While completely expected given the steady march of New World screwworm (NWS) from Central America into Texas, the recent finding of an infected dog in New Mexico has focused more attention on this parasitic infestation. I wrote a recent Clinician’s Brief article about it if vets that have access are looking for broader details, but I’ll focus on the drug aspect here…what drugs do we have for prevention or treatment of New World screwworm? I’ll focus just on drugs here. A core component of treatment is removal of larvae, but antiparasite drugs are used concurrent with that, and for prevention.

For an emerging/re-emerging disease, we actually have quite few options. Data supporting them is variable and is a combination of field studies and basic information about the drugs. In the US, the FDA has given conditional approval or Emergency Use Authorization (EUA) to a few drugs. Beyond that, there are some others that should work. (Lack of conditional authorization or EUA may be more a reflection of some companies not bothering to pursue the process vs some drugs not being effective).

The main drug class is isoxazolines. This includes lotilaner, afoxalaner, fluralaner and sarolaner. This drug class has proven efficacy against NWS and some products have approval in the US (more on that below).  Lotilaner’s approval is probably based in large part on a small study where naturally infested dogs were given a single dose, and all larvae died within 24 hours. Afoxolaner and sarolaner have also been shown to have 100% field efficacy in small studies.

In terms of specific products for dogs and cats…
Conditional FDA Approval for treatment

Credelio Quattro CA-1 (lotilaner, moxidectin, praziquantel), dogs

Emergency Use Authorization for treatment

Credelio (lotilaner), dogs and cats

NexGard (afoxolaner), dogs

NexGard COMBO (esafoxolaner, eprinomectin, praziquantel): cats

Other

Nitenpyram had 100% efficacy in a small study of naturally infected dogs within 24 hours, using normal doses. (Larvicidal efficacy of nitenpyram on the treatment of myiasis caused by Cochliomyia hominivorax )

Spinosad might be less effective but still potentially useful. One study reported 80% efficacy in dogs.( Efficacy of spinosad on the treatment of myiasis caused by Cochliomyia hominivorax () I’d be wary comparing drugs and studies since they are small and heterogenous, but wouldn’t use spinosad as a first line option if other drugs were avaiable since I’d see no advantage and there’s the potential it’s less effective.

Fluralaner (Bravecto) has Conditional Approval for prevention and treatment in cattle. It’s reasonable to assume similar efficacy in dogs and cats, where it’s used at similar doses.

Macrocyclic lactones are common parasite prevention drugs in dogs and cats. Doramectin and conditional approval for prevention and treatment in cattle, while ivermectin has EUA for prevention in cattle. However, it is critical to note that the doses of these drugs that are used for heartworm prevention in dogs and cats are very low (e.g.4-6 micrograms/kg vs 200 micrograms/kg They are not comparable to what is used for NWS. Therefore, while ivermectin, moxidectin, selamectin and milbemycin might be useful at high doses, we cannot assume that dogs/cats getting products like Heartgard, Inteceptor, Sentinel, Milbemax, Proheart and Advantage Multi for routine parasite prevention have any protection against NWS.

Imidacloprid (in Advantage II, Advantage Multi, Seresto collars, K9 Advantix II) is also unlikely to have any reasonable effect based on its mechanism and the low drug levels in tissue (vs skin, where’s best effective).

None have approval for prevention. That’s likely a labelling/regulatory issue rather than a concern about efficacy. Treatment data are easier to get. You take some infested animals, treat them, and see what happens. Prevention is tougher, especially in field situations. You’d take a bunch of animals and put them on a preventive, and compare the infestation rate to untreated controls. However, for a rare disease, you’d need a massive number of dogs or cats to show efficacy. You could also use an experimental model whereby animals are given preventives or placebo and then experimentally exposed to flies. Those are a lot of work and a lot of money, and there’s less appetite for using animal infection models when we don’t need to. Here, given what we know about the parasite, the drugs and the disease, I think it’s fair to assume that a drug that works for treatment will be useful for prevention. 100% protection? That’s hard to say so I’d never completely guarantee protection, but I’d be quite confident in this group of drugs.

The bad news for the southern US

  • NWS is here and will probably be here for a while as eradication efforts will take time.

The good news

  • compared to many other emerging or re-emerging diseases, we have effective treatments already in place.

We don’t have any approved products in Canada for treatment or prevention of NWS, and I suspect there won’t be much push by companies to get authorization since there’s no threat of it becoming an endemic issue here. However, the drugs and parasite are the same as elsewhere, so we can approach management of travelling or imported infected dogs as per what’s done in other countries. Treatments are extra-label but that’s true about a lot of what we do.