What is a Reptile?

In comparative medicine, we often lack information in a given species, including which infectious agents are present and their clinical significance. When information is lacking in a species, the best model to use is typically the closest relative from which data are available. This requires knowledge of species relationships; many commonly used terms, such as reptile or lizard, may lead to erroneous understanding of relationships.

A monophyletic group is defined as a group that contains all descendants of a common ancestor. In comparative medicine, understanding what constitutes a monophyletic group is needed to understand relationships and choose appropriate models. Primates constitute a monophyletic group; when the primates, except for humans, are referred to, the term nonhuman primates is generally used. This qualification in the term helps the reader understand that a group is being referred to, with the exception of one member.

When the evolution of the terrestrial vertebrates (tetrapods) is examined, the fossil record and far more complete and convincing nucleic acid sequence phylogeny analyses are in agreement on relationships.⁹ The earliest divergence among the tetrapods is between the amphibians and amniotes, who are fully terrestrial (Fig. 33-1). The amniotes are then further divided into mammals and sauropsids. Within the sauropsids, the next group to diverge is the squamate (lizards and snakes)–sphenodontid (tuatara) clade. Following this, the Testudines (turtles) diverged, and the last two major sauropsid groups to diverge were the crocodilians and dinosaurs. The only surviving group of dinosaurs is the birds. This is supported by the fossil record and by sequence data.²³ Although birds themselves constitute a monophyletic group, if birds are not considered as part of the reptiles, then reptiles are not a monophyletic group. What we really mean by reptiles is sauropsids, and birds are a group of reptiles. Crocodilians and birds share a number of medically relevant similarities, including a four-chambered heart with the right aortic arch being the major outflow for oxygenated blood, and lungs with unidirectional rather than tidal air flow.⁵ However, the significant differences between birds and crocodilians are also obvious and the clinician needs to be careful not to overextrapolate. The obvious differences between a caiman and sparrow underscore the more cryptic but greater differences between a caiman and monitor lizard.

Figure 33-1 Phylogenetic tree of the tetrapods. Branch lengths are not to scale.

Another common error is the idea of lizards as a group distinct from snakes. In squamate evolution, the earliest divergence is the geckos, followed by the divergence of the skinks, night lizards, plated lizards, and girdled lizards (Fig. 33-2). The next groups to branch off were the teiids, lacertids, and amphisbaenids and the remaining group, containing snakes, iguanids, agamids, chameleons, monitors, helodermatids, and anguids, is known collectively as the Toxicofera, named for the commonality of the presence of venom glands.⁶ Snakes diverge in the middle of the squamates and, if snakes are removed, then lizards are not a monophyletic group. Snakes are a group of lizards and a ball python is a better model for an iguana than a leopard gecko.

Figure 33-2 Phylogenetic tree of the squamates. Branch lengths are not to scale.

Novel Viruses

Because reptile virology is still a nascent field, it is not uncommon to find previously unknown viruses. Assessing risk and responding appropriately to the identification of a novel virus requires understanding of context, viral ecology, and evolution. In an ideal situation, we would have information including the results of experimental infection with the virus in a set of animals from the species of concern. This is rarely the case. However, even if it is, it is important to recognize that Koch’s postulates represent a concept that should not be overinterpreted. No infectious agent is either a pathogen or not a pathogen. There have been many asymptomatic human Ebola virus infections, and people have died of septicemia caused by Lactobacillus acidophilus.^4,14 It is important to correlate infection with clinical patient evaluation and histology before infectious disease is considered as a significant differential. All clinical infectious diseases have multiple contributing factors, including those from the infectious agent, the host, and the environment.

With respect to virus factors affecting infectious disease, even with a novel agent, the biology of a given virus may be used to make clinical predictions. A virus with a lipid envelope will be less stable in the environment than a nonenveloped virus and disinfection protocols should be adjusted accordingly.

Another important viral factor with clinical implications is genome organization; some viruses have segmented genomes. Throughout biology, hybridization is a factor allowing rapid nondetrimental change, allowing species to invade novel habitats.²⁰ This is considered the main reason for the evolution of sex. Segmentation of viruses, allowing reassortment, is essentially the viral equivalent of sex, providing a hybrid advantage for crossing host species. This has been best studied in the Orthomyxoviridae.

Large biologic differences of significant clinical importance are seen between DNA viruses and RNA viruses. DNA viruses generally replicate more accurately and have proofreading systems for error correction. This enables them to be larger and more complex, and to have a more intricate relationship with the host. A delicate balance such as chronic or latent infection is more commonly seen with DNA viruses. This balance is generally specific to an endemic host and, although infection may be possible in aberrant hosts, aberrant host infections lack balance and are generally rapid in course. This may be associated with significant pathology. As examples, human herpesvirus 1 and Macacine herpesvirus 1 often cause relatively benign cold sore lesions or be asymptomatic in their endemic hosts, which are humans and rhesus macaques, respectively. Infections in aberrant hosts, such as human herpesvirus 1 in a white-handed gibbon or Macacine herpesvirus 1 in a human, result in overwhelming viral encephalitis that is usually fatal and does not generally result in successful further viral transmission. It may actually be advantageous for endemic host populations to be infected with some of these agents, because disease may eliminate competitors and predators, and not the endemic host. DNA viruses do not tend to evolve as rapidly as RNA viruses and are less likely to switch hosts successfully.

Because of lack of proofreading by their polymerases and constant selection by the host immune system, RNA viruses have the fastest mutating genomes found in nature, and tracking evolution of these viruses is highly clinically significant. One recent meta-analysis has found that of the 20 virus families infecting humans, four RNA virus families—Reoviridae, Bunyaviridae, Flaviviridae, and Togaviridae—accounted for more than 50% of emerging and reemerging viruses.³¹ Emerging disease is also frequently associated with host switches. The human disease meta-analysis found that 816 of 1407 (58%) are zoonotic and, of human diseases, zoonotic diseases are significantly more likely to be emerging. The simpler nature of smaller RNA viruses and their ability to change rapidly makes them more adaptable to new hosts. This ability to move successfully and rapidly to new hosts removes the selective pressure not to harm the host, and RNA viruses generally tend to result in more explosive epidemiology.

There is generally little that a clinician may do about host factors when dealing with novel viruses, other than to advise avoiding mixing species and resultant aberrant host infections. The areas that the clinician can affect most are environmental factors. Many factors, including but not limited to coinfections, stress, nutritional status, temperature, reproductive status, age, and noninfectious disease processes, all play significant roles in viral disease. Of these, temperature is the area that differs most in nonavian reptiles from what is seen in mammals. Poikilothermy provides the patient with the ability to tolerate a wider range of body temperatures; viruses often tolerate a narrower temperature range, or at least disease may only manifest in a narrower temperature range. Patients with a wide thermal gradient may choose a febrile or other thermal response.¹⁷ Providing appropriate thermal opportunities is often a highly effective therapy.

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