks are the main vectors of animal and human organisms. Ticks transmit several viral agents, called tick-borne viruses (TBV), such as tick-borne encephalitis virus and Crimean-Congo hemorrhagic fever virus, which have reemerged in multiple areas of the world²⁷. TBV have a natural cycle between ticks and wild animals in nature, with humans as accidental hosts^27,28. Emerging TBVs are continually discovered, probably related to the increase of tick populations in different regions of the planet and invasion of human beings into areas infested by ticks^27,28.

Detection and sentinel surveillance of TBD require molecular tools for diagnosis²⁹, for example, serological tests have proven to be inconclusive in diagnose Lyme disease³⁰. The use of molecular biology tests in recent years has increased the sensitivity and specificity of the diagnosis of infections in the group of Rickettsiales. Molecular diagnosis enables the accurate identification not only at genus level, but species, providing additional characterization on the epidemiology and the evolution of clinical disease. Furthermore, PCR as well enzyme restriction tests of the vector blood meal can be employed to analyze their feeding source and possibly identify the ecological reservoir of the organisms³¹. Etiological agents of the group of Rickettsial, including those in the genuses Anaplasma, Neorickettsia, Ehrlichia, and Rickettsia, are relevant and often vector-borne organisms of canines and felines, but also of bovine, livestock and other animals, which appears to be a wide range of hosts^10,32.

Conclusions

Besides the limited number of studies in Latin America on TBDs, the prevalence of these diseases is increasing, triggered by globalization, as well the impact climate change and variability. Tick and TBDs investigators, vet doctors, medical and public health practitioners should work to share their expertise on different aspects of TBDs, such as tick ecology, disease transmission, diagnostics, and treatment, in order to face the challenges of scientific, political, and public engagement for TBD research and control in this region³³. Systematic reviews as well as observational analyses are necessary in order to understand the current situation of TBDs. Molecular tools can provide valuable information for understanding the evolution of their etiological agents, as well as provide insights into host-pathogen-vector-environment interactions. Probably, what we have seen till now is just the tip of an iceberg and there is a need for more studies in Latin America about TBDs.

Data availability

No data is associated with this article.